Simulation of Evapotranspiration and Its Response to Environmental Changes in the Semi-Humid and Semi-Arid Regions of Northern China

Abstract: The semi-humid and semi-arid region of Northern China is a typical climatic transition zone characterized by water scarcity and ecological fragility. As water resources constitute the critical constraint on sustainable development in this region, investigating evapotranspiration (ET)—the primary pathway of water loss—is imperative. This study employed a Soil-Plant-Atmosphere Continuum (SPAC) model to characterize hydro-thermal transfer processes within the vegetation-soil system from 2001 to 2020, quantifying both total ET and its components. Model validation demonstrated robust performance against in-situ observations, with coefficients of determination (R²) for latent heat flux, net radiation, and land surface temperature ranging from 0.48 to 0.95 across four representative sites. Spatially, ET decreased from southeast to northwest, with a multi-year regional average of 439.53 ± 32.80 mm. ET exhibited distinct seasonal variability, peaking in summer (235.61 ± 24.15 mm) followed by spring (111.13 ± 13.02 mm), autumn (74.13 ± 9.07 mm), and winter (18.38 ± 3.46 mm). Partitioning analysis revealed that the multi-year average vegetation transpiration (T) and soil evaporation (Es) were 244.53 ± 30.16 mm and 195.00 ± 15.52 mm, respectively, yielding a mean transpiration fraction (T/ET) of 0.54 ± 0.04. The spatial pattern of T/ET was demarcated by the Greater Khingan – Taihang Mountains, showing higher values in the east, lower values in the west, and peak values along the boundary line and its vicinity. Seasonal divergence was pronounced: transpiration dominated in summer (T/ET reaching 0.64 ± 0.05), whereas soil evaporation prevailed in other seasons, reducing T/ET to 0.17 ± 0.04 in winter. ET and its components showed significant sensitivity to environmental changes. Spearman analysis indicated strong correlations (r > 0.8) with downward shortwave radiation (Rs), vapor pressure deficit (VPD), and leaf area index (LAI). Random Forest and SHAP analyses further revealed different key factors influencing the processes: Rs, VPD, and LAI were the primary drivers for total ET; soil evaporation was mainly influenced by Rs, relative humidity, and VPD; and transpiration was mainly driven by LAI and Rs, with importance values of 0.35 and 0.17, respectively. Notably, LAI was crucial in controlling the T/ET ratio, with an importance value of 0.46. These findings offer vital scientific insights for ecosystem conservation and water resource management in the semi-humid and semi-arid regions of Northern China.

Keywords: Evapotranspiration; Numerical simulation; SPAC model; T/ET ratio; Climate change; Leaf Area Index; Semi-humid and semi-arid regions of Northern China.