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item Moran, Mary
item Keefer, Timothy
item Scott, Russell - Russ
item Emmerich, William
item Cosh, Michael
item O'NEILL, P. - NASA

Submitted to: Interagency Conference on Research in the Watersheds
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2006
Publication Date: N/A
Citation: N/A

Interpretive Summary: Encroachment of woody plants in grasslands is becoming a common phenomenon across the Western U.S. This transformation alters the balance of water loss through transpiration (T) from plants and evaporation (E) from the soil. In semi-arid, water-limited environments, this can result in a reduction in total vegetation growth and a disruption in natural carbon cycling. At a recent conference on Ecohydrology, scientists identified the partitioning of E and T as one of the most important ecohydrological challenges. In this study, daily evapotranspiration (ET) was partitioned into E and T using measurements of diurnal soil surface temperature. The difference between the mid-afternoon and pre-dawn soil surface temperature, termed Apparent Thermal Inertia (IA), was used to identify days when E was negligible. For dates when E'0 and ET was known, T was determined as the residual. This approach was validated at two sites dominated by woody and herbaceous vegetation using conventional instrumentation that was maintained in place continuously for two years. These preliminary results show promise for addressing important hypotheses posed for semiarid sites, such as 1) woody plant encroachment should increase potential soil evaporation (E) and 2) the T/ET ratio is sensitive to changes in woody plant cover.

Technical Abstract: An approach is proposed in which daily ET, measured with a conventional Bowen ratio technique, is partitioned into E and T using coincident measurements of diurnal soil surface temperature. Sites dominated by woody and herbaceous vegetation in the USDA ARS Walnut Gulch Experimental Watershed were instrumented with automated sensors as part of the NASA/USDA Soil Moisture Experiment SMEX04. At each site, surface temperature, soil moisture, soil temperature and meteorological data (including solar radiation, precipitation and evapotranspiration) were measured at 1 to 20-minute intervals over an eighteen-month period in 2004 and 2005 encompassing the dry/hot season, the North American monsoon and the dry/cool season. Results showed that apparent thermal inertia (IA), defined as the difference in soil surface temperature at 2:00 pm and 5:00 am, was related to soil evaporation. The IA values exceeding a nominal threshold were used to identify days when E was negligible, and consequently, ET'T. Further work is planned to derive a total annual T/ET ratio to better understand the ecohydrological consequences of woody plant encroach