Effects of forest evapotranspiration on soil water budget and energy flux partitioning in a subalpine valley of China

Jiuzhaigou Valley (JZG) has been rated as World Heritage, National Nature Reserves and world-class places of interests, due to its fantastic beauty of water. In recent years, however, JZG is losing its beauty with runoff decreasing and stream drying up. An increase of evapotranspiration (ET) over the rehabilitating forest is assumed to be the main reason for runoff decreasing, however, this assumption remained untested for lack of long-term observing data. To test this assumption, a long-term field experiment was carried out from August 2013 to December 2015 in JZG over a typical secondary forest. Meteorological parameters and energy fluxes were measured by eddy covariance system, soil moisture by time-domain reflectometry probes, and ET by eddy covariance method and soil water budget. Results showed that (1) despite the limitation of high elevation, high atmospheric humidity and low annual air temperature, annual ET was quite large. It was 700 mm and 739 mm, respectively for 2014 and 2015, while the corresponding precipitation (P) was 1003 mm and 782 mm. ET/P was close or over 1, which meant that P was mostly used for forest ET and only a very small proportion of P contributed to runoff. (2) The ratio of latent heat flux (LE) to net radiation (R-n) was 0.69 and 0.75 in the growing season of 2014 and 2015, respectively. These high LE/R-n ratio values were considerably higher in JZG than in other areas of the Tibetan Plateau, which could be attributed to better forest rehabilitation in JZG. Our results also showed that ET at the forest site was limited by available energy. We conclude that ET in JZG was energy limited, whereas forest ET consumed most precipitation. (3) The above assumption is verified to be true. The high ET rate after forest rehabilitation in JZG comes at a cost of decreasing runoff and leads to streams drying up. These results will be useful for sustainable managements of water and forest in JZG.