Impact of Land Use on Water Quality

In their attempt to maximize productive agricultural space and protect cultivated fields from innundation, farmers within SMC Watershed have struggled to strike an efficient balance between their agricultural practices and the natural processes of the watershed. Plowing up prairie to uncover rich soils, installing drainage tiles, and maximizing crop cover has come to bear some negative impacts.

Overland Erosion

Infiltration rate, a value attributed to land cover and soil, explains how fast water is able to seep into the ground. When more water is applied to the land surface (rainfall or irrigation) than the soil can handle over a given period of time, overland erosion occurs. The potential for overland erosion is enhanced by the current dominant land cover. Loosely compacted topsoil, covering 86% of the watershed, provides little resistance to erosion. A prairie, for example, is loaded with root structures that hold soils in place, and the physical presence of plant structure above ground greatly reduces the potential for significant overland flow. When application of water exceeds infiltration rates, puddles accumulate as sources for overland flow. As illustrated in the picture below, channels are created in the soil where this flow is attempting to travel topographically down gradient. The water in these runoff channels is loaded with suspended sediments and dissolved nutrients accumulated in the field. The negative implications of overland erosion are twofold. Firstly, the removal of sediment and nutrients from cultivated lands is a concern to farmers, who spend a lot of money to purchase and apply fertilizers and who put in a lot of effort to retain their fertile soils. On the flip side, overland erosion leads to increased contamination of the ditches and streams that runoff channels ultimately discharge into.

Open Intakes

The image to the right shows an open tile intake in a cultivated plot. Farmers install drains like this in order to provide drainage to topographically low areas of their plots, where water would naturally accumulate and puddle. These drains are connected directly to the tile network below the surface, and everything entering the pipe is carried out to open ditches. The orange basket mounted on top of the intake is supposed to prevent large objects, like cornstalks, from entering and clogging the lines. Open tile intakes, while logically designed, compound the issues associated with overland erosion. With the exception of large debris, nearly everything from the field (soil, nutrients, manure) is susceptible to being washed down the drain during a storm event. Again, the implications of such are negative both for the farmer and for the watershed as a whole. Below, the severity of erosion that can be associated with open intakes is illustrated. Large channels are created as water is funneled toward the intake. Because the tile line provides absolutely no resistance to the flow of water, the potential for erosion increases as higher overland flow velocities are achieved.

Looking down into intake.         Open intake in the field.

 

No Buffers

In an effort to maxmize productivity, farmers often cultivate every acre of fertile soil they own. The problem with this practice is that there is no buffer zone between the fields and the open ditches, as seen below. This is primarily a concern because soil and fertilizers from the field can easily be washed into the drainage ditch. In addition, fertilizers are sprayed onto fields, either in powder or liquid form. With no buffer zone, there is no avoiding spraying chemicals directly, though unintentionally, into open ditches. While this is only a fractional loss of fertilizer to the farmer, the level of contamination in the water in nearby ditches skyrockets during times of application. Compounding the problem, application of fertilizers is often concurrent with annual periods of increased rainfall. Thus, more water is falling on the fields, increasing the potential for overland erosion, and more water is flowing in the ditches, accelerating the transportation of contaminants down to the creek.

 

Tiling has also had an impact on water quality in the watershed. At the next stop, we will examine this relationship.