Abstract
Realistic modeling of infiltration, runoff and erosion processes from watersheds requires estimation of the effective hydraulic conductivity (Km) of the hillslope soils and how it varies with soil tilth, depth and cover conditions. Field rainfall simulation (RS) plot studies provide an opportunity to assess the surface soil hydraulic and erodibility conditions, but a standardized interpretation and comparison of results of this kind from a wide variety of test conditions has been difficult. Here, we develop solutions to the combined set of time-to-ponding/runoff and Green– Ampt infiltration equations to determine Km values from RS test plot results and compare them to the simpler calculation of steady rain minus runoff rates. Relating soil detachment rates to stream power, we also examine the determination of “erodibility” as the ratio thereof. Using data from over 400 RS plot studies across the Lake Tahoe Basin area that employ a wide range of rain rates across a range of soil slopes and conditions, we find that the Km values can be determined from the combined infiltration equation for ~80% of the plot data and that the laminar flow form of stream power best described a constant “erodibility” across a range of volcanic skirun soil conditions. Moreover, definition of stream power based on laminar flows obviates the need for assumption of an arbitrary Mannings “n” value and the restriction to mild slopes (<10%). The infiltration equation based Km values, though more variable, were on average equivalent to that determined from the simpler calculation of steady rain minus steady runoff rates from the RS plots. However, these Km values were much smaller than those determined from other field test methods. Finally, we compare RS plot results from use of different rainfall simulators in the basin and demonstrate that despite the varying configurations and rain intensities, similar erodibilities were determined across a range of infiltration and runoff rates using the laminar form of the stream power equation.
Highlights
Modeling watershed runoff and erosion processes realistically requires field determination or model calibration estimation of surface soil infiltration rates or effective hydraulic conductivities (Km ) and erosion rates because they vary with soil cover/tilth/slope conditions, and seasonally with changing water contents
Rainfall simulators (RSs) are essential tools for investigating the dynamic processes of infiltration, runoff and erosion under a variety of field conditions [1,2,3,4], and information from rainfall simulation (RS) test plots can provide the infiltration/runoff parameterization required for watershed modeling
We focus on a direct comparison of the infiltration and erosion results obtained from three RSs used in the basin that have rain drop energies of < 14 and about 34 of maximum values (i.e., short and tall drop-former (DF) simulators, respectively) and the greater energy sprinkler RS used by the USFS
Summary
Modeling watershed runoff and erosion processes realistically requires field determination or model calibration estimation of surface soil infiltration rates or effective hydraulic conductivities (Km ) and erosion rates because they vary with soil cover/tilth/slope conditions, and seasonally with changing water contents. The field methods often used to measure in situ saturated or effective hydraulic conductivities (Ks or Km , respectively, where Km is some fraction of Ks ) include surface (e.g., disk, single/double-ring infiltrometers, and rainfall simulators), or subsurface techniques (e.g., bore-hole methods). Hydrology 2016, 3, 23 tilth and surface cover while providing insights into the effectiveness of soil restoration methods in the field They are not without complications associated with surface disturbance/roughness, type and configuration of surface cover, steep slope (double-ring methods require mild slopes), disk plate hydraulic contact and estimation from rainfall minus runoff rates using rainfall simulators.
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