An assessment of a conceptual rainfall‐runoff model's ability to represent the dynamics of small hypothetical catchments: 2. Hydrologic responses for normal and extreme rainfall

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Abstract
Results from five numerical experiments to test a conceptual rainfall‐runoff model (the soil moisture accounting component, SMA, used by the U.S. National Weather Service), comprising combinations of three sets of input data (hourly precipitation and evapotranspiration data) and four hypothetical two‐layered small catchments (areas ≤0.2 km2, saturated hydraulic conductivity from 0.02 to 0.2 m/h, soil depth from 0.8 to 1.6 m, hillslope length from 100 to 250 m, and slope from 0.04 to 0.10), representing a wide range of flow mechanisms, are presented. The calibrated SMA model could simulate flow volumes corresponding to broad‐scale flow mechanisms but generally performed poorly during dry to wet catchment transitions. SMA was found to be unreliable in forecasting extreme floods, especially under dry antecedent moisture conditions, and for catchments where calibration data were comprised primarily of base flow. For each case, the sum of the SMA conceptual storages differed from the maximum physical water storage capacity of the hypothetical small catchment. The calibrated SMA model parameters were climate dependent indicating that a calibrated SMA model should be used with caution when predicting hydrologic response to a changed climate.