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Competency Area 2: Soil hydrology AEM

PO 14. Understand how seasonal soil conditions and landscape position affect runoff and leaching.

Seasonal soil conditions change in response to the seasonal distribution of precipitation relative to the distribution of the potential evapotranspiration. In the Northeast, the distribution of the annual precipitation is similar from month to month, but the evapotranspiration peaks in July and diminishes significantly in the colder months. During the months when the rainfall is higher than the evapotranspiration, the soil will be gaining water. When the rainfall is lower than evapotranspiration, the soil water is extracted. As the soil water content increases, there is a limit to how much it can hold the field capacity.

Although some surface runoff can occur anytime in response to large storms, most of the runoff (and tile drain discharge) begins after evapotranspiration ceases in the fall, and it peaks in late winter/early spring in response to all the additional water from snowmelt. Runoff starts to decrease in late spring in response to increasing evapotranspiration, which again begins to extract water from the soil. Runoff and leaching are minimal during the late summer months because the soil water content is usually less than field capacity, and the soil can store rains that occur at this time. In fact, during the summer months, irrigation may be needed to maintain the soil water content from dropping below critical thresholds. The following figure shows the distribution of the average water balance for Central NY.

water loss.JPG

 image source: NRCCA Soil and Water Management Study Guide