Channel Ground-Water Interaction (CGI)
CGI in HMS is designed to simulate the interaction between the stream and groundwater. The leakage between them, which is treated as either source or sink in GHM, is calculated using Darcy's Law. The approach of CGI is similar to the river package in MODFLOW (McDonald and Harbaugh, 1988). A layer of low permeable material at streambed is assumed to separate water at channel from the ground-water system at each stream cell. Conceptual representation of channel ground-water interaction is shown in above figure. Assuming that all parameters within a cell are constant, the leakage at a given stream cell is sumulated.
The calculated leakage is a function of the hydraulic difference between the ground-water hydraulic head (from GHM) and river stage (from THM) at a given stream cell. When h(i,j) is smaller than B(i,j) and d(i,j) is greater than zero, leakage from stream to the ground water becomes constant (Cd * thickness of the low permeable layer). When h(i,j) is smaller than B(i,j) and d(i,j) is zero, there is no channel ground-water interaction. Often this idealized geometry attributed to the stream differs from the reality. Thus, in practice, Cd usually becomes a calibration parameter.