Community Agriculture Alliance: The reservoir we already have
Community Agriculture Alliance
Living in the western United States means that water — how it is used, and how it is stored — is a contentious topic. When there are talks about managing our water everyone seems to pay attention.
With all of this there is little talk of improving the reservoir that is available to every ag producer regardless of their water rights. This reservoir doesn’t require any infrastructure to move water and is free to use. The reservoir that I am talking about is in the topsoil that our lives depend on. That’s right, topsoil can store water just like any other reservoir.
The term we use to refer to how much water a soil can hold is “water holding capacity” and can be affected by several things. The physical characteristics of the soil refers to the ratios of sand, silt, and clay.
This affects water holding capacity because the ratio determines how much void space is available to fill and how much surface area there is for the water to cling to. Too much space and the water just moves through without stopping. Not enough space and water can’t move into the soil.
While management can’t impact the physical makeup of the soil, it can change how the soil functions. Soil organic matter can be as important as the physical properties. Every 1% change in organic matter for the first foot of soil can result in around a 0.15 inch change in water holding capacity. That would mean that with a 3% increase in soil organic matter, 0.45 inches of snowmelt could be stored in the soil instead of running off. On a 100 acre field, this would be almost 50 acre-feet of water. Soil organic matter can change based on land management practices.
Management can also affect how much water infiltrates into the soil. The soil’s ability to catch the water is almost more important than how much it can store. Management has much more of an impact on a soil’s infiltration ability than the physical characteristics of the soil.
This is especially true when the water comes fast like it does in a summer thunderstorm. Having plenty of residue and minimizing disturbance helps to maximize infiltration. A highly disturbed soil can collapse and seal up when water hits it. Think of trying to pour a pitcher of water on a pile of dust. It just gets the top wet and runs off.
Management that affects soils includes proper grazing management, ensuring a diverse range of plants, and maintaining residue to keep the soil covered. Leaving enough stubble on dryland hay fields will help to improve the next year’s crop. Leaving 50% of the forage in a grazed field will ensure maximum water available the next year.
The Natural Resources Conservation Service has resources available to help evaluate management systems and make recommendations that could ensure soils function at a high potential. For more information, contact Clinton Whitten at Clinton.Whitten@usda.gov or 970-879-3225 ext. 3246.
Clinton Whitten, USDA Natural Resources Conservation Service
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