Grassed waterways have been installed on many fields in the past to prevent erosion on the bottoms of drainage channels. Since the use of glyphosate has boomed in South Dakota, many of these grassed waterways have been sprayed out and farmed over. In most years this has proven profitable with the addition of croppable acres, however, heavy rain last spring has shown how delicate many of these drainages are.
Erosion can be seen in the form of headcuts and gullies across South Dakota and neighboring states. This erosion is taking tons of valuable top soil with it, lowering the productivity of the eroded lands.
Additionally, many growers have taken out grassed waterways after putting tile in their fields. The thought is that the tile could substitute for the waterway. By lowering the water table, drain tile does help reduce potential for surface runoff and soil erosion by creating more room to store water in the soil.
However, if the intensity of a storm is high enough that rainfall rate exceeds the infiltration rate of the soil, runoff on the surface will occur. A tiled field only drains out water that has already infiltrated and will have no effect on water that is unable to infiltrate the soil once surface runoff begins.
There are many factors that may keep water from entering into the soil, creating runoff. These factors are:
Intense Storms: High intensity storms may exceed the infiltration ability of the soil. Even soils with very high infiltration rates can have surface runoff from very high intensity storms.
Reduced infiltration: Heavy rain has a large impact on the soil surface. This impact breaks up soil particles so they fall into the voids of other soil particles. This starts to create a seal on soil surface and discourages infiltration and raises the risk of runoff.
Soil structure is also an important factor for infiltration. The more tillage that is conducted on agricultural lands the more the soil structure is diminished and the passages for water movement through the soil are closed off.
Soil water holding capacity: During the spring of the year, rain often falls on multiple days of the year; often back to back. In wet soils, there is little additional room for additional infiltration. While drain tile can help by removing excess water, it can only drain the drainable pore space, which is relatively small in heavy-textured soils. When large rain events occur, the soil profile can quickly become saturated and any additional rain water has to stay at the soil surface.
Erodible Soils: Some soils naturally erode more easily than others, based on their chemistry. Positive ions like magnesium and calcium help hold soil particles together into aggregates while ions like sodium disperse the soil. Highly erodible soils need to be protected by other means to keep them in place.
Topography: The slope of the land is an intuitive contributor to erosion. The greater the slope the more energy the running water has to dislodge soil particles as it flows. Vegetation can go a long way to absorb that energy and slow water down while shielding the soil at the same time.
Tile and grassed waterways function well in combination, because the tile helps protect the waterway from excess wetness and the grassed waterway prevents gully erosion from surface runoff that might expose the tile.