HR: 0800h
AN: H31D-0420    [Abstracts]
TI: Quaternary Geology, Unsaturated Soil-Moisture Measurements, and Evidence for Overland Flow in the Globe Piedmont, Mojave Desert, CA
AU: * Schmidt, K M
EM: kschmidt@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Nimmo, J
EM: jrnimmo@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Miller, D
EM: dmiller@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Stock, J
EM: jstock@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Winfield, K
EM: kawinfie@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Perkins, K
EM: kschmidt@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Belknap, S
EM: sbelknap@email.unc.edu
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: We hypothesize that the evolution of desert surfaces by pavement development and pedogenesis strongly controls soil moisture properties including water infiltration rates and moisture retention. This leads to strong spatial variation in rainfall-runoff relationships and availability of moisture and nutrients for biota. Correlating these hydrologic properties with surficial geologic units based on pedogenesis may provide the only practical means of estimate runoff generation and soil-water-plant dynamics over broad areas. To illustrate these relationships, we installed and monitored a variety of sensors in undisturbed soils within different aged deposits across the arid Globe piedmont, northeast of Kelso, Mojave National Preserve, California. Since 2002 we have monitored rainfall, soil moisture, and unsaturated soil matric potential using weather stations, and depth-stratified heat-dissipation and dielectric-constant probes. Our studies focus on two adjacent alluvial fans (700 to 950 m altitude) that contain active washes draining bedrock highlands, and intrafan regions of desert pavement. In this region, old (Pleistocene) deposits have advanced desert pavements capping silt-rich, vesicular A (Av), argillic, and petrocalcic horizons. Old deposits support sparse vegetation cover and generally small canopy volumes. In contrast, coarse-textured, weakly developed soils of active and young (Holocene) deposits correlate with greater cover of certain plants such as creosote bush ({\it Larrea tridentata}). We estimated transient initial infiltration rates by measuring the time elapsed draining a given water volume in a bottomless bucket that served as an infiltration ring. Infiltration experiments and depth-stratified probe data indicate that young deposits with weak soils have higher infiltration rates than older deposits. We measured the lowest infiltration capacities (70-420 mm/hr) on Pleistocene fan deposits and on an active wash where the surface was capped by a muddy depositional crust. Young alluvial fan and colluvial deposits were highly conductive with initial rates of 1000 to 2700 mm/hr. In active washes, cm-thick depositional laminae caused significant lateral flow. Macropore flow was only locally observed. Experimental wetting fronts within active washes terminated within silty-sand lenses above coarse sand to gravel layers. In older deposits, lateral flow followed Av horizon boundaries. Field-measured infiltration rates were strongly time-dependent with initial values exceeding final values. This decrease may reflect wetting from initially dry conditions and the sponge-like influence of cyanobacteria, lichen, and clay. Storm-averaged rainfall intensities (~14 mm/hr) associated with a monsoonal storm in August 2003 did not exceed the lowest experimental infiltration capacities. Averaged over 5 minute intervals, peak rain intensities were 67 mm/hr, similar to the lowest measured infiltration rates of 70 mm/hr on older deposits. In addition, field observations reveal that intrafan overland flow, erosion, and sediment transport occurred during the storm downslope of desert pavement on older deposits. The correlation of peak rainfall intensities, experimental infiltration rates, and field observations of runoff support our hypothesis that advanced pedogenesis promotes runoff generation. Future infiltration experiments using shallow water ponded in a 1-m diameter ring will constrain how pedogenesis limits long-term infiltration and influences the location and timing of runoff, wetting front depths, and lateral flow.
DE: 1815 Erosion and sedimentation
DE: 1860 Runoff and streamflow
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting