Abstract:
Agricultural drainage system modeling plays a significant role in managing the excess water disposal in agricultural fields that hinder the crop productivity. Due to agricultural drainage, problem sugarcane fields are prone to waterlogging and physical degradation of topsoil. This study aimed to analyze the shallow groundwater table, drainage, and nitrogen load of farming fields, using a process-based, and field-scale hydrological mode1(DRAINMOD), on sugarcane plantation in Wonji-Shoa, Ethiopia. The effect of climate change on the agricultural drainage were investigated using the ensemble of four downscaled model outputs (CanESM2, HadGEM2, CNRM-CM5, and MIROC5) of RCP4.5 and RCP 8.5 scenarios. Baseline scenario (from 1976 to 2005) and future scenario (2025-2045, 2046-2075, and 2076-2100) analyzed for study area. Calibration and validation conducted in a 20-year (1976–1995) and 10-year (1996 to 2005) respectively. Daily weather records, daily water table depth, soil data and daily drainage discharge data used as an input data to calibrate and simulate the DRAINMOD model. The model performance criteria (MAE, R2, and RMSE) showed that a good agreement in simulating daily drainage discharge and water table depths. The largest seasonal rise in maximum temperature for the RCP4.5 scenario was 2.1°C, while 2.50°C for RCP8.5 scenario. Seasonally, rainfall rises in the Belg and Kiremt seasons while falling in the Bega season. The greatest percent seasonal increment in rainfall for the RCP8.5 and RCP4.5 scenarios was 44.4 % and 31.50% respectively. The calibration and validation of daily drainage result with R2 of 0.805 and 0.79, MAE of 0.14 and 0.07 cm/day, and RMSE of 0.21 and 0.073 cm/day, respectively. Similarly, the calibration and validation periods of the daily water table depth simulation, shows an acceptable result of R2 (0.89 and 0.88), MAE (34 and 2 cm), and RMSE (44 and 5 cm), respectively. In both the RCP8.5 and RCP4.5 scenarios, the downscaled temperature output shows rise in all future-projected period. The mean annual evapotranspiration predicted to rise in future period, which reduce the amount of drainage discharge from agricultural land. This study examines how future climatic change will affect subsurface drainage by designing the drain spacing and drain depth that correspond to the maximum sugarcane yield.
