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-km2 (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 five-year 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.