WATER ENFILTRATION INTO SONLS Thesis for the Dam” of M. S. MICHEGAN STATE UNIVERSETY Ramadhar Singh 1961 LIBRARY Michigan State University ABSTRACT WATER INFILTRATION INTO SOILS by Ramadhar Singh The great importance of surface-water management and conservation is evident because of widespread use of the water for many purposes. water is not only necessary for human and animal consumption but it is equally important for plant growth and for industrial purposes also. In spite of all its importance, it is unfortunate that this natural resource is not equally distributed all over the world throughout the year. Hence, proper management is necessary for both the arid and humid regions of the world to make the best use of water and at the same time to save our precious soils from going into the sea. With this in view this study about the water infiltration into the soil, which is essentially the entry of water from the soil—surface into the soil, has been done. It is believed here that by prOper manip- ulation of water infiltration into soils, most of the problems of water nmnagement and conservation can be overcome. But for proper manipulation, it is necessary to know the various factors of this phase of soil-water. It is an example of the phenomena of water movement in porous media. water may move in the larger pores through the action 0f gravity or in the smaller pores through the action of capillary and gravitional forces. The various methods used for the measurement of infiltration- Capacity of soils are: 1. Artificial rainfall device. (a) Sprinkling device to produce artificial rainfall- (b) Artificial rainfall applied by Drip Screen and Drip Towers. R) 4:" U0 Ramadhar Singh Cylinder Infiltrometers. Basin Method. watershed Hydrograph Method. Soil Cores. The various factors which modify the rate of infiltration into soils are: Soil Cover and Vegetation. Physiographic factors. (a) SlOpe and Surface roughness, (b) Altitude. Soil Characteristics. (a) Porosity, (b) Temperature, (c) Structure, (d) Colloidal Content of the soil. (I) Organic Matter, (II) Exchangeable bases, (III) Inwash of Silt. (e) Initial Soil Moisture and Duration of Wetting of the Soil. (f) Soil air. (g) Sub-Soil. (h) Visible Holes. (I) Root Holes, (II) Animal and Microbial activities, (III) Shrinkage Cracks. Climatological factors. (a) Rainfall intensity and raindrop characteristics, (b) Temperature, (0) Season. water Characteristics. Time water is on the surface. Cultural Practices. Ramadhar Singh (a) Tillage, (b) Use of Fertilizers and Manures, (c) CrOpping Patterns, etc. Hydraulic engineers View the increase in rate of infiltration as a loss. Agriculturalists promote an increase in the rate of infiltration as a conservation measure. The various techniques which can be used for this purpose are: 1. Breaking of Surface Crusts. 2. Sub-Soiling or Deep Ploughing. 3. Detention of running water by- a. Using vegetation, b. Incorporating Organic Matter, c. Contour- cultivation, d. Strip cropping, and e. Terracing, etc. WATER INFILTRATION INTO SOILS By Ramadhar Singh A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Resource Development 1961 ACKNOWLEDGMENTS The author wishes to express his sincere gratitude and apprec- iation to Dr. Clifford R. Humphrys for his guidance and inspiration throughout the course of this study. Sincere thanks are also extended to Dr. Raleigh Barlowe, Dr. M. H. Steinmuellen Mr. Paul J. Schneider, and Dr. William J. Kimball of the Department of Resource Development, Dr. H. M. Singh of the Department of Soil Science, Mr. B. K. Mukherjee of the Department of Farm Crops, and fellow graduate students for their help and inspira- tion. Last, but certainly not least, the author is indebted to his parents, his wife, his father-in-law and other family members for their continual moral and financial support, without which this study would never have been completed. ii TABLE OF CONTENTS ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . LIST OF II-I‘IJS 'I'IDATI OBIS O O O O O O O O O O O O O O 0 CHAPTER I . . . . . . . . . . . . . . . . . . l. 2. Introduction . . . . . . . . . . . . . . . . Definition of Terms . . . . . . . . . . . . . CMR II 0 O O O O I O I O O I O O O O O O O O O 1. Review of the Literature . . . . . . . . . CHAPTER III . . . . . . . . . . . . . . . . . . . . 1. Theoretical Considerations . . . . . . . . . 1. Movement in Saturated Soils . . . . . . . 2. Movement in Unsaturated Soils . . . . . . Instrumentation for Infiltration Measurements Artificial Rainfall Method . . . . . . . Cylinder Infiltrometers . . . . . . . . . Basin Method . . . . . . . watershed Hydrograph Method . . . . . . . Soil Cores . . . . . . . . . . . . . . \I‘l-F'UOIUH CHAPTER IV . . . . . . . . . . . . . . . . . . . . 1. Factors Affecting Infiltration Capacity . . Soil Cover and Vegetation . . . . . . . . Physiographic Factors . . . . . . . . . . Soil Characteristics . . . . . . . . . . Climatological Factors . . . . . . . . . water Characteristics . . . . . . . . . Time water is on the Surface . . . . . . Cultural Practices . . . . . . . . . -\]O\\n.F"U)l\)l—‘ iii Page ii Page CWR V O O O O O O O O O O O O O O O O O 0 O O O O O O O O O O 5 8 1. Possible Interest in Increasing Infiltration Capacity of Soils . . . . . . . . . . . . . . . . . . . . . . . . . . 58 1. Breaking of Surface Crusts . . . . . . . . . . . . . . 59 2. subsoiling . . . . . . . . . . . . . . . . . . . . . . 59 3. Detention of Running Water . . . . . . . . . . . . . . 60 2. Summary and Conclusions . . . . . . . . . . . . . . . . . . 62 BIBLImRAPI-H O O 0 O O O C O O O O O O O O O O O O O O O O O O O O 70 1. Literature Cited . . . . . . . . . . . . . . . . . . . . . 7O 2. Other References . . . . . . . . . . . . . . . . . . . . . 76 iv Figure 1. Diagram showing the movement of water due to gravitational forces . . . . . . . . . . . . . . 2. Soil water distribution. . . . . . . . . . . . . . . . . 3. .Artificial rainmaking device . . . . . . . . . . . . . . A. Pearse Square-Foot Apparatus . . . . . . . . . . . . . . . Double-ring infiltrometer . . . . . . . . . . . . . . . 6. Basin method . . . . . . . . . . . . . . . . . . . . . 7. Effect of removing surface protection on rate of intake of water by soil . . . . . . . . . . . . . . . . . . . 8. Protection of soil surface from the beating affect of raindrOps on flat surface . . . . . . . . . . . . . . 9. Effect of raindrOps on steep SlOpe . . . . . . . . . . . 10. The relation of permeability to porosity . . . . . . . . ll. Infiltration capacity in surface inches in Clarion loam 12. The effect of the initial soil moisture content on the rate of infiltration during first ten minutes of rain. 13. Rate of infiltration with the increase in temperature. 1h. Seasonal variation of maximum, minimum, and average apparent infiltration capacities, North Concho Drainage; Basin, Texas . . . . . . . . . . . . . . . . . . . . 15. Comparative infiltration with clear and turbid water- LIST OF ILLUSTRATIONS Ruston sandy loam . . . . . . . . . . . . . . . . . Page It 18 2O 23 25 26 33 31+ 35 37 Al ”5 51 52 51+ CHAPTER I 1. INTRODUCTION The economics of soil conservation are, perhaps, more con- fusing than of any other human activity. The classical concepts of production, demand and prices of individual intrepreneurs in a society wedded to individual enterprise have already been compli- cated by problems of controlled money and the discovery of the overwhelming need for employment. Agriculture is the only way of living open to many people of the world and, by no means a compet- itive industry. But over a great part of the land surface of the earth, water limits plant growth either because there is too much or too little of it in the soil. Hence, careful water management is essential to a stable and efficient agriculture, and efforts by a number of agencies are being directed toward water management and conservation activities such as irrigation, drainage, flood pre- vention, and erosion control. The realistic approach toward these water management and conservation activities requires accurate in- formation on the rate at which different soils will take in.water under different conditions. Conservation of water is possible by improving the rate of entry of rain or irrigation water into the soil and retaining it for a relatively longer period of time by improving the physical conditions of the soil. Increasing the 2 rate of entry of water or what is called the rate of infiltration, is a primary necessity in water conservation. Infiltration is often viewed by the hydraulic engineers as a loss and by the agriculturists as a gain. It is the process which provides water for nearly all terrestrial plants and for much of the animal life; it furnishes the ground water for wells and most of the stream flow in periods of fair weather; it reduces floods and soil erosion. Infiltration is, therefore, a process of vital economic importance. The subject of infiltration is very important as far as its importance in soil and water conservation work is concerned, but it is very complicated as far as the study of its various phases is concerned. Although any single phase of infiltration can be a good subject for study, here attempts have been made toward an all round study of the subject in question. 