Spatiotemporal patterns of ecological condition under water-salt stress in the yellow river delta: an improved remote sensing ecological index approach

Abstract

Accurately assessing ecological condition is crucial for ecosystem conservation. The Remote Sensing Ecological Index (RSEI), which incorporates soil moisture (WET), was widely used to assess regional ecological condition. However, in coastal wetlands, the performance of the WET index is often undermined due to high soil salinity caused by seawater intrusion. Therefore, a Water-Salt Stress (WSS) index was developed by integrating WET and salinity (SI) to quantify the combined water-salt stress on vegetation. Replacing WET with WSS, an improved index, Water-Salt Stress Remote Sensing Ecological Index (WSRSEI), was established for ecological condition assessment. Results show higher WSS (> 0.7) in tide-influenced coastal areas and lower WSS (< 0.4) near inland channels. In high-WSS zones, WSRSEI was significantly lower than RSEI. Independent field validation demonstrate that WSRSEI significantly outperforms RSEI in capturing actual vegetation conditions. Long-term analysis (1985–2025) revealed a slight overall decline in the delta's WSRSEI (from 0.631 to 0.621). Ecological condition improved in the Qingshuigou Nature Reserve (QSGNR) (WSRSEI: 0.565 to 0.627) due to sediment input and post-2008 ecological water supplementation, but declined in the Diaokou River Nature Reserve (DKRNR) (0.621 to 0.554) due to coastal erosion and saltwater intrusion. Engineering structures such as levees and farm dikes effectively mitigated seawater intrusion. Since 1985, the expansion of aquaculture ponds, bare land, and built-up areas has been a major factor in the overall decline of ecological condition. This study provides a refined methodological framework for coastal ecological condition assessment.