Zambezia (1982), X (i).RESEARCH REPORTSOIL MOISTURE AND SOIL TEMPERATURE EEGIMESIN ZIMBABWEESTIMATED FROM CLIMATIC DATATHE PURPOSE OF this paper is to present Information on soil temperature and soilmoisture regimes in Zimbabwe In order to facilitate classification of soil accordingto the United States system (United States, 1975).The terms of 'soil moisture regime' and 'soil temperature regime' refer tovariations in soil moisture and soil temperature with time. Moisture regimes offreely drained soils are defined in terms of the presence or absence of water held at atension of less than 1500 kPa in the moisture-control section. The upper boundaryof the moisture-control section is the depth to which a dry soil is moistened by 25 mmwater within 24 hours and the lower boundary is the depth to which a dry soil ismoistened by 75 mm water within 48 hours. The soil temperature regime is definedin terms of the mean annual soil temperature, and the difference between meanwinter and mean summer soil temperatures at a depth of 50 cm,ESTIMATION OF MOISTURF AND TEMPERATURE REGIMESOwing to the scarcity of data on variations in soil moisture and temperaturethroughout the year in Zimbabwe, it is worth considering the possibility of usingclimatic data. Newhall (United States, 1975) has developed a procedure forcalculating soil-moisture regimes of freely drained soils from mean monthlyprecipitation, mean monthly temperature and latitude. The model assumes thathalf the monthly precipitation is depleted at the fall rate of potential evaporationand half is evaporated at a rate linked to the amount and the location of availablewater remaining in the soil. He estimates eYaporation by a slightly modifiedThomthwaite (1948) procedure.One of the authors of this study, Van Wambeke, has transcribed parts of theoriginal Cobol programme of Newhall into Fortran. It estimates soil moisture andtemperature regimes. The mean annual soil temperature is obtained by adding2,5°C to the mean annual air temperature. Seasonal amplitude in soil temperatureat 50 cm depth is estimated from the difference between mean winter (average ofJune, July and August) air temperature and mean summer (average of December,January and February) air temperature multiplied by a factor (0.66).The climatic data used in the analysis was taken from seventy-two meteor-ological stations listed in the Climate Handbook Supplement, No. 5 (Rhodesia,1968).CLASSIFICATIONThe udic, ustic and aridic classes of soil moisture regime are defined in detail in SoilTaxonomy (United States, 1975), and the following short descriptions are givenonly as a guide to their meaning. The udic soil moisture regime is common to soils ofhumid climates that have well distributed rainfall. Water moves down through the41Table INUMBERS AND NAMES OF METEOROLOGICAL STATIONSNo.I234567QO9101112131415161718192021222324NameBanket Research StationBeitbridgeBinduraBingaBirchenough BridgeBuheraBulawayo AirportBulawayo Goetz ObservatoryChegutuChiberoChinhoyiChipingeChipinge Experimental StationChipuriroChiranduChivhuGokweGrand ReefGutuGwebiGweru ThornhillHarare AirportHarare BelvedereHarare KutsagaNo.1252627282930313233343536373839404142434445464748NameHarare Research StationHendersonHope MapopoHope PatrolHot SpringsHwangeHwange Main ParkInyanga ExperimentalStation OrchardInyangani LulecheKadomaKariba GorgeKaroiKeziKwekweLupaneLusuluMakoholiMaronderaMartin ForestMatopos NurseryMatopos SandveldMondoroMount DarwinMutare Fire StationNo.495051525354555657585960616263646566676869707172NameMutokoMvumaMvurwiNcemaNkayiNuanetsiNyamandhlovu Experiment StationNyandaNyangadziPlumtreeRusapeSabi Valley Experiment StationShamva PanmureSouthdownTjolotjoTrelawneyTriangle MillTuli EstateVictoria Falls PoliceVumba National ParkWedzaWest NicholsonZakaZvishavaneTable IISOIL MOISTURE AND SOIL TEMPERATURE AT TEN SITESAS ESTIMATED BY THE NEWHALL PROCEDURENo. ofStation231221253032485665Name of StationBeitbridgeBulawayo GoetzLaboratoryChipingeGweru ThornfaillHarare ResearchStationHwangeInyanga ExperimentalStation OrchardMutare Fire StationNyandaTriangle MillSoil Moisture Control SectionCumulative DaysDry3401400504124803796238in a YearPart