Evaluating the effects of revegetated shrub on land surface ET in arid and semiarid areas using SEBAL model

Abstract: Caragana liouana is one of the most common planting tree species in the desert region of the Inner Mongolia Plateau in northwest China. The cultivated shrub has greatly enhanced the vegetation coverage and ecological restoration to keep from desert encroachment in the regional ecosystem in Yanchi County since 1970. Correspondingly, the vegetation structure has also been changed eventually to dominate the water vapor interactions between land surface and atmosphere. However, only a few studies were focused on the quantitative evaluation of anthropogenic measurement on the function of the original desert steppe ecosystem, especially from the viewpoint of the water cycle. Therefore, this study aims to evaluate the effect of anthropogenic revegetation on evapotranspiration (ET), a key process of the water cycle in a terrestrial ecosystem, in order to reveal the coupling mechanism between vegetation change and hydrological process in most arid and semiarid areas. The spatial ET was retrieved for the Yanchi County in different stages in the whole year using a surface energy balance algorithms for land (SEBAL) model driven by Landsat-8 OLI/TIRS and meteorological data. The specific ET was also measured in the anthropogenic planted shrub and nearby grassland plot (as control plot) using the visual interpretation of high-resolution remote sensing images including GF-2 and Google Earth platform. The results showed that: 1) there was a high accuracy in the ET retrieved by the SEBAL model in Yanchi County, especially in agreement with the observation of sites. The model was therefore suitable for retrieving and spatial characteristics analysis of regional ET in arid and semiarid areas; 2) The daily average ET in the planted shrub areas was 1.20 mm/d higher than that (1.17 mm/d) in the compared grassland. As such, the planting shrub effectively tailored the surface hydrological process, while increased the water consumption of the ecosystem. A critical change of ecosystem function occurred in arid and semi-arid areas, where the water supply was limited. Nevertheless, there were great variations in the enhancement of ET for the different seasons and biogeographical conditions. The most obvious enhancement of ET was found in August, but a negative effect was observed in March and April. The reason was that the surface hydrological process was independent of plants before the growing season. 3) A stronger effect occurred to enhance land surface ET in the higher density and vegetation coverage of shrubs. The most remarkable effect was performed in the case of NDVI>0.4, where the shrub needed much more water to grow. The specific mechanism should be paid attention to implement grassland ecological restoration, and combatting desertification. As a result, a reasonable eco-hydrological threshold can be determined for the anthropogenic revegetation mostly with shrub species in arid and semiarid areas. Particularly, the water supply of ecosystem from precipitation was more or equal to the water consumption for shrub plant growth. Therefore, it is essential to construct a stable and sustainable artificial ecosystem via the anthropogenic revegetation using the water balance of land and atmosphere in arid and semiarid areas. The finding can provide a scientific guidance for regional ecological and water resources management.