DEVELOPMENT OF A MODFLOW TOOL FOR DELINEATION OF STREAM DEPLETION ZONES

A new tool has been developed for calculating stream depletion (SDZTool) from hypothetical wells using a MODFLOW groundwater flow model. The SDZTool produces a three dimensional map of stream depletion that can be used in developing water management strategies. A stream depletion zone (SDZ) as defined by the 2013 Montana Legislature is “an area where hydrogeologic modeling concludes that as a result of a ground water withdrawal, the surface water would be depleted by a rate equal to at least 30% of the ground water withdrawn within 30 days after the first day a well or developed spring is pumped at a rate of 35 gallons a minute” (MCA 2013: 85-2-380). New wells within the established stream depletion zones that are exempted from the permitting process (“exempt wells”) are limited to a maximum pumping rate of 20 gallons per minute and limited to a total withdrawal of 2 acre-feet per year. Exempt wells that are outside of stream depletion zones can pump at up to 35 gallons per minute, and can withdraw up to 10 acre-feet per year (MCA 2013: 85-2-306). Similar laws have been adopted by other states. If a properly developed and calibrated numerical model is available for an area, the stream depletion that would result from pumping a well at any particular location can be calculated. However, manually testing the effects of a well in all possible locations would be extremely inefficient and time consuming. Following the basic procedure outlined by Leake et al. (2010) a generic tool has been developed for MODFLOW that sequentially simulates a pumping a well in featureless grid cells. Because the testing of each cell is completely independent of the results from other cells, the desktop program runs in parallel, using all available processors. In addition, several options for increasing the efficiency have been developed, including a recursive search method and a version capable of running on a high performance computer.