50-year evapotranspiration declining and potential causations in subtropical Guangdong province, southern China

Background: Evapotranspiration (ET) is an important flux term in the terrestrial hydrologic cycle system that integrated atmospheric and land surfaces, hydrological and biological processes. Any variations of the hydrological processes induced by climate change and forest management would be significantly reflected in ET component. Methods: In this paper, we firstly calculated Guangdong's 50-year annual, humid- and dry-season ET jointly based on the water balance model and a physical-based ET model. Then, the separate contributions of climate change and reforestation on ET changes were evaluated by the double mass curve method. Results: The time-series annual ET declined significantly (p < 0.001) during the past 50 years. The year 1980 showed to be an important change point. Both humid- and dry-season ET kept much stable during the former period 1956-1979, while decreased significantly (p < 0.001) during the later period 1980-2006. Climate change and reforestation contributed for about -34.02 and 31.03 mm/year for the annual ET variations, while about -28.18 and 27.25 mm/year for the dry seasons, and about -5.84 and 3.78 mm/year for the humid seasons, respectively. Discussions and conclusions: By comparison, the positive contributions of reforestation on ET changes were completely offset by the negative contributions of climate changes. Climate changes were the key factors responsible for the reduction of Guangdong's ET. In particular, the regional ET in dry seasons was quite more sensitive to climate change and land cover changes, while much less sensitive in humid seasons. Our results also confirmed that land cover changes would have little impacts on the regional Er changes when P/PET > 1, but might cause greatly influences on ET when P/PET < 1. These conclusions indicate that afforestation might be actively encouraged in humid regions, but should be treated cautiously in non-humid regions or even be limited in arid regions. It will be directly useful for guiding government in future forest management. (C) 2015 Elsevier B.V. All rights reserved.