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 partial or complete access to groundwater to maintain their ecological structure and functions, providing 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, geographic information system (GIS) data dredging, and hydrogeological survey data. Firstly, the potential GDE distributions are preliminarily delineated by the topographic features and the differences in terms of vegetation water situation and soil moisture at the end of the dry and wet seasons. On this basis, according to the given GDE 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, riverbed elevation, and vegetation root development depth and through surveys of karst springs and aquifers. Following this, 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, with 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. In order to verify the reliability of GDE distributions, the study verified the determination of GDEs through hydrological rhythm analysis, as well as through the analysis of the hydrochemical characteristics of various waterbodies in the basin and of ecohydrological signals such as groundwater invertebrates. The hydrological rhythm analysis in the Shuyuan section showed that the proportion of base flow to river flow is about 54.15 % and that S-GDEs still receive spring water recharge even in the extremely dry season. Furthermore, the analysis of hydrochemical sampling from the karst aquifer, the Quaternary aquifer, the spring water, and the surface reservoir water reveals that GDEs are also relished by groundwater. More importantly, we also found a distinctive ecohydrological signal of GDEs is the presence of millimeter-sized groundwater fauna living in the different types of GDEs. In addition, the study suggests that the use of isotope and environmental DNA technology to analyze the hydrological–sediment–biological connectivity between groundwater and GDEs is the future development direction of this field.