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Title: Evaluation of the Compound Topographic Index (CTI) for the Location of Ephemeral Gullies on Olive Orchards at a Microcatchment Scale

item Bingner, Ronald - Ron

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/15/2007
Publication Date: 9/26/2007
Citation: Taguas, R.E.V., Yuan, Y., Bingner, R.L., Pena A.A., Munoz, J.L.A., Alcantara, R.P. 2007. Evaluation of the Compound Topographic Index (CTI) for the Location of Ephemeral Gullies on Olive Orchards at a Microcatchment Scale. In: Proceedings of the XI International Congress on Project Engineering, September 26-28, 2007, Lugo, Spain. IV:1312-1323.

Interpretive Summary: Erosion is a serious issue in the olive orchards grown in Spain. Surface runoff in Mediterranean areas has been shown to produce excessive sheet/rill and gully erosion. Technology is needed to address both types of erosion, with the USDA RUSLE program typically used to analyze sheet and rill erosion. Technology has been limited that is needed to assess the impacts of conservation practices used to control gully erosion. Most of the gully erosion in these regions are a result of ephemeral gullies, where the gullies are repaired through tillage practices, but reform after subsequent rainfall. The capability of addressing ephemeral gullies has recently been incorporated into USDA’s AnnAGNPS watershed pollutant loading model, but requires information about the location of potential ephemeral gullies scattered throughout a watershed. GIS technology has been developed to provide AnnAGNPS the information needed to simulate the effect of conservation practices on ephemeral gully erosion by automatically locating the mouths of the gullies. Studies were performed on a watershed in Spain containing observed ephemeral gullies over several years. The GIS approach adequately located most of the extent of the ephemeral gullies, but several improvements to the approach were described. This mainly involved the need to include the impact of soil properties on the formation of ephemeral gullies. This information can be used in the development of improved watershed conservation management plans internationally, and especially by Federal action agencies, such as NRCS and the US Army Corps of Engineers.

Technical Abstract: Spain has over 2.1 million ha dedicated to olive crop (Civantos, 2001). Several studies about sheet and rill and inter-rill erosion at plot scale in olive orchards have been done; however, it has been proved that flow concentrated erosion in Mediterranean areas with other land uses contributes significantly to total soil loss. Diverse topographic indexes have been used to describe and to locate ephemeral gullies since they can be used to identify risk zones of gully erosion. In this case, the automated Compound Topographic Index mapping was applied to predict the location of ephemeral gullies in an olive orchard microcatchment of 6 ha. Numerical criteria that consider predicted and observed meeting pixels of gullies were used and a CTI threshold of 3 was determined. Despite the suitability of CTI on most of gullies, several limitations were observed related to soil properties. Thus, the spatial distributions of the soil organic matter content (%OM), soil texture, bulk density (BD) and saturated hydraulic conductivity (Ks) derived from the interpolation methods: Kriging and Inverse Distance Weigh; and field considerations such as soil depth and parent material were examined. Although the improvements of predictions were not noticeable, different mechanisms of development have been identified in badly-located gullies, where infiltration conditions (BD, Ks) and shallowness of the soil profile have larger influence than drainage area. The limitations of the use of topographic indexes should be considered. However, the concept of CTI improves our understanding of the erosion process in ephemeral gullies.