Spatial Variability of Soil Water Storage in a Karst Savanna On the Edwards Plateau, TX.

On the karst landscapes such as those on the Edwards Plateau in central Texas, rocks occupy a significant fraction of the soil volume, reducing water-holding capacity and restricting root growth. Hydrologic studies in these ecosystems are challenging because of the difficulty in measuring water content in rocky soils.  We evaluated spatial and temporal variability in soil water storage in a savanna with ~50% woody cover (Juniperus ashei, Prosopis glandulosa) on the Plateau.  The soil is a Rumple gravelly clay loam (Clayey-skeletal, mixed, active, thermic Typic Argiustolls) with chert fragments occupying ~50% of the soil volume between depths of 0.2 and 1.0 m.  Limestone residuum occurs at depths of 1 to 1.5 m.  Water contents to a depth of 1.6 m were measured in a 25 m × 25 m grid (5-m node spacing) by neutron scattering, and time domain reflectometry.  Bulk density profiles were measured by gamma densitometry.  Rock created high spatial variability in water storage capacity, ranging from 185 to 401 mm, and coupled with heterogeneous distribution of trees led to high spatial variability in root water uptake.  Most of the water uptake came from the upper 1 m of the soil profile.  Water extracted from below 1.6 m, the maximum depth of our measurements, accounted for approximately 10% of Evaporation (ET), indicating that roots had access to water stored within the bedrock, possibly in soil pockets.  Because spatial and temporal variability in water storage and retention are high, a large number of measurements are required to quantify water dynamics in karst terrain.   Hydrologic and vegetation models must account for this variability, and need to include the impact of water storage within the rock matrix if they are to provide realistic simulations of ET and water dynamics.