An assessment of a conceptual rainfall‐runoff model's ability to represent the dynamics of small hypothetical catchments: 1. Models, model properties, and experimental design

A set of tests is proposed to determine the reliability of soil moisture accounting rainfall‐runoff models for prediction beyond the calibration experience and for extreme event forecasts under dry and wet antecedent conditions. The tests are developed for small (less than 0.5 km 2 ) hypothetical hillslope catchments whose hydrologic response to rainfall and evapotranspiration (ET) fluxes is modeled with an accurate physically based model (S‐H) developed by Smith and Hebbert (1983). The tests are based on the assumption that the climatic and resulting S‐;H streamflow time series are error free. The conceptual model explored is a modified version of the Sacramento model (used by the U.S. National Weather Service) reprogrammed for inputs at various time increments as small as 1 min. A Nelder and Mead (1965) direct search optimization scheme is also included in this modified model (denoted SMA) to assist in model calibration. The set of tests is designed to illuminate differences in modeled ET fluxes, streamflow time series, and the relative amounts of Hortonian and saturated overland flow and subsurface flow under a wide variety of climate and hillslope combinations. The test scheme ensures that the conceptual model is tested for a number of hydrodynamic conditions where flow responses range from primarily surface, to primarily subsurface, to relatively even amounts of surface and subsurface flow for humid and subhumid environments. It also provides direct comparisons of the physical (hillslope catchments) and conceptual model storages, and explicit evaluation of the combined influence of conceptual model structure and calibration errors. Specific model structure differences between S‐H and SMA are detailed to elucidate any differences between model responses reported in the companion paper. Hydrologic responses are evaluated with statistical measures and graphical comparisons for time increments ranging from minutes to years.