Effects of evapotranspiration and precipitation on dryness/wetness changes in China

Based on daily meteorological data from 693 weather stations for the period 1960–2017, the characteristics of dryness/wetness trends and their relation with reference crop evapotranspiration (ET 0 ) and precipitation changes were assessed in China. The results showed that the semi-arid/semi-humid areas and humid areas of southwestern China experienced a drying trend, while the arid areas in northwestern China and the humid areas in southern China became wetter over the past 58 years. The dryness/wetness trends were highly sensitive to the variability of ET 0 in the arid and semi-arid areas and to precipitation in the humid and semi-humid areas. Regional differences in the contributions of ET 0 and precipitation to the dryness/wetness trends were significant. In the arid areas, the average contribution of ET 0 was 7% larger than that of precipitation, except in winter. A decreased ET 0 due to a reduction in wind speed and the increase in precipitation led to a wetting trend in these areas. In the semi-arid/semi-humid areas, ET 0 had a significantly larger effect on the dryness/wetness trends than precipitation. Contribution of ET 0 was about 21% larger than that of precipitation in spring and about 7% larger at the annual timescale. Due to an increase in ET 0 because of rising temperature and decrease in precipitation, the areas tended to be drier. In humid areas, precipitation was the dominant factor for the wetting trends. Contribution of precipitation was about 4 to 10% larger than that of ET 0 in summer and at the annual timescale and even completely dominated the trend of SPEI in winter. However, contribution of ET 0 in spring was slightly larger than that of precipitation in the humid areas, and a significant increase in temperature led to an increase in ET 0 and resulted in a tendency of dryness. Aside from the reduction in precipitation, increased evapotranspiration had comparable contributions to the trends of dryness, even greater than that of precipitation in the drying season and in drying areas. Our findings suggested that the effects of ET 0 against the background of global warming deserve more attention in future studies of dryness/wetness or drought.