HR: 14:40h
AN: H53H-05    [Abstracts]
TI: Simple Ecohydrological Models: Is Average Root Zone Soil Moisture an Adequate Driver in the Functions for Evaporation and Assimilation?
AU: * Kurc, S A
EM: kurc@ag.arizona.edu
AF: University of Arizona School of Natural Resources, 325 BSE PO Box 210043, Tucson, AZ 85721-0043,
AU: Small, E E
EM: eric.small@colorado.edu
AF: University of Colorado Dept of Geological Sciences, Campus Box 399 2200 Colorado Ave., Boulder, CO 80309-0399,
AB: Dryland ecosystems are typically characterized by low annual precipitation, much of which is delivered in the form of small rainfall events that may only wet the top portion of the root zone. In these areas, evapotranspiration (ET) is limited by the availability of soil moisture rather than by atmospheric demand, i.e. ET << potential ET. Likewise, when optimal temperatures and soil nutrients are not limiting, the uptake of carbon by vegetation via photosynthesis, i.e. assimilation, is also limited by the availability of soil moisture. Though soil moisture is largely depth dependent, only average root zone soil moisture is used in typical simple models of ecohydrological processes. Here, we show that in semiarid grassland and shrubland, the surface soil layer is the primary source of water for ET, at least throughout the monsoon season. Conversely, we show that only large precipitation events (or series of small events) generate enough soil moisture below the influence of atmospheric demand to trigger carbon assimilation in these dryland ecosystems. From this we hypothesize that a realistic representation of ecohydrological processes in semiarid areas can not be made solely using average root zone soil moisture. In this study we utilize records of ET, assimilation, and soil moisture at several depths collected during 3 summer monsoons at the Sevilleta National Wildlife Refuge in central New Mexico using eddy covariance methods. Additionally we employ a simple bucket model of ecohydrological processes (e.g. Rodriguez-Iturbe et al. 1999, Daly et al. 2004) driven by average root zone soil moisture. We compare bucket model predictions of ET and assimilation to the actual data records. We show that (1) bucket model predictions of ET lack the dynamic temporal variability of actual observations, (2) declines in ET following peaks are significantly steeper in observed than in predicted times series of ET, and (3) peaks in bucket model predictions of assimilation occur before peaks in observed assimilation. Our results suggest that two or more layers of soil moisture are necessary for making more accurate predictions of ET and assimilation in semiarid ecosystems.
DE: 1813 Eco-hydrology
DE: 1818 Evapotranspiration
DE: 1847 Modeling
DE: 1878 Water/energy interactions (0495)
DE: 4806 Carbon cycling (0428)
SC: Hydrology [H]
MN: 2007 Fall Meeting