Investigation of partial contributing areas in watersheds with a conjunctive surface subsurface flow model

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 the North Creek 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. This research also showed that the SA in a real watershed was often larger than the PCA. Theoretically, the PCA-2 and SA values would be identical if the entire SA were to contribute flow to the total runoff. Most importantly, the comparison of PCA from the CFM and from the TOPMODEL indicated that TOPMODEL PCA belongs to PCA-2. The SA is often used as the actual PCA in many hydrologic models, which can potentially overestimate the runoffs.