Identification, Mapping and Eco-hydrological Signal Analysis for Groundwater-dependent Ecosystems (GDEs) in Langxi River Basin, North China

Abstract. Groundwater-dependent ecosystems (GDEs) refer to ecosystems that require access partially or completely to groundwater to maintain their ecological structure and functions, provisioning very important services for the health of land, water, and coastal ecosystems. However, regional identification of GDEs is still difficult in areas affected by climate change and extensive groundwater extraction. To address this issue, taking the Langxi River Basin (LRB), one of the lower tributaries of the Yellow River in North China, as an example, we propose a four-diagnostic criteria framework for identifying the GDEs based on remote sensing, GIS data dredging and hydrogeological surveys. Firstly, the potential GDEs distributions are preliminarily delineated by the topographic features and the differences of vegetation water situation, soil moisture in the end of the dry and wet seasons. On this basis, according to the given GDEs identification criteria, three main types of GDEs in the basin including the stream-type GDEs (S-GDEs), vegetation-type GDEs (V-GDEs) and karst aquifer-type GDEs (K-GDEs) are further determined by comparing the relationship between groundwater table and riverbed elevation, vegetation root development depth, and though surveys of karst springs and aquifers. And then the GDEs are mapped using the spatial kernel density function which can represent the characteristics of spatial aggregation distribution. Results show that the potential GDEs are mainly distributed in plain areas and a small part in hilly areas, reflecting the moisture distribution status of waters, vegetation and wetlands in the basin that possibly receive groundwater recharge, however, the true GDEs are concentrated in the riverine and riparian zone, the vegetation-related wetland and the scattered karst spring surroundings which groundwater directly moves toward and into. To verify the reliability of GDEs distributions, ecohydrological signal analysis were also performed in this paper. The analysis of river hydrological process curve and karst spring hydrograph in Shuyuan section showed that the proportion of base flow to river flow is about 54.15 % and S-GDEs still receive spring water recharge even in the extremely dry season. And the analysis of hydrochemical sampling from the karst aquifer, Quaternary aquifer, spring water and surface reservoir water reveals that GDEs are also relished by groundwater. More important, we also found a distinctive ecohydrological signal of GDEs is the presence of millimeter-sized groundwater fauna living in the different types of GDEs. Finally, the validity of the method proposed in the study for identification and mapping of the GDEs is also discussed. It still has some room for improvement if the water, sediments and biotic connectivities between groundwater and GDEs are analyzed by using isotopes and DNA technology under the recommended four-diagnostic criteria framework.