2. DEFINITION OF TERMS Infiltration A drop of rain may (a) fall on a water surface, (b) be inter- cepted by vegetation, or (c) fall directly to the soil. A particle of water reaching the soil may (a) be returned directly to the at- mosphere by evapo-transpiration, (b) flow overland toward a stream, or (c) absorbed by the earth and become subterranean water. The absorption of water into the earth is called infiltration. It may also be defined as the movement of water from the surface of the ground into the soil. The role of infiltration in the hydrologic cycle was first discussed by Horton in 1933 (25). As conceived by Horton, infil- tration is the passage of water through the soil surface into the soil. .As defined by the section on Hydrology, American GeOphys. Union-Trans., 193A (60) infiltration is "the process by which liquid water enters the zone of aeration. It includes the formation and upbuilding of films around soil grains through wetting by rainfall, melting snow or temporary street water and the subsequent progressive downward movement of mobile water through the soil by film-flow after the demand for pellicular water has been satisfied". Infiltration capacity or rate According to the Soil Science Society of America, subcommittee on permeability and infiltration (58), the term infiltration capacity of a soil is the same as the infiltration rate. Various investigators (28), (70), (58) have reported that some confusion exists in the use and definition of the term infiltration rate. In order to meet on a common ground in this matter, this thesis will follow the concepts of Richards (58) and his definition of this term. Richards states that infiltration rate is the maximum rate at which a soil, in a given condition at a given time can absorb rain. It may also be defined as the maximum rate at which a soil will absorb water impounded on the surface at a shallow depth when ade- quate precautions are taken regarding border or fringe effects. Quantitatively, infiltration rate is defined as the volume of water passing into the soil per unit of area per unit of time. It has the dimensions of velocity. Initial dry infiltration The term initial dry infiltration is the maximum infiltration capacity of a soil at the start of the dry run. Initial wet infiltration is the maximum infiltration capacity of a soil at the start of a wet run. Percolation Percolation is the movement of water within the soil. It may also be defined as the movement of moisture through saturated soils due to gravity, hydrostatic pressure, or both. Although the phenomena of infiltration and percolation are not alike, they are closely related to each other as the infiltration cannot continue unimpeded unless percolation provides sufficient space in the surface layer for infiltration water. Permeability In specific application to soil moisture movement, permeability is the hydraulic conductivity of saturated soil. Qualitatively, per- meability is the capacity of porous medium to transmit water under pressure. Quantitatively, it is the specific property designating the rate or readiness with which a porous medium transmits fluids under standard conditions. Porosity Porosity is the property of the soil to contain pores. It is the percentage of the total volume of soil which is not occupied by solid materials. CHAPTER II 1. REVIEW OF THE LITERATURE The investigation of the processes involved in the infil- tration of water into the soil is not new. Its investigation started as early as 1898 when King (39) made comprehensive studies on the flow of air and water through a column of soil. .Again in 1911 Green and Ampt (22) made a study on the effect of capillary pull on the downward movement of a moisture front in a column of soil. King (32) further stated that rainfall can enter the soil only as fast as the escape or displacement of an equal volume of soil air. Many other investigators have worked on effect of'trapped air on the rate of infiltration. IFree and Palmer (20) attempted to correlate the inter-relationships between air movement, pore size, and infil— tration so that the results could be applied to field soil under natural rainfall. When they maintained a constant head of water on sands of different texture, they found that in open columns infil- tration decreased with depth and time. Infiltration proceeded slowly in the closed columns at first and only increased when entrapped air had sufficient pressure to lift a layer of saturated sand and escape through the top of the column. This was observed to be true only for the columns which contained the finer sand size. They felt that re- tarded infiltration was due to the small capillaries which were blocked by compressed air and a saturated layer at the tOp of the soil column which resisted normal air passage. No definite conclu- sions were drawn but tendencies were noted. They further concluded that in undisturbed soils, infiltration was dependent upon the inde- pendent interaction of cracks, worm and root holes, degree of aggre- gation, degree of shrinkage, swelling, number of pores, size and distribution of pores and other factors found in a normal soil. King (32) and Slitcher (61), as well as Mavis and Wilsey (38), have shown the effect of grain size, and particle arrangement on the permeability of sands. Muskat (uh) has dealt in some detail with the factors that govern the permeability of various natural media, showing the complexities of this problem in contrast to the more simple one found in studying the permeability of graded sands of known physical properties. The role of infiltration in the hydrologic cycle was pointed out by Horton (25). He stressed the importance of infiltration to rainfall - runoff data and that for a given soil, the infiltration rate varied between the maximum value when the soil was dry and a min- imum value after wetting and packing. Through a series of papers, (26), (27), (28), Horton suggested that soil structure, texture, pro- file, initial moisture content, temperature, porosity and vegetal cover, are main factors which influence infiltration rate. He further stated that infiltration capacity was usually less than transmission capacity because of the related effects of packing and plugging by rainfall on the surface of a soil mass. He believed that infiltration rate is governed mainly by conditions at or near the soil surface. European investigations of the relation of soil structure to water movement in soils as measured by noncapillary porosity and in- filtration are reviewed by Sokolovsky (6A) and Williams (66). In general, the data shows that treatments increasing the number of large stable aggregates increase the noncapillary porosity and the infiltration rates. Middleton (39) found the dispersion ratio and erosion ratio to be among the best indices of the erosional behavior of soils. These determinations are measures of the ease with which a soil goes into suspension to be carried away in the runoff water or to clog the pores and reduce infiltration as shown by Lowdermilk (35) and other workers. Musgrave (Al), in making infiltration capacity determinations of soils in the field, employed single unbuffered steel rings long enough to reach the "B" or imperVious horizon as an infiltrometer, and one thousand cubic centimeter burrettes for the maintenance of water heads. With similar apparatus ZMerman. (70) found that on Duffield silt loam in four stages of erosion (slight, moderate, severe, and a virgin soil) infiltration rates were unexpectedly high for the moderately eroded soil with lower infiltration rates for the vergin soil next in order. Zwerman argued that this unexpected order was due to the impedence of water flow by entrapped air. The air was said to be trapped between particles of 2mm and less. An aggregate analysis of the moderately eroded and virgin soils showed that parti- cles greater than 2mm in size were about the same for both. Since the infiltration cylinders were jacked down to the "B" horizon and the moderately eroded soil had more favorable conditions for air perme- ability as well as water permeability, it showed a much higher infil- tration capacity over a seven and one-half hours run. zwerman felt that Musgrave's method gave much higher infiltration values than would be found in natural and artificial rainmaker type infiltro- meters. Lewis and Powers (3%) listed porosity, chiefly noncapillary, texture, structure of the soil, cracks, worm holes, animal burrows, shrinkage, root channels, temperature, colloidal content, organic matter, PH, slope, etc., as the factors affecting the infiltration capacity of soils. They further discussed the moisture pressure gradients of various kinds that were found to affect the infiltration capacity. .As a unit volume of water entered the soil, conditioned by its physical prOperties, it became subject to the force of capillary pull or particle field forces. In an evaluation of surface factors affecting the rate of in- take of water by soils, Duley (l3) Observed that cultivated and bare soils had a high rate of water runoff and soil erosion. He stated that the rapid reduction in the rate of intake by cultivated