Wednesday, November 15, 2006 - 8:15 AM

Energy Balance and Water Use of Savanna Ecosystems on the Edwards Plateau, Texas.

James L. Heilman1, Kevin J. McInnes1, Marcy E. Litvak2, M. Keith Owens3, and James F. Kjelgaard4. (1) Texas A&M Univ, 2474 TAMU, College Station, TX 77843, (2) Univ of New Mexico, Albuquerque, NM 87131, (3) Texas Agricultural Experiment Station, Uvalde, TX 77880, (4) Natural Resources Conservation Service, Half Moon Bay, CA 94019

The Edwards Plateau is a large, distinct ecoregion in south and west-central Texas that contains the watersheds and recharge zone for the Edwards Aquifer that provide water for local municipalities, farms, and ranches.  Historically, extensive portions of the Plateau were dominated by live oak-Ashe juniper savannas underlain by perennial, mixed C3/C4 grasslands, but much of the grasslands have been replaced by juniper-dominated scrubland due largely to suppression of wildfires.  Because of increasing demands for water along the Interstate Highway 35 corridor from Waco through Austin to San Antonio, juniper is being cleared in hope of enhancing recharge of the Aquifer, even though evidence suggests that water yield from such clearing may be minimal.  While it is not clear if removal of juniper increases Aquifer recharge, it is clear that removal results in loss of sequestered carbon and perhaps in increased temporal variability in carbon, water, and energy exchange.  Studies were initiated in 2004 on the Freeman Ranch near San Marcos, TX, to evaluate how carbon, water and energy exchange in Plateau savannas were affected by the vegetative structure of the ecosystems.  Fluxes of CO2, sensible heat, and latent heat were measured using the eddy covariance technique above grassland, oak-juniper forest, and grassland in transition to a juniper-dominated scrubland.  At the forest site, 90% of the net radiation could be accounted for in other measured energy fluxes, while at the grassland site, 80% could be accounted for in other fluxes. We compare energy balances among the three sites, and discuss the impact of energy balance closure on the estimation of water loss through evapotranspiration.