Assessing the effects of water table depth on water use, soil salinity and wheat yield: Searching for a target depth for irrigated areas in the upper Yellow River basin
The irrigation districts of the upper Yellow River basin face a progressive reduction of water allocation and the need to apply water-saving practices due to increasing water scarcity. The adoption of such practices will lead to lower water tables, hopefully in conjunction with controlled soil salinity levels and improved crop yields. However, excessive water saving associated with excessive increase of the water table depth may decrease capillary rise and affect crop production. In view of understanding the related processes in Qingtongxia Irrigation District, the physically based agro-hydrological model, SWAP, was adopted to explore the response of soil water and solute dynamics, and crop yield to water table changes. With this purpose, SWAP was modified through the inclusion of a simplified crop growth module, a method of variable active-nodes, and a nonlinear osmotic head-dependent function for a better description of the effects of salinity stress on root water uptake. The model was calibrated and validated using wheat's experimental data from 2007 and 2008. Simulations of soil water content, salinity concentration, biomass, and crop yield fitted well with field observations. The calibrated model was then used to predict changes in crop yield, soil water dynamics and soil salinity considering scenarios with different groundwater depths and irrigation strategies. The present irrigation strategy favors salt leaching even when considering the increase of water table depth that results in small crop yield reduction (<6%) due to salt stress. The 80% and 60% reduced irrigation strategies led to increased soil salinity and, eventually, crop yield reductions of 6–14% and 13–21%, respectively, when the depth of the water table increased. A target groundwater depth of 1.0–1.5m is suggested to be optimal for wheat's growth season with the aim of maintaining crop yields under the present conditions.