soils, as rainfall on the surface, was accompanied by the formation of a thin, compact layer at the soil surface and that it was able to pass through this layer only very slowly. Duley postulated that this thin, compact surface layer was partly due to the beating effect of rain drops and partly due to runoff. Duley and Russell (15) studied the effect of vegetation on in- filtration. They found that due to presence of cr0p residues on the soil surface - (l) Infiltration was greatly increased. (2) 'Water and wind erosion was reduced. Lutz and Leamer (37) evaluated the effect of texture and swelling on the pore size distribution and permeability of a series of sand separates and on several subsoils. Soil permeability in relation to noncapillary porosity has been investigated by Baver (2). He concluded that soil permeability is some function of the noncapillary porosity at the flex-point in the soil moisture-tension curve. Also, Nelson and Baver (#6) evaluated the movement of water through soils in relation to the nature of the pores. By making studies of soil structure and its corresponding perme- ability characteristics, O'Neal (#8) and Upland and O'Neal (66) con- cluded that structure was probably the most important factor in eval- uating permeability. They further suggested that other character- istics of the structural aggregation and their relation to one another must be considered. Nelson and.Muckenhirn (A7) in determining field percolation rates on Wisconsin soils obtained data which agreed with those cited by Zwerman and Musgrave. They found large variations in the amount of water taken in to soil columns enclosed by the long steel cylinder type of infiltrometers. They felt this was due to the disturbance of the soil by the cylinders. They decided that this method was unsat- isfactoryand settled on the type of apparatus used by Katchinskii (31) and Kohnke (33) as initially designated by Nestrov which con- sisted of two concentric steel squares, driven to a shallow depth in the surface soil. 10 The importance of chemical factors related to infiltration rates of soils was suggested by Pillsbury and Richards (56). They conducted a study of the effects of different amounts of ammonium sulphate and organic matter added to the soil on its infiltration rate. They found infiltration rates to increase progressively as the amount of surface organic matter increased. This was observed in combination with both urea and ammonium sulphate. However, the increase in infiltration rate was less with urea. Jehnson (30) conducted field eXperiments to test the effec- tiveness of chlorination of the applied water in preventing the de- cline in infiltration rate associated with microbial activities. He observed that the decline in rate was not eliminated, but as the microbial population in the soil was reduced by chlorination, the infiltration rate leveled off at a higher than normal rate. He further observed that when chlorination was stopped, numbers of micro- organisms increased rapidly and the infiltration rate dropped sharply. Hopp and Slater (23) studied the influence of earthworms on infiltration rates. They found that earthworms increased infiltra- tion rates by a factor of four, on light fine-textured soils. Moore (#0) investigated the effect of temperature on infil- tration rate in soils and found that as temperature increased between 5°C and 30°C there was a corresponding increase in infiltration rate. There was a rapid increase in infiltration rate from 30°C to 35°C after which a rapid decrease occurred. Duley and Domingo (1%) studied the effect of water temperature on infiltration rate. They found that under natural rainfall condition, any change in infiltration rate, due to change in water temperature, would be too slight to have any practical significance. 11 By making Pedological studies in relation to infiltration phenomena, Smith (63) emphasized the importance of soil genesis and morphology. Tindall (65) observed that the lower the initial soil moisture, the higher the infiltration rate and vice versa. Bertoni et_§l (A) observed that final infiltration rates varied with the season of the year. Higher infiltration rates during the summer than during the cooler seasons of the year are in agreement with observations of other workers (5, 6, 2h). Much work has been done in determining infiltration capacity of soils primarily from the standpoint of the soil conservationist and the irrigation engineers. Their interest lies in the answer to the question, "How much rain or sprinkler irrigation can be applied to a given area or soil type and for how long a period before the soil becomes saturated and precipitation or irrigation rates exceed infil- tration.capacities?" The apparatus generally used by these workers has been the F or FA infiltrometer, of the rainmaker type. This equipment is expensive, has a high Operating cost and is cumbersome. CHAPTER III 1. THEORETICAL CONSIDERATIONS Infiltration is an example of the phenomena of water move- ment in porous media. The porous flow problem can be discussed from two points of view according to the dominance of moving force. - (1) That which moves in the larger pores primarily through the action of gravity, or movement in a saturated soil. (2) That which moves primarily through the action of capillary plus gravitational forces, or movement in an unsaturated soil. 1. Movement in Saturated Soils The media in which all the pores are completely filled with one homogenous liquid is called a saturated media. Although it is difficult to remove all the air and water vapor from a soil column, even under controlled laboratory conditions, in this discussion soil is supposed to be fully saturated. For such cases Darcy's law (t) which states that "the volume A of water passing downward through a sand filter bed in unit time is (*) The original equation given by Darcy was - . Q = KS (H + e), where Q is the volume of water passing downward e through the filter in unit time, S is the area, e is the thickness of the sand layer, H is the height of water on tOp of the sand layer, and "K un coefficient dependant da la nature due sable" i.e., a coefficient depending on the nature of the sand. l3 prOportional to the thickness of the bed.", can be applied. According to this terminology - V=K§ 1 Where - V is the flow in cubic centimeters per second. is the length of the column in centimeters. is the difference in hydraulic head in centimeters. is the coefficient of permeability or permeability constant. NEED-4 Many investigators have found that the size, density of packing, and hydration of soil particles greatly influence permeability or water entry into the soil. Hence, the downward movement of water into the soil takes place through different zones that have varying physical character- istics. The porosity and permeability of each zone may also vary greatly. If a soil is saturated with water, the rate of downward movement is restricted by the permeability of the least pervious zone. In brief, it can be stated that the downward movement of water by gravitational forces in natural soils, is related to the amount, size and continuity of the larger pores as determined by soil structure, texture, volume changes, and biological channels (root channels, worm holes, etc.), to the hydration of the pores, and to the resistance offered by the entrapped air. .A conventional diagram for the movement of water due to gravitational force is shown in fig. (1) 1h SOIL PARTICLE ENTRAPPED AIR WATER HANGING TO sou. PARTICLES Fig. I.--Diagram showing the movement of water due to gravita- tional forces. 2. Movement in Unsaturated Soils When insufficient liquid is present to fill all the pores so that gas or vapor fills the remaining space, the medium is said to be unsaturated. The distribution and movement of water may be in any direction, depending upon the forces involved. The dominant moving forces are capillary or capillary potential and gravitational forces. Buckingham (8) defined the flow of water under these condi- tions as follows: "we must think of it as a current of water through the soil; and as a measure of the strength of the capillary current at any point we may take the amount of water which passes in one unit 15 of time through a unit area of imaginary plane surface perpendicular to the direction of motion". According to this definition, movement of water in the soil is a diffusion phenomenon. Later on this concept was supported by Gardner and Widstoe (21), Richards (57), Childs (9), and Childs and Collins (10). This concept is similar to those characterizing the flow of water through pipes, the flow of electricity, and the flow of heat. Flow of water in Tubes - If water be allowed to flow through a pipe from an elevated tank, the volume of water that comes out of the pipe depends on - the difference of pressure between the two ends of the pipe, and the conductance of the pipe. Supposing that diameter of the pipe is very small in compari- son to its length, the rate of flow will be proportional to the fall in pressure head per unit length times the gravitational constant "g". A potential gradient has both magnitude and direction. (poten- tial has only magnitude). This means that