PARENT SESSION
Poster Session #72: Water Relations II.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Partitioning evapotranspiration fluxes and sources in desert floodplain ecosystems .

Yepez-Gonzalez, Enrico^*,1, Ellsworth, Patrick¹, Cable, William¹, Hultine, Kevin¹, Williams, David¹, ¹ School of Renewable Natural Resources, Tucson, Arizona

ABSTRACT- The dynamics and sources of evapotranspiration in desert floodplain ecosystems is dpendent on the relative abundance of grasses and deeply rooted woody plants. We used stable isotope techniques to assess a) the relative contribution and timing of transpiration and soil evaporation to the total evapotranspiration flux (ET), and b) partition the water sources of the dominant vegetation cover, the woody legume Prosopis velutina and the C₄ perennial grass Sporobolus wrightii. Measurements were conducted in a mesquite woodland along the upper San Pedro River Basin during June (pre-monsoon), July (monsoon) and September (post-monsoon) 2001. We identified the isotopic signal of component ET fluxes (ET) using Keeling-type plots of water vapor collected within the vegetation boundary layer. Transpiration dominated the ET flux throughout the growing season. In fact, during the monsoon when shallow soil layers were relatively moist, transpiration accounted for 93% of total ET. The stable isotope ratio of plant xylem water and from soil cores and groundwater show that during the driest periods of the growing season (pre-monsoon and post-monsoon), Prosopis derives the bulk of its water from deep soil layers. Only during the wettest periods of the monsoon did Prosopis shift to shallow soil water. Sporobolus took water mainly from shallow soil layers wetted by monsoon storms. The encroachment and establishment of Prosopis in these ecosystems apparently has significant impacts on local and potentially regional water balance by decoupling ET from monsoonal precipitation.

KEY WORDS: Stable Isotopes, Evapotranspiration, Prosopis velutina, Water soruces