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

• Annual, humid- and dry season ET declined significantly during 1980–2006 period.
• Changes of precipitation patterns and amounts are major reasons for ET declining.
• ET is little sensitive to climate change and land cover changes in humid seasons.
• Reforestation has little impacts on ET changes when regional P/PET > 1.
• Positive contributions of reforestation are offset by negative of climate change.

BackgroundEvapotranspiration (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.MethodsIn 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.ResultsThe 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 conclusionsBy 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 ET 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.