Accurate phosphorus (P) load estimation in subsurface drainage water is critical for assessing the field-scale efficacy of conservation practices and minimizing the environmental impact of P loss from subsurface-drained fields to freshwater bodies like the Great Lakes. Also, to build a more resilient crop production system in a changing climate, it is crucial to understand how future weather patterns affect subsurface drainage design and whether subirrigation will be needed in the future for crop production. In this study, we used high-frequency P concentration measurements to investigate P transport dynamics and evaluate the effects of water sampling strategies on the uncertainty of P load estimation. We also evaluated the change in the optimum drain spacing from using historical (1994 - 2023) and future (2030 - 2059) weather data and assessed the efficacy of subirrigation to alleviate yield reduction due to drought stress in southeast Michigan, USA.We used the HydroCycle-PO4 instrument to measure total reactive P (TRP) concentration at a high resolution from a subsurface-drained field. Hourly TRP concentration and hourly drainage discharge measurements formed the reference P load dataset. Four hypothetical water sampling strategies were evaluated: (a) time-proportional discrete sampling, (b) time-proportional composite sampling, (c) flow-proportional discrete sampling, and (d) flow-proportional composite sampling. All sampling strategies underestimated TRP load compared with the reference dataset, regardless of whether the underestimation was statistically significant. Total reactive P load underestimation changed from 0.2 to 51% as time-proportional discrete sampling intervals increased from 3 h to 14 d. Total reactive P load underestimation changed from 12 to 43% as the time-proportional compositing scenario increased from 1 to 7 d, each with one aliquot per day. In the case of the flow-proportional discrete sampling scenario, the lowest (0.6%) and the highest (–5.1%) uncertainties were observed when 1- and 5-mm flow intervals were used. The relative error based on the results provided by the flow-proportional composite sampling ranged from 0.2% when using a 1-mm flow interval to –6.7% when using a 5-mm flow interval. In conclusion, the flow-proportional sampling strategies provided a more accurate estimate of cumulative P load with fewer samples because a greater portion of samples were taken at higher flow rates compared with time-proportional sampling strategies.Results showed that there was a good relationship between TRP concentration and drainage discharge (R-squared = 0.60) such that TRP had a transport-limited chemodynamic pattern, that is TRP concentration tended to increase with an increase in flow during events. A 1% increase in drainage discharge resulted in a 1.36% increase in TRP load, indicating a significant increase in P concentration during high flows. We found that flow events substantially contributed to P loss (89%) because of capturing the rapid increase in P concentration during high flows. The rate of increase in P concentration during the rising limb ranged from 0.02 to 0.66 mg/L per hour. The highest 7.7% of drainage flow transported 75% of the TRP load during the monitoring period. The hysteresis pattern tended to be positive (clockwise) during the study period, indicating that preferential flow was a pathway for TRP loss. Most flow events (30 out of 36) displayed a flushing effect in which P concentration increased with a rise in drainage discharge. In conclusion, high-frequency P sampling showed that management and conservation practices should target flow events to reduce P loss.A total of 27 general circulation models with a moderate greenhouse gas emission scenario (shared socioeconomic pathway 2-4.5) were used for climate projections. Simulations were performed using the DRAINMOD model, and the optimum drain spacing was determined based on the maximum average annual return on investment. Results showed that the projected 30-year average annual precipitation is not expected to change significantly while that of the temperature will increase by 2.5°C in the future. Future optimum drain spacings for depths of 75 cm and 125 cm were found to be 300 cm and 600 cm wider than historical spacings, respectively. On average, there was a 23% decrease in 30-year average annual drainage discharge, attributed to an average 17% increase in evapotranspiration. Drought stress is projected to be the primary cause of yield loss in the future, due to increased temperatures and an average 8% deeper water-table depth. Subirrigation shows high potential in reducing year-to-year crop yield variability in the future (decreasing the coefficient of variation for the yield from 0.26 to 0.06, on average) and increasing yield by up to 31%. In the past, subirrigation initiation was feasible in late June with a weir depth of 70 cm. However, in the future, subirrigation is anticipated to be more advantageous when starting sooner in early to mid-June, coupled with a shallower weir depth ranging from 65 cm to 55 cm. In conclusion, a wider drain spacing, providing reduced drainage intensity, along with subirrigation may be needed in the future to mitigate crop yield loss from drought stress.
Read
