基于SEBAL模型评估干旱半干旱区人工灌丛植被对陆表蒸散的影响

评估人工灌丛植被重建对干旱半干旱区陆地生态系统蒸散的影响,不仅能揭示植被变化与水文过程的耦合机理,又可为区域生态治理与水资源管理提供科学指导。该研究利用Landsat-8OLI/TIR遥感影像及气象数据等驱动SEBAL模型,反演宁夏盐池县的年内不同日期的陆表蒸散,结合目视解译选取的人工灌丛区与对照草地,评估了人工灌丛植被对陆表蒸散的影响。结果表明:1)SEBAL模型的蒸散反演精度与站点观测结果较为一致,可用于干旱半干旱区蒸散反演及空间特征研究;2)盐池县人工灌丛植被区日平均蒸散为1.20mm/d,高于对照草地1.17mm/d的日平均蒸散量,即干旱半干旱区人工种植灌木林增加了生态系统水分消耗,但不同季节和不同生物地理条件下的蒸散增强作用存在差异,蒸散增强在8月份最大,而3、4月份呈现负效应;3)人工灌丛的密度越大、植被盖度越高,对陆表蒸散的增强作用越强,特别在NDVI0.4的高盖度情况下蒸散增强作用更加明显。由此可知,在水资源紧缺的干旱半干旱区开展以灌木树种为主的植被重建需在合理的生态水文阈值范围内开展,才能构建出稳定可持续的人工生态系统。

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 1.2 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.