Investigation of partial Contributing areaS in watersheds with a conjunctive surface-subsurface flow model

Yanqing Lian, Ph.D.

Associate Professional Scientist

Watershed Science Section

Illinois State Water Survey

Abstract:

The partial contributing area concept (PCA) has been used in watershed hydrology for several decades; however, there has never been any clear definition of it prior to this study. This research proposed three definitions of PCA (PCA-1, PCA-2, and PCA-3) to cover the major PCA categories used in watershed hydrology, which emphasize that a PCA has to contribute flow to the total runoff and is referred to a specified location at a specified time. The PCA-1 characterizes the instant value observed at the watershed outlet, the PCA-2 identifies the instant value observed within the watershed, and the PCA-3 is the total area contributed to runoff during a rainstorm event.

This research developed a physically based conjunctive flow model (CFM) coupling two-dimensional surface three-dimensional subsurface transport to simulate the total runoff in watersheds. In conjunction with the CFM, an overland flow-tracking program (OFTP) was developed to compute the PCA under the proposed definitions. The model can compute the saturated area (SA), and can separate the total PCA-1 and PCA-2 into the fraction due to saturation overland flow and Hortonian overland flow mechanisms.

Application of the CFM and the OFTP to a real watershed in Illinois proved that the CFM is applicable to a real watershed with reasonable accuracy for flow simulations, and also that the PCAs in a real watershed can be computed with scientific and meaningful definitions. In practice, PCA-1, PCA-2, and PCA-3 values can help with cost-effective engineering designs such as detention ponds or treatment plants.