Development and application of long-term root zone salt balance model for predicting soil salinity in arid shallow water table area

A simple root zone salt balance model was developed for long-term soil salt prediction. A groundwater balance module was integrated to obtain the water flux at the bottom of the root zone, since groundwater level is easily and reliably measured data. An assumption that the net percolation at the bottom of root zone equals to the net recharge from root zone to unconfined groundwater system was used, which makes capillary rise and infiltration water leaching of root zone obtainable. The model accuracy and limitation were evaluated by comparing the simulation results with those from HYDRUS-1D under various soil texture and bottom boundary conditions. The mean relative error (MRE) of soil salt content ranged from - 6.25% to 9.95%, and the root mean square error (RMSE) from 0.02 kg/100 kg to 0.05 kg/100 kg, which suggest the model accuracy. The determination coefficient (R-2) of the cumulative percolation at the bottom of root zone and recharge to groundwater at the end of each calculation period was 0.99, which validates the model assumption. The model was then applied to predict soil salinity in the Longsheng irrigation district of Hetao Irrigation District, China. The soil salinity and water table depth data from 1999 to 2016 were used to calibrate and validate the model parameters. The MRE values of water table depth and average soil salt content were 10.08% and 10.84%, and RMSE values 0.37 m and 0.04 kg/100 kg in the validation period. Subsequently, the validated model was used to simulate the soil salinity in future 100 years under various water saving conditions. Water table depth and autumn irrigation for salt leaching were the two major factors to control soil salinity. The required autumn irrigation to keep root zone in mild salinity level (0.3 kg/100 kg) can decrease by 25.1 mm with 0.1 m water table depth increase, while it should increase by 64 mm with 1.0 kg/m(3) groundwater salinity increase in the crop growing season. Moderate groundwater exploitation to increase water table depth is recommended to save autumn irrigation water and control soil salinity.