The relationship between vegetation and groundwater in the hyper-arid Ejina Delta of northwestern China

Abstract:

Vegetation in the hyper-arid Ejina Delta of northwestern China plays irreplaceable role in preventing desertification and maintaining sustainable development in inland Heihe river basin . The groundwater mainly recharged by Heihe river provides the main water source for the vegetation survival in Ejina Delta. While due to extensive water use in mid and upper stream for local economic development, the Ejina Delta had experienced long-term no flow coming in each year and result in groundwater table great decline, which further lead to severely vegetation degradation during the 1990s. Ecological water allocation therefore was implemented in several arid inland basins to prevent ecological degradation. In order to wisely allocating water for ecological use, the relationship between vegetation and groundwater needs to be well understood, that is the aim of this paper. With the GMS software package, a groundwater flow model for the Ejina oasis was established and the spatial distributions of groundwater dynamics during 2000-2011 were simulated. Combing annual average groundwater table and NDVI during growing season on a per-pixel base, it was found that vegetation grew well and had a high biomass when the groundwater was in 1.5-3.8m and vegetation showed the best when the groundwater was about 2m. Further studies conducted at some locations with long-term ground water monitoring showed that when the groundwater table depth was more than 5 and 6 m, the vegetation conditions did not show an obvious response to groundwater table fluctuations; when the groundwater fluctuated between the proper intervals mentioned above, the correlation between vegetation condition and groundwater table were remarkably spatially heterogeneous. This result indicated that future ecological water allocation should consider more spatial heterogeneity of vegetation response to water environment for efficiently using the limited water.