The relationship of δD and δ18O in soil water and its implications for soil evaporation across distinct rainfall years in winter wheat field in the North China Plain

Abstract

Soil evaporation plays a key role in regulating local climate and water loss. Stable isotope ratios of water (²H/¹H and ¹⁸O/¹⁶O) are effective tracers for studying water flux. This study examines three isotope-based indicators deuterium excess (d-excess), the slope of the soil water evaporation line (SEL), and line-conditioned excess (lc-excess) across three wheat growing seasons: wet, ordinary, and dry years. The influencing factors of d-excess, SEL, and lc-excess, respectively, soil, vegetation, and meteorology, were analyzed using various methods. Wheat yields varied significantly, reaching 6.69 t ha⁻¹ in wet years, 8.66 t ha⁻¹ in dry years, and 9.28 t ha⁻¹ in ordinary years. The lc-excess was highest in ordinary years, and d-excess peaked during dry years. A negative correlation between d-excess and SEL slope, and between lc-excess and SEL slope, was observed in dry and ordinary years (P<0.05), but not in wet years (P>0.05). Multivariate regression showed that net radiation (Rn) was the primary factor influencing SEL, contributing 54.19 %, 11.58 %, and 29.27 % in wet, dry, and ordinary years, respectively. Leaf area index (LAI) was the most significant factor affecting lc-excess (37.91 % in wet years, 32.22 % in dry, and 30.92 % in ordinary years). Vapor pressure deficit (VPD) affected d-excess in wet and ordinary years, while air (Ta) and soil temperature (Ts) were key in dry years. Variation partitioning revealed meteorological factors primarily influenced SEL, lc-excess, and d-excess in wet years, while soil, vegetation, and climate interactions had greater effects in dry and ordinary years. The lc-excess, integrating multiple factors, is a better indicator of soil evaporation than SEL.