Water Supply in the Mississippi Delta: What the Model Has to Say

Author(s): Mason, P.

A regional groundwater flow model has been built as a tool to better understand the system flows and to project future water levels in the Missisippi River Valley alluvial aquifer (MRVA). This is a highly productive aquifer which supports vast amounts of agriculture and aquaculture in northwest Mississippi. Water levels are declining in this aquifer and will be of increasing concern in the future.

To quantify discharge, the model incorporates a method of estimating pumpage for agriculture and aquaculture, based on crop distribution patterns and rainfall-response factors.

Recharge to the aquifer is complex and unusual, since a widespread impermeable surficial unit restricts rainfall infiltration in most of the Delta plain. Good calibration was achieved only when the model fully accounted for recharge data from several sources. Positive recharge sources are: groundwater in the adjoining formations on the eastern bluff hills line, rain infiltration through the alluvial fans along the bluffline, and rain infiltration through sandy areas along the Mississippi River.

Other sources serve as both discharge and recharge areas for the aquifer, depending on season and/or location. These are: the Mississippi River, the underlying Tertiary aquifers (Cockfield and Sparta), the major rivers and the bluffline streams.

The base model period, built from known data for streams, precipitation, crops, and water levels, etc. ran 1996 through 2006. On average, the aquifer lost about 230,000 acre-ft of water per year from 1996 to 2006. During this time, pumpage per season averaged about 3 million acre-feet, with a minimum of 1.7 million acre-feet in 2002 and a maximum of 4.5 million acre-feet in 2000. Rainfall infiltration averaged about 2.4 million acre-feet per water-year, with a low of 1.9 million acre-feet in 1998 to a high of 3 million acre-feet in 2003. Over the ten year period, there were 2 years during which rainfall infiltration exceeded pumpage. In 8 of the years pumpage exceeded rainfall infiltration.

Several scenarios have been run from 2009 water levels forward, simulating conditions 20 years into the future, and the results of these are presented.

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