MODELING SURFACE-WATER INTERACTION WITH GROUND WATER USING MODFLOW2000

A new streamflow routing package (SFR1) has been written for MODFLOW2000 that improves the simulation of surface-water interaction with ground water. SFR1 uses continuity of mass to route flows into one or more streams and can compute stream stage (depth) as a function of streamflow. The following five options are available to compute the depth of a stream reach in a model cell: (1) specifying a depth; (2) computing depth from Manning’s formula assuming a wide rectangular channel or (3) an eight-point cross section; (4) a power function that relates flow to depth and width, and (5) rating tables that relate flow to depth and width. A different option can be used for each stream. For example, an irrigation canal may be best represented using the first option, whereas a stream may be best represented using option 5. For the last three options, leakage between the stream and ground water changes as a function of stream width. The program also allows for direct precipitation and overland runoff as inflow to a stream and evapotranspiration as outflow from a stream. In addition to routing flow along streams, the program allows for several types of diversions from a stream. Options for diverting flow include specifying a flow rate, specifying a fraction of flow in a stream, and specifying a flow when water is diverted into a flood-control channel. SFR1 is fully connected to a lake package (LAK3) allowing for integrated water budgets of streams, lakes, and ground water.

In areas of the desert Southwest, a thick unsaturated zone can separate streams from the underlying ground water. The time for water to travel through an unsaturated zone to the water table can be months, but presently is assumed to be instantaneous in MODFLOW. A kinematic wave approximation to vertical flow through an unsaturated zone beneath a stream is being implemented to account for residence time of water in the unsaturated zone.