Both groundwater and surface water resources are sensitive to unsaturated flow. Important relevant processes include subsurface infiltration, stormflow, partitioning of rainfall into infiltration and runoff; flow anisotropy and preferentiality.
Aquifer recharge—water that moves from the unsaturated zone into the saturated zone—is of key importance for groundwater. Quantitative estimation of recharge rate is important for evaluating the sustainability of groundwater supplies, though this rate does not equate with a sustainable rate of extraction. Where contamination of an aquifer is a concern, estimating the recharge rate is a first step toward predicting solute transport to the aquifer. Recharge may cause a short- or long-term rise of the water table. Recharge rates vary considerably in time and space. Recharge often occurs episodically in response to storms and other short-term, high-intensity inputs. For a given amount of infiltration, concentration short, intense pulses usually enhances recharge because it entails shorter residence times for water in the portions of the soil from which evapotranspiration takes place. This input-intensity relationship has important climate change impacts; most global warming scenarios predict effects such as increased average storm intensity and sudden or early snowmelt, either of which would likely cause substantial increases in recharge. Similarly, a larger fraction of infiltrated water will become recharge if it is concentrated in narrow channels such as fingers or macropores, not only because this tends to hasten its passage through the unsaturated zone, but also because the water then occupies less of the volume of soil from which evapotranspiration takes place.
Our research emphasizes measurement and estimation methods for aquifer recharge rates and their interpretation with respect to hydrologic, geologic, and climatic variables. RELATED PAPERS
Unsaturated-zone transport processes control many aspects of the introduction, persistence, and spread of agents that degrade water quality—toxic substances, salinity, and high populations of certain microorganisms. Agricultural practices degrade water quality by mechanisms such as surface water runoff and ground water recharge. Subsurface waste disposal usually resulted in the transport of hazardous and radioactive wastes. The first approximation of the transport of harmful substances is that they go as the water goes. Many cases additionally require accounting for processes such as chemical reactions and phase changes that retard or accelerate the transport of toxins and contaminants, though a major part of essentially all unsaturated-zone water quality investigations is the evaluation of water flow. Our studies focus on transport of contaminants through the unsaturated zone at sites contaminated by waste disposal, spillage of hazardous materials, and agricultural practices.
Water is a critical element of all ecosystems. Its quality and availability are major determining factors of ecosystem health. Likewise, ecosystems have a dominating influence on water resources. Biota interact with the physical and chemical processes of water and geologic materials in ways that are essential to the hydrologic cycling that sustains the amount and maintains the quality of surface- and groundwater resources. Research areas of ecohydrology include soil moisture availability, transpiration and plant water use, adaption of organisms to their water environment, influence of vegetation on stream flow and function, and feedbacks between ecological processes and the hydrological cycle.
Our main ecohydrologic studies have concerned soil moisture relations in arid regions. Plants and animals use different strategies to survive and flourish when water is minimally and erratically available. The persistence of soil water within a particular depth range and its characteristic time scales of fluctuation play a large role in determining the type of biotic community that can thrive at that location. For example, the depth to which infiltration percolates and the degree to which water is retained at particular depths affect the relative advantage of shallow-rooted grasses compared with deep-rooted shrubs. The deeper unsaturated zone below the upper soil layers, including bedrock, can supply much of the water needed by many plants. In the Mojave National Preserve, California, we conducted field studies and numerical modeing to evaluate soil-moisture dynamics with respect to habitat quality.
Unsaturated flow entails complex interactions of widely diverse materials—minerals, air, water in all its phases, living organisms with their products and remnants, chemicals both natural and anthropogenic. These interactions are important over a huge range of scales. A result of this complexity is that unsaturated flow is still a poorly understood phenomenon of nature. The generally accepted theory of unsaturated-zone flow routinely falls short of adequately predicting key processes, for example preferential flow, the dynamics of unsaturated hydraulic properties, and the effects of the typically extreme heterogeneity of natural unsaturated-zone materials. As a consequence, important problems are unavoidably approached with gross simplifications. Our project aims to develop quantitative generalizations and predictive techniques that are more widely applicable, more far-reaching in their results, more closely tied to the critical issues we need to address, and more closely representative of the phenomena that are observed to occur.
Unsaturated zone processes are notoriously difficult to quantify. Soil and rock materials are opaque to visible light, spatially variable in the extreme at all scales, and easily disrupted by intrusive instrumentation. Relevant conditions and properties are rich in nonlinearities, discontinuities, hysteresis, and coupled interactions. Limitations of present laboratory and field techniques are a major hindrance to large-scale hydrologic application and further development of unsaturated flow theory.
Some of the developments we are working toward are: