An entropy-based investigation into the impact of ecological water diversion on land cover complexity of restored oasis in arid inland river basins

Abstract

An improved understanding of the response of oasis ecosystems to ecological water diversion is essential for sustainably managing water resources in arid inland river basins. This entropy-based investigation provides new insight into the ecohydrological impacts of ecological water diversions through a case study of Qingtu oasis in the lower Shiyang River basin in Northwest China. Since 2010, an ecological water diversion project was put in place by Water Resources Bureau of Shiyang River Basin to restore the degraded Qingtu oasis. Annual cumulative water releases from this water diversion project had reached 0.25 billion m3 in 2018. This contributed to an increase in shallow groundwater level of 1.2 m. A combination of land cover classification indices and an entropy approach were used to assess the ecohydrological response to water diversion inputs. Land cover was classified as sparsely, lowly, moderately, and highly vegetated areas using Normalized Difference Vegetation Index (NDVI) calculated from Landsat images and the Excess Green minus Excess Red Index that was estimated from unmanned aerial vehicle images. Land cover complexity was evaluated using component complexity and spatial complexity. The component complexity corresponds to land cover composition and was assessed using the Shannon's entropy. The spatial complexity corresponds to spatial configuration of different land cover compositions and was analyzed using spatial entropy methodology. The rise in groundwater level caused by ecological water diversion drove the land cover transition and enhanced the component complexity. Field survey data showed that tendency and randomness co-existed in the spatial pattern of groundwater depth and its relationship with NDVI, which jointly affected spatial complexity (i.e., dependence and independence in spatial co-occurrences of different land cover categories). Results from this study improve understanding of the impacts that ecological water diversions can have on land cover evolution in natural oasis in arid inland river basins.