Identification and modeling of important stream flow producing processes in watersheds

Date of Completion

January 2002


Hydrology|Engineering, Environmental




The need to identify runoff mechanisms and simulate surface water flows in watersheds other than those dominated by the Hortonian mechanism has prompted the development of a new physically-based hydrologic model called Gridded Surface Subsurface Hydrologic Analysis (GSSHA). The GSSHA model is capable of simulating stream flow generated from Hortonian runoff, saturated source areas, exfiltration, and groundwater discharge to streams. The model employs mass conserving solutions of partial differential equations (PDEs) and closely links the hydrologic compartments to assure an overall mass balance and correct feedback. The model is tested on two very different watersheds: a primarily Hortonian basin with fine-textured soils and deep water table, and a watershed with stream flow generated from a variety of sources, Hortonian runoff, saturated source areas, and groundwater discharge to the stream. These two diverse watersheds provide a difficult test of the model formulation. Spatial convergence studies and sensitivity analysis indicate that very small vertical cell sizes, on the order of 1 cm, in the 1-D Richards' equation solution are required to accurately simulate hydrologic fluxes at the watershed scale. ^ The model is able to efficiently forecast outlet discharge at the Hortonian basin, with very little bias in the predictions. While the new GSSHA model produces good predictions of discharge at the outlet of the Hortonian watershed during the summer growing season, simpler methods of calculating infiltration can also be used successfully in Hortonian watersheds. The model was found capable of simulating both the trends and magnitude in soil moisture with a root mean square error, within approximately 20% of estimated porosity, during the entire year at various depths in two locations in the Hortonian watershed, even though soil moisture data were not used to calibrate the model. The GSSHA model closely reproduces discharge during the calibration and verification periods in the non-Hortonian basin. The new model formulation represents an advance in the state-of-the-art in distributed physically-based watershed scale hydrologic modeling because it incorporates the latest technology and is verifiable at both the process and watershed scale. ^