the potential gradient is the change in poten- tial per unit distance in the direction of the maximum rate of increase of the potential. In other words, a relation of the -- Current = Conductance x Difference of pressure) x constant i.e., Rate of flow = -K K', in which K is the conductance of the pipe and N is the potential gradient. Similar analogies can be developed for the flow of heat or of electricity. Flow of Heat - According to Fourier's law - H = KG 16 where, H - is the number of heat units which pass in one second through 1 sq. cm. perpendicular to the current and G is the tempera- ture gradient at a given point in the given direction, or this equation can be written as - g3 HS = -K dS where T is the temperature and S is the direction of the current. Flow of Electricity_- Similarly, if L be the conductivity and C be the current, according to Ohm's Law, we can have the equation C = LE Where E is the potential gradient dV or CS = L ES, where V is the potential and S is the direction of the current. Flow of water through Soil column - In a similar fashion the capillary flow of water through soils can also be analyzed. The flow of capillary water may be eXpressed as follows: Q=-K0) - (1) Where 'Q' is the capillary current density, or the mass of water which passes in one second through 1 sq. cm. of an imaginary plane perpen- dicular to the direction of flow, 'K' is the capillary-conductivity, and 'o' is the capillary-potential gradient. But equation (1) applies only to horizontal flow. .As the force of gravity acts verti- cally, this equation may be modified as - V = 4K4) - (2) where 'V' is the volume of flow, '0‘ is the change in the total water moving force per unit distance, and 'K' is the specific conduc- tivity. 17 In equation (2), o = $4.). in which 'A' is the gravitational potential gradient. Equation (2) shows that the movement of water by capillarity occurs under the combined effects of the capillary poten- tial gradient and the gravitational potential gradient. It is a function of the driving forces and the conductivity of the systems Rapid flow takes place only when both conductivity and potential gra- dients are large. The greater the difference in moisture content be- tween any two points in the soil, the greater will be the potential _and the more rapid will be the movement of water, if capillary con- ductivity is not zero. If a dry soil is placed in contact with moist soil with different degrees of wetness, water will move into the dry soil faster from the wetter soils than from the drier. Philip (#9, 50, 51, 52, 53, 5h, 55) points out that increasing water depth over the soil surface produces much the same effect as de- creasing the initial moisture content. Initially the potential gra- dients attributable to water depth will be much greater than the gravitational potential gradient, but will decrease inversely with the distance of penetration of the saturated zone. Therefore, the effects of water depth decreases with time for essentially the same reason as capillary effects do. .A conventional distribution of soil water during infiltration is shown in Fig. 2. INFILTRATION WATER HYGROSCOPIC NOISTURE FILMS AND PORE ANGLE WATER CAPILLARY WATER "'4" GROUND WATER 18 G R OUND SURFACE TEST HOLE Fig; 2.--Soil Water Distribution m z 3... F— 2 9 .— < a: m < m o z a: IL 2 >- 9 5 '5 .a a: .1 a a. 4 ( mg” *0 a: (N .1 :’ 0. <1 0 6 \ F'- can “£1.14 Sol-'0 t-N“ < 3|— en 19 2. INSTRUMENTATION FOR INFILTRATION - MEASUREMENTS In the past investigators have used different devices to measure infiltration capacity of different soils. Methods most commonly used are:- (1) Artificial rainfall devices- I. Sprinkling device to produce artificial rainfall.-(a, b, c, d, e, f.) II. Artificial rainfall applied by Drip screen and drip towers. (2) Cylinder Infiltrometers. (3) Basin Methods . (A) Watershed Hydrograph Methods. (5) Soil Cores. 1. Artificial Rainfall Method (I) Sprinkling device - Investigators have used spray nozzles to produce artificial rainfall to show the effect of different factors like vegetative cover, rainfall intensity, slope, soil structure, mul- ching and method of cultivation, etc. On intake of water. In this method artificial rainfall is produced through spray nozzles hanging in air at the desired height on stands (as in Fig. 3). 20 V ‘TO NOZZLE WATER suppo\s .__ —— TIL.— - —-——-—-’ TIL—:1”-\ ’do' 'r-a—n‘ v“ _'— a, ----- ”— :.~’—r"' ’. >- *- -~' 0- #‘fi--p-. a—\ - ——— ""‘;"~—- 7?. - —~°\ " ' — “‘2 d":-,- -' ""/ " I " fl METAL CONTAINER ‘ RUNOFF TROUGH ‘t Fig. 3. .Artificial Raindmaking Device water is applied in the field through the nozzles which are connected to the power pump through a metal pipe. Nszles face downward or upward or at an angle according to the suitability of the eXperiment. The experimental plot is equipped with metal boarders and runoff collection troughs. Rainfall measurements are made by standard rain gauge placed inside and outside the plot. Inathis way runoff and rainfall are measured to determine the infil- tration of water as the difference between applied rainfall and runoff. This is the general principle of sprinkler type infiltrometer, but 21 different workers have used this device in a variety of different ways. Some have even used ordinary garden can spray also for this purpose. Some of the devices based on this principle are: a) Type D-1 Infiltrometer - Beutner gt al_(5) used the type D-l infiltrometer to evaluate surface soil conditions and various types of native plant covers and their management in terms of their in- fluence on erosion and infiltration. The type D-l infiltrometer used four 1.5 inch Mulsifyre nozzles spraying downward and mounted on an overhead frame. They applied the artificial rainfall on 6 x 2h feet plots at a rate of 3 to 6 inches per hour, and recorded the runoff to find out the infiltration rate. I b) North Fork Infiltrometer - Rowe (59) described this apparatus in 19h0. It consisted of four brass MSG angle fog nozzles of a type similar to those supplied with standard garden pressure Sprayers. The nozzles were pointed upward and the simulated rain was allowed to fall upon a l x A foot plot. A buffer area was provided and runoff was collected. A.canvas was spread over the framework to protect the artificial rainfall from wind distortion. .A hand pump was used to operate the pressure equipment. The type of nozzle used produced smaller drops than natural rainfall, resulting in higher infiltration rates. c) Type 'F' and 'FA' Infiltrometers - Diebold (12) used type F and FA infiltrometers to find out certain infiltration values in forest areas. Type 'F' nozzle which produces relatively large drops, varying from 3.2 to 5.0 mm. in diameter, is used for both these infiltrometers. Type 'F' uses a plot 6 x 12 feet with a boarder area 3 feet in width and 22 type 'FA' uses a plot 1 x 2.5 feet with a border- area of 1.5 feet in width, with both the instruments tents are used to protect the spray pattern from the wind. In the past all the instruments using type 'F' nozzles had projected the spray upward or had indicated the nozzle at an angle. The type'FA'instrument is a modified North Fork infiltrometer. The main changes are substitutions of'F'nozzles in place of the standard fog nozzles and replacement of the hand Operated with a power pump. d) Type E - Infiltrometer - Type E-infiltrometers are similar to type 'F' but they produce smaller drops. e) Rocky Mountain Infiltrometer - Wilm (68) described this instru- ment in 19A3. It is originated from the type 'FA' infiltrometer but it is considerably different from that. This instrument uses 3 type 'F'nozzles, which applies artificial rainfall in an arching pattern on to a 2 x h foot plot. Rainfall and runoff is measured to calcu- late the infiltration rate. f) Pearse Square-foot Apparatus - This apparatus consists of a light metal frame, one foot square, with a runoff plate along the lower edge. Water is applied from a graduated cylinder to the upper edge of the plot, through a perforated pipe. water applied and runoff is measured in cubic centimeter to calculate the infiltration rate. (As shown in fig. u) 23 ".419, WATER 52,-- CONTAINER :3 12.; "W’- PERFORATED "L RUNOFF P'P£\ ‘ VALVE COLLECTOR f sou. ‘ ‘V 'l . 4"“ ~ W ~ METAL ravage-r " CONTAINER - _--.- ~ u..- a“..- - -, I -l "--- -—.¢‘.. w" ~--- Fig. h.