Hillslope hydrology in stepped limestone terrains of the Central Texas Hill Country

Hillslope hydrology in stepped limestone terrains of Central Texas is strongly governed by riser/tread microtopographic landforms. Medium to fine-textured, skeletal, carbonatic Mollisols are derived from weathering of indurated Glen Rose Limestone of differing physical/chemical composition, and thus, stability. The thickest and most permeable soils form on high-gradient risers; the less permeable (generally thinner) counterparts occur on low-gradient steps. Risers serve as local recharge loci and treads are discharge sites for vadose-zone water. Pedogenic translocation of carbonates from upper to lower horizons within the soil/regolith has formed multiple aquitards in subsoils and at the contact with limestone bedrock. These aquitards limit root development and vertical water transport. Development of ephemeral water tables at aquitard interfaces occurs during periods of seasonal wetness. Between the depths of 30 and 120 cm, the probability of a perched water table increases from 7% near the surface to 16% at depth. Hydraulic connectivity of perched water tables from step to step is limited with no systematic increase in soil wetness from upslope to downslope risers. Some water tables are confined with weak artesian or hydrostatic heads. Transfer of water through these soil/regolith systems is augmented by low to very low water retention coupled with moderate to high infiltration rates. However, water retention is greater than commonly evoked because soft limestone clasts and soft bedrock strata (commonly excluded from totals) contribute up to 10-30% by volume. The hydrological cycle proposed for steps consists of infiltration, recharge, evapotranspiration, lateral interflow, restricted drainage, and discharge to the next downslope step. Hence, this compartmentalized hillslope hydrology appears to be a cascading, step-to-step downslope process resulting in multiple stages of water and solute buffering, bioremediation, and probable increase in mean residence time from summit to valley floor. These processes result in positive impacts on ecohydrology, resource management, and water quality.