Determination of Watershed Infiltration and Erosion Parameters from Field Rainfall Simulation Analyses

Abstract

Realistic modeling of infiltration, runoff and erosion processes from watersheds requires estimation of the effective hydraulic conductivity (K m) 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 K m 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 K m 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 K m 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 K m 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.

Surface Runoff in Watershed Modeling—Turbulent or Laminar Flows?

“…An even smaller 'n' value (~0.02) is required to match turbulent and laminar flow celerities (see Figure 3). The range of surface runoff rates considered and listed in Table 1 readily exceed those commonly found in the field (e.g., Tahoe Basin; [40]), or predicted in rainfall-runoff modeling, suggesting that assumption of laminar flow conditions for all but the defined channel flows are likely more appropriate in hillslope modeling efforts rather than assuming application of the Manning's equation and guesstimation of 'appropriate' roughness 'n' values. Nonetheless, for all surface conditions considered, measured flow depths were consistent with those predicted by laminar flow assumption embodied in Equation (2), rather than the turbulent flow conditions represented by Equation (3) and used in physical watershed modeling efforts to date.…”

“…This project originated from an effort to relate observed field simulated runoff rates from Rainfall [40] and Runoff Simulators to model-predicted values so as to better infer soil properties associated with hillslope soils restoration efforts and estimation of watershed sediment loading rates [5]. Such modeling effort was hampered by the inability to a priori predict when surface runoff generation would result from infiltration excess, rather than saturation excess (more readily simulated when available soil water storage is filled).…”

“…An equivalent graph of the ratios of turbulent to laminar flow mean velocities would be the same as that shown in Figure 5 for the ratio of depths. This distinction in velocities is important towards estimating stream power associated with soil detachment and definition of "erodibilities" from RS studies where Grismer [40] found that laminar flow-determined mean velocities better represented the dependence of soil detachment on stream Similarly, anticipating the later discussion of the experimental methods, Figure 5 illustrates the ratio of flow depths determined assuming turbulent and laminar flow conditions, respectively, on a 20% slope at a range of unit width flow rates and 'n' values. The overland flow rates shown equate to Reynolds numbers that range from 15 to 45 and corresponding laminar flow depths of 0.31-0.44 mm, respectively.…”

Groundcover influence on physical and chemical soil properties, runoff and nutrient loss and grape yield and quality in semiarid Mediterranean vineyards

“…Rainfall simulators have been widely used in order to investigate hydrological and erosional processes at watershed and vineyard scales or to compare soil management strategies (Grismer, 2016;Marques et al, 2007;Rodrigo Comino et al, 2016a). On the other hand, runoff simulators contributes to the understating of erosion processes because the effect of runoff in particle detachment, transport and nutrient loss is analyzed separately from the raindrops impacts, which makes the difference with rainfall simulators.…”

“…In agricultural land under semiarid conditions, due to the compaction of the soils and the lack of organic material, the rainfall intensity exceeds the infiltration rate, and then surface runoff takes place (Esteves and Lapetite, 2003). Rainfall simulation is a useful methodology to measure the loss of water, sediments and nutrients because it allows repetitions of the experiments and the accurate control of the rainfall intensity and distribution (Iserloh et al, 2013;Lassu et al, 2015;Grismer, 2016).…”