-- Pearse Square-Foot Apparatus (II) Artificial rainfall applied by Drip Screen and Drip Towers- For the study of erosion and infiltration, Ellison (l6) and associates (Ellison and Pomerance (17), Ellison and Slater (18) ), used Drip Tower rainfall applicator. .A tank with holes drilled on h-inch centers supplied the water to the drip tower. .A screen of l-inch mesh chicken wire was placed directly below the tank and cheesecloth was spread loosely over the wire so that it would be pressed in to the openings. A short piece of yarn was hung from each pocket to form water drOps of uniform diameters. Different yarn sizes were used to obtain various uniform drop sizes. Drop velocity was controlled by varying the height of the apparatus. Intensity was controlled in the supply tank by varying the head of 2A water or by reducing the size of the holes supplying water to the drip screen. The drip screen was oscillated to distribute drops evenly over the plot. Runoff and rain applied was measured to cal- culate infiltration rate. 2. Cylinder Infiltrometers Various types of cylinder infiltrometers have been used in the study of infiltration rates and permeability of soils. Usually metal rings of known diameter are driven into the soil to depths ranging from a few inches to more than a foot, so that lateral or divergent flow of water from the rings could be reduced to a minimum. The methods of adding water to these cylinders include such principles as constant head, falling heads, and sprinkling application. Previ- ously, only single rings were used to study infiltration rates, but now to check the lateral movement of water to a still higher degree, double ring or multiple ring devices, which create a buffer area sur- rounding the central compartment, are used. In this device two or more rings are driven into the soil surrounding each other, isodia- metrically; Measurements of infiltration rates in the central com- partment are supposed to be indicative of the vertical component of flow. A metal plate is used to drive the rings into the soil. Water is applied in the central ring and its level is periodically measured with a hook gauge. Thus, the area of the ring, quantity of water applied and time to absorb a certain amount of water, are noted to cal- culate the infiltration rate of that particular soil. .A double ring infiltrometer is shown in Fig. 5. Limitations of cylinder infiltrometers are method of placement of the rings into the soil and entrapped air inside the soil column. Most of the time soil is disturbed at the time of placement of the rings which causes high infiltration rates. En- trapped air also affects infiltration rate. 3. Basin.Method This method is similar to the large ring devices, except that here large area is utilized. Plots usually range in area from 10 square feet up to 0.25 acres, and are provided with a border arrangement so that water can be applied to the basin, providing an 25 26 inpounding condition. A given amount of water is applied to the basin, and the rate of infiltration is recorded by measuring the water level in the basin and time elapsed. .A conventional basin arrangement to measure infiltration rate is shown in Fig. 6. MEASURING SC)“. SURFACE Fig. 6.--Basin Method The small basins have some limitations as found with the ring infiltrometer device, i.e., the factor of entrapped air, which can re- sult in retarded vertical movement of water. Beside this limitation, the larger basins are cumbersome to handle. A. watershed Hydrograph Method The Hydrograph method is used to determine average infiltration capacity over a drainage basin during large storms where runoff records together with adequate rainfall data, are available. The basis of the determination of infiltration capacity by this method is the fact that surface runoff approximately equals the difference between the rainfall and the infiltration during the part of the storm in which the rain intensity exceeds the infiltration capacity. 27 Rainfall intensity, total amount of rainfall and total amount of runoff of a drainage basin during a storm period are recorded at two or three stations. Then with these data rain graphs are prepared to deter- mine infiltration capacity. The determination of infiltration capacity 'f' consists of find- ing a value of 'f' such that the product of 'f' and the duration of rainfall at intensities in excess of 'f' subtracted from the total rain- fall for the same interval will leave a remainder equal to the surface runoff. Since the values at different stations on the same drainage basin will vary, the average is taken to get the average infiltration capacity. Here it is assumed that surface runoff equals the rainfall excess, but surface runoff cannot occur unless there is water standing on ground surface. It means actual rainfall excess is less than the measured run- off. There is, however, always some residual rain at intensities less than infiltration capacity, part of which is contributed to surface run- off, so that actually the surface runoff ranges from values a little less than to values a little greater than the total rainfall excess. In gener- al, the error resulting from the assumption that rainfall excess is equal to runoff is very small. Infiltration capacities obtained by means of watershed hydrographs are of limited value from the agronomic standpoint since direct inter- pretation of data is virtually impossible. .Also, simultaneous compara- tive studies regarding such factors as type and densities of vegetal cover and tillage practices are not possible since no two watersheds are similar enough for such a study. 28 5. Soil Cores Several attempts have been made to study permeability and infil- tration by laboratory studies using soil-cores. Some workers have used undisturbed soil-cores in which natural structural conditions were main- tained, and others have attempted to characterize permeability by means of disturbed soil columns. It is believed that a study of the internal physical conditions of a soil cOuld be of great value in the interpre- tation of infiltration data obtained by any of the methods already dis- cussed. Permeability or infiltration study can be conducted by taking a column or core of soil A to 5 inches in diameter and 9 to 12 inches long by means of a hand operated soil core auger or by such other device in such a way that the natural characteristics of soil-core sample is not disturbed. Then the cores are maintained in a natural condition by treatment with melted parafin or by keeping them in split-plastic tubes. In the laboratory, the cores are placed in a special apparatus which have water collecting devices at the bottom, and a constant head of water is maintained on them by means of Mariotte-bottles or by such other device. water passing through the soil-core is collected at the bottom and meas- ured. The time of passing that much amount of water through soil-core is also noted. Then the rate of infiltration is calculated by dividing the amount of water by time. The percolation rates of the cores serve as a means of studying field permeability, but sometimes the rates vary too widely for cores of the same soil to warrant an attempt to fix field per- colation rates by this means only. The other difficulty is that completely undisturbed samples cannot be taken. 29 Permeability measurements are also conducted on disturbed soil samples to study the relative changes in percolation rates brought about by specific chemical and physical soil treatments. Permeability measure- ments are made in the similar way as in the case of undisturbed soil sample by using constant head of water and recording the time of perco- lation of a certain amount of water through the soil sample under con- sideration. CHAPTER IV 1. FACTORS AFFECTING INFILTRATION CAPACITY There are numerous factors which affect the rate of infiltra- tion into soils. It would be difficult to list them in the order of their importance. Almost any one factor may be of controlling importance in a given case. Although it is not possible to list all the factors governing infiltration rates, however, a list of the main factors which modify the rate of infiltration is given below. (1) Soil cover and vegetation. (2) Physiographic Factors. a. SlOpe and surface roughness. b . Altitude . (3) Soil characteristics. a. Porosity. b. Texture. c. Structure. d. Colloidal contents of the soil. I. Organic matter. II. Exchangeable bases and colloidal clay membrane. III.Inwash of silt. e. Initial soil moisture and duration of wetting of the soil. f. Soil air. g. Subsoil characteristics. 31 h. Visible holes. I. Root holes. II. Animal and microbial activities. III.Shrinkage cracks. (h) Climatological Factors. a. Rainfall intensity and raindrops characteristics. b. Temperature. c. Season. (5) Water characteristics. (6) Time water is on the surface. (7) Cultural practices. a. Tillage. b. Use of fertilizers and manures. c. Cropping pattern. 1. Soil Cover and Vegetation Soil cover and vegetation have a remarkable affect on water in- filtration into soils. Vegetation Operates both at and below the soil surface to influence infiltration forces. At the soil surface, plant stems and root crowns, and litter accumulations that lie partly on and partly within the soil, all serve to break on otherwise smooth surface into one that is rough and pitted. Water that reaches the surface is held in depressions, with the resulting build up of ephemeral ponds that exert some hydrostatic pressure upon water at the immediate soil surface. Because these ponds are usually very shallow the pressures they develop are small; the degree to which they supplement other infiltration forces cannot be very great. The action of vegetation in detaining and retaining 32 water on the soil is more important in holding water in place longer for infiltration, than in developing hydrostatic pressure. Grass cover has a very good effect on runoff water, causing it to move in thin sheets across the land surface and more slowly than it does across bare land. Because of slower movement, water gets more time to infiltrate into the soil. The crown canOpy and ground litter accumulations due to the vegetal cover, shield the soil against heat and wind, and in this way decrease the rate at which the soil dries and hence water on the soil surface gets more time to infiltrate in the soil. The crown canopy and ground litter accumulations also shield the soil surface against the beating effect of raindrops which otherwise pack the soil surface and make the rainwater muddy to reduce infiltration. Below the soil surface, vegetation exerts more important influences upon infiltration forces. Attractive forces drawing water into the soil are greater in dry than in moist soil, and hence infiltration is more in dry than in wet soil. Transpiring vegetation reduces the water content of subsoil and hence as soon as water comes in contact with the soil surface, force of attraction for water or capillary forces act from be- low to take water from the upper surface, as a result infiltration in- creases. There is an increase in soil organic matters by root growth, especially in the instance of a permanent cover, improves the water holding capacity of the soil and favors infiltration of water. Duley (13) has shown experimentally that if surface is protected by straw or other materials, a high rate of intake may be maintained for a considerable time. A comparison of the intake of water when soil is covered and when it is bare is shown in Fig. 7. 33 Effect Of Removing Surface Protection on Rate of Intake of Water by Soil I.6 L Burlap fiemoved m' D Straw Removed \ 0| .2 ~ /' . “ or as x \I 2 (E. q; Q 5a 8. :5 -U) \c Q u: S 3: __ V) g) .4 E 1 L 1 1 L J 1 L L O 20 4O 60 80 O 20 4O 60 80 I00 TIME IN MINUTES Fig. 7.--Curve showing effect of sprinkling a sandy loam soil on intake of water when soil is protected compared with rate when soil is bare (After Duley, 1939). 2 . Physiographic Factors a. Slope and surface roughness - The slope Of the land surface is an important factor determining the proportions of infiltration and run- off. On a flat surface where there is no opportunity for runoff, the water may linger until the ground can absorb it. With increasing steep- ness of slope the impelling forces of gravity become more encouraging for runoff. When the soil surface is flat, then, a short time after the start of the storm, water accumulates on the soil surface and the falling rain- 3h drops dissipate their energy in the water rather than by dispersing in the soil. In this way the rainwater remains clear to increase infiltra- tion and packing affect of raindrops is also lost which causes a reduc- tion Of infiltration capacity. ”pa/Vi CLOUD /’///’///,// WI/ ///////// ///////_//// /////// / / /’/l I // // /// / / // 77/] RAINDROPS //// /// 7 ., / //////////// 77/727”? ' /,'/7// fl’fl I, //:,7/ ACCUNULgTEO /77/ ’ //7 / , / 77 7/7/// ////’//’,/ M I ,4 //~ // WWII/07,77; ,/ “"5 I I / 7 ’ //0 fl/ /ZM /A&fl%// /é§;/“a//&&%%Z4// fiflC’L//// :::;I: s O I L "TSURFACE Fig. 8.--Protection Of soil surface from the beating affect of raindrops on flat surface. Also, a sheet of water over the surface creates a pressure head which induces a higher rate of infiltration. But with the increase Of the steepness of slope, the beating affect of raindrops is increased because they fall directly on the soil surface. This causes not only soil erosion, but also the compaction of the soil surface to reduce infiltration. 35 //’\‘ //~"”‘\/'*\./’—\ (V \-~—2 CLOUD ’A/ $\"J/&k¢‘,h::v/ “/ g’ ") / ///7 ’ l/ // / , I /& 1V / // I [/I I ’ [1],, If, I / ///, /’ I //’ / / x, ’ 1' / I / I // I / ' //l t I, [1],," 7’\ son. /// ’ ,' , -//// /,',/ // // RAINDROPS SURFACE / / ;€//',/’ , a , , / [7/19 / ’ , / // ’ 0 ’ ’ 'IQ/’ACCUMULATED WATER out. ................................... Fig. 9.--Effect of Raindrops on Steep Slope. Roughness of the soil surface causes infiltration to increase. Roughly, infiltration is equal to total rainfall minus runoff. When the soil surface is smooth, a little amount of water suffices the need of filling up the whole area and the rest of the water runs off or evap— orates from the soil surface. But when the soil surface is rough, numerous small ponds are cre- ated on the soil surface and more water is required to fill them up. .As a result, less water is left for runoff and at the same time due to in- crease in pressure head on the soil surface as a result of more water, infiltration rate is increased. water in these small ponds gets more 36 time to infiltrate. Due to surface roughness velocity of flow of water is also reduce and hence water gets more time to infiltrate into the soil. b. Altitude - The altitude of the land surface may also be a factor in infiltration, chiefly because of its effect upon climatic fac- tors of precipitation and temperature, and upon the factors of vegetation and soil development. Higher the altitude, lower the temperature and hence the viscosity of water is increased which causes reduction in in- filtration. Several other climatic factors are also changed at higher altitudes which affect the rate of infiltration directly or indirectly. 3. Soil Characteristics a. Porosity - Of the various factors which may modify the rate of infiltration of water into field soils, the percentage of porosity is one of the most dominant. Porosity determines the storage available for infiltered water and also affects resistance to flow. Both these effects are such that infiltration increases with porosity. Musgrave, G. W. and Free, G. R. (#3) have found out that different soils vary in their infiltration capacity. They found wide differences in rates of water intake between Davidson clay loam, a friable, relatively pervious soil of Southeastern Piedmont (derived from basic igneous rocks), and the regionally associated Iredell loam (also derived from basic igneous rocks) which has a dense and much.less permeable clay subsoil. The average infiltration for the former was 0.82 inch an hour over a 3-hour period, whereas for the latter the corresponding rate was only 0.01 inch an hour. These measurements were all made on firm, bare ground, under comparable initial conditions of soil moisture. 37 These variations in rates of infiltration for different soil types are closely correlated with (1) total amount of pore space and (2) aver- age size of the individual Openings. Other things being constant, soils having a large combined pore space (cavity), as well as large individual space between the soil particles or aggregates of particles, have in- filtration rates much higher than soils whose individual openings are small. Musgrave, G. W. & Free, G. R., have (h2) shown experimentally that increasing the average percentage of pore space of cores of field struc- ture through surface cultivation has markedly increased the rate of in- filtration. The relation of permeability to porosity has been shown in Fig. 10. 3.0 “J 3.5 *— 4 a: 2.0 2 52 2 _’ l.5 O L) (I tn l.0 0.. (3 O o -J 0.5 O C) 0.2 0.4 (315 0.8 II) ‘l.2 LOG POROSITY FACTOR Fig. IlO.--The Relation of Permeability to Porosity (After Baver, 1938). 38 b. Texture - Texture or size of particles of soil modify the rate of infiltration due to its affect on capillary or gravitational flow of water in the soil. In soils of fine texture, not only is the amount of film surface eXposed greater than in coarse soils, but the curvature of the films is also greater due to the shorter radii. The effective pressure exerted by the films is consequently much higher in fine grained soil. The greater eXposure of surface and the increased pressure both serve to raise the friction coefficient and retard the rate of flow. The finer the texture of the soil, other factors being equal, the slower is the movement of water. The percolation in a heavy soil takes place largely through lines of seepage, which are really large channels develOped by various agencies. Besides this, moisture holding capacity of finer textured soil is also greater than coarser textured soil, which ultimately reduces the attractive capacity of the former soil for further moisture as compared to the latter one and hence the flow of soil water is less in finer tex- tured soil than in coarser textured one. So the rate of infiltration is more in coarser textured soil as compared to the finer textured one. c. Structure - The structure of the soil or, in other words, the arrangement of the particles, will become a factor in water movement inso- .far as it affects the amount of effective capillary surface and the poro- sity of the soil. Any arrangement of particles that will increase the runnber of angles of contact and porosity of the soil will evidently in- creuase the amount of movement of water. The compacting of a loose soil wilLL increase the possible capillary moisture until all the interstitial Spacne becomes capillary in its nature; further compacting will then cause 39 a marked decrease. The granulation of a clay soil, by producing a crumb structure and by actually increasing the effective surface eXposure, tends to increase its water holding capacity. Water fails to penetrate highly colloidal soils of poor structure, and all moisture movement is extremely slow. Highly colloidal soil of good structure, on the other hand, are readily permeable to water. The