Stronger control of surface conductance by soil water content than vapor pressure deficit regulates evapotranspiration in an urban forest in Beijing, 2012-2022

With urbanization rapidly increasing, evapotranspiration (ET) in urban forests plays an increasingly important role in urban hydrology and climate. However, large uncertainty remains regarding the regulating factors of ET in urban area. Using the eddy-covariance technique, we investigated the temporal variations of ET in an urban forest park in Beijing for 2012-2022. Daily ET was close to zero during winter and reached 3-6 mm day-1 in summer. Daily ET increased with vapor pressure deficit (VPD) and soil water content (SWC). When binning daily data into 10 bins according to percentiles of either SWC or VPD, surface conductance (gs) increased with SWC for all VPD bins, but showed weak responses to VPD for SWC bins. Monthly ET increased linearly with normalized difference vegetation index, showed a strong correlation with gs, and exhibited saturated responses to increasing monthly precipitation (PPT). Annual ET ranged from 326 to 566 mm (with the eleven-year mean of 428.66 +/- 83.83 mm; mean +/- SD), and varied from 55 % to 148 % of annual PPT. Aggregated ET over the eleven years (4715 mm) accounted for 82 % of the aggregated PPT. Soil water replenishment through PPT from mid-summer to late autumn of the previous year was responsible for the generally higher monthly ET in spring relative to PPT, resulting in a weak correlation between annual ET and PPT in the same calendar year (R2= 0.28). The PPT in the early growing season partly promoted annual ET through its effect on summer gs. Our results suggest that biotic factors and PPT seasonality played an essential role in regulating ET at the seasonal and interannual scales, respectively. Soil dryness imposed additional constraints on gs at the finer timescale (i.e., days). Urban land surface models should adequately address the soil moisture carryover effect to improve simulations.

Automated summary

With the rapid increase of urbanization, evapotranspiration (ET) in urban forests has become increasingly important in urban hydrology and climate. However, there is still a large uncertainty regarding the factors that regulate ET in urban areas. This study investigates the temporal variations of ET in an urban forest park in Beijing using the eddy-covariance technique. The results show that daily ET is close to zero during winter but reaches 3-6 mm day-1 in summer. Daily ET increases with vapor pressure deficit (VPD) and soil water content (SWC). Monthly ET increases linearly with normalized difference vegetation index and shows a strong correlation with surface conductance (gs), while exhibiting saturated responses to increasing monthly precipitation (PPT). Annual ET ranges from 326 to 566 mm, and soil water replenishment through PPT from the previous year is responsible for the generally higher monthly ET in spring relative to PPT. Biotic factors and PPT seasonality play essential roles in regulating ET at different scales.