Dry/Part'Moist209642878811231101120122Moist012431822323103292221440MaximumiMoist inSome Part102203603103197736032326483Consecutive Daysn a YearDry afterSummerSolstice9500002800022MoistafterWinterSolstice0093283601042700MoistureRegimeAridicUsticUdicUsticUsticAridicUdicUsticUsticAridicSoil TemperatureMeanAnnual°C25.621.721.420.220.927.217.221.721.724.8SeasonalAmplitudeat 50 cmDepth °C6.44.43.74.84.04.43.34.65.26.0TemperatureRegimeHyperthermicIsothermicIsothermicIsothermicIsothermicIsohyper-thermicIsothermicIsothermicThermicHyperthermic44RESEARCH REPORTv 4|Figure 1: SOIL MOISTURE REGIMES IN ZIMBABWE.J.P. WATSON AND A. VAN WAMBEKE45"r vFigure 2: SOIL TEMPERATURE REGIMES IN ZIMBABWE.46RESEARCH REPORTsoil at some time in most years. Soils with aridic moisture regimes are found in aridclimates. They undergo little or no leaching and soluble salts sometimesaccumulate in the proile. The ustic moisture regime is intermediate between theudic and the aridic regime, and is one of limited moisture; but the moisture ispresent in summer when conditions are suitable for plant growth.The classes of soil temperature regime applicable to Zimbabwe are defined asfollows. The thermic soil temperature regime has a mean annual soil temperature of15°C or higher but lower than 22°C and the difference between mean winter andmean summer soil temperature is more than 5°C. The hyperthemic soil temperatureregime has a mean annual soil temperature of 22°C or higher and the differencebetween mean winter and mean summer soil temperature is more than 5°C. Theprefix 'iso-' to the name of a soil temperature regime indicates that the meansummer and winter soil temperatures differ by less than 5°C.MAPPINGThe placement of boundaries in Figures 1 and 2 has been guided by reference tocontours on the 1:2,50G,GGO map of Zimbabwe (Rhodesia, 1976), natural regionboundaries of the 1:1,000,000 Natural Regions and Farming Areas map ofZimbabwe (Zimbabwe, 1980) and rainfall class boundaries shown in the Mean-Annual Rainfall map of Zimbabwe (Rhodesia, 1968).The percentage areas of Zimbabwe covered by the three moisture regimeclasses shown in Figure 1 are as follows: udic 2 percent, ustic 79 per cent and aridic19 per cent.The only boundary drawn on Figure 2 is between thermal and isothermaltemperature regimes on one side of the line and hyperthemic and isohyperthermictempeFature regimes on the other side of the line. The percentage area of Zimbabweoccupied by thermic and isothermic temperature regimes is 23 per cent, and byhype fine rmic and isohyperthermic, 77 per cent.Table I gives the names of the meteorological stations listed in Figures 1 and 2and Table II gives some of the data from the computer printout.CONCLUSIONNewhall's soil moisture model has been tested against observations on soilmoisture on the Great Plains of the United States over a period of 20 years. Thecorrelation between calculated and observed soil moisture is about 0.8.Although the methods of estimating soil moisture and soil temperatureregimes from climatic data have not been tested in Zimbabwe they are believed tobe reasonable ones for use in classifying soil according to the United Statessystem.University of ZimbabweCornell UniversityJ.P.WATSONA.VAN WAMBEKEAcknowledgementWe thank Mr R.G. Wheeler and Mr C. Togarepi of the Geography Department,University of Zimbabwe, for making fair copies of Figures 1 and 2.J.P. WATSON AND A, VAN WAfVlBEKE47ReferencesRHODESIA 1968 Rhodesia: Mean Annual Rainfall. Scale 1:2,500,000 [mapj(Salisbury, Go¥, Printer for Dep, of Meteorological Services),1976 Rhodesia. Scale 1:2,500,000 [map] (Salisbury, Gov. Printerfor Surveyor General),1978 Climate Handbook Supplement, No, 5 (Salisbury, Gov.Printer for Dep. of Meteorological Services),THORNTHWAITE, C.W. 1948 'An approach toward a rational classification ofclimate', Geographical Review, XXXVIII, 55-94.UNITED STATES 1975 Soil Taxonomy: A Basic System of Soil Classificationfor Making and Interpreting Soil Surveys (Washington, U.S. Gov. PrintingOffice for Soil Survey Staff, Dep. of Agriculture, Handbook No. 435),ZIMBABWE 1980 Zimbabwe: Natural Regions and Farming Areas, Boun-daries as at 1st January, 1930. Scale 1:1,000,000 [map] (Salisbury, Gov,Printer for Surveyor General).