Using Root Zone Water Quality Model to Optimize Subsurface Drain in Hetao Irrigation District

Abstract

【Background】 Hetao irrigation district (HID) is an important agricultural production base in northeastern China but faces soil salinization due to the long-term irrigation using the Yellow river water and poor drainage management. Subsurface drain had been increasingly implemented in the district, but how to improve its effectiveness such that it not only controls groundwater table below a critical depth but can also keep soil sufficiently moist to facilitate crop growth remains obscured. 【Objective】 The aim of this paper is to investigate how to achieve this goal through optimizing subsurface drains. 【Method】 We took sunflower as an example and simulated water flow and solute transport in the soil using the root zone water quality model (RZWQM2). The model parameters were estimated by calibration against experimental data measured from 2018 to 2020, and the calibrated model was then used to evaluate how an change in the drain design, including their burying depth and spacing, affects soil water flow and crop growth under different scenarios. 【Result】 ①Comparison between the calibrated model and the experimental data showed that the RMSE of the model for soil water was 0.049~0.065 cm3/cm3 for sandy soil and 0.012~0.037 cm3/cm3 for other soils, and that the MRE for cumulative drainage and crop yield was below 5.88% and 3.40% respectively; its R2 was 0.798 for groundwater, 0.817 for soil water storage in 0~100 cm soil, and 0.912 for leaf area index. ②Simulations revealed that raising the bury depth of the drain from current 1.5 m to 1.4 m with the spacing kept at 45 m as used by local farmer can raise the groundwater table by 5.2 cm, reduce drainage loss by 40.0%, and increase crop yield by 85.3 kg/hm2. ③Keeping the drain exit 1.5 m deep in rainy season and raising it to1.2 m deep in other seasons can raise the groundwater table by 2.2 cm, reduce drainage loss by 46.0%, and increase crop yield by 66.4 kg/hm2. 【Conclusion】 RZWQM2 is accurate to simulate water flow and sunflower growth in areas drained by subsurface drain, and its application to Hetao irrigation district indicated that burying the drains 1.4 m deep with a spacing of 45 m was optimal. For areas with drains already installed at 1.5 m deep and 45 m spacing, keeping the drain exit 1.5 m deep in the rainy season and raising it to 1.2 m deep in other seasons worked the best.

Keywords

• rzwqm2
• subsurface drainage
• controlled drainage
• sunflower
• hetao irrigation district