very porous structure allows the gravity water to disappear very quickly and so the infiltration rate of such structured soil is greater than massive, compact structured soil. d. Colloidal contents of soil - The movement of water in soils is determined by a combination of many factors, among which resistance offered by the soil and influenced by amount, composition and state of colloids is of chief importance. I. Organic matter — Organic matter increases the rate of in- filtration by its pronounced effect on soil granulation and by serving as soil protective cover on the soil surface. Organic matter in sufficient quantity protects the soil surface from the beating effect of rain which otherwise makes the soil surface compact and rain water muddy which reduces the rate of infiltration considerably. Organic matter decreases materially the dispersion ratio and increases the number of large sized aggregates when applied to soils of relatively poor structure containing only a small amount of cementing materials necessary for the formation of stable aggre- gates. Water moves readily into these non-capillary pore spaces and such soils have a high infiltration capacity. When relatively large quantities of organic matter are present in a soil, the 1+0 physical condition is favorable for the absorption of water re- gardless of soil granulation because the organic matter occupies several times as much volume as an equivalent weight of soil. Three percent of organic matter in a soil by weight equal approx- imately 15 percent of the total mass of the soil by volume. The flocculating effect of soluble material which is slowly liberated when organic matter decays has also a pronounced effect on soil structure which facilitates water infiltration into soils. Recent microsc0pic studies reveal that part of the organic matter in many soils appears not as a film surrounding the particles, but as individual units between soil aggregates or sand grains. Not much work has been done to show the effect of these large masses of organic material between mineral aggregates on mois- ture movement, but it is reasonable to believe that water will move through the pore space in organic matter more rapidly than it will move through the fine pores between the clay particles. In some instances, the masses of organic matter appear to act as wedges which separate the mineral particles. The effect of different amount of organic matter on infil- tration capacity in Clarion Loam is shown in Fig. ll. 5 2 o’ / m / > -; I6 TONS or MANURE L/ 2 PER ACRE \/ 3 o E a 3 / -9 o o/ m .0 Lu I 2 ’o/ 0 — 8 TONS or. E / MANURE PER ACRE :3 -L-‘—w“‘°‘ ° E 0".”o’ \ a: ,. CHECK D U) 0 20 4O 60 80 IOO I20 TIME (in minutes) Fig. ll.--Infiltration capacity in Surface Inches in Clarion Loam. (After Smith, Brown, and Russell, 1937) 42 II. Exchangeable bases and colloidal clay membrane - Generally two types of basic elements are recognized in the soil or soil solution which affect water infiltration into it - the alkaline, which includes sodium, potassium and ammonia; and the earthy, which includes calcium, magnesium, iron and aluminum. The elements of both groups may take part in exchange reactions between the solution and the soil. The effect of such reactions on the side of the soil is to produce one set of physical conditions when the com- bination is with the alkaline bases and a very different set of conditions when these are replaced by the earthy bases. These differences in the physical condition of the soil are manifested conspicuously in at least three ways - in perme- ability to water, in turbidity of water extract, and in cementation or drying. When the preponderance of combined bases is with the alkaline group the soil is less permeable to water, the soil ex- tract is more turbid, and the soil tends to cement together on drying and hence less infiltration. When the preponderance is with the elements of the earthy group these symptons are manifested in the opposite direction. The presence of acid electrolytes, such as chlorine, sulphate, or nitrate, in the solution tends to prevent the manifestation of the physical effects that follow the replacement of the earthy bases by the alkaline bases in the soil combination and hence increased infiltration. _Another factor that affects the permeability of water through soils is the state of hydration of its colloids. If part 1+3 of the water entering the soil pores is held on the surface of the particles forming the pores and causes swelling, the effective pore size will decrease. Thus pores, and the non-capillary dimen- sion may be reduced to capillary pores, and capillary pores may become essentially sealed to water movement. 0n the other hand, soils that are capable of holding water by capillarity without swelling may permit rapid downward movement of water. Permeability measurements of the clay membrane have shown that the highly dis- persed and hydrated systems are more impermeable than the floc- culated and only slightly hydrated systems. The H-ion clays of all soils are more permeable than the ca-clays. Lulz, J. F. (36) found experimentally that the swelling properties of four clay colloids are in the order-Bentonite) Putnam) Iredell) Davidson and hence the permeability in the re- verse order. He further demonstrated that the K, Na and Li cations showed no definite order of effect on the swelling of the colloids. The Ca, Ba, and H ions decreased swelling in the order named. Permeability of the different clay'membranes was in the order Methylene blue) H) Ba) Ca) K) Na) Li. Bennett (3) observed that soils with a low Silica- Sesquioxide ratio were more friable, permeable, and resistant to erosion than those with a high ratio, and that the friable soils showed no visible swelling or shrinkage even at extreme moisture variations. Soils with a high 3102 - R203 ratio were subject to wide changes in volumes during drying or wetting. Anderson (1) found that swelling increased with the Si02 - R203 ratio and that nu Na-saturated clay swelled about twice as much as K-clay. Addition of Ca-ions to the Na-clay decreased the amount of swelling. Winterkorn and Baver (69) have reported that the total amount of water taken up by the colloidal clays increased with the Si02- R203 ratio. They also show that water intake extends over a longer period of time in soils with a high 5102 - R203 ratio. III. Inwash of Silt — Presence of silt with irrigation water due to river flood or with runoff water, affects infiltration rate considerably. Particles of less than 0.5 mm. or silt materials, like clay, include that part of the soil in which such pheno- mena as swelling and shrinking, cohesion, plasticity, and cementing of particles principally occur. It includes the colloidal fraction as one of the most active constituents. Silt contents choke the pore spaces and cement the passage of water through soil. Soils high in silt content have massive structure which is not favorable for water infiltration into the soils. Hence with the increase of the silt content of the soil, its infiltration capacity de- creases. e. Initial moisture content and duration of wetting of the soil. .NEal (#5) observed that initial soil moisture content had a greater effect on the infiltration capacity during the first 20 minutes than any other fructor. The rate of infiltration varied approximately inversely as the sqluare root of the soil moisture content at the beginning of the rain. (Fiég. 12). After a period of 30 minutes, the rate of infiltration became VEIT)’ slow, and after one hour, it was approximately uniform. Musgrave 1+5 (#1) also found that the infiltration was very small when the initial moisture content of the soil was high. A possible explanation is that initial moisture contents of the soil fill the voids between the particles and reduce the capacity for further additions of water. It also causes swelling of the soil colloids which disturbs pore size distribution and hence water infiltration into the soil is reduced considerably. Wetting of the soil for a long time affects the infiltration rate in a similar fashion. g... :30b4 '<:z m— t}:E(l(33 \\\\\\\ .4: _° LLUJ , 55“ 0132 i LL“) (at, or): r :27: o ‘ (I oc>o o 5 no :5 2025 3035 INITIAL MOISTURE CONTENT OF SOIL-PERCENT __ __ . .. , _ - _ _-.._ .fl- . ...-..- _ . --—4-—o-v— .___-.-v— - . . Wm”- —. F543. 12.