Subsurface drains layout in arid areas with purposes of salt control and discharge

Abstract: Subsurface drainage has been widely used to prevent saline-alkali soil. However, it remains a concern to determine the layout of subsurface drains in arid areas while considering its complex hydrogeological conditions, e.g., deep groundwater table, arid climate, and strong spatial soil heterogeneity. Therefore, it is necessary to investigate the method of determining layout parameters, e.g., spacing and depth of subsurface drains in arid region. The leaching salt will move upward due to strong evaporation in arid areas, which limits the evaluation index of desalination rate to evaluate the effect of subsurface drains. So, there are 3 major objects in this study: 1) to define an additional evaluation index to evaluate the effect of subsurface drains; 2) to determine relationship between the layout parameters of subsurface drains and the effects of subsurface drains for controlling or discharging soil salt; and 3) to evaluate the impacts of soil characteristics on effects of subsurface drains for controlling or discharging soil salt. The evaluation index, ratio of salt discharge to leaching, is defined and used with desalination rate to evaluate the effects of subsurface drains for controlling or discharging soil salt. The experimental data of salt leaching under the subsurface drainage condition in Xinjiang was used to calibrate and validate the HYDRUS-2D model. 16 scenarios of numerical experiments were implemented to calculate the soil salt transport under different layout parameters of subsurface drains with 6 different spacing (100, 200, 300, 500, 1 000, 1 500 cm) under 3 different depths (60, 100, 150 cm). 1 scenario without subsurface drain was carried out for comparison. The soil salinity and desalination rate as well as ratio of salt discharge to leaching were calculated for evaluation. In addition, 4 different soil types (loam, silt, silt loam, sandy clay loam) under the same depth of 1 m and spacing of 10 m were used to analyze the effect of soil texture on the effects of salt leaching and discharge of subsurface drains. The results showed that the simulation values were in good agreement with observed ones, and the established model could simulate soil water and salt transport under subsurface drains. The spacing of the subsurface drains showed strong impact on the desalination rate while no obvious impact was found from the depth of the subsurface drains and soil characteristics. There was an exponential relationship between the desalination rate and spacing of subsurface drains. The spacing and depth of the subsurface drains and the soil characteristics had significant effects on the ratio of salt discharge to leaching. The exponential and linear relationship between ratio of salt discharge to leaching with the spacing and depth could be established, and also the ratio of salt discharge to leaching was exponentially related to saturated hydraulic conductivity. The empirical formula of the desalination rate and ratio of salt discharge to leaching with the layout parameters of subsurface drains under the specific soil and leaching schedule was established. The ratio of salt discharge toleaching was considered as a more appropriate evaluation index in arid areas because it could clearly distinguish leaching and discharging salt and it was sensitive to the layout parameters and soil characteristics. Furthermore, an empirical formula representing the relationship between the ratio of salt discharge to leaching with layout parameters and saturated hydraulic conductivity was established, which could be used to determine the layout parameters according to the different objects of controlling or discharging soil salt. The calculation results were compared with those from the Professional Standard. The results showed that the calculated spacing from the proposed formula equaled to that from the Professional Standard formula with the ratio of salt discharge to leaching equaled to 500%. This study provides a possible quantitative evaluation method to determine the layout parameters in arid areas.