Catchment Hydrology and Landcover in Karst Savannas: A Case Study in East Sumba, Indonesia

Abstract

Shallow groundwater systems are a critical water source to communities in resource-poor tropical savanna regions, where karstic landscapes are particularly prone to water insecurity. There has been limited investigation into, and development of water resources in, these areas and many peoples’ livelihoods are constrained by the need to travel significant distances from home in order to obtain water for household and productive use. Springs are particularly important sources of perennial household water supply. It is thus crucial to understand controls on groundwater availability in these contexts in order to support sustainable use. The aim of this study is to support land-use planning by increasing the understanding of landcover as a control on recharge to shallow groundwater in a karstic catchment of the tropical savanna in East Sumba, Indonesia, with a focus on two key research questions. (1) What are the key environmental controls on groundwater recharge in tropical karst savannas? (2) What is the influence of catchment revegetation on groundwater recharge? To address these questions, the study employs remote sensing approaches to identify trends in NDVI and its relationship with available rainfall and soil moisture; hydrochemical and isotope analyses to study flowpaths and recharge rates; and field and lab analyses of soil characteristics and infiltration potential tests in order to measure the influence of landcover on infiltration to the vadose zone.

The study finds that epikarst and groundwater storage are both used by woody vegetation in the Larawali catchment, particularly during the dry season, while soil moisture is the key water source during the wet season. Shrink-swell soil characteristics are found to be an important control on infiltration and subsequent deep drainage, and are more important than landcover during the early wet season. Following soil saturation, however, the high macroporosity that results from woody vegetation root channels promotes rapid infiltration and drainage through preferential pathways. Analyses of major ions and δ²H and δ ¹⁸O established that recharge to epikarst and shallow groundwater storage is from local meteoric sources and occurs within a period of a few months. All three springs and two wells were identified as being of a Ca-HCO₃ water type. Low Mg/Ca ratios of spring and well water provide evidence of short residence times within the karst groundwater storage and likely rapid conduit flow. The chloride mass balance found that the recharge fraction of rainfall for groundwater in the Larawali catchment is between 15% and 30%, with annual average recharge of 172 mm.

The study provides a conceptual model of the ecohydrological system of the Larawali catchment and concludes that landcover provides some control over rates of infiltration through the soil layer and into the epikarst. It is determined that woody vegetation in the catchment has, in general, a greater evapotranspiration potential than herbaceous vegetation, however, the characteristics of precipitation and the lithosphere may be more significant controls of groundwater recharge than the structural differences between woody and non-woody vegetation.