Evaluating the effects of different irrigation water sources on soil temperature using HYDRUS (2D/3D) and considering the coupled movement of water and heat

Drip irrigation with alternate use of surface water and groundwater (ADI) has been widely applied in arid regions to relieve the effects of heat stress on crop growth. However, the heat dynamics under ADI are still unclear, especially concerning the impacts of ADI on daily and seasonal fluctuations of soil temperature (T_s). Thus, a two-year experiment was carried out during 2019–2020 in the Jiuzhuang comprehensive saving-water experimental station to continuously monitor soil water content (SWC) and T_s variations. Moreover, the HYDRUS (2D/3D) software was used to simulate T_s fluctuations under various evaluated scenarios involving a) surface water irrigation (SW), b) groundwater water irrigation (GW), c) alternate use of groundwater and surface water irrigation (1G1S), d) two groundwater irrigations and one surface water irrigation (2G1S), and e) three groundwater irrigations and one surface water irrigation (3G1S). The result showed that the HYDRUS (2D/3D) software could precisely simulate soil water content and T_s dynamics under all irrigation treatments, with the root mean square error of 0.01–0.06 cm³ cm⁻³ and 1.25–1.57 °C for SWC and T_s in the verification period, respectively. Apparent spatial-temporal differences in diurnal T_s fluctuations under different ADI treatments were found, especially in the 5 cm soil depth. In general, T_s decreased in response to an increase in the frequency of groundwater irrigation. The lowest T_s occurred in the 3G1S treatment under different ADI treatments. The average T_s in both years under 3G1S was 12.2 % lower than under SW and 4.4 % higher than under GW. However, the highest T_s occurred in the 1G1S treatment under different ADI treatments. Average T_s in both years under 1G1S increased by 8.6 % and 15.4 % compared to 2G1S and 3G1S, respectively. Meanwhile, the difference in T_s fluctuations under different ADI treatments during daytime was substantially higher than during nighttime. The largest area (1271.8 cm²) of “moderate T_s” (20–22 ℃) occurred in the 2G1S treatment. Moreover, the longest “optimal T_s” duration occurred for the 22.5 mm irrigation depth under 2G1S. Therefore, the irrigation depth of 22.5 mm and the 2G1S treatment is recommended as the optimal irrigation strategy in this region.