INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Predicting Runoff for a RUSLE2 Ephemeral Gully Calculator
Submitted to: International Soil Conservation Organization Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 3, 2008
Publication Date: May 23, 2008
Citation: Dabney, S.M., Yoder, D.C., Bingner, R.L., Vieira, D.A. 2008. Predicting Runoff for a RUSLE2 Ephemeral Gully Calculator. In: Proceedings of the International Soil Conservation Organization Conference, May 18-23, 2008, Budapest, Hungary. 2008 CDROM.
Interpretive Summary: RUSLE2 is the erosion model that the Natural Resources Conservation Service (NRCS) uses to estimate soil erosion on farmland and to determine farm program eligibility. RUSLE2 does a good job of estimating erosion on hillsides, but it cannot estimate erosion in concentrated flow channels like the “ephemeral gullies” that form in cropped fields during intense storms that the farmer smooths over the next time he tills his field. Adding the ability to account for this kind of soil erosion has been identified as a priority need by NRCS. In this paper we explore ways that rainfall runoff, and the ephemeral gully erosion it causes, can be estimated using the information already available in the RUSLE2 databases. The results are encouraging. We suggest following the outline presented in this paper will allow a reliable estimate of concentrated flow erosion to be made for all farm fields in the eastern part of the United States.
RUSLE2 provides robust estimates of average annual sheet and rill erosion from a wide range of land use, soil, and climatic conditions, but it cannot calculate channel erosion, including ephemeral gully erosion. Estimation of ephemeral gully erosion within RUSLE2 has been identified as a priority need by user agencies. RUSLE2 currently uses runoff from the location-specific 10-yr 24-hr precipitation depth (P10y,24h) estimated using aUSDA curve number (CN) method to calculate sediment transport capacity. Because this approach assumes a 10-yr transport event occurs every day, it gives a conservative estimate of sediment deposition along RUSLE2 hillslope profiles and in the channel at the bottom of the hillslope. However, applying the same approach to calculation of erosion in the channel could lead to excessive sensitivity of ephemeral gully erosion estimates to tillage frequency, since the storm simulated may be large enough to cause the channel to erode to its ultimate shape on the first day after each tillage event. We therefore sought a smaller suitable index storm for ephemeral gully erosion estimation. This paper describes steps toward the end of estimating index storm size and frequency using only information already contained within the RUSLE2 databases. We then outline a possible method of using the index storms to calculate ephemeral gully erosion within a RUSLE2 context as a scaled sum of a location-specific number and distribution of erosion events each year, whose sizes and durations are estimated from the existing RUSLE2 databases and whose total runoff volume approximates local average annual runoff.