--The effect of the initial soil moisture content on the rate of infiltration during first 10 minutes of rain. (After Neal, 1938) Moisture content of the soil is increased gradually due to pro- longfhfl wetting which decreases the further infiltration of water into the soil- M6 f. Soil Air — The soil is a porous medium, the voids around and between the soil particles being filled with air and water. The air is the lighter of the two fluids and acts as a cushion, filling pores as they become voids of water and retreating to the atmosphere above when water flows into the pores. The presence of air in the soil results in permeability values being affected by variations in pressure and temperature. The increase in volume of entrapped air with increase in temperature results in a relative decrease in flow, which partly compensates for the increase in flow due to the decrease in viscosity of the water. During the movement of soil water, sufficient air pressure builds up to hinder infiltration. g. Subsoil characteristics - Rate of infiltration depends upon subsoil - characteristics also. More impermeable subsoil like dense clay or rock material decreases the rate of percolation of water which even- tually affects the rate of infiltration. More permeable subsoil like sandy or sandy loam soil percolates water more rapidly to make sufficient room for infiltrating water and consequently the infiltration rate is increased. When water table of an area is sufficiently near the ground surface, a little addition of water increases the height of the water table which ultimately reduces the absorbing capacity of the soil surface. A rock material covered with a very thin layer of soil or exposed hcaavy subsoil due to excessive soil erosion have very low infiltration ztrte due to their incapability to percolate or transmit sufficient quantity of water . it? h. Visible Holes - Visible holes in the soil surface adds in infiltration rate of the soil due to increament in the absorbing capa- city of the soil. A considerable quantity of water is filled in these holes during a storm which infiltrates slowly even if the rest of the water runs off from the soil surface. I. Root holes - In addition to the surface effect of vege- tation in retaining runoff and increasing infiltration, other benefits results from underground effects, such as increased organic supply and channels opened by ramifying root penetration. When plants die, roots are left in the ground which finally de- compose, leaving the root channel in the ground. These channels increase the water absorbing capacity of the soil which increases the infiltration rate. II. Animal and Microbial activities - Animals and micro- rganisms affect infiltration rate due to their pronounced affect on soil structure. Earthworms by carrying materials to the surface, exert a mixing effect, while lines of seepage and visible holes develop which increase the infiltration capacity of the soil. Insects, especially ants and other burrowing creatures, increase the rate of infiltration by loosening the soil and making holes. Soil flora and fawna make the soil more friable, loose and more permeable to increase infiltration. While these animals and micro-organisms increase the rate of infiltration in many different ways, cattle decrease the rate of infiltration sometimes in a certain area. Due to their con- tinuous trampling, soil surface is compacted and soil structure is disturbed which finally decreases the rate of infiltration. #8 III. Shrinkage Cracks - The process of swelling and shrinkage which accompanies wetting and drying is an important property of some soils. It largely contributes to structure formation in clay soils and also appears to be associated with the genesis of gilgai formation. The cracking resulting from shrinkage makes soil much more permeable to water and assists in aerating the deeper horizons of the profile. An increase in water entry due to extension of cracking would occur as the soil dries out, but large increase would not be eXpected until the water content is reduced below the wilting point and large cracks are present. Although cracking due to shrinkage increases the rate of infiltration in clay soils of massive structure, but it cannot compensate for their normally low permeability and poor aeration. h. Climatological Factors a. Rainfall intensity and raindrops characteristics - The effect of drop size on infiltration capacity is mostly due to its effect on the rate of rain packing and breaking down of soil structure. Therefore, excessively large drops tend to reduce the amount of infiltration during the first few minutes of rainfall until such time as a sheet of water collects over the soil surface. Effective size of pores are reduced due 'to infiltration of small soil particles with the percolating water. As tine water moves downward through the orifices it frequently carries small derbached particles of silt and sediment of various sizes, which lodge at ¢a_I)oint of constriction, tending thus to effect greatly the rate of in- filtrati on . 1+9 The slacking of aggregates during the course of the storm, causing their disintegration, likewise is effective in reducing the effective size of pores which ultimately reduces the rate of infiltration. The formation of surface crusts by heavy rain is a common occurrence, partic- ularly on soils which have been intensively cultivated. These crusts seal the soil surface to reduce the rate of absorption of water. The rate of infiltration varies directly with rainfall intensity when intensity is less than the infiltration capacity. However, the in— tensity has little noticeable effect on the rate of infiltration when it exceeds the capacity rate. 'With sufficiently numerous small dr0ps the entire soil surface will be continuously absorbing water at its maximum infiltration capacity with a lower rain intensity than is required for complete absorption over the whole surface with large drops with greater intensity. b. Temperature - Temperature is considered to affect to a greater or less degree practically every physical process in the soil. Some of the most important processes thus influenced include the gravitational flow of water, capillary movement and retention of moisture, diffusion and flow of gas, etc. Temperature of both soil and water has some effect on infil- 'tration through its effect on the general level of biotic activities, vis- ‘cosity'of water, viscosity and permeability of soil colloids, and perhaps 1x) some degree through its effect on the viscosity and volume of soil air. The: effect of temperature on infiltration is, therefore, undoubtedly com- p1e=3<. The surface tension of water is an inverse function of tempera- ture - Hence, a decrease in temperature to a certain degree increases the 50 surface tension and decreases the capillary potential. In other words, cooling the soil increases its attraction for water. However, in pres- ence of other variables, this process becomes negligible to increase in- filtration. With the decrease in temperature viscosity of soil water in- creases which hinders the rate of infiltration. But on the other hand with a rise in temperature of the soil not only varies the amount of capillary'water and thus increases the possible free water present, but at the same time it increases the fluidity and thus facilitates percola- tion. The expansion of the soil air also tends to increase such movement. .According to ordinary law of convection water moves from higher to lower temperature. Hence, when upper surface of soil is more heated than the lower surface or soil water at the upper surface has more temperature than the lower one, it moves quickly from upper to the lower surface and so the rate of absorption of water is increased. Bouyoucos (7) found experimentally that rate of percolation of water in sandy loam, silt loam, clay loam, and clay and muck soils increased with the rise in temper- ature up to about 30°C. and then decreased with further rise in tempera- ture. In case of sand, however, the rate of flow increased with a con- stantly rising of temperature. In the former soils results are explained under the hypothesis that the colloidal material present swells with in- crease in temperature and tends to close the channels through which the water flows. In a warm soil temperature micrdbial activities are also increased which adds in infiltration rate indirectly by increasing the porosity of the soil. With the increase in soil and water temperature evapotrans- piration rate is also increased but that is negligible as compared to 51 the rate of absorption. Keeping all the other factors constant, an over- all affect of soil and water temperature on the rate of absorption of water is shown in Fig. 13. With the fall of temperature below the freezing point, soil water also freezes. The upper surface of soil converts into a compact mass due to constant freezing and thawing. The upper horizon; of the frozen soil is very often saturated with water almost to its full moisture capacity, hence it cannot take more water. Due to cementing effect of frost on soil, water absorption is considerable decreased. 8C)? 0! O l TEMPERATURE c° A O I 20s } 4 L l J o no 20 30 4o INFILTRATION RATE, INCHES PER HOUR ; ‘u—. Fig. l3.--Rate of Infiltration with the increase in temperature. (After Fletcher, l9h9) 52 c. §e§§gg - Seasonal variations in infiltration accompany seasonal variations in temperature of air, soil and water, variations in soil mois- ture, in vegetal cover and farm practices, in intensity duration and fre- quency of rainfall and in biological activities, etc. In summer months temperature rises and soil surface becomes comparatively dry which adds in increment of water infiltration into the soil but on the other hand in cold months soil surface is frozen which decreases the rate of water entry into the soil. Horton (28) made a study of the seasonal variation of maximum, minimum and average apparent infil- tration capacities for the North Concho River Basin in Texas - Fig. 1h. 30 F }‘\ >- / ‘ l- I \ ._ _ . <3 | :53 I, | L,IZZO ' a: 2m c>a.b5 F10