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Irrigation and Aquifer Depletion

It is common knowledge that Midsouth producers who irrigate crops rely on groundwater from the Mississippi River Valley Alluvial Aquifer (MRVAA), which is part of the Mississippi Embayment Aquifer system. Over the last 2-3 decades, increased irrigation in the region has resulted in a decline in the water level in this shallow aquifer, and this decline has generated concern to the point that remedial measures are being proferred and discussed.

The MRVAA comprises an area of about 32,000 sq. miles, and is present in 7 states–the major portion is in Arkansas, Mississippi, and Louisiana. It is the uppermost aquifer in the Mississippi Embayment system. Six major rivers drain the relatively flat alluvial plain that comprises the land area above the aquifer. Aquifer thickness ranges from 25 to greater than 150 ft., and averages about 100 ft. in thickness. It thins from North to South.

Land use within the MRVAA is about 80% agricultural. Due to the land use and the increase in irrigated agriculture, large-scale pumping has had a negative effect on the system; i.e., the water level in the MRVAA is declining. Before this recent development, rivers received most of the outflow from the MRVAA, but now they have become a major source of inflow due to this large-scale well withdrawal of water from the aquifer

In Feb. 2019, the Council for Agricultural Science and Technology (CAST) published Issue Paper 63 titled “Aquifer Depletion and Potential Impacts on Long-Term Irrigated Agricultural Productivity” that reviews the causes and consequences of groundwater depletion. Pertinent excerpts from that paper follow, with special emphasis on depletion of the MRVAA.

•   The main subjects of this issue paper are the causes and consequences of groundwater or aquifer depletion, which is defined as the continuous reduction in the volume of water stored in an aquifer. Specifically, the subject is the year over year trends of diminishing aquifer amounts that jeopardize the capacity of the aquifer to supply stored water for necessary activities such as irrigation of crops.

•   Groundwater is the earth’s most extracted raw material, and approximately 70% of groundwater withdrawals in the world are used to support irrigated agricultural production. In the US, about 71% of groundwater withdrawals are used to irrigate crops.

•   An aquifer’s ability to provide significant water quantities to a well depends on the porosity and permeability of the aquifer material–e.g., rock, gravel, sand.

•   When water is pumped from a well, the result is a lowering of the water level in the well and the adjacent aquifer. This “drawdown” causes a hydraulic gradient that draws water from the aquifer toward and into the well to replace the water that is pumped from the well. Steeper hydraulic gradients result in faster groundwater movement to the area where drawdown occurs.

•   In regions where irrigated cropland occupies a significant acreage, the use of groundwater is exceeding the rate at which its sources are replenished by natural processes. Thus, these groundwater sources are slowly being depleted. This is true for the MRVAA.

•   The most obvious consequences of aquifer depletion to agriculture are the loss of a long-term water supply for irrigation, the loss of well productivity, and the increased costs of drilling deeper wells to reach a lowered water table in the aquifer. If an aquifer is hydrologically connected to surface water bodies, the overdraft of the groundwater source will increase the infiltration of the surface water to recharge the depleted aquifer, thus leading to a decline in both surface water and groundwater resources over time.

•   Once aquifer depletion is recognized, methods such as using more efficient irrigation systems to deliver water to a crop, the development/use of crops that require or use less water, artificial replenishment of groundwater systems, and economic incentives that will encourage water conservation must not only be considered but implemented by the users.

•   Groundwater in aquifers is a non-exclusive or common resource because it flows across ownership or arbitrarily-defined water management district boundaries. Thus, it is difficult to exclude users from obtaining benefits from its use, even if they do not contribute to the sustainment of the resource. This, then, is the difficulty in managing a common-use resource such as the MRVAA; i.e., it is difficult to sustain the conservation of this water resource if some users benefit from its use without regard to the long-term maintenance of the resource. This ultimately results in the overuse of the resource and a decline in its overall value or contribution over time unless measures are enacted to restrict use of the resource. Such is the case with the MRVAA; the majority of crop irrigators who use water from this aquifer have not adopted verified conservation practices known to reduce water withdrawal from the aquifer.

•   If unabated, an approach to addressing the overdraft problem could be the privatization of groundwater pumping rights, which is an allotment or allocation process whereby private entities or users have a fixed allocation of the finite groundwater over a specified period of time. This practice is already in place in parts of the US where regional water management plans have been created to ensure the future condition of an aquifer.

Of course, the most obvious solution to decreasing aquifer depletion is to extract less groundwater from it. Since agriculture is the single largest user of groundwater in the US, it follows that a reduction in groundwater pumpage for crop irrigaton could decrease groundwater depletion. Networks of automated weather stations such as that described and tested in the article found here and used in the article found here could be used with research-based crop coefficients (click here) to estimate crop water requirements so that irrigation applications could more accurately match crop water needs without over-application of water and crop yield loss, resulting in decreased withdrawal from the aquifer.

Another solution to mitigate aquifer depletion that is not as obvious or as discussed is enhancement of groundwater replenishment by direct input or by increasing recharge from other water sources. Click here for an article that discusses this and provides results from case studies of several small-scale evaluations of techniques that can be used in the Midsouth for this purpose.

The facts are clear. The continued population growth in the US and increasing irrigation of cropland will increase the stresses on maintaining sustainable water resources. This in turn will increase reliance on groundwater sources as a direct supply for this increased need for water, which will then result in groundwater depletion. For agriculture, this continued decline in water tables will result in decreasing yield of wells used to provide irrigation water to crops and create the need to drill deeper wells that will cost more and cost more to operate.

All of this will have to be mitigated by a combination of increasing the recharge to an aquifer and decreasing the demand on groundwater. For irrigated agriculture to continue in the Lower Mississippi River Basin, it is likely that both measures should or must be implemented now rather than later to protect the MRVAA resource. To ignore this urgent need will likely lead to reduced crop productivity in the region with unwanted and irreversible consequences to its agricultural sector.

Composed by Larry G. Heatherly, Mar. 2019, larryheatherly@bellsouth.net