Modeling of Soil Water and Salt Dynamics with Shallow Water Table and Its Effects on Root Water Uptake in Heihe Arid Wetland, China.

Abstract: It is critical to understand the interactions between soil water and salts of wetland and their effects on plant growth under the influence of shallow groundwater and irrigation measures regarding climate warming. In this study, the Hydrus-1D model was used in an arid wetland of the Middle Heihe River, China to investigate the effects of the dynamics of soil water, soil salinization, and depth to water table (DWT) as well as groundwater salinity on Chinese tamarisk root water uptake. The modeled soil water and electrical conductivity of soil solution (EC_sw) are in good agreement with the observations, as indicated by RMSE values (0.031 and 0.046 cm³·cm⁻³ for soil water content, 0.037 and 0.035 dS·m⁻¹ for EC_sw, during the model calibration and validation periods, respectively). The calibrated model was used in scenario analyses considering different DWTs, salinity levels and the introduction of preseason irrigation. The results showed that (I) Chinese tamarisk root distribution was greatly affected by soil water and salt distribution in the soil profile, with about 73.8% of the roots being distributed in the 20–60 cm layer; (II) root water uptake accounted for 91.0% of the potential maximal value when water stress was considered, and for 41.6% when both water and salt stress were considered; (III) root water uptake was very sensitive to fluctuations of the water table, and was greatly reduced when the DWT was either dropped or raised 60% of the 2012 reference depth; (IV) arid wetland vegetation exhibited a high level of groundwater dependence even though shallow groundwater resulted in increased soil salinization based on future 30-yr climate shifts; and (V) preseason irrigation, as one mitigation strategy, could effectively increase root water uptake by leaching salts from the root zone. We concluded that a suitable water table and groundwater salinity coupled with proper irrigation are key factors to sustainable development of arid wetlands.