Temporal and spatial variability of shallow soil moisture across four planar hillslopes on a tropical ocean island, San Cristóbal, Galápagos

Abstract

Study region: This paper provides a summary of findings from temporal and spatial studies of soil water content on planar hillslopes across the equatorial island of San Cristóbal, Galápagos (Ecuador). Study focus: Soil water content (SWC) was measured to generate temporal and spatial records to determine seasonal variation and to investigate how the behavior of surface and near-surface root-zone soil water may support island-wide hydrogeology models. SWC probes were installed at four weather stations in a climosequence to generate a temporal record, and spatial surveys of shallow SWC across the selected sites were completed during wet and dry seasons. Temporal differences in SWC were driven by seasonal variations in rainfall and evapotranspiration, while spatial variability remained high during both wet and dry seasons. Unsaturated hydraulic conductivity determined by mini-disk infiltrometers was highly variable across the slopes, as were other hydrologic variables. New hydrological insights for the region: The high heterogeneity of soil water and hydrologic characteristics provides a means to explain why little runoff is observed at the study sites: soils do not saturate uniformly across hillslopes, allowing for runoff generated in one part of the hillslope to be conducted into the soil in adjacent parts of the hillslope. The lack of connected surface runoff helps explain how water enters the groundwater system of the island.

Keywords

• Temporal soil water record
• Spatial soil water distributions
• Hillslope soil water connectivity
• Runoff generation
• Galápagos climosequence