At the local scale, artificial impounded reservoirs in dry regions exert influence on the surrounding local climate. Impounded reservoirs have been found to alter precipitation patterns and increase temperature, specific humidity and surface evaporation. The consequences of impoundment or its related climatic changes on the surrounding vegetation are still not well understood. We here examined the potential effect on surrounding tree growth of the impoundment of the Ertan Reservoir, China, in 1998. We measured Yunnan pine tree rings at three sites on a slope close to the reservoir and two control sites far away from the reservoir. We compared the annual radial growth of tree rings at all sites with air temperature, precipitation and air humidity in the region. We also used cumulative distribution functions to quantify the probability of attributing radial growth changes to background climate variability. We find an enhancement of tree growth from 2000 to 2002, right after the reservoir's impounding in experimental and control sites. Hence, the tree growth can initially be explained by favourable conditions benefiting tree growth. However, we cannot entirely attribute such enhancement to the background climate variability when studying the relationship between tree growth and climatic variables over the 36 years and their probability of occurrence. Tree growth in the three years following impoundment can only be attributed to the simultaneous effect of favourable regional climate conditions and the reservoir's impoundment in itself. These conditions decrease vegetation stress by decreasing air temperature and increasing air relative humidity. Although the findings of this study shed more light on the environmental and climatic changes induced by the impoundment of reservoirs, they also call for the need of monitoring climatic variables in the vicinity of reservoirs.

Water management by the impoundment of reservoirs has been found to influence evapotranspiration not only locally but also at the basin scale. Highly regulated hydrological basins generally show the effect of a net increase in evapotranspiration accompanying the successive impoundment of reservoirs. However, understanding and isolating the effect from a particular single impounded reservoir remains a challenge due to the lack of long-term observation data required and the existence of many other drivers present at the basin scale. Focusing on the hydrological basin having the largest hydropower potential in China, we isolated in time and space and quantified the effects of a single impounded reservoir on evapotranspiration and the evaporative ratio (i.e., the ratio of actual evapotranspiration to precipitation) before and after the construction of the Ertan Dam in 1998. We find that the dam has increased evapotranspiration in the smallest subbasin by 46±15 mm/yr and the evaporative ratio by 0.05±0.015, from the period before impoundment (1983-1997) to that after impoundment (2000-2012). This increase is found only within the smallest differential subbasin holding the impounded reservoir and cannot be explained by other changes in land use or vegetation. We use this result from our hydrological basin-constrained approach to calculate the water footprint of the hydroelectric project as 16.5 m³/GJ, which accounts for additional hydroclimatic effects of the impoundment of the reservoir beyond the water surface. Hence, this study finds that when runoff data is available, the water consumption and the water footprint of hydropower projects can be calculated by water mass balance at the scale of their hydrological basins.

The relationship between climate and forest is critical to understanding the influence of future climate change on terrestrial ecosystems. Research on trees at high elevations has uncovered the relationship in the Hengduan Mountains region, a critical biodiversity hotspot area in southwestern China. The relationship for the area at low elevations below 2800 m a.s.l. in the region remains unclear. In this study, we developed tree ring width chronologies of Pinus yunnanensis Franch. at five sites with elevations of 1170–1725 m in this area. Monthly precipitation, relative humidity, maximum/mean/minimum air temperature and the standardized precipitation evapotranspiration index (SPEI), a drought indicator with a multi-timescale, were used to investigate the radial growth-climate relationship. Results show that the growth of P. yunnanensis at different sites has a similar response pattern to climate variation. Relative humidity, precipitation, and air temperature in the dry season, especially in its last month (May), are critical to the radial growth of trees. Supplemental precipitation amounts and reduced mean or maximum air temperature can promote tree growth. The high correlations between chronologies and SPEI indicate that the radial growth of trees at the low elevations of the region is significantly limited by the moisture availability. Precipitation in the last month of the previous wet season determines the drought regime in the following dry seasons. In spite of some differences in the magnitudes of correlations in the low-elevation area of the Hengduan Mountains region, chronologies generally matched well with each other at different elevations, and the differences are not evident with the change in elevation.

Climatic variation within a typical dry-valley area located in the southern Hengduan Mountains of China is studied, and the potential regional climate influences of large reservoirs in the area are discussed. Six meteorological stations near a reservoir are identified and classified into two categories (dry and non-dry valleys) to compare their level of climate change. Temperatures tended to increase since 1990 with a precipitation shift toward the dry season in both dry and non-dry valleys. Wavelet analysis shows that temperature and precipitation have significant variation with periods of 3.6 and 16.5 years, respectively. The standardized precipitation evapotranspiration index (SPEI) shows that dry valleys have multiple drought trends. Temperature in non-dry valleys changed more than that in dry valleys, but the variations of other indices in the two categories of valleys are not statistically different. The climatic variation of one station is in accordance with the reservoir filling, which is related to the orientation of the reservoir in the prevailing wind direction especially during summer. This study provides a profile of the climate change of dry valleys and documents the influence of large artificial reservoirs on regional climate.