Simulation and evaluation of soil water and salt transport in desert oases of Hetao Irrigation District using HYDRUS-1D model

Abstract: Great changes have occurred in the agricultural ecology, due mainly to the ever-decreasing water diversion from the Yellow River, particularly for the desert oases in the Hetao Irrigation District of China. Taking a sand dune-wasteland-lake system as the research object, this study aims to simulate and evaluate the soil water and salt transport during the growth period in the desert oases after autumn irrigation using HYDRUS 1D model. Two-year experimental data was selected to simulate the water and salt dynamics of the sand dune, wasteland, and the junction of sand dune-wasteland. As such, the characteristics of water and salt transport were revealed to evaluate the agro-ecological situation in desert oases. The results showed that a highly accurate assessment was achieved in the soil water content, where the mean relative error (MRE) was -1.30%-1.52% in calibration and validation, the root mean square error (RMSE) was 0.01-0.03 cm3/cm3, the coefficient of determination (R2) was 0.82-0.93, and the regression coefficient (b) was 0.98-1.01. A simulated accuracy was also acceptable in the soil salt content, where the MRE was -0.43%-5.71%, the RMSE was 0.06-0.23 dS/m, the coefficient of determination (R2) was 0.76-0.82, and the regression coefficient (b) was 0.98-1.03. Specifically, the water content at the soil layer of 20, 40, 60, 80, and 100 cm in the sand dune decreased by 25%, 32%, 34%, 37%, and 35%, respectively. The storage of soil water also decreased by 102-114mm. The soil water loss of sand dune still remained 67-103mm, where there was less recharged flow from groundwater. It inferred that the sand dune was in a state of water deficiency during the whole year. The water content at the soil layers of 60-100cm in the sand dune-wasteland junction decreased by 18%-22%, whereas, the soil water storage increased by 61mm after autumn irrigation. The water content at the soil layers of 20, 40, 60, 80, and 100cm in the wasteland decreased by 16%, 17%, 27%, 42%, and 34%, respectively, while the soil water storage decreased by 160-228 mm, but the soil water storage increased by 2-55 mm after autumn irrigation. The soil water balance remained on the wasteland and sand dune-wasteland junctions during the whole year. The evapotranspiration of wasteland was twice that of the sand dune, whereas, that of sand dune-wasteland junction was between them. Furthermore, the groundwater recharge of wasteland was 3-5 times as much as that of the sand dune. Additionally, the salt accumulation rates at the soil layers of 0-100cm in the sand dune, sand dune-wasteland junction, and wasteland were 34%-51%, 14%-17%, and 25%, respectively in the growth period. After autumn irrigation, the salt accumulation rates of the sand dune and sand dune-wasteland junction were 47%-59% and 3%-6% respectively, whereas, the desalination rate of wasteland was 0.7%~5.0%. The sand dune and sand dune-wasteland junction were in the salt accumulation state all year round, whereas, the wasteland was in the desalination state after autumn irrigation. Consequently, the rainfall presented little effect on the agro-ecological balance of desert oasis, while the autumn irrigation played a key role in improving the agricultural ecology of the desert oasis. The findings can provide a theoretical reference for the agricultural ecology management of desert oases in Hetao Irrigation District of China.