Modeling Coupled Water-Salt-Heat Transport in a Rainwater-HarvestingSubsurface Irrigated Wolfberry Field Using HYDRUS-2D

To investigate the soil water, salt, and heat dynamics in a wolfberry field under runoff-collecting infiltration irrigation in an arid region, this study conducted an orthogonal experiment at the wolfberry planting base in Gangou Village, Yuanzhou District, Guyuan City, Ningxia, to determine the optimal layout pattern for the runoff-collecting infiltration irrigation system. Subsequently, a simulated rainfall device was used to replicate natural rainfall conditions in the experimental area, while actual variations in soil water content, salinity, and temperature at depths of 20–50 cm within a horizontal distance of 0–80 cm from the infiltrator were monitored. The HYDRUS-2D model was then employed to simulate and predict the two-dimensional transport and distribution patterns of soil water, salt, and heat in the experimental field. The long-term application effects of the runoff-collecting infiltration irrigation technique were also analyzed. The results indicated that: (1) The order of influencing factors on the system layout pattern was: catchment area > rainfall amount > catchment material > slope of the catchment film. The optimal configuration consisted of ordinary plastic film, a catchment film slope of Grade III, and a catchment area of 1.6 m². (2) The HYDRUS-2D model simulations showed good agreement with measured values for soil water, salt, and heat. The Nash-Sutcliffe efficiency (NSE) ranges for the model evaluations were 0.63–0.88 (water), 0.39–0.81 (salt), and 0.043–0.45 (heat), respectively. The root mean square error (RMSE) ranges were 0.001%–0.013% (water), 22.29–75.67 mg/L (salt), and 0.07–0.33 °C (heat), respectively. (3) Predictions from the HYDRUS-2D model revealed that by the end of the irrigation period, compared to initial values, soil water content and salt concentration in the wolfberry root zone increased by 3.37% and 1.86%, respectively, while soil temperature decreased by 22.34%. During the irrigation period, the distribution characteristics and variation patterns of soil moisture and salinity in the two-dimensional profile of the root zone exhibited significant spatiotemporal heterogeneity. In contrast, soil temperature decreased linearly and uniformly without showing distinct spatial variation. This research can provide a theoretical basis and scientific guidance for the promotion and application of runoff-collecting infiltration irrigation technology in arid and semi-arid regions.