Effect of Controlled Irrigation and Drainage on the Capillary Rise in Paddy Fields Simulated Using the HYDRUS-1D Model

Abstract

【Objective】 Water flow between soils and groundwater not only affects regional hydrological cycle but also groundwater use by plants. The aim of this paper is to investigate how shallow groundwater and soil water in paddy fields exchange under different combinations of irrigation and drainage. 【Method】 We considered four irrigation treatments by keeping the low-limit of soil saturation to resume irrigation at 50% (I1), 60% (I2), 70% (I3) and 80% (I4), and three drainages with the critical depths for grounder water table not to rise beyond kept at 30 cm (D1), 50 cm (D2) and 70 cm (D3). Water flow in the soil under each scenario was simulated using the Hydrus-1D. 【Result】 The model was able to reproduce the change in soil moisture measured in the field, with RMSE for soil water content at different depths varying from 0.010 4 to 0.088 4, and their associated NSE in the range of 0.041 5 to 0.761 2. The relative error between the simulated and measured water exchange between unsaturated soil and the groundwater was 4.6%. Water ascendence from the groundwater to the soil increased as the low-limit soil saturation set for irrigation increased and the critical groundwater depth for drainage decreased. The average groundwater discharge to soil peaked in I1, increasing by 50.42%, 50.42% and 92.93% compared to that in I2, I3 and I4, respectively. The peaked and total groundwater discharge to the soil both decreased with the decrease in groundwater depth. The average discharge peaked in D1, being 2.30 times and 4.73 times that in the D2 and D3, respectively. The total groundwater discharge in D3 was 48.47 mm and 34.22 mm less than that in D1 and D2, respectively. 【Conclusion】 Groundwater depth and irrigation scheduling both affected the total amount of capillary rise in paddy fields at significant level, especially the groundwater depth. Dropping the groundwater depth coupled with water-saving irrigation in paddy fields could effectively enhance capillary rise and improve groundwater use efficiency of crop.

Keywords

• paddy fields
• water-saving irrigation
• capillary rise
• hydrus-1d
• groundwater depth