To provide an improved methodology for characterizing the field‐saturated hydraulic conductivity (Kfs) over broad areas with extreme spatial variability and ordinary limitations of time and resources, we developed and tested a simplified apparatus and procedure, correcting mathematically for the major deficiencies of the simplified implementation. The methodology includes use of a portable, falling‐head, small‐diameter (∼20 cm) single‐ring infiltrometer and an analytical formula for Kfs that compensates both for nonconstant falling head and for the subsurface radial spreading that unavoidably occurs with small ring size. We applied this method to alluvial fan deposits varying in degree of pedogenic maturity in the arid Mojave National Preserve, California. The measurements are consistent with a more rigorous and time‐consuming Kfs measurement method, produce the expected systematic trends in Kfs when compared among soils of contrasting degrees of pedogenic development, and relate in expected ways to results of widely accepted methods.
Kerie J.; and Wolock, David M., "Factors influencing ground-water recharge in the eastern United States" (2007). USGS Summary Ground-water recharge estimates for selected locations in the eastern half of the United States were obtained by Darcian and chloride-tracer methods and compared using statistical analyses. Recharge estimates derived from unsaturated-zone (R UZC ) and saturated-zone (R SZC ) chloride mass balance methods are less variable (interquartile ranges or IQRs are 9.5 and 16.1 cm/yr, respectively) and more strongly correlated with climatic, hydrologic, land use, and sediment variables than Darcian estimates (IQR = 22.8 cm/yr). The unit-gradient Darcian estimates are a nonlinear function of moisture content and also reflect the uncertainty of pedotransfer functions used to estimate hydraulic parameters. Significance level is <0.001 for nearly all explanatory variables having correlations with R UZC of <À0.3 or >0.3. Estimates of R SZC were evaluated using analysis of variance, multiple comparison tests, and an exploratory nonlinear regression (NLR) model. Recharge generally is greater in coastal plain surficial aquifers, fractured crystalline rocks, and carbonate rocks, or in areas with high sand content. Westernmost portions of the study area have low recharge, receive somewhat less precipitation, and contain fine-grained sediment. The NLR model simulates water input to the land surface followed by transport to ground water, depending on factors that either promote or inhibit water infiltration. The model explains a moderate amount of variation in the data set (coefficient of determination = 0.61). Model sensitivity analysis indicates that mean annual runoff, air temperature, and precipitation, and an index of ground-water exfiltration potential most influence estimates of recharge at sampled sites in the region. Soil characteristics and land use have less influence on the recharge estimates, but nonetheless are significant in the NLR model. ª
To assess the effect of pedogenesis on the soil moisture dynamics influencing the character and quality of ecological habitat, we conducted infiltration and redistribution experiments on three alluvial deposits in the Mojave National Preserve: (i) recently deposited active wash sediments, (ii) a soil of early Holocene age, and (iii) a highly developed soil of late Pleistocene age. At each, we ponded water in a 1‐m‐diameter infiltration ring for 2.3 h and monitored soil water content and matric pressure during and after infiltration, using probes and electrical resistivity imaging (ERI). Infiltration and downward flow rates were greater in younger material, favoring deep‐rooted species. Deep‐rooted species tend to colonize the margins of washes, where they are unaffected by sediment transport that inhibits colonization. The ERI results support important generalizations, for example that shallower than 0.5 m, infiltrated water persists longer in highly developed soil, favoring shallow‐rooted species. Soil moisture data for the two youngest soils suggested that saturation overshoot, which may have significant but unexplored hydroecologic and pedogenic effects, occurred at the horizontally advancing wetting front. Spatial heterogeneity of soil properties generally increased with pedogenic development. Evidence suggested that some early‐stage developmental processes may promote uniformity; the intermediate‐age soil appeared to have the least heterogeneity in terms of textural variation with depth, and also the least anisotropy. Lateral heterogeneity was pronounced in older soil, having a multitude of effects on the distribution and retention of soil water, and may facilitate certain water‐conserving strategies of plants over what would be possible in a laterally homogeneous soil.
Over historic time Hawai‘i's dryland forests have been largely replaced by grasslands for grazing livestock. On‐going efforts have been undertaken to restore dryland forests to bring back native species and reduce erosion. The reestablishment of native ecosystems on land severely degraded by long‐term alternative use requires reversal of the impacts of erosion, organic‐matter loss, and soil structural damage on soil hydraulic properties. This issue is perhaps especially critical in dryland forests where the soil must facilitate native plants' optimal use of limited water. These reforestation efforts depend on restoring soil ecological function, including soil hydraulic properties. We hypothesized that reforestation can measurably change soil hydraulic properties over restoration timescales. At a site on the island of Maui (Hawai‘i, USA), we measured infiltration capacity, hydrophobicity, and abundance of preferential flow channels in a deforested grassland and in an adjacent area where active reforestation has been going on for fourteen years. Compared to the nearby deforested rangeland, mean field‐saturated hydraulic conductivity in the newly restored forest measured by 55 infiltrometer tests was greater by a factor of 2.0. Hydrophobicity on an 8‐point scale increased from average category 6.0 to 6.9. A 4‐point empirical categorization of preferentiality in subsurface wetting patterns increased from an average 1.3 in grasslands to 2.6 in the restored forest. All of these changes act to distribute infiltrated water faster and deeper, as appropriate for native plant needs. This study indicates that vegetation restoration can lead to ecohydrologically important changes in soil hydraulic properties over decadal time scales.
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