Effects of soil water content on forest ecosystem water use efficiency through changes in transpiration/evapotranspiration ratio

Ecosystem water use efficiency (WUE), defined as the ratio of gross primary production (GPP) over evapotranspiration (ET), is a crucial variable that describes the tradeoff between ecosystem carbon uptake and water loss. However, the effects of soil moisture on WUE remain unclear. Based on hourly observations from 36 forest eddy-covariance sites globally and two percentile-based statistical models, we investigate the effects of soil water supply (i.e., volumetric soil water content, SWC) and vapor pressure deficit (VPD) on carbon-water coupling at the hourly timescale. We further decompose WUE into two components, i.e., the ratio between GPP and plant transpiration (T), WUEt (GPP/T), and the ratio of plant transpiration to total evapotranspiration, T/ET. Results show that the sensitivities of WUE to SWC significantly increased with the increase of VPD. The sensitivities of WUE to SWC are high at high VPD, due to a rapid decrease of T/ET with SWC at high VPD. At low VPD, WUE is largely independent of SWC. The contribution linked to WUEt is much weaker and no significant relationships are observed between the sensitivities of WUEt to SWC and VPD. Our study suggests that the change of ecosystem WUE with SWC is largely due to changes in T/ET, while WUEt remains rather constant.