Subsurface Flow and Drainage
In some situations, downward movement of water through the soil is restricted. This may be due to a rock layer or to the interface of one type of parent material with another (Fig. 10). This is illustrated here by a non-irrigated watershed in eastern Nebraska with primarily silty clay soils, where downward movement of soil water is hindered by an interface of loess overlying glacial till. Some of the water then slowly flows laterally below the surface to the hillside, where it seeps to the surface and eventually enters a stream flowing into a lake. The soils of the watershed generally have agronomically moderate STP. Near the point of entering the lake, the stream formed from these seepage waters has a bio-available P concentration of 0.06 to 0.22 parts per billion. With natural subsurface flow, more transport of P to surface waters can be expected with the following conditions: high STP; sandy soil which allows more downward movement of P; shallow depth to restricted downward water movement; and short distance to seepage of the water to the surface. Even in a situation lacking all of these conditions, the amount of P transported may be significant.
Figure 10. Subsurface flow and subsurface drainage can deliver significant amounts of P to surface waters. When soil overlies a less permeable layer, soil water may move laterally and emerge as seepage or a spring which flows into a surface water body. (Illustration by Charles Wortmann)
Subsurface drainage can be a significant transport factor with installed subsurface drainage systems that have conduits near the soil surface. These systems remove large amounts of water from agricultural fields, and the soluble P content can be high.