Abstract
The ability of the atmosphere to remove water from land surface is measured by potential evapotranspiration (ETp), which is often inferred by the reference evapotranspiration (ETo). Therefore, ETp is often considered only influenced by the above-ground meteorological conditions. Based on its concept, ETp should also link with surface conditions that influence the surface resistance. Such linkages differ in dry and wet regions with different surface covers. Here, we calculated ETo and especially analyzed the effects of surface conditions including vegetation cover indicated by NDVI (Normalized Difference Vegetation Index) and root-zone SWC (soil water content) in a humid subtropical province of China. Results show that ETp, NDVI, wind speed, temperatures have increased significantly during 1982–2015 and relative humidity (RH) has decreased significantly. Linear trends of these variables varied across seasons, but similarities were found between spring and winter and between summer and autumn. Summer saw the greatest changes in ETp per unit of environmental variable change. Solar radiation, RH, and precipitation exerted overall stronger influence on ETp (R2 > 0.50) than other factors. NDVI and SWC were found positively and negatively affecting ETp at all time scales. Partial correlation analysis showed significant influence of NDVI and SWC at the monthly scale; moreover, SWC influenced ETp more significantly in summer than other seasons (p < 0.05). Since actual evapotranspiration is often deducted from ETp by multiplicative stress functions in many hydrologic models, understanding the relationships between ETp and environmental changes can help improve the formulation and estimation of actual evapotranspiration.
Highlights
Potential evapotranspiration (ETp) is defined by many researchers as the water escaping rate from a well-watered vegetated surface (e.g., Xiang et al, 2020)
The changes in ETp and the environmental variables presented different seasonal features, e.g., rainfall and soil water content (SWC) increased only in summer, different from the result in Pearl River basin during 1961–2007 by Gemmer et al (2011); Rs increased in spring and winter while decreased in summer and autumn; ETp and NDVI increased in all seasons and higher increase rate was in spring and winter
This study examined the variations in potential evapotranspiration (ETp) and its relationships with the associated environmental factors across time scales
Summary
Potential evapotranspiration (ETp) is defined by many researchers as the water escaping rate from a well-watered vegetated surface (e.g., Xiang et al, 2020). The characteristics of vegetated surface can vary in different definitions, for example, Rosenberg (1974) defined ETp as “the evaporation from an extended surface of a short green crop which fully shades the ground, exerts little or negligible resistance to the flow of water, and is always well supplied with water,” in which the vegetation features are implicit; while Allen et al (1998) defined a reference grass with a fixed height of 0.12 m, ETp and Environmental Impacts a surface resistance of 70 s m−1, and an albedo of 0.23, and their ETp is known as the reference evapotranspiration (ETo). The main difference between ETp and ETo lies in the parameterization of the surface conditions vegetation type and characteristics. ETp measures the ability of the atmosphere to remove water from land surface through evaporation and transpiration (Kirkham, 2014). Quantification of ETp determines the performance of such models regarding the water flows, mixing and balance to better understand the interactions between climate and hydrology
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