Simulating and projecting phreatic evaporation in Huaibei Plain using CMIP6 multi-model ensemble

Phreatic evaporation can constitute a critical vertical linkage in the vertical interaction between surface water and groundwater systems. Accurate simulation and quantification of phreatic evaporation are of great significance for the assessment and sustainable management of shallow groundwater resources in the Huaibei Plain. This study aims to simulate and project the phreatic evaporation using the CMIP6 multi-model ensemble. The measurements were performed to extract the data from the Wudaogou Hydrological Experimental Station. The commonly used formulas were applied to evaluate the regional features of phreatic evaporation. The Ye Shuiting formula was further optimized after evaluation. An advanced iterative algorithm was employed to integrate the spatial distribution of the groundwater depth data over the plain. The historical variation and dynamics of phreatic evaporation were simulated, particularly focus on the lime concretion black soil and yellow fluvo-aquic soil areas. Future projections of precipitation and evaporation from five CMIP6 climate models were used as the primary climatic forcing factors. Multi-model ensemble approaches were also employed to integrate these climate projections using long short-term memory (LSTM). As such, the phreatic evaporation iterative algorithm was combined with the optimal Ye Shuiting formula. Future trends of phreatic evaporation were predicted under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. The results show that: 1) The Ye Shuiting formula shared the high applicability to both lime concretion black soil and yellow fluvo-aquic soil in the study area. The optimal Ye Shuiting formula outperformed the rest empirical ones. The annual average phreatic evaporation was ranked in the descending order of: yellow fluvo-aquic soil area (254.5 mm)> areal average of Huaibei Plain (179.3 mm)> lime concretion black soil area (108.5 mm). Phreatic evaporation in all subregions also exhibited an increasing trend during the historical period. 2) The LSTM multi-model ensemble demonstrated strong performance in reproducing the variations in precipitation and evaporation during the baseline period. Projections indicated that the future precipitation and evaporation exceeded the historical levels under the three emission scenarios, with the ranking order of SSP5-8.5, SSP1-2.6, SSP2-4.5. The phreatic evaporation iterative algorithm showed that the amplitude of groundwater depth fluctuations was smaller under the three future emission scenarios than that during the historical period. Specifically, the groundwater depths under the SSP1-2.6 and SSP2-4.5 scenarios were lower than those during the historical period, whereas those under SSP5-8.5 exhibited a significantly larger relative to the historical period. 3) The magnitudes of phreatic evaporation variations under the three future emission scenarios exceeded those in the historical period, indicating an overall increasing trend. The amplification amplitudes and change rates were ranked in descending order of the SSP1-2.6, SSP2-4.5, and SSP5-8.5. Notably, the change rate and uncertainty in winter were significantly higher than those in spring, summer, and autumn. The high accuracy of phreatic evaporation can greatly contribute to the data reference for the simulation and prediction of the phreatic evaporation and water cycle components in the Huaibei Plain.