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[1] One of the most challenging parameters in hillslope-and watershed-scale, distributed, hydrologic models is the lateral saturated hydraulic conductivity (K s ). In this paper, we present a methodology to determine the hillslope-scale lateral K s above a moderately deep sloping restrictive layer in an 18 Â 35 m hillslope plot using perched water level measurements and drain tile outflow data. The hillslope-scale lateral K s was compared to small-scale K s measured with small soil cores and the Guelph permeameter. Our results show that small-scale K s measurements underestimate the actual hillslope-scale K s . The hillslope-scale K s measurements were 13.7, 4.1, and 3.2 larger than small soil core measurements in the A, B, and E horizons, respectively. We argue that the gap between small-scale and hillslope-scale K s within the same porous medium is foremost a measurement problem. Data analysis provided the K s distribution with depth, showing a sharp decrease in K s within the first 0.1 m of the soil and an exponential decline in K s below 0.1 m. The distribution of K s with depth was best described by a double-exponential relationship. Overall, results indicate the importance of macroporosity, perhaps of biological origin, in determining K s at a hillslope scale.

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Abstract:Verification of distributed hydrologic models is rare owing to the lack of spatially detailed field measurements and a common mismatch between the scale at which soil hydraulic properties are measured and the scale of a single modelling unit. In this study, two of the most commonly calibrated parameters, i.e. soil depth and the vertical distribution of lateral saturated hydraulic conductivity K s , were eliminated by a spatially detailed soil characterization and results of a hillslope-scale field experiment. The soil moisture routing (SMR) model, a geographic information system-based hydrologic model, was modified to represent the dominant hydrologic processes for the Palouse region of northern Idaho. Modifications included K s as a double exponential function of depth in a single soil layer, a snow accumulation and melt algorithm, and a simple relationship between storage and perched water depth (PWD) using the drainable porosity. The model was applied to a 2 ha catchment without calibration to measured data. Distributed responses were compared with observed PWD over a 3-year period on a 10 m ð 15 m grid. Integrated responses were compared with observed surface runoff at the catchment outlet. The modified SMR model simulated the PWD fluctuations remarkably well, especially considering the shallow soils in this catchment: a 0Ð20 m error in PWD is equivalent to only a 1Ð6% error in predicted soil moisture content. Simulations also captured PWD fluctuations during a year with high spatial variability of snow accumulation and snowmelt rates at upslope, mid-slope, and toe slope positions with errors as low as 0Ð09 m, 0Ð12 m, and 0Ð12 m respectively. Errors in distributed and integrated model simulations were attributed mostly to misrepresentation of rain events and snowmelt timing problems. In one location in the catchment, simulated PWD was consistently greater than observed PWD, indicating a localized recharge zone, which was not identified by the soil morphological survey.

Seasonally perched water tables (PWTs) are common in loess‐derived Argixerolls and Fragixeralfs of the Palouse region of northern Idaho and eastern Washington. However, little is known about the short‐term PWT dynamics in these rolling to hilly landscapes and how they are influenced by a regional climatic gradient. In this study, PWTs on an Argixeroll hillslope receiving 700 mm of mean annual precipitation (MAP) and a Fragixeralf hillslope receiving 830 mm of MAP were monitored hourly for four seasons. Results demonstrate that timing of PWT formation may vary considerably from year to year, and may occur up to 3 wk earlier in Fragixeralfs than in Argixerolls. Once formed, the PWTs respond rapidly to precipitation and snowmelt in both soils, with PWT levels increasing as much as 60 cm within a period of <24 h. Water table levels are at or near the soil surface numerous times during the season following periods of rainfall or snowmelt. Perched water table dynamics are remarkably consistent across the region, with similar responses observed in hillslopes located 28 km apart. Relatively dense, light‐colored E horizons overlying the restrictive horizons remain continuously saturated for up to 6 to 7 mo yr−1 and develop redox potentials sufficiently low for Fe reduction to occur. Results suggest that seasonal PWTs drive the processes of ferrolysis and hydroconsolidation, and these processes are responsible for many of the E horizon properties common to Argixerolls and Fragixeralfs of the region.