U.S. flagAn official website of the United States government
                    Here's how you know **Official websites use .gov**
                            
                            A **.gov** website belongs to an official government organization in the United States.
 **Secure .gov websites use HTTPS**
                            
                            A **lock** (                            
                              A locked padlock                                                      
                            
                            ) or **https://** means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.


Menu Agricultural Research Service
						
                    U.S. DEPARTMENT OF AGRICULTURE
 Southwest Watershed Research Center: Tucson, AZ
 close



    Research

                    Research Projects
                                    Publications
                                    Collaborations
                                    News
                                    Software
                People

                    Staff Listing
                                    Careers


        


                    About SWRCSearch small
                    Search
            
            ARS Home
                                About ARS
                                Contact Us




ARS Home Pacific West Area Tucson, Arizona SWRC Research Publications at this Location





- ARS National Programs
- Search for a scientific manuscript
- Search for a research project
- ARS Software
- Programs and Projects
- Collaborations
- Subjects of Investigation
- Publications

Title:   Author  item
                                            Moran, Maryitem
                                            Keefer, Timothyitem
                                            PAIGE, G. - UNIVERSITY OF WYOMINGitem
                                            Scott, Russell

                                                         - *Russ*item
                                            Emmerich, Williamitem
                                            Cosh, Michaelitem
                                            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/AInterpretive 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


Return to top


Connect with ARS Sign up


ARS Home
                            USDA.gov
                            Plain Writing
                            Policies & Links
                            Civil Rights Statements
                            FOIA
                            Accessibility Statement
                            Privacy Policy
                            Non-Discrimination Statement
                            Quality of Information
                            USA.gov
                            WhiteHouse.gov