Vegetation dynamics induced by groundwater fluctuations in the lower Heihe River Basin, northwestern China

Since 2000, the environmental flow controls project has been implemented in the lower Heihe River Basin, a typical arid inland river basin in northwest China, to restore the deteriorated ecological environment in this region. The aim of this study was to explore the impacts of groundwater fluctuations on vegetation dynamics. Our results can be used as a reference for water resources planning and management to maintain proper environmental flows in arid areas. The location (by Global Positioning System) and depth of the monitoring wells, as well as groundwater table depth and salinity were measured in situ at each site from July to August 2009. Based on the measurements of the groundwater table depth and salinity following the implementation of environmental flow controls project (EFCP) in the lower Heihe River Basin, the groundwater fluctuations during the period from 2001 to 2009 were analyzed. Descriptive statistics and Pearson's correlation were used to analyze the relationship between vegetation changes and groundwater table fluctuations. Additionally, the spatial distributions of the groundwater table depth and salinity were interpolated using the simple kriging method. Trend analysis was applied to the time series of integrated Moderate Resolution Imaging Spectroradiometer normalized difference vegetation index data to identify interannual vegetation dynamics. The relationship between vegetation status and groundwater environment was investigated at different spatial scales by analyzing and comparing the time series and trends. (i) The groundwater table and salinity increased significantly in most of the study area with spatial heterogeneity. On average, the groundwater table rose similar to 0.5 and 1.5 m in the upper and lower Ejina Basin, respectively, and the groundwater salinity increased across the study area by 0-4%. (ii) A notable correlation between the vegetation status and the groundwater table was revealed when the groundwater table depth fluctuated between 1.8 and 3.5 m, whereas the vegetation did not show an obvious response to groundwater table changes when the groundwater table depth was more than 5-6 m. (iii) Vegetation restoration mainly occurred in riparian areas within 500-1 000 m of from natural rivers, where the groundwater table depth varied from 2 to 4 m, and salinity was < 5%, whereas vegetation degradation appeared at some locations where groundwater environment had deteriorated.