Effects of land cover on variations in stable hydrogen and oxygen isotopes in karst groundwater: A comparative study of three karst catchments in Guizhou Province, Southwest China

Abstract

Studying the spatial and temporal variations of stable hydrogen and oxygen isotopic values (δD and δ18O, respectively) in precipitation and groundwater in catchments with different land cover is of great significance to understanding the hydrologic cycles within the catchments. This study is focused on three karst catchments, Banzhai, Chenqi and Dengzhanhe, in Guizhou Province, Southwest China, a region with a subtropical humid monsoon climate. We analyzed the spatial and temporal variations in the δD and δ18O of precipitation and groundwater in these areas from September 2007 to September 2009. Local meteoric water lines (LMWLs) for the study areas and their relationships with groundwater were established. The seasonal variations of δD and δ18O for both precipitation and groundwater were similar, being depleted in the heavier isotopomer in the rainy season and enriched in the heavier isotopomer in the dry season. The similarity of patterns between groundwater and precipitation indicates short groundwater residence times in the three catchments, typical of karst terrains. The isotopic values of the Banzhai catchment, which is covered by a thin soil layer beneath virgin forest floor, had the largest variation among the three catchments. The isotopic compositions of the Banzhai catchment groundwater were close to those of precipitation in the rainy season and responded rapidly to it, indicating that precipitation quickly recharges groundwater in this area. In comparison, in the Chenqi and Dengzhanhe catchments, the thicker soil layers and large numbers of paddy fields and dry lands in the catchments resulted in more intense evaporation, and therefore relatively higher isotopic values in the infiltration water. Consequently, the isotopic values of spring water in these two areas were higher and varied to a lesser extent in the rainy season than those of precipitation. The Rayleigh fractionation model based on the correlation between the deuterium excess (d) and evaporation was employed in this study. The model calculation results showed that the catchment evaporation was more intense in Dengzhanhe than in Chenqi, with 14 ± 1% and 6 ± 4% of the water evaporated in Dengzhanhe and Chenqi, respectively. The higher percentage of evaporation in Dengzhanhe was mainly due to the larger percentage of paddy fields in the catchment. In contrast, the effects of evaporation from free water surfaces or soil surfaces on groundwater in the virgin forest-covered Banzhai catchment were extremely weak. Based on the isotopic and high-resolution continuous meteorological and discharge data, the transpiration rates were estimated to be 78% for Banzhai, 10 ± 4% for Chenqi, and 24 ± 1% for Dengzhanhe, and the differences among these values are mainly attributed to differences in vegetation types in those areas. These results show that the variation in stable isotopes in groundwater can be used as a key index in evaluating the effects of different land cover changes and environmental changes on the water cycle in a catchment.