Measuring Autogenic Recharge over a Karst Aquifer Utilizing Eddy Covariance Evapotranspiration

Abstract

Autogenic, or direct aquifer recharge can best be measured as the remainder of a water balance utilizing precise measurement of precipitation, evapotranspiration (ET) and runoff. ET is the largest component of a precipitation water balance and can be measured within 5% using an eddy covariance system with Bowen-ratio energy balance corrections. Water balance components of precipitation, evapotranspiration, internal runoff, soil moisture were measured using a eddy covariance system, tipping bucket and visual rain gauges, flumes, and soil-moisture sensors. The research site was located within a 0.19-km 2 (46-acre) internal drainage sinkhole basin where runoff never flows beyond the basin, but potentially reaches a cave serving as a drain to the sinkhole. Other than the cave drain, the basin slopes are indistinguishable from other slopes across the Barton Springs Segment of the Edwards Aquifer. Over a 505-day water balance interval where change in soil moisture was negligible and precipitation was 42% above average, ET was 68% of precipitation, discrete internal runoff was 6%, and remaining component of diffuse autogenic recharge was measured as the residual of total rainfall as 26% of rainfall. Over a longer period of average rainfall, internal runoff diminished to 3%, but was as high as 42% of precipitation during single storms when the soils were near saturation. These results closely match results from a fiveyear water balance over the Trinity Aquifer of Central Texas where ET was measured to be 65% of precipitation using a Bowen-ratio climate tower, runoff was measured to be 5% of precipitation, and recharge was calculated as the residual at 30% of rainfall. ET flux tower data from other sites across Central Texas indicate that under average precipitation conditions, autogenic recharge is about 28% and intervening recharge area runoff is about 3% of precipitation. During years of higher than average precipitation, authogenic recharge and intervening recharge area runoff combined increase within the range of 30% to 45% of precipitation. N. M. Hauwert, J. M. Sharp 870

Stream Recharge Water Balance for the Barton Springs Segment of the Edwards Aquifer

“…Autogenic recharge on the recharge zone is estimated as 23% to 28% of precipitation in this study, with 15% to 20% occurring directly over the intervening area and 8% originating from intervening area runoff that recharges within the major stream channels (Figure ). For comparison, 26% of rainfall was measured to directly infiltrate through soil‐covered areas of an upland research site in the center of the Barton Springs Segment, using a climate tower and flumes (Table ; Hauwert and Sharp ). This 1.4 year water balance period was selected as an interval where starting and ending volumetric soil moisture was 20% to eliminate the variable of changes in soil moisture storage.…”

“…Autogenic recharge originates from direct precipitation over the recharge zone, a portion of which recharges within the intervening area (upland autogenic recharge) and the remaining recharges via runoff to the major stream channels (stream autogenic recharge). In karst areas of the world, autogenic recharge is typically 20% of precipitation or greater (Hauwert and Sharp ). Allogenic recharge sources originate as runoff from upstream of the outcrop of a karst aquifer.…”

“…Wilcox () also noted a 35% precipitation water balance discrepancy between evapotranspiration calculated by stream gauging in a channel water balance study (Maclay ) and direct climate tower measured evapotranspiration (Dugas et al ) within the same Seco Creek watershed. In their compilation of published annualized Central Texas climate tower and runoff data, including data from one station within the Barton Springs Segment, Hauwert and Sharp () note that of average annual precipitation over Central Texas Edwards and Trinity Aquifers, 69% evapotranspirated, 28% recharged, and 3% ran off upland areas. These data also show that recharge and evapotranspiration vary directly with annual precipitation, but the difference in values between sites across Central Texas karst aquifers was not so great as to account for the variation in results derived from channel water balance and climate tower methods.…”

Surface water–groundwater interactions along the Blanco River of central Texas, USA

“…1) comes from infiltration of water in the stream beds, with the remaining 25 % infiltrating in the uplands between the streams. Other studies indicate more recharge occurs in the uplands (Hauwert and Sharp 2014). Most studies of the Trinity describe recharge as broadly distributed and relatively low, about 4 % of total rainfall (Ashworth 1983).…”

Recharge processes of Matese karst massif (southern Italy)

“…This is computed generally at annual scale and assuming that cross boundary flow does not occur (Drogue 1971;Bonacci and Magdalenic 1993;Bonacci 2001). This ''rough'' estimation can be improved considering the evapotranspiration processes (Fiorillo and Pagnozzi 2013;Hauwert and Sharp 2014). Fiorillo et al (2014) proposed an annual model to estimate the groundwater recharge at the long-term scale, especially for large areas with strong morphological irregularities, and not completely covered by hydrological monitoring.…”