Identification And Assessment Of Ecological Drought Of Zhalong Wetland Based On Multi-source Remote Sensing Data

Wetlands are valuable ecosystems providing a variety of important ecosystem services such as Hydrology regulation and storage,water purification,climate regulation.However,due to the dual influence of global climate change and human activities,the increasing frequency and intensity of wetland ecological drought are threatening the global wetland ecological security,resulting in the shrinking of wetland areas and the decline of the ecological service function.Clarifying the process of wetland ecological drought is the premise and foundation to guide the restoration of wetland hydrological situation.However,the wetland is a special natural complex formed by the interaction of water and land.How to scientifically identify and evaluate wetland ecological drought and drought characteristics is a key scientific problem to be solved urgently in wetland hydrological regulation and water resources management to deal with climate change.Zhalong wetland is one of the important wetlands in the world.It has vital ecological environment and social service functions.The drought caused by climate change and human activities intensifies the risk of wetland degradation.Under the guidance of optimizing wetland protection and comprehensive managementof water resources,this paper accurately extracts the ecological drought indicator factors of Zhalong wetland,including wetland water area,water level,vegetation growth,and soil humidity,by using multi-source long-time series remote sensing data.At the same time,this study reveals the temporal and spatial evolution trend of the three wetland elements such as hydrology,vegetation,and soil,and constructs the wetland ecological drought index coupling various factors based on the local ordered weighted average model.Finally,the ecological drought of Zhalong wetland was identified and evaluated accurately,the temporal and spatial variation characteristics of the drought were analyzed,and drought development regular pattern was revealed.The research results can not only provide scientific basis and technical support for wetland ecological water replenishment but also promote the development of theoretical methods of wetland ecological drought research,to ensure the maximum benefit of wetland water resources.The main research results are as follows:(1)Evolution characteristics and rules of eco-hydrological factors in Zhalong Wetland based on multi-source remote sensing data Based on multi-source remote sensing data,Normalized Difference Vegetation Index (NDVI)is used to analyze the water area and water level of Zhalong Wetland from 1984 to2021.Temperature Vegetation Dryness Index(TVDI)was calculated and analyzed.The results showed that the water area of Zhalong wetland decreased before ecological water replenishment and increased after ecological water replenishment since 1984.The monthly NDVI in the growing season showed a decreasing and increasing trend before and after ecological water replenishment.The correlation coefficients between water area and water level of sub-basins with measured stations in Zhalong Wetland reached a significant correlation level.The correlation coefficients between water level data based on Sentinel-3 altimetry satellite and corresponding water area were 0.71 and 0.82,respectively.The results show that the stability and consistency of water level data based on Sentinel-3 are good.The correlation coefficient R~2 between TVDI and measured soil moisture data from 1984 to 2021 was 0.63,indicating that TVDI could well represent the change of soil moisture in non-water area of Zhalong Wetland.(2)Driving factors and correlation analysis of eco-hydrological factors in Zhalong WetlandThe contribution rates of ecological water replenishment,runoff and precipitation to water area change in Zhalong Wetland were 46%,37%and 17%,respectively,and to NDVI change were 25%,11%and 64%,respectively.Furthermore,the correlation analysis of NDVI and Normalized Difference Water Index(NDWI)with rainfall and runoff in Zhalong Wetland was conducted,and the correlation between NDVI from May to October in Zhalong wetland and accumulated rainfall in the previous 60 days was strong,R~2 was 0.53.The correlation coefficient R~2 between NDVI and precipitation in non-water area is 0.89,indicating that NDVI in non-water area is more sensitive to precipitation response.The correlation coefficient between NDWI and runoff in water area reached a significant level.It can be shown that it is of practical significance to use NDVI and NDWI to represent the drought caused by rainfall and runoff deficit of wetland,which leads to the deterioration of wetland vegetation growth,and the decrease of water index.The decrease of NDWI can not only indicate the decrease of wetland water area,but also indicate the deterioration of aquatic vegetation growth.Therefore,the characterization of wetland drought by NDVI and NDWI has certain physical significance.(3)Construction of wetland ecological drought index by coupling three factors of wetland water,soil and vegetationFirstly,Standardized Precipitation Index(SPI)and Standardized Runoff Index(SRI)are used to analyze the meteorological and hydrological drought in Zhalong Wetland.The results show that:The meteorological drought and hydrological drought in Zhalong wetland showed a slightly serious trend since 1984.Secondly,the wetland ecological drought index(WEDI)with 30 m spatial resolution covering multi-source spatio-temporal information from 1984 to2021 was constructed based on local OWA method.The correlation analysis between WEDI and the measured values of precipitation,runoff and soil moisture in the same period was conducted.The case data of each month were selected to calculate WEDI,and the comparative analysis was conducted with meteorological drought.The results showed that the correlation coefficient between WEDI and rainfall,runoff and soil moisture was 0.73,reaching a significant correlation level.The occurrence time and severity of wetland ecological drought reflected by WEDI are consistent with meteorological drought.WEDI can reflect the spatial heterogeneity of wetland ecological drought in detail and analyze the complexity of wetland drought in detail.WEDI of the same period as the measured water level were selected to classify the drought grades of Zhalong wetland,and compared with the change of water level.The results showed that the fluctuation of water level was consistent with the change of WEDI drought grades corresponding to the spatio-temporal scale.(4)Identification and assessment of ecological drought in Zhalong Wetland based on WEDIUsing wetland ecological drought index,variation coefficient,trend analysis and drought frequency analysis,this paper identified and evaluated the ecological drought in Zhalong Wetland from May to October in 1984-2021.The results showed that the spatial variation of WEDI was more obvious in May,followed by September and August.The variation coefficient fluctuated the most in July.In Zhalong wetland,there was a great difference in the variation trend of drought in each month.The drought situation in June,July and October had a trend of getting better,while the drought situation in May and August had a trend of getting worse,and the trend of getting worse was concentrated in the concentrated distribution area of reed marsh.In July,the overall drought frequency was the highest,and the distribution of drought frequency at all levels differed greatly from month to month.The area without severe drought and extreme drought was large.The drought frequency at all levels was the highest in July,and the drought frequency at all levels was the lowest in June.The results provide a scientific basis for ecological water replenishment in Zhalong wetland and for drought cognition and water resource management in other wetlands.