PS 24-46 - Hydrological losses and soil moisture carryover affected the relationship between evapotranspiration and rainfall in a temperate semiarid shrubland

Background/Question/Methods

Evapotranspiration (ET) represents a major pathway of water loss from terrestrial ecosystems. However, large uncertainty exists regarding the relationship between ET and precipitation at seasonal and interannual timescales in drylands. Using the eddy-covariance technique, we investigated temporal variations of ET in a semiarid shrubland of northern China during January 2012–December 2020. This study aimed to characterize the temporal variations in ET, and to examine the biophysical controls over ET at seasonal and interannual timescales. In particular, we tested the hypotheses that (1) factors related to water stress (rainfall, soil moisture, and vapor pressure deficit) are dominant over biotic factors in explaining ET dynamics at seasonal and interannual timescales, and (2) the degree of coupling between ET and rainfall depends on their seasonal patterns, which are associated with potential hydrological losses and inter-seasonal soil moisture carryover.

Results/Conclusions

Monthly ET was positively and linearly correlated with the normalized difference vegetation index and surface conductance (g_s), and showed a saturating relationship with monthly rainfall. Analyses based on bin-averaged half-hourly values revealed increases in midday ET and g_s with increasing soil water content, and decreases in midday g_s and saturated ET with increasing vapor pressure deficit. Annual ET for ecohydrologic years (July 1–June 30) ranged 207–297 mm yr^-1 (with a multi-year average of 242 mm yr^-1). Aggregated ET over the study period (2163 mm) accounted for 78% of total rainfall. Rainfall amount was a poor predictor of year-to-year differences in annual ET, wet-season (July 1–November 30) ET, and dry-season (December 1–June 30) ET. Wet-season ET (mm season^-1) was generally lower than rainfall while dry-season ET tended to be higher than rainfall. Our results suggest that biotic factors played an essential role in regulating seasonal variations in ET, while air and soil dryness imposed additional constraints on ET at finer timescales. Hydrological losses (e.g., to subsurface runoff and/or deep drainage) during wet seasons and soil moisture carryover from wet to dry seasons may be nonnegligible in the studied semiarid shrubland, and thus affected seasonal and interannual relationships between ET and rainfall.