The ecological stability of restored vegetation by ecological water conveyance project in a typical terminal wetland in an arid inland river basin, northwestern China

Abstract

Overexploitation of water resources has led to severe ecological degradation and even desertification in many terminal wetlands in arid inland river basins, northwestern China. To restore the degraded vegetation ecosystem, ecological water conveyance projects (EWCPs) have become an important measure. Scientific assessment of the ecological stability of restored vegetation is of great importance for formulating reasonable ecological water management. Considering this, a systematic study was conducted in a typical terminal wetland of the Qingtu Lake Wetland (QLW) in Shiyang River Basin, northwestern China. The pixel-scale restored vegetation area (RVA) each year since the start of EWCP was extracted based on remotely sensed vegetation index. RVA increased dramatically in the first five years and became stable from 2016. The time lag of the response of RVA increase to ecological water conveyance was about 2 years. A bell-shaped function between RVA and groundwater depth was obtained based on the micro terrain of QLW via UAV. Five groundwater depth thresholds were then determined. The optimal groundwater depth in the hydrometric station was 2.91±0.09 m for the maximal RVA (17.08±3.25 km2). The optimal ecological water volume into Qingtu Lake was further estimated (the volume is 2224.4×104 m3) for the maximal RVA by a polynomial function between ecological water volume and groundwater depth. With the help of remotely sensed soil salinization and water surface, the vegetation restored during the first few years of EWCP in the southwest, west and north of QLW was found to degrade again due to the aggravation of soil salinization in these regions since 2019. Soil salinization was accelerated by the low groundwater depth with high mineralization and high evaporation capacity of climate without adequate inundation of ecological water by the unreasonable water allocation strategy. Under current simple and lax ecological water management in QLW, soil salinization will intensify and vegetation will further degrade. In other words, the ecological status of restored vegetation in QLW is unstable. Based on the optimal ecological water volume, even distribution of ecological water is helpful to maintain the stability of restored vegetation.