More effective water management strategies for irrigation in the Delta area of western Mississippi — using the most efficient irrigation method for a particular crop and greater use of surface water resources — could help to halt the decline in the shallow alluvial aquifer and maintain its sustainability into the second half of this century.
That’s the conclusion of a study by the Mississippi State University Department of Geosciences, which evaluated 48 years of crop, irrigation, and climatological data for Sunflower County, Miss., which makes heavy use of water from the aquifer for irrigation of crops and aquaculture.
“Climate, crop acreage, irrigation water use, and groundwater decline data were used to construct a model that simulates the effects of climatic variability, crop acreage changes, and specific irrigation methods on consequent variations in the water volume in the aquifer,” says Charles Wax, state climatologist for Mississippi and professor in MSU’s Department of Geosciences.
The model was then used to determine the impact of those variations on the aquifer through the year 2055.
Working with Wax on the study were Jonathan W. Pote, associate director, Mississippi Agricultural and Forestry Research Station, and Tia L. Merrell, graduate research assistant, Department of Geosciences.
The most recently documented water volume decline in the aquifer was estimated at 500,000 acre feet.
“Use of the model to simulate changes in irrigation methods and crop acreages from 2008 through 2055 shows potential to stabilize the water volume in the aquifer through implementation of various management strategies,” Wax says.
Four water management scenarios were simulated in the study: static land use/water use in 2006; total efficient irrigation methods; total inefficient irrigation methods; and enhanced surface water use, when available, instead of groundwater.
Results of the 48-year model simulations showed how irrigation systems and management can affect the water volume in the aquifer.
• In the first scenario, based on 2006 Sunflower County crops, irrigation methods, rainfall, and other data, water volume in the aquifer begins at little more than a negative 200,000 acre feet and consistently drops to about negative 600,000 acre feet in the first eight years. Then, drawdown stabilizes and water volume even rises between about 2015-2040, but then drops consistently to about negative 1.6 million acre feet during the period 2041-2055.
• In the second scenario, with water use practices changed to reflect the most conservative water use method for each crop (100 percent pivot irrigation for cotton; 100 percent zero grade for rice; 100 percent pivot for corn; 100 percent zero grade for soybeans; and 100 percent 6/3 management for catfish), there was consistent recovery of water volume beginning in the first year of these practices, ending in 2055 with a positive aquifer volume of about 2.9 million acre feet.
• In the third scenario, based on practices using the least conservative water use methods for each crop (100 percent furrow irrigation for cotton; 100 percent straight irrigation for corn; 100 percent contour irrigation for rice; 100 percent pivot irrigation for soybeans; and 100 percent maintain full for catfish), there was a consistent water volume decline in the aquifer over the 48-year period, ending at about negative 4.2 million acre feet in 2055.
• The fourth scenario used surface water in lieu of groundwater when growing season rainfall was 30 percent or more above average. This resulted in consistent declines in water volume from the beginning of the period to about 2017.
During that 10-year period, however, there were no years in which growing season precipitation met the 30 percent above normal threshold.
But, from about 2017 to 2044, water volumes in the aquifer increased or stayed level, well above what the volume would have been each year if no surface water had been used.
Beginning in 2044, another group of years occurred when the precipitation didn’t meet the 30 percent threshold and water volumes declined until the end of the period, but still ended about positive 800,000 acre feet.
Results of the modeling indicate, the researchers concluded, that “the aquifer responds to small changes in water use associated with crop type, irrigation methods, and use of surface water, when available. Results also show that the aquifer water volume is apparently very strongly related to changes in water use methods associated with climatological variability.”
Wax says the study is being expanded in 2010 to cover all of the Mississippi Delta region.
Mississippi farmers are increasingly utilizing various forms of irrigation to insure that crops get the needed amount of water at the right time to maximize yields, the researchers note, and demand for groundwater resources continues to grow at a rapid rate.
“The shallow alluvial aquifer is the most heavily developed source of groundwater in the Delta and the entire state,” Wax notes. “Water volume in the aquifer is subject to seasonal declines and annual fluctuations caused by both climatological and year-to-year water use variations.
