Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Other
Publication Acceptance Date: 8/2/2001
Publication Date: N/A
Interpretive Summary: Ephemeral gully and sheet-rill erosion are serious problems on corn and soybean fields in the Deep Loess hills bordering the Missouri river valley from Western Iowa to west-central Missouri. Terraces are popular, but they are expensive to build and maintain and sometimes result in lower efficiency of farm operation. In China, India, and Indonesia, earthen terraces are formed by the natural process of trapping sediment with narrow-row, stiff-stemmed grass hedges. While quantitative information on the mass of sediment deposition is relatively limited, observations indicate that sediment buildup behind the grass hedges is quite high, often a meter or so every decade. In 1992, rows of stiff-stemmed grass hedges were planted within a 5.6-ha watershed located in western IA, near Treynor. Runoff and sediment yield data were separated into two periods, one without hedges (1975-1991) and the other with hedges (1992-1999). Comparing watershed responses with the USDA-Water Erosion Prediction Project (WEPP) watershed model showed that the grass hedges reduced sediment yield by 88% for the 1992-1997 period, a period characterized by normal runoff. However, excessive runoff events in 1998 and 1999 caused portions of the hedges to be undercut and washed away, particularly in areas on the landscape where runoff concentrated. For these years, measured sediment yields were higher than those predicted with the WEPP model because some of the previously trapped sediment was eroded off the watershed. The solution is to plant more hedges in areas of the watershed where runoff concentrates. Our results will be useful to soil conservationists and erosion control planners who are interested in using stiff-stemmed grass hedges to reduce sediment yields leaving farm fields.
Technical Abstract: Stiff-stemmed grass hedges planted in narrow rows within a field offer an opportunity for effective erosion control at a reasonable cost. In this research, the effect of grass hedges in controlling soil erosion was studied on a 6-ha watershed located in the deep loess hills region of western Iowa. Runoff and sediment yield data were measured from the outlet tof the watershed for a 25-yr period (1975-1999). The USDA-Water Erosion Prediction Project (WEPP) watershed model (Vers 99.5) was used to evaluate the effect of grass hedges on surface runoff and sediment yields. The model was first calibrated and evaluated using runoff and sediment yield data measured during the period before grass hedges were established in the watershed (1975-1991). For this period, the model underpredicted mean annual surface runoff by 12% and overpredicted mean annual sediment yield by 13%. The model was then run for the grass hedge period (1992-1999). During the first 6 years after grass hedge establishment, predicted sediment yield was 8 times higher than that measured, indicating that grass hedges were effective in trapping sediment from the hillslopes and reducing sediment loss from the ephemeral gullies. In 1998 and 1999, several extreme runoff events damaged the hedges and resulted in substantially higher sediment yields from the watershed. For these two years, predicted sediment loss was 32% lower than that measured. Grass hedges were effective in reducing soil erosion during normal runoff years. Extreme runoff events, however, undercut and washed out portions of the hedges, particularly in the ephemeral gullies, causing some of the previously deposited sediment to be transported from the watershed.