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Title: Evapotranspiration partitioning in semiarid shrubland ecosystems: A two-site evaluation of soil moisture control on transpiration

Author
item Cavanaugh, Michelle
item KURC, S. - University Of Arizona
item Scott, Russell - Russ

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2011
Publication Date: 9/21/2011
Citation: Cavanaugh, M.L., Kurc, S.A., Scott, R.L. 2011. Evapotranspiration partitioning in semiarid shrubland ecosystems: A two-site evaluation of soil moisture control on transpiration. Ecohydrology. 4:671-681.

Interpretive Summary: The productivity of vegetation in drier parts of the world is highly sensitive to timing and amount of precipitation. Future climate scenarios suggest that the frequency and magnitude of precipitation events will change in these regions. How much and to what extent will these changes impact the water cycle in creosotebush shrublands that dominate the three North American hot deserts? In this study, we measured and examined how precipitation is transformed into moisture in the soil and then how that moisture is lost back to the atmosphere as evaporation from bare soil and transpiration from plants (T) within two creosotebush ecosystems in southern Arizona. During the dry period preceding the summer rainy season, both E, T and soil moisture were very low. With the onset of summer rains, E dominated ET; shrubs did not respond to increases in soil moisture for approximately three more weeks. A series of large precipitation events increased moisture at deeper soil layers, and triggered plant transpiration. Overall, ET was largely related to moisture levels in the shallow soil layers while T was related to moisture deeper in the soil. Under the current precipitation regime, characterized by many small storms and few large storms, soil moisture is low with most precipitation inputs lost as E. However, if climate changes lead to less frequent but larger precipitation events, these types of vegetation communities could experience shifts to more plant water use and productivity.

Technical Abstract: Vegetation of dryland ecosystems is sensitive to precipitation pulses. Future climate scenarios suggest that the frequency and magnitude of precipitation events will change. How much and to what extent will these changes impact the hydrological cycle in creosotebush (Larrea tridentata) shrublands that dominate the three North American hot deserts? In this study, we examine the partitioning of precipitation inputs into evaporation (E) and transpiration (T) within creosotebush ecosystems at sites characterized by bimodal precipitation regimes: the Santa Rita Experimental Range (SRER) and the Walnut Gulch Experimental Watershed (WGEW). At both sites, during summer 2008, we measured ET using eddy covariance, whole plant T using the heat-balance sap flow, and soil moisture at several depths. During the dry period preceding the summer monsoon, both ET and soil moisture were very low. With the onset of summer rains, E dominated ET; shrubs did not respond to increases in soil moisture for approximately 3 more weeks. Series of large precipitation events increased moisture at deeper soil layers, and triggered T. Overall, ET was largely correlated to moisture levels in shallow soil layers typical of dryland ecosystems dominated by dry conditions, high evaporative demand, and poor soil infiltration. Under the current precipitation regime, characterized by many small storms and few large storms, soil moisture is low with most precipitation inputs lost as E. However, if climatic changes lead to less frequent but larger precipitation events, dryland communities could experience shifts in the partitioning of ET affecting the hydrologic budget of the ecosystem.