Surface and subsurface flow relationships to Ogallala aquifer recharge on the Southern High Plains of Texas
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Playa lakes, the common geomorphic features on the Southern High Plains, are important sites for Ogallala aquifer recharge. Playas are closed depressions on the landscape that collect surface runoff from the surrounding area. Water in playas is lost through evaporation, evapotranspiration and recharge. Thus, playa watershed characteristics, playa morphology and playa basin soil characteristics become important considerations of playa hydrology. Understanding the playa hydrology is imperative to develop playa management decision support tools to enhance recharge. This research focused on the surface and sub-surface hydrology of playa basins. Four playas on the Texas High Plains with grassland outer basins were studied. Three of them were formed in geographic areas in which the surrounding soils are fine, and the other one was surrounded by loamy soils. Playas studied ranged from 7 to 12 ha. in size and were 9 to 18 meters deep within the landscapes. Playa floors were 0.5 to 0.75 m below the annulus. Playa watersheds were as large as 6.5 km2 and had drainage channel inputs measuring up to 18.5 km long at 1% contributing watershed area. Playas formed from fine sediment areas had extensive surface drainage networks with high drainage density as compared to the one formed from loamy sediments. Sedimentation from drainage channels impacted the typical playa structure. Erosional and depositional effects of drainage deformed the playa annulus to a more flat structure. This structural change in playas increased the surface area and the potential for water evaporation. Playa basin soils were indicative of the hydrological processes. Soil morphological features such as gleying and pedogenic carbonate distribution were self explanatory for the water inundation and playa recharge. Clayey playa floor soils were noncalcareous, indicative of flushing carbonates from the profiles by the recharging waters. Slickensides developed within the deep playa floor soils similarly illustrated the deep water percolation. Playa annulus soils were coarse in texture as compared to the playa floor soils. Redoximorphic features appearing in playa annulus soils provided evidence of either extended hydroperiods in playa annulus areas and/or the effect of lateral water movement from the playa floor. Few drainage input channels flowing on low relief landscapes could explain the non ponding nature of some playas. Absence of redoximorphic features in those playa floor soils suggests similar non-ponding hydrologic history. Noncalcareous playa floor soils at the drainage input zone is consistent with the recharge through this type of playas. High intensity rainfall events, soil stratification, and permeability contrasts, along and slope of playa basins suggest potential through-flow for these playas. Variability in earth conductivity, as measured by electromagnetic induction (EMI), was an effective tool to diagnose the through-flow areas. Spatial variability patterns of soil apparent electrical conductivity refined the focus areas for potential through-flow within the playa basins. Pedological evidence identified the soil textural contrasts of through-flow. Depending on the depth to sand lens, soil ECa vertical heterogeneity as defined by the profile ratio reflected specific signatures for potential through-flow areas. The profile ratio proved to be an effective indicator to identify the through-flow areas within the playa basins. This research proposes that through-flow in playa basins has the potential to be a significant source of recharge. Flow verification and assessment,as a source of recharge is still needed. Electromagnetic induction in combination with pedological analysis and Geographic Information Systems can be a powerful through-flow analysis tool.