Hydraulic Properties of a Desert Soil Chronosequence in the Mojave Desert, USA

packed gravel that overlies a thin (3–10 cm), fine-grained, gravel-poor, vesicular A (Av) soil horizon.1 Desert paveDesert pavements are prominent features in arid environments ments are prominent features in arid environments and and consist of a surface layer of closely packed gravel that overlies a thin, gravel-poor, vesicular A (Av) soil horizon. Well-developed Av can be found on a variety of landforms of significantly horizons form distinct and highly structured columnar peds. These diverse ages ranging from Holocene to Tertiary (Bull, structures, along with their siltand clay-rich texture, are hypothesized 1991; Cooke et al., 1993). Pavements have been used in as controlling infiltration and hence the overall hydrologic conditions subdividing and correlating Quaternary alluvial fans for in the soil profile. The objectives of this study were to (i) evaluate studying neotectonics and Quaternary climate change how pedological development in near-surface soil horizons in an arid (McFadden et al., 1998; Bull, 1991; McDonald et al., alluvial fan complex affects the soil hydraulic characteristics, and (ii) 2003). Other research has shown that age is an important to compare the use of Wooding’s equation and inverse modeling for consideration in development of desert pavements, esevaluating hydraulic conductivity in highly layered, near-surface soils. pecially in areas downwind of source zones for aeolian These objectives were approached through field tension infiltrometer deposited material (McDonald, 1994; McDonald et al., studies, soil sampling, and laboratory analyses of soil texture, water content, and soluble salt concentrations. Soils at five sites were studied 1996; McFadden et al., 1998). Pavements tend to be more at the Mojave National Preserve, California, representing a soil chroprevalent and more strongly developed on older surnosequence (50–100 000 yr) with varying degrees of desert pavement faces where aerosolic clayand silt-sized particles are development. Results indicated 100-fold and threefold declines, redeposited on the surface and are subsequently transspectively, in saturated hydraulic conductivity (Ks) with both analytical located downward into the soil profile. Increasing accumethods, and w using Wooding’s method, as the soils aged. No clear mulation of aerosolic fines with time enhances the develtrends in Ks or w were detected in the underlying horizon, indicating opment of a highly structured Av horizon consisting of that the controlling feature at these sites, in terms of water entry, was distinct columnar-shaped peds, ranging in diameter from the Ks of the surface (Av) horizon. Soluble salt concentrations within about 3 to 8 cm, that part to platy peds, ranging in thickthe profile indicated reduced infiltration with increased pavement ness from 0.2 to 1 cm. The formation of desert pavement development. Results showed that surface age can be used as an excellent predictor of saturated hydraulic conductivity (r 2 0.9254). and the Av horizon is extremely slow and can take from Further, results suggest that Av horizon development represents a key 4000 to 10 000 yr to become established. process controlling water cycling, potentially influencing ecosystem Hydraulic properties of structured soils have been function in arid lands. studied for some time, primarily in environments different from the arid Mojave Desert. Some studies have shown that pedological development of clayey soils can S ecosystems in arid and semiarid environexplain dynamic water flow through cracks and other ments require detailed knowledge of the dynamic macropores (Lin et al., 1998, 1999; Bouma and Wosten, relationships among geomorphology, soil hydrology, 1979; Jarvis and Messing, 1995). Others have sought to surface characteristics, and plant cover. This is especially understand the pore classes that contributed the majorimportant given the limited occurrence of rainfall events ity of flow (e.g., Watson and Luxmoore, 1986), many that produce significant local recharge or surface runoff of which were also in clayey soils. The study described that could lead to recharge at other focus areas (microhere is unique with respect to the significantly drier topographic lows or ephemeral washes). It is also imporclimate where the soils developed, the general lack of tant that these dynamic relationships be investigated swelling clays in these arid climates, the difference in the before development in undisturbed arid areas is undertime needed to develop the structure, and the fragility of taken. Timely availability of new data will be useful in these surfaces. Their disruption can have a potentially future decision-making processes because development dramatic impact on water balance. of these fragile lands is likely to occur with or without The understanding of how soil hydrology affects soil the results of scientific research. development spans a number of practical environmental Desert pavements consist of a surface layer of closely applications. Knowledge of soil dynamics and the consequent linkages to ecosystem development will facilitate M.H. Young, S.G. Benner, and D.G. Meadows, Desert Research improved designs of evapotranspiration disposal covers, Institute, Univ. and Community College System of Nevada, Las Vegas, ultimately leading to stable and maintenance-free feaNV 89119; E.V. McDonald and T.G. Caldwell, Desert Research Institures on the landscape (Shafer et al., 2004). Soil hydroltute, Univ. and Community College System of Nevada, Reno, NV ogy plays an extremely important role in near-surface 89512; D.G. Meadows, currently, Hydrologic Sciences Program, Univ. of Nevada, Reno, NV 89532; S.G. Benner, currently, Dep. of Geowater balances and the associated responses in watersciences, Boise State Univ., Boise, ID 83725. Received 2 Nov. 2003. limited ecosystems. McDonald et al. (1996) demonOriginal Research Paper. *Corresponding author (michael.young@ dri.edu). 1 The lower case letter v formally stands for the presence of plinthite (Soil Survey Staff, 1998); however the v has been extensively used in Published in Vadose Zone Journal 3:956–963 (2004).  Soil Science Society of America a substantial number of studies of desert soils that possess horizons with abundant vesicular pores. 677 S. Segoe Rd., Madison, WI 53711 USA