Abstract
Ecosystem water-use efficiency (EWUE) is an indicator of carbon–water interactions and is defined as the ratio of car-bon assimilation (GPP) to evapotranspiration (ET). Previous research suggests an increasing long-term trend inannual EWUE over many regions and is largely attributed to the physiological effects of rising CO2. The seasonaltrends in EWUE, however, have not yet been analyzed. In this study, we investigate seasonal EWUE trends andresponses to various drivers during 1982–2008. The seasonal cycle for two variants of EWUE, water-use efficiency(WUE, GPP/ET), and transpiration-based WUE (WUEt, the ratio of GPP and transpiration), is analyzed from 0.5°gridded fields from four process-based models and satellite-based products, as well as a network of 63 local fluxtower observations. WUE derived from flux tower observations shows moderate seasonal variation for most latitudebands, which is in agreement with satellite-based products. In contrast, the seasonal EWUE trends are not well cap-tured by the same satellite-based products. Trend analysis, based on process-model factorial simulations separatingeffects of climate, CO2, and nitrogen deposition (NDEP), further suggests that the seasonal EWUE trends are mainlyassociated with seasonal trends of climate, whereas CO2and NDEP do not show obvious seasonal difference inEWUE trends. About 66% grid cells show positive annual WUE trends, mainly over mid- and high northern latitudes.In these regions, spring climate change has amplified the effect of CO2in increasing WUE by more than0.005 gC m2mm1yr1for 41% pixels. Multiple regression analysis further shows that the increase in springtimeWUE in the northern hemisphere is the result of GPP increasing faster than ET because of the higher temperature sen-sitivity of GPP relative to ET. The partitioning of annual EWUE to seasonal components provides new insight intothe relative sensitivities of GPP and ET to climate, CO2,and NDEP.