“These declines can be dramatic, and are most notable during the April-October period, particularly in years when normal crop water demands are accentuated by concurrent abnormally dry climatic conditions.”
Recharge during the remainder of the year has recently been insufficient to restore water volume, he says, the result being that “the aquifer is now being mined at the approximate rate of 300,000 acre feet per year, and the most recently documented water volume decline — October 2005 to October 2006 — is estimated at 500,000 acre feet.
“This may represent a worst case scenario, of severe drought combined with consequent increased demand for irrigation. It is estimated that water use for row crops doubled during this period.”
It is of “critical importance,” Wax says, to understand how climatological variability and agricultural uses cause the groundwater volume in the aquifer to vary, and equally critical to “discover and implement management strategies to change irrigation methods” to use precipitation and other surface water sources as substitutes for aquifer withdrawals in order to reduce the use of groundwater.
“Stopping the consistent drop in water volume in the aquifer will require a curtailment averaging about 300,000 acre feet of groundwater use each year, and the highest priority of this research is to find and recommend solutions to the problem.”
Farmers in the region and planners in the Yazoo Mississippi Delta Joint Water Management District can employ the findings, Wax says, to “design sustainable water use scenarios” that will allow continuation of the region’s agricultural productivity.
Agriculture is the major consumer of water in the Southeast states, the researchers note, and “has the potential to become disproportionately consumptive.”
Research to reduce reliance on groundwater in aquaculture, for example, “has shown remarkable potential reductions through use of management strategies to create storage capacity to capture rainfall to keep ponds filled.” For Delta catfish ponds, studies indicate that groundwater consumption can be reduced by nearly 70 percent annually by capturing precipitation.
In rice production, straight levee systems and use of multiple inlets have been shown to significantly reduce water use. In Mississippi field trials, intermittent (wet/dry) irrigation has been shown to reduce water use and non-point source runoff by as much as 50 percent, with no yield loss.
The model used to analyze Sunflower County data is interactive, allowing the researchers to change input values and alter final output in order to simulate specific scenarios and determine possible methods and strategies to aid in groundwater conservation and management.
The data included daily precipitation from 1949 through 2008; crop data for cotton, rice, soybeans, corn, and catfish; total irrigated acres for each type of irrigation for years 2002-2008, the only years for which water use data were available; water use data for 2005 through 2008 for each irrigation method and crop, based on 140 sites; and total average water use for each crop 2005-2008. A model for catfish was based on climate data and average water use by management scheme. Water use data for row crops, rice, and aquaculture were then combined with acreage data to calculate the total amount of water used for irrigation for each crop in the county in 2006.
“This analysis provided an evaluation of water use by crop type, which the basis for developing a static model,” Wax says. The model was used as a standard against which all other scenarios of climatic variability, land use, and management changes were compared.
A method to account for rainfall variability during a growing season was also incorporated.
“The climate data, crop data, water use data, and rainfall/water use relationships were used to develop a model that could assess water volume declines in the aquifer over a growing season,” he says. “Based on crop average water use relationships in effect in Sunflower County in 2006, we could calculate the amounts of water taken from the aquifer by each specific irrigation method and management method for each of the five crops, then total the specific water uses for each year, resulting an a total annual reduction in volume of water in the aquifer.”
Using the 2006 Sunflower County land use and crop water use relationships with rainfall/water use relationships developed for each crop, growing season precipitation for the 48-year period 1961-2008 was used as a variable in the model to estimate total yearly water use 48 years into the future, 2008-2055.
The average of the annual aquifer recharge volumes between 1989-2008 was used with yearly modeled water volume declines to characterize cumulative water volume changes over the 48-year period. Then, the model was used to simulate different scenarios of water use by changing crop acreages or irrigation methods from the 2006 data, allowing assessment of changes in water volumes over time under different land use and management conditions. The model was then used to formulate recommendations for monitoring and managing water volume changes in the aquifer.