It can be difficult for water experts in other parts of the world to understand why farmers in the Mid-South need to irrigate when they receive 50 inches of rainfall annually (more than that probably in 2019).
Dr. Michele Reba, research hydrologist and lead scientist at the USDA-ARS’ Delta Water Management Research Unit, says explaining that seeming contradiction to people who get far less rainfall can be challenging.
“When I give talks internationally — which doesn’t happen very often — but when I do, I tell them we have all these water problems; all these challenges; and we get 50 inches of rain every year, said Reba, speaking at the Arkansas Soil & Water Education Conference at Arkansas State University at Jonesboro.
“And the audience is like, ‘What? How is that possible?’” she said. “You know we get all this rain, but it doesn’t always rain at the right time or in the right places. So what we need to do is take advantage of what we have and use it a bit more effectively.”
She said she remembered talking to an audience member about rainfall the first time she attended the Arkansas Soil & Water Education Conference, which is held annually at Arkansas State University in Jonesboro.
“My daddy would always say that when we would get rain in certain parts of the farm it would go away,” she quoted the participant as saying. “There has to be a way that we could put water in certain places and get it back into the aquifer.”
Managed Aquifer Recharge
She discussed several methods for Managed Aquifer Recharge or putting water back into an aquifer, preferably faster than it is being withdrawn by pumping in areas like the Delta region of northwest Mississippi or the Grand Prairie of Arkansas.
“We talk about how we have this supply and all this space in the aquifer,” she said. “Why don’t we artificially put water back into it? With a lot of work, the U.S. Geological Survey and our unit have done, we’ve been slowly progressing toward that.”
One method — direct injection — would appear to be a relatively simple solution for the problem of declining aquifers: Simply pump water from a water source into the ground.
“The problem with direct injection and where we are with the Mississippi River Valley Aquifer,” said Reba, “is that the MRVA is used as a source for drinking water. If you want to put water into the MRVA, you have to bring it up to drinking water standards. That’s going to be prohibitively expensive.”
Infiltration basins are an avenue that could be used in places like Australia where land is abundant and water is scarce. “This is a very likely implementation of Managed Aquifer Recharge in that kind of space where you have a lot of land that you’re not using,” she said. “We don’t have that here. Available cropland is really at a premium.”
River diversion and river bank infiltration are other possibilities. “That’s something the state of Mississippi is working on — they have a pilot project that a colleague of ours is heavily involved in,” she said. “So, we are excited to see what will happen with that.”
And so is a device called an infiltration gallery, which involves a gravel-filled trench that is cut through the confining of layer of clay found on the soil surface in many parts of the Mid-South.
“One of the challenges is that no one has done it in this part of the world,” she said. “While that is a challenge, it could also be an opportunity that we could test for all the reasons I list here. Those include:
• It would not interfere with surface activities so there would be limited removal of productive ground.
• It could be built with common, cheap materials.
• It may be resistant to more common managed aquifer recharge challenges.
• It would allow farmers to take advantage of existing reservoirs.
“In the 1960s and the 1980s, the U.S. Geological Service tried to use direct injection, but we have so much fine material in our surface water,” she said. “If you tried to take water from a ditch and pump it through a well, what happens in their study is their screens got clogged very quickly. They determined the cost of cleaning that water was prohibitive.
“What we have now that we didn’t have in the 1960s and the 1980s are things like the 43 reservoirs in the Cache River basin alone. Think about that as sort of an initial settling pond in this project. After it settles out, we can use that water for direct injection or the infiltration galleries. That way we could take advantage of those existing reservoirs.”