Below the earth’s surface, below the slow-moving creeks and placid bayous, aquifers store immense quantities of water.
“In Mississippi, 90 percent of drinking water comes from groundwater,” says U.S. Forest Service research hydrologist Ying Ouyang. “Aquifers also provide water to farmers.”
Over the past 50 years, groundwater levels in the Mississippi Delta have declined by more than 20 feet. “The decline is likely a result of using groundwater to irrigate crops,” says Ouyang.
Farmers have responded by building water storage ponds. But how big should these ponds be? How many acres of cropland could such a pond irrigate? Until recently, no one had answers.
“We developed a model to show how many acres of soybeans can be irrigated from a one-acre agricultural pond in Mississippi,” says Ouyang.
Ouyang and his colleagues used a programming language called STELLA to develop the model. The researchers mapped the flow of rainwater into the pond, evaporation out of the pond, and a host of other hydrological processes. The study was published in the journal Water Resources Management.
The scientists measured pond water level and water quality. They also used local weather station data on rainfall, solar radiation, air temperature, and other factors. “We used these observations to validate the model predictions,” says Ouyang.
The researchers simulated pond hydrology with and without pumping. “The scenario without pumping reflected the natural field conditions,” says Ouyang. “The second scenario shows the amount of water available for irrigation.”
The results showed that the ratio of pond size to irrigated area is about 1:18, if the pond is six feet deep and the irrigation rate is one inch per day.
The ratio also assumes the pond water level stays above 1.2 feet. If almost all the water were to be pumped out, the ratio of pond size to irrigated area would decrease to 1:20.
“The ratio is for irrigating soybeans, but the model could be applied to other crops,” says Ouyang.
Using pond water to irrigate 2.5 acres of soybeans (at the 1:18 ratio) would save over 700 cubic yards of groundwater each year.
The researchers also evaluated daily and seasonal pond hydrology.
“Rainfall is a major driving force at Metcalf Farm,” says Ouyang. “Rainfall and runoff water – mostly runoff – was a huge source of pond water.” Groundwater that had been pumped onto fields was also a source of runoff. The researchers suggest considering surface water runoff when designing the size of a pond.
The pond collected the most rainwater in winter, followed by spring. “This corresponds well to the timing of seasonal rainfall in Mississippi,” says Ouyang.
Only small amounts of water evaporated from the pond, and the heavy clay soils at the farm prevented water from seeping out.
The researchers did not consider soil cracking, which could be a problem in Mississippi’s heavy clay soils. However, the study strongly suggests that irrigation ponds can help conserve groundwater.
“Our results could have substantial impacts on the approximately 4.5 million acres of cropland in Mississippi,” says Ouyang. “Other regions of the U.S. with similar groundwater management challenges would also benefit from using this predictive tool.”
The Mississippi Soybean Promotion Board funded this study.
For more information, email Ying Ouyang at email@example.com.