Submitted to: Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/28/2004
Publication Date: 1/1/2005
Citation: Seyfried, M., Schwinning, S., Walvoord, M.A., Pockman, W.T., Newman, B.D., Jackson, R.B., and Phillips, F.M., 2005, Ecohydrological control of deep drainage in arid and semiarid regions. Ecological Society of America. 86(2):277-287. Interpretive Summary: Two important and related issues facing rapidly growing communities in the arid and semiarid portions of the US are: Can current and projected withdrawals from the groundwater be maintained? and, Can groundwater quality be maintained in light of the use of these regions for storage and disposal of hazardous waste? These issue are related because the rate of groundwater recharge determines sustainability of water supply and the rate of hazardous contaminant transport to the groundwater. Until very recently, estimates of recharge in these regions have been very low but positive, indicating slow but steady recharge which may be critical for both issues. We analyzed water and chloride concentrations in deep arid and semiarid soils with shrub vegetation using computer simulation models. The results show that, under the specified conditions, there has been no recharge for five to ten thousand years. We then analyzed plant the physiology of dominant shrubs in the region and showed that they are capable of creating and maintaining such dry environments. This has at least two important implications for management of arid and semiarid lands. First, where the critical conditions exist, it is not reasonable to expect any resupply of the groundwater, therefore it is important to look in other areas. Second, in considering storage of hazardous waste, these sites are potentially useful, but their effectiveness is assured only if the native vegetation can be maintained.
Technical Abstract: The amount and spatial distribution of deep drainage (downward movement of water through the bottom of the root zone) and groundwater recharge affect water supply and quality in the rapidly growing, semiarid USA. We synthesize research from the fields of ecology and hydrology to address the issue of deep drainage in semiarid regions. We start with a recently developed hydrological model that accurately simulates soil-water potential and geochemical profiles measured in thick (>50 m), unconsolidated vadose zones. Model results indicate that, since the climate change that marked the onset of the Holocene period 10 to 15,000 years ago, there has been no deep drainage and that continuous, relatively low (< -1 MPa) soil-water potentials have been maintained at depths of 2-3 m. The scenario derived from these results proposes that the native, xeric shrub dominated plant communities that gained dominance during the Holocene, generated and maintained these conditions. We present three lines of ecological evidence that support the scenario. First, that xeric shrubs have sufficiently deep rooting systems with low extraction limits to generate the modeled conditions. Second, the characteristic deep-rooted soil-plant systems store sufficient water to be effectively buffered against climatic fluctuations. And third, adaptations resulting in deep, low extraction limit rooting systems confer significant advantages to xeric shrubs in semiarid environments. We then consider conditions in semiarid regions in which the model scenario may not apply, leading to the expectation that portions of many semiarid watersheds supply some deep drainage. Further ecohydrologic research is required to elucidate critical climatic and edaphic thresholds, evaluate the role of important physiological processes (such as hydraulic redistribution), and evaluate the role of deep roots in terms of carbon costs and whole plant development.