The maximum threshold of vegetation restoration (EVI-Area) in typical watersheds of arid regions under water constraints

Accurate ecological water regulation to achieve the optimal coupling between ecological water supply and vegetation water demand is the key challenge for ecological conservation in arid regions. Previous studies have mainly focused on the relationship between vegetation and specific water supply methods in arid regions. However, there has been limited attention to the maximum thresholds of Enhanced Vegetation Index (EVI) and area for vegetation protection and restoration under limited water conditions. Therefore, this study targets the mainstream of the Tarim River and conducts a systematic analysis of the maximum thresholds of EVI and area for vegetation protection and restoration under water constraints. Firstly, vegetation change patterns were discriminated using logistic and Gaussian functions. The results indicated that with enhanced watershed water resources management, both vegetation EVI and Evapotranspiration (ET) showed an increasing trend, which was projected to continue in the future. Based on the discrimination of vegetation change patterns, this study innovatively defined regions with linear, logarithmic, logical, and Gaussian patterns as area expansion zones requiring priority in ensuring ecological water supply, and regions with exponential patterns as quality improvement zones for improving vegetation restoration quality. By extracting vegetation EVI and ET indicators and constructing copula function in the two zones, the joint distribution function revealed that under water constraints, the range of natural vegetation expansion area-EVI threshold in the Tarim River was 17959.02 (EVI: 0.137) similar to 18279.75 (EVI: 0.137) km(2). The maximum threshold of vegetation EVI in the quality improvement zone was 0.223. Under current conditions, the average vegetation EVI in the area expansion zone was 0.137 (stable level), indicating a threshold for natural vegetation restoration of 0.137. During normal water years, besides increasing vegetation EVI to the threshold of natural vegetation restoration, the area could still expand by 665.06 similar to 985.79 km(2), corresponding to a vegetation area threshold of 17959.02 similar to 18279.75 km(2). In wet water years, the natural vegetation area could expand by 5172.53 similar to 5493.26 km(2), with a vegetation area threshold of 22466.49 similar to 22787.22 km(2).