Stable isotopes as tools for watershed hydrology model development, testing and rejection

Process representation in the watershed hydrology model structure continues to be a vexing issue in hydrology. Complex hydrological descriptions at the hillslope scale have been difficult to incorporate within a watershed modeling framework due to the disparity between the scale of measurements and the scale of model sub-units. As a result, parameters represented in many conceptual watershed hydrology models are often not physically-based or related to physical properties, and therefore cannot be established prior to a model calibration. While tolerable for predictions involving water quantity, water quality simulations require additional attention to transport processes, flowpath sources and water age. This paper examines how isotope hydrology studies, especially those where residence time can be quantified in surface waters and soil/ground waters may be used to subsume flowpath process complexity and provide a simple, scalable evaluative data source for water quantity-quality based conceptual models at the catchment scale. We present a set of simple distributed hydrologic models (from simple to more complex) along with a methodology to evaluate, through direct simulation, the residence time distribution of the model sub-units. Our results show that models that might otherwise be acceptable for flow may be wholly rejected for an inability to capture residence time dynamics. This process-based rejection framework can be very helpful in the evaluation of conceptual models that include chemistry. (author)