|Hernandez, M. - UNIVERSITY OF ARIZONA|
|Nearing, G. - UNIVERSITY OF ARIZONA|
|Paige, G. - UNIVERSITY OF WYOMING|
|Cosh, M. - USDA-ARS HYDROLOGY LAB|
|O'Neill, P. - NASA|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 14, 2008
Publication Date: January 9, 2009
Citation: Moran, M.S., Scott, R.L., Keefer, T.O., Emmerich, W.E., Hernandez, M., Nearing, G.S., Paige, G.B., Cosh, M.H., O'Neill, P.E. 2009. Partitioning evapotranspiration in semiarid grassland and shrubland ecosystems using time series of soil surface temperature. Agricultural and Forest Meteorology. 149:59-72. Interpretive Summary: Encroachment of woody plants in grasslands has become a common phenomenon across the Western U.S. over the past 150 years. Shrublands have a different water demand from grasslands, manifesting in different water availability for plant transpiration (T). In turn, these shifts in evaporation (E) versus T related to vegetation change can impact management strategies in dryland ecosystems associated with land use and climate change. To study this phenomenon, a network of towers is in place to measure evapotranspiration (ET) in grass- and shrub-dominated ecosystems throughout the Western U.S. A method is described and tested here to partition the daily measurements of ET into E and T based on simple measurements of surface temperature. The method was tested at Walnut Gulch Experimental Watershed in southeast Arizona over three years, 2003-2005. Results showed that reasonable estimates of daily T were obtained for a multi-year period with ease of operation and minimal cost. With known season-long daily T, E and ET, it is possible to determine the soil water availability associated with grass- and shrub-dominated sites and better understand the hydrologic impact of regional woody plant encroachment.
Technical Abstract: Information about the ratio of transpiration (T) to total evapotranspiration (T/ET) is related to critical global change concerns, including shrub encroachment and non-native species invasion. In this study, a new approach was used to partition measurements of ET into daily evaporation (ED) and daily transpiration (TD) in a semiarid watershed based on the low-cost addition of an infrared thermometer and soil moisture sensors to existing eddy covariance and Bowen ratio systems. The difference between the mid-afternoon and pre-dawn soil surface temperature, termed Apparent Thermal Inertia (IA), was used to identify days when ED was minimal (EDmin), and thus, TD=ETD- EDmin. For other days, an empirical approach was used to partition ETD into ED and TD based on root zone soil moisture. The method was tested using Bowen ratio estimates of ET and continuous measurements of surface temperature with an infrared thermometer (IRT) at a grassland and shrubland site within the Walnut Gulch Experimental Watershed in southeast Arizona USA in years 2004-2006. Validation was based on a second dataset of Bowen ratio, IRT and shrub sap-flow measurements in 2003. Results showed that reasonable estimates of TD were obtained for a multi-year period with ease of operation and minimal cost. Estimates of TD and ED were summed over the study period when plants were actively transpiring for years 2004, 2005 and 2006 to estimate totals over the study period, TS and ES respectively. Preliminary analysis suggests that the accuracy and precision of TS are on the order of about 5% of TS. For this study period, TS was related strongly to ETS, with a slope of 0.79 for the grass-dominated site and 0.64 for the shrub-dominated site for the three years. Thus, for these sites during the study period in these years, the TS/ETS was higher for the grass-dominated site than for the shrub-dominated site, and did not vary systematically with variation in amounts and timing of rainfall. These results offer some insight into the hydrologic impact of woody plant encroachment in semiarid regions.