It is a foregone conclusion that global crop production will have to increase during the next decades to supply an increasing world population with food and agricultural products. It is also a foregone conclusion that water will play a key role in that effort. This is especially true in the US, which will be expected to supply a large portion of this increased crop production.

During the past year, the Mississippi Soybean Promotion Board (MSPB) has promoted and conducted activities related to its SIP initiative. The ultimate goal of this program and its components is to foster the adoption of tools that will eventually result in lowering the amount of water that is withdrawn from the Delta’s alluvial aquifer to irrigate crops, while at the same time maintaining or increasing the yield level of those irrigated crops.

In a September 9, 2014 article in Agronomy Journal, Dr. Jerry Hatfield outlines how water use in agriculture affects plant productivity and the surrounding environment. The most pertinent points in that article are summarized below.

• Agriculture comprises 70% of the total global water use. This raises the issue of improving the efficiency of agricultural water use in order to decrease its water demand and reduce the environmental impact of this use.


•  Water use in agriculture is directly related to plant productivity and indirectly related to animal productivity through the consumption of plant products.


•  The environmental impacts of agricultural water use include both onsite effects at the point of application and the offsite effects arising from surface runoff and transport of materials (sediment and chemicals) away from the application point.


•  A challenge to improved water use efficiency in agriculture applications is to balance the environmental impacts with the need to maintain or enhance plant productivity. The increasing demand for fresh water makes this a key component in the sustainability of crop irrigation.

In his article, Dr. Hatfield has delineated the key components of how water is partitioned in agricultural settings.

Evapotranspiration (ET)

Water that is transpired through plant leaves or that is evaporated from plant surfaces, the soil, and water surfaces affects the surrounding environment. ET is an often overlooked component in the maintenance of cooler air temperatures. Thus, a reduction in the amount of water used in agriculture could have an impact on the earth’s overall temperature.

Surface Runoff

Surface runoff from agricultural settings occurs when incoming water from either rainfall or irrigation exceeds the soil’s ability to absorb it. The environmental implications of this process arise from the movement of soil, nutrients, and pesticides from these settings into adjoining water bodies.

Soil management practices that will impede or reduce runoff will have a positive impact by reducing the amount of water from rain and irrigation that leaves the site of application. These include maintaining residue cover and growing cover crops in the off-season of the main crop when ET is lowest and the soil has the least capacity to store additional water.

It is important to remember that water runoff is the main source of water for lakes, streams, and rivers. Thus, a reduction in runoff is not necessarily the main point. Rather, it is more relevant to reduce the velocity of runoff to minimize its erosive forces, and ensure that runoff water that does leave a site contains minimal soil and chemical contaminants.

Ground Water Recharge and Leaching

When water from rain or irrigation exceeds the storage capacity of the soil, movement of water in the soil will be downward due to gravitational force. This is commonly referred to as leaching or percolation. The soil’s water holding capacity, the ET rate, and soil texture largely control leaching. A coarse-textured (sandy/loamy) soil will be subject to more leaching than will a fine-textured (clayey) soil.

When leaching occurs, soluble materials such as nitrate-N in the soil profile are subject to being carried with the water as leachate. These materials thus can enter the groundwater and become contaminants.

Understanding the connections among crop water use, precipitation patterns, soil water holding capacity, a soil’s propensity for leaching, and irrigation water application are necessary for proper irrigation management. This is especially important to guard against over-irrigation, especially at the end of the season.

Environmental Impact

Irrigation produces environmental impacts beyond the site of application, and these effects must be considered when evaluating the effect of agricultural water use and management on environmental quality.

• Direct impacts are increased evapotranspiration in areas with large irrigated acreage, increased surface runoff of water, increased deep percolation of chemicals from the root zone, and a possible change in groundwater recharge patterns.


• Indirect impacts are the changes to the landscape brought about by construction of reservoirs, alteration of canals and drainage ditches, and land forming to facilitate surface watering. These changes will alter the natural flow of water, and could potentially change the position of wetlands in the landscape.

To reduce the environmental impact from the potential water imbalance that will be created by irrigation, management practices should be used to reduce the potential impacts. These include:

• Use limited tillage to maintain crop residue cover to facilitate increased infiltration of rainfall that can be used in lieu of irrigation.


• Use cover crops to extract excess nutrients from the soil, and prevent both leaching and surface runoff.


• Use crop rotations that will result in a lower total irrigation water requirement over the span of the rotation.


• Use irrigation water conservation tools that will increase irrigation water use efficiency.

For Delta soybean irrigators, here are some points that will specifically apply to not only improve irrigation efficiency but also reduce irrigation’s potential negative environmental impact.

• Do not start irrigating too soon. Remember that early-season water use by soybeans is well below that during reproductive development, and irrigating before soil water has been used for early growth and development will promote runoff and leaching and do little to increase production.


• Cease irrigating in time for the crop to use what is applied in the last irrigation. Over-irrigating at the end of the growing season will promote leaching and result in a smaller reservoir for the infiltration of off-season rainfall.

MSPB’s SIP initiative was started to promote and conduct activities that will foster the adoption of tools that will eventually result in lowering the amount of water that is withdrawn from the Delta’s alluvial aquifer to irrigate crops. According to the above points, this initiative can also be used to foster and promote activities and tools that will lessen the potential environmental impact that may result from irrigation of the large acreage in the Delta.

Composed by Larry G. Heatherly, Sept. 2014, larryheatherly@bellsouth.net