Physically Based Approach in Hydrology-What Is the Benefit?

Advances in hydrology have traditionally relied heavily on empirical relationships that were derived from experimental data. With the advent of digital computers in the 1950s and 1960s, there was great optimism that modeling the entire hydrological cycle would allow significant advances to be made. The Stanford Watershed Model is such an example. However, the Stanford Watershed Model is a conceptual model that still relies on experimental data. As digital computers became more powerful in the 1970s and 1980s, governing differential equations such as the Saint Venant equations could readily be solved numerically. This advance sparked a proliferation of hydrological models that are called physically based models such as the Storm Water Management Model. This type of modeling was expected to facilitate monumental advances in hydrology. The outstanding feature in these models is that the values of the parameters are meant to be determined a priori i.e., without any experimental data . However, over the past two decades, hydrologists have slowly realized determining the values a priori with the current generation of physically based models is difficult if not impossible. Some of the difficulties are as follows. 1. A lack of areal measurement technique Grayson et al. 1992 ; 2. Equifinality Beven 1993 ; and 3. Aggregation approach and overparameterization Sivapalan 2003a,b . In fact, Beven 1989 concluded that the current generation of physically based models are essentially of conceptual models. Woolhiser 1996 and Renard 1997 also expressed doubts about whether true physically based models are attainable. In view of the preceding, is there any difference between the