Field data and flow system response in clay (vertisol) shale terrain, north central Texas, USA

The water budget in clay shale terrain is controlled by a complex interaction between the vertisol soil layer, the underlying fractured rock, land use, topography, and seasonal trends in rainfall and evapotranspiration. Rainfall, runoff, lateral flow, soil moisture, and groundwater levels were monitored over an annual recharge cycle. Four phases of soil–aquifer response were noted over the study period: (1) dry-season cracking of soils; (2) runoff initiation, lateral flow and aquifer recharge; (3) crack closure and down-slope movement of subsurface water, with surface seepage; (4) a drying phase. Surface flow predominated within the watershed (25% of rainfall), but lateral flow through the soil zone continued for most of the year and contributed 11% of stream flow through surface seepage. Actual flow through the fractured shale makes up a small fraction of the water budget but does appear to influence surface seepage by its effect on valley-bottom storage. When the valley soil storage is full, lateral flow exits onto the valley-bottom surface as seasonal seeps. Well response varied with depth and hillslope position. FLOWTUBE model results and regional recharge estimates are consistent with an aquifer recharge of 1Ð6% of annual precipitation calculated from well heights and specific yield of the shale aquifer. Copyright  2005 John Wiley & Sons, Ltd.