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Water yield (WY) refers to the difference between precipitation and evapotranspiration (ET), which is vital for available terrestrial water. Climate change has led to significant changes in precipitation and evapotranspiration on a global scale, which will affect the global WY. Nevertheless, how terrestrial WY has changed during the past few decades and which factors dominated the WY changes are not fully understood. In this study, based on climate reanalysis and remote sensing data, the spatial and temporal patterns of terrestrial WY were revisited from 1981 to 2018 globally using an improved Mann-Kendall trend test method with a permutation test. The response patterns of WY to precipitation and ET are also investigated. The results show that the global multi-year mean WY is 297.4 mm/a. Based on the traditional Mann-Kendall trend test, terrestrial WY showed a significant (p < 0.05) increase of 5.72% of the total valid grid cells, while it showed a significant decrease of 7.68% of those. After correction using the calibration method, the significantly increasing and decreasing areas are reduced by 10.52% and 10.58% of them, respectively. After the correction, the confirmed increase and decrease in WY are mainly located in Africa, eastern North America and Siberia, and parts of Asia and Oceania, respectively. The dominant factor for increasing WY is precipitation, while that for decreasing WY was the combined effect of precipitation and evapotranspiration. The achievements of this study are beneficial for improving the understanding of WY in response to hydrological variables in the context of climate change. [ABSTRACT FROM AUTHOR]

Ecosystem-scale water-use efficiency (WUE) is an important indicator for understanding the intimately coupled relationship between carbon and water cycles in ecosystems. Previous studies have suggested that both abiotic and biotic factors have significant effects on WUE in forest ecosystems. However, responses of WUE to phenology in the context of climate change remain poorly understood. In this study, we analyzed the sensitivity and response patterns of seasonal WUE to phenology in Zhejiang Province where typical subtropical forest ecosystems are located, and discussed potential causes of the changes of the sensitivity and response patterns along different climate gradient during 2000–2014. The results of interannual partial correlation analysis showed widespread negative correlations between WUE and the start of growing season (SOS) in spring. This is because the increase in gross primary product (GPP) is larger than that of evapotranspiration (ET), resulting from an advanced SOS. The positive correlation between WUE and SOS was widely observed in summer mainly because of water stress and plant ecological strategy. The autumn WUE enhanced with the delay in the end of growing season (EOS) mainly because of the increase in GPP meanwhile the decrease or steadiness in ET, resulting from a delayed EOS. In space, the sensitivity of spring WUE to SOS significantly decreased along the radiation gradient, which might be related to strong soil evaporation in high radiation area; the sensitivity of WUE to SOS in summer showed a positive correlation with precipitation and a negative correlation with temperature, respectively, which might be attributed to the compensation of GPP to the delayed SOS and water stress caused by high temperature. The sensitivity of WUE to EOS increased significantly along the radiation and precipitation gradients in autumn, which may be because the increase of radiation and precipitation provides more water and energy for photosynthesis. [ABSTRACT FROM AUTHOR]

• The explanatory power of EOS to the interannual variations in GPP, ET and WUE were quantitatively assessed. • The autumn phenology played a crucial role in regulating GPP, ET and WUE in autumn on the Tibetan Plateau. • The delayed EOS increased the daily mean GPP but meanwhile increased the ET in autumn, rather than enhanced the WUE. Vegetation phenology is a commonly used indicator of climate-vegetation interactions, which also plays a crucial role in regulating gross primary productivity (GPP), evapotranspiration (ET) and ecosystem water use efficiency (WUE, defined as the ratio between GPP and ET). Actually, quantitative evaluation of the impact of phenology on WUE is critical yet lacking. In this study we detected the interannual variations of autumn WUE, GPP and ET and conducted Redundancy analysis (RDA) to quantitatively assess their responses to autumn end of growing season (EOS) in the grassland ecosystem across the Tibetan Plateau. Our study found that the delayed EOS increased the daily mean GPP but meanwhile increased the ET in autumn, rather than enhanced the WUE. Over the entire Tibetan Plateau, the daily GPP and ET in the autumn of the steppe were dominated by the delayed EOS, with the explanatory power of EOS to GPP and ET of 75.90% (P < 0.01) and 21.80% (P < 0.05). An one day delay of the EOS increased the daily GPP and ET by 0.05 × 10−1 gC m−1 day−1 (P < 0.01) and 0.27 × 10−1 mm day−1 (P < 0.01), respectively. Furthermore, the EOS agreed on a positive correlation with daily GPP and ET in different ecoregions and the interannual variations of WUE for the meadow and steppe in the different ecoregions across the Tibetan Plateau did not show significant trend because of this same increasing pattern of GPP and ET form the delayed EOS. This study highlights the impacts of EOS on the interannual changes of the GPP, ET and WUE in autumn in a large cold and semiarid/arid area, which indicated that phenology should be regard as a crucial indicator in the process of carbon water coupling cycles and their biological regulatory mechanisms in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Improving understanding of changes in intra-annual variability (IAV) of evapotranspiration (ET) and the underlying drivers is an essential step for modeling hydrological processes in response to global change. Previous studies paid special attention to climatic regulations of IAV of ET. However, ignoring the role of landscape characteristics (e.g., vegetation coverage) can introduce great uncertainty in the explanation of ET variance. In this work, the Poyang Lake Basin, which is a typical humid basin in China, was taken as the study area. It has experienced an obvious climate change and revegetation since the 1980s. Here, trends of IAV of ET and their responses to four climatic variables (i.e., air temperature, precipitation, downward shortwave radiation and wind speed) and vegetation coverage were explored from 1983 to 2014. The results show that IAV of ET exhibited contrary trends during the past decades. It significantly (p < 0.05) declined with a significant linear slope of −0.52 mm/year before 2000, and then slightly increased (slope = 0.06 mm/year, p > 0.05) over the basin, which was generally consistent with the IAV of temperature and radiation. The proposed variables could well capture the change in IAV of ET, while their dominators were different during the two contrasting phases mentioned above. The IAV of radiation and temperature dominated the change of the IAV of ET over 77.82% and 35.14% of the basin, respectively, before and after the turning point. Meanwhile, the rapid increase in vegetation coverage, which was associated with afforestation, significantly (p < 0.05) reduced IAV of ET over about 35% of the study area. The achievements of this study should be beneficial for a sophisticated awareness of responses of intra-annual variability of ET to climate and land cover changes at the basin scale. [ABSTRACT FROM AUTHOR]