Canopy greenness, atmospheric aridity, and large rain events jointly regulate evapotranspiration partitioning in a temperate semiarid shrubland

Evapotranspiration (ET) partitioning is critical for evaluating ecosystem productivity, water balance, and plant vs. ecosystem water use efficiency. However, large uncertainties remain regarding the dynamics and regulating factors of the ratio of transpiration (T) to ET (i.e., T/ET) in semiarid shrublands. Using eddy-covariance and sap flow measurements, we investigated the canopy and climatic controls on T/ET at seasonal and interannual timescales in a shrubland of northern China. Transpiration accounted for 0.59 +/- 0.12 (mean +/- SD) of ET over the main-growing seasons (May-September) of 2012-2017, with seasonal T/ET ranging from 42% to 77% across years. The T/ET ratio generally increased over the main-growing season, being low in May/June and high in August/September. At the seasonal scale, monthly T/ET increased linearly with normalized difference vegetation index (NDVI) and surface conductance, decreased linearly with increasing vapor pressure deficit (VPD), and showed saturated responses to increasing monthly rainfall. Bin-averaged daily T/ET increased linearly with NDVI, decreased asymptotically to about 0.50 with increasing VPD, and showed a weak positive correlation with soil water content (SWC) at 30 cm depth. At the interannual scale, aggregated main-growing-season T/ET increased with mean NDVI and decreased with increasing mean VPD, but was independent of seasonal mean SWC and rainfall amount. Nevertheless, large storms favored higher T/ET, with the amount and proportion of rainfall delivered in large events (i.e., > 25 mm d-1) explaining about 80% of the variability in seasonal T/ET across years. Our results suggest that canopy greenness, atmospheric aridity, and large rain events jointly regulated ET partitioning in the studied semiarid shrubland. Changes in precipitation regime and atmospheric aridity under climatic warming may have consequences for T/ET, ecosystem productivity, and water use efficiency in temperate semiarid drylands.