Estimation of water use efficiency in the terrestrial ecosystems of the Yangtze River Delta region, China

Understanding the characteristics of water use efficiency (WUE) could enhance our understanding of the integrated response of terrestrial ecosystems to climate change. In this study, the vegetation photosynthesis model (VPM) and the revised remote sensing-Penman Monteith (RRS-PM) model were used to characterize the spatial-temporal dynamics of gross primary productivity (GPP) and evapotranspiration (ET) in the Yangtze River Delta (YRD) region. Our predicted GPP and ET results were in the range of those reported by other models. To be specific, the RRS-PM model could explain 85% of the variability in ET at the Anji site, China. The root mean square error between the simulation value and the measured ET was 8.543 mm month(-1). For the evergreen broadleaf forest, deciduous needle leaf forest and deciduous broadleaf forest, GPP decreased and ET increased, thus the WUE = (GPP/ET) inevitably decreased. The increase in WUE in grassland was due to the decreased ET, for which the rate of decrease was greater than for GPP. For the spatial distribution of WUE, the maximum value occurred in the southern and northeastern parts of the YRD region, while the minimum value was distributed in the middle and northwest. This is likely due to the different land-cover types. In the south, there is more forest, while in the north the land-cover type is almost entirely cropland. In our study, GPP increased with increasing LAI across all of the plant function types. However, ET only increased with LAI when LAI was less than 2 m(2) m(-2) across all of the plant function types, once LAI was larger than 2 m(2) m(-2), ET did not continue to increase with LAI.