WATER RESOURCES RESEARCH GRANT PROPOSAL
Project ID: 2004NV65B
Title: Small Scale Variability of Soil Ped Hydraulic Properties: Potential Impact on Soil Recharge and Ecosystems
Project Type: Research
Focus Categories: Hydrology, Geomorphological Processes, Drought
Keywords: spatial variability, groundwater recharge, soil hydraulic properties, soil recharge, ecosystems, soils development, plant water availability
Start Date: 03/01/2004
End Date: 02/28/2006
Federal Funds: $7,000
Non-Federal Matching Funds: $14,125
Congressional District: Nevada 01
Desert Research Institute
Spatial variability of soil properties has significant impacts on desert ecosystems that are highly water limited. Coupling that observation with the drought that the southwestern United States has been experiencing for the past several years, we need to better understand how water moves through the upper soil surface and into deeper horizons. Previous work has shown that soils on alluvial fans have hydraulic properties that are significantly dependent on surface age, though other factors play some role. Thus, water percolation through older soils, specifically those with desert pavements, will reduce percolation to depths where the bulk of plant roots are located, and into deeper soils where it can recharge groundwater supplies.
Desert pavements are common landscape features of arid environments worldwide. Desert pavements consist of a surface layer of closely packed gravel embedded in structured, fine-grained, gravel-poor vesicular A (Av) soil horizons. The surficial layer is typically 1-10 cm thick. It easily breaks into irregularly shaped, columnar polygons called peds. The Av horizon is perhaps the most significant, and least understood feature in the hydrologic regime of these desert environments. Once water penetrates the Av horizon, and percolates the subsoil, the moisture can be held for extended periods (pavements can act as a mulch). Thus, desert pavements and the associated Av horizons are potentially very ecologically significant.
The objectives of this study are 1) to fully investigate the efficacy and limitations of a new method for the determination of soil hydraulic properties of individual soil peds, 2) to determine the degree of heterogeneity in soil hydraulic properties on a small (cm) scale and 3) to determine if the commonly used 20-cm diameter tension disc infiltrometer is sufficient to capture the variability on these alluvial surfaces. Because desert pavements are such prominent landscape scale features, this work has important implications not only for the state of Nevada, but also for arid regions worldwide. Results from this study would have direct application to a variety of salient issues such as aquifer recharge, waste management and landfill design, ecosystem health and conservation, and contaminant transport.
Our approach is to use tension disc infiltrometer experiments (20-cm diameter) to estimate the soil hydraulic properties on pavement surfaces ranging in age from 4,000 to 100,000 years old. The infiltrometer will provide a set of average soil hydraulic properties over the disc area. Small soil caissons containing the individual soil peds will be recovered, and transported to DRI in Las Vegas. Each ped will then be individually analyzed for hydraulic properties using a laboratory method that was modified from a previous study. Once identified, the spatial variability of property values will be evaluated using geostatistical techniques, so that the results can be subsequently scaled up to the pedon level. We believe this is the first time that such measurements have been made.
The expected results from this study will be an assessment of the spatial variability of soil hydraulic properties in highly structured, desert pavement soil environments. Previous work in this area by the author and his student indicates that soil properties can change by two orders of magnitude within very short distances, on the order of 10s of cm. However, the measurement scale of commonly used devices in soil physics appears to capture that variability, allowing the spatial structure of the soil surface to be applied to larger land areas. If this is true, hydraulic property estimates of larger soil areas can be statistically validated. This means that predictions of deep percolation, potential groundwater recharge, and soil water balance, can be made using numerical models that also capture the statistical variability.