Development and application of a monthly water and salt balance model for seasonally frozen agricultural and non-agricultural areas with shallow groundwater table

Long-term water and salt predictions in seasonally frozen arid agricultural regions with shallow groundwater tables are important for water conservation and soil salinity control. In this study, we developed a monthly water and salt balance model. The water balance model was built to obtain the water flux in the vertical layers and the water exchange between agricultural and non-agricultural areas. In the frozen period, the upward water flux from the groundwater was calculated using a function which considered the water table depth and the negative accumulated temperature, and the volume of groundwater entering the root zone was estimated by the thickness of the root zone and frozen depth. With regards to frozen soil, the model considered salt diffusion and expulsion as well as convection. Only four additional parameters were required to estimate the water and salt processes during the freeze-thaw period. Soil salinity and water table depth data from 1999 to 2018 were used to calibrate and validate the model in a seasonally frozen agricultural district in the Hetao Irrigation District (HID), China. The mean relative error (MRE) of water table depth and averaged soil salt content was-4.29 % and 8.65 %, and the root mean square error (RMSE) was 0.38 m and 0.02 g/100 g during the validation period of 2010-2018. The validated model was used to simulate future long-term soil salinity under different autumn irrigation. The results showed that increasing autumn irrigation water can decrease root zone soil salinity during the autumn irrigation period, but it has a limited effect on long-term annual average soil salinity. The groundwater table becomes shallower with increasing irrigation water in autumn, which results in greater upward salt in the adjacent freeze-thaw and crop-growing periods. This study offers a new perspective on reducing autumn irrigation in HID.

Automated summary

This study developed a water and salt balance model to predict long-term water and salt dynamics in seasonally frozen arid agricultural regions. The model was validated in a region in China and showed that increasing autumn irrigation water can decrease soil salinity during the irrigation period, but has limited impact on long-term average soil salinity.