Seven inches of rain fell in the Crookston, Minn., area in July, creating opportunities for ponding and surface runoff.
Heavy rainfall in a short window of time can lead to measurable nutrient loss. by Lindsay Pease
B
etween May 1 and July, 13 inches of rain fell at the University of Minnesota’s Northwest Research and Outreach Center in Crookston. This was about 3.5 inches greater than the 30-year average. What started out as a near-normal season for rain took a sudden turn with 7 inches of rain in July. Quite literally, when it rained, it poured.
Falling with intensity Many factors play into whether heavy rains lead to nutrient loss. Some of these factors are how much it rained, what soil type you have, what crop you grew, how much fertilizer you applied,
how quickly it rained after you applied the fertilizer, and whether you have tile (or irrigation). One of the key factors that determines if water will be absorbed into the soil or runoff the soil surface is rainfall intensity. The soil can only absorb water so fast. This is the soil’s infiltration rate. If rainfall intensity exceeds the soil’s infiltration rate, you will get ponding or runoff. Infiltration rate depends on both the soil’s physical properties (texture, bulk density, and so forth) and the soil’s existing moisture content when it starts raining. Soils that are more coarse in texture absorb water faster than finer
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textured soils. Drier soils absorb water faster than wetter ones. Management practices can have an effect on infiltration rate, too. Less compacted soils will absorb water faster than more compacted ones. In general, no matter what the soil properties are, if soil moisture is high and you get a high-intensity rainfall event, then conditions are right for surface runoff. By my estimation, from May through July, rainfall intensity in Northwest Minnesota’s Red River Basin region likely exceeded infiltration rate on six days for sandier soils and up to 21 days on our more clay soils. That means between six and 21 chances for ponding and surface runoff.
Impacts the environment Nutrient loss in surface runoff tends to be more of an environmental concern than a soil fertility concern. Surface runoff, ponding, and flooding can both damage crops and erode the soil surface, but it will not collect plant-available jofnm.com
10/22/20 10:03 AM
