Decreasing Runoff and Soil Erosion
Runoff from high intensity rainfall is a smaller, but potentially important, loss of water. In eastern Colorado, a majority of the annual precipitation comes in the form of brief, high intensity, summer thunderstorms, often resulting in runoff and erosion. The amount of runoff depends on soil type, slope length and steepness, and on soil surface conditions. Management practices that reduce this runoff will improve precipitation use efficiency.
Runoff represents a short-term water loss to the cropping system, while soil erosion induced by runoff can cause long-term and permanent damage to agricultural systems. It is estimated that between 2 and 6.8 billion tons of soil per year is lost from cropland in the United States due to erosion. While wind erosion may dominate in dryland cropping systems, water erosion rates can also threaten the soil's ability to sustain crop production in the long-term. Management practices that protect the soil surface from crusting and runoff can greatly reduce soil erosion rates. Soil erosion can be decreased by 80 to 90 percent in no-till systems when compared to conventionally tilled land.
Recently, a coupled analysis of historical hourly rainfall intensity data and field measurements from Sterling and Stratton, Colorado were used to estimate potential runoff and soil erosion from dryland agroecosystems (Table 4.3). The estimates were made separately for wet years (average to above average rainfall) and for dry years (below average rainfall) and for scenarios with low and high runoff probability. Low probability runoff scenarios correspond to flatter land with good residue cover, while high runoff probability corresponds to steeper slopes with little residue cover. Runoff was estimated to range between 0.3 inches for drought years and management with good surface protection to 3.2 inches for wet years and management with poor protection of the soil surface. The potential to capture as much as three inches of precipitation through improved management practices will translate into greater crop yield and higher profitability. Annual rates of erosion by water were estimated to range between about 0.4 tons per acre to as high as 4.1 tons per acre (Table 4.3). Soil erosion rates are too high for long term sustainability of crop production if management practices do not provide soil surface protection. Management that protects the soil surface and reduces the probability of runoff is an effective means of soil erosion control. Residue management achieved through no-till or minimum till practices is the most effective means of reducing runoff and soil erosion.
Table 4.3 Average annual amount of high intensity rainfall (>0.5 in/hr) and estimates of runoff and soil erosion at Sterling and Stratton, Colorado for years with average to above average annual precipitation (wet years) and years with below average precipitation (dry years).
1Poor surface protection with little crop residue
2Good surface protection with adequate crop residue
3Average annual amount of high intensity rainfall (>0.5 in/hr)
Table 4.4 Typical soil water loss from different tillage operations 1 and 4 days after tillage.