HR: 17:30h
AN: H42M-07    [PDF]
TI: Ecotonal Control on Vadose-Zone Fluxes in Arid Regions Over Very Long Time Scales
AU: * Phillips, F M
EM: phillips@nmt.edu
AF: New Mexico Tech, 801 Leroy Place, Socorro, NM 87801
AU: Walvoord, M A
EM: walvoord@usgs.gov
AF: U.S. Geological Survey - WRD, Denver Federal Center, Box 25046, MS 413, Lakewood, CO 80225-0046
AU: Sandvig, R
EM: rsandvig@nmt.edu
AF: New Mexico Tech, 801 Leroy Place, Socorro, NM 87801
AB: Recent studies indicate that vegetation plays an important role in regulating recharge in semiarid and arid basins over very long time scales. Several lines of evidence from desert floor environments in the southwestern United States suggest that vegetation has established essentially permanent upward hydraulic gradients, effectively precluding diffuse recharge since the transition from woodland to xeric scrub in the early Holocene. However, less xeric vegetation (such as the pygmy pi\~{n}on and juniper forest) may support significant diffuse recharge. We show comparative water potential and porewater chemistry profiles collected from various vegetation communities in the Chihuahuan Desert of west Texas. The modeled soil water (vapor and liquid) flux regimes illustrate a conversion from substantial downward fluxes under the mixed woodland to upward fluxes under grassland and xeric scrub. Model results also indicated a trend in increasing drying front propagation depth from the grassland to recently-encroached xeric scrub to well-established xeric scrub. Drying fronts are the result of upward soil water fluxes initiated up to 16 thousand years ago in the xeric scrub community. In contrast, the nearby woodland community supports active, and likely episodic, recharge on the order of 5 to 15 mm yr$^{-1}$. The mechanism by which some vegetation takes up essentially all seasonally available moisture within the root zone, preventing downward soil water fluxes for periods of thousands of years, but adjacent vegetation communities regularly permit downward fluxes, remains to be determined. Nevertheless, these results suggest that understanding the relation between vegetation community and vadose-zone hydrological processes may be the most profitable avenue toward quantifying diffuse groundwater recharge. We hypothesize that vegetation type may be a reasonable proxy for estimating recharge in semiarid and arid basins. Ongoing research is intended to test the hypothesis of ecotonal control on soil water fluxes by means of a vadose-zone drilling project along an aridity index gradient in central New Mexico.
DE: 1615 Biogeochemical processes (4805)
DE: 1818 Evapotranspiration
DE: 1851 Plant ecology
DE: 1875 Unsaturated zone
DE: 1878 Water/energy interactions
SC: Hydrology [H]
MN: 2003 Fall Meeting