Physically based hydrologic modeling: 1. A terrain‐based model for investigative purposes

THALES, a simple distributed parameter hydrologic model is presented and applied to two catchments in Australia and the United States, each with different dominant hydrologic responses. The model simulates Hortonian overland flow and runoff from saturated source areas and is used to identify some of the barriers to modeling the hydrology of small catchments. At Wagga Wagga in New South Wales, Australia, runoff is produced from saturated source areas, whereas on the Lucky Hills catchments at Walnut Gulch in Arizona, Hortonian overland flow processes dominate. Simulations at Wagga Wagga are based on published parameters and field data measured as part of an intensive field program and result in a relatively poor fit of the outflow hydrographs for a series of storms. The simulated position and growth of saturated areas coincides with the limited available information, indicating that at least the gross effects of subsurface water movement are being represented. For the Lucky Hills catchments, the hydrographs at the catchment outlet and points within the catchment are simulated for a storm series. The results are highly dependent on the parameter values, which are poorly defined, highlighting the lack of measured field data and lack of methodology for the collection of data at a scale appropriate for such models. The model structure is also shown to have a major influence on the output. The influence of simulating surface flow as sheet flow or rill flow or through a series of ephemeral gullies, as well as the choice of the surface roughness parameter and antecedent soil water conditions, is shown to have a profound effect on the distributed flow depth and velocity predictions. By fitting model parameters, a simulation assuming Hortonian overland flow produced similar results at the catchment outlet to those based on partial area runoff. These results are of concern since it is common to calibrate and verify hydrologic models based on the accuracy with which the catchment outflow is predicted. The internal estimates of flow characteristics following such a calibration often provide the input to sediment and nutrient transport models. Models such as THALES produce an enormous amount of information and have the theoretical potential to provide a “universal” tool for the representation of hydrologic response. However, problems of verification and validation of such models are acute. These problems relate to the difficulty in measuring/deriving parameters a priori, measurement of the catchment response in sufficient detail for testing, and the validity of the fundamental assumptions and algorithms used in model development.