Water Erosion of Agricultural Land
Total P concentrations in soils are often strongly related to sediment concentrations found in runoff. Therefore, total P loss is often closely related to the rate of erosion; and P losses can be great, even when STP is agronomically moderate. This is because a large proportion of total soil P is in non-labile forms, and so is not measured with agronomic soil tests used to determine P availability. As discussed in Lesson I, much of the total P in water bodies is not immediately available to aquatic vegetation, but a large proportion of it may become available over time. Therefore, additions of P to surface waters need to be considered with both medium- and long-term perspectives. The close relationship between the rate of erosion and total P loss presents an opportunity to reduce P loss. Often, where water erosion is the dominant transport factor, a 50% reduction in erosion should reduce total P loss by at least 40%.
Figure 1. The 'critical source area' concept for loss of agricultural P to surface waters. (From Sharpley and Sheffield, Livestock and Poultry Environmental Stewardship Curriculum)
Discussion Question: Why does a 50% reduction in erosion load cause a reduction in total P loss of at least 40%?
Answer: Total P loss is generally closely related to erosion lost because of the close relationship between sediment and total soil P. The reduction in total P loss is expected to be similar to the reduction in erosion load.
Generally speaking, there are three key steps for transport of particulate P by water erosion:
- Energy of falling raindrops causes 'detachment' of particulate inorganic or organic P as a function of soil texture and structure.
- Flowing water transports detached particulate P. This is a function of soil particle size and sediment carrying-capacity of the flowing water.
- Sedimentation and resuspension of particulate P can occur in streams, ponds and lakes, both during and after initial transport process Several processes are involved in water erosion. The raindrop splash effect, sheet erosion, rill erosion, and gully erosion are briefly discussed.
Raindrop Splash Effect
The raindrop splash effect (Fig. 2) is very important to the disruption of soil aggregates, as well as to the movement of sediment down slope as a contributor to sheet erosion, which will be discussed later. The raindrop splash effect is a result of the energy of falling raindrops causing detachment of particulate inorganic P or organic P and down-slope movement of sediment. Maintenance of ground cover, such as in reduced- or no-till operations (Figs. 3 and 4), use of cover crops, and enhancement of the stability of soil aggregates can be important in reducing detachment of soil particles. The effect of manure application in enhancing soil aggregation discussed in Lesson I is relevant to reducing soil erodibility due to the raindrop splash effect.
Figure 2. The raindrop splash effect is an important erosive force. Weak soil aggregates are broken, releasing easily carried soil particles. Next, P is desorped, creating potential to carry more P in sheet flow of runoff water.
Figure 3. Ground cover reduces the raindrop splash effect and slows runoff to reduce the potential for sheet erosion, as in the third frames.
Figure 4. Well-maintained ground cover absorbs much of the energy of falling raindrops, minimizing the erosive impact of the splash effect. It also slows water movement associated with sheet erosion, reducing the erosive capacity of runoff water and allowing for more sedimentation.
Sheet Erosion
Sheet erosion, although less noticeable than other types of erosion, typically is the main erosive force. Sheet erosion is less noticeable, as it does not leave obvious cuts in the soil surface as with rill or gully erosion. Sheet erosion is the removal of a relatively uniform, although thin, layer of soil from the land surface by unchanneled runoff, or sheet flow. Soil particle size and the erosive capacity of the flowing water influence the amount of detached sediment P transported in sheet flow. Sediment transport is greater with smaller colloid size and greater erosive capacity. As sheet erosion works at the soil surface, it affects the soil that is typically highest in total P, as well as solution and labile P. Sedimentation, adsorption, and resuspension of P occur as it flows across the field in sheet erosion. The velocity of sheet flow is reduced by ground cover, i.e., cover crops and crop residues (Fig. 3.) When the velocity of sheet flow is reduced, its sediment carrying-capacity is reduced, and more sedimentation and resorption of P occur. Physical and vegetative barriers (Figs. 5, 6 and 7) which intercept or slow water flow reduce erosive capacity and increase sedimentation and P adsorption.
Figure 5. Physical barriers, such as terraces, that intercept or slow water flow, reduce erosive capacity and increase sedimentation and P adsorption. (Photo by Bahman Eghball)
Figure 6. Vegetative filter strips, or buffers, are barriers that intercept or slow water flow, thereby reducing erosive capacity and increasing sedimentation and P adsorption. (Photo by Bahman Eghball)
Figure 7. Narrow vegetative filter strips are an alternative to terraces in creating barriers that intercept or slow water flow, thereby reducing erosive capacity and increasing sedimentation and P adsorption.
Rill Erosion
Rill erosion is the process by which numerous small channels--less than three inches in depth--are formed. This type of erosion results from concentration of overland water flow associated with sheet erosion (Fig. 8). As with sheet erosion, rill erosion removes soil that is relatively high in soil P as compared to deeper soil. Rill erosion can be especially serious on recently cultivated land. Rill erosion is best minimized by minimizing sheet flow, such as by maintaining crop residues and utilizing cover crops. Physical barriers, such as terraces, and vegetative barriers can be effective in stopping or reducing rill erosion. Narrow filter strips are less effective in checking rill erosion as compared to sheet erosion, as the runoff associated with rill erosion is concentrated flows through the strips with greater velocity.
Figure 8. The erosive capacity of sheet flow of runoff water increases as the flow becomes concentrated, resulting in both rill erosion and sheet erosion.
Gully Erosion
Gully erosion, including ephemeral gully erosion, refers to the cutting of narrow channels resulting from concentration of sheet and rill flow of runoff water. Ephemeral gullies are small channels of approximately 3 to 12 inches deep. Gullies may be one to several feet deep. Gully erosion cuts deep and removes the surface soil as well as deeper soil that may still have substantial amounts of total P but relatively less solution and labile P as compared to the surface soil. Gully erosion needs to be prevented, as it is difficult to check once started. In many cases, the flow is dispersed before it reaches the surface water body. These situations offer an opportunity to effectively use vegetative buffer strips to slow the rate of flow for increased sedimentation and P adsorption. Vegetative barriers are even less effective against gully erosion than rill erosion, as the flow is concentrated when it enters the strip, and often has enough velocity to flatten and flow over the vegetation of the buffer strips.