EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Evaluation of the Compound Topographic Index (CTI) for the location of gullies in cultivated areas of Córdoba (Spain)

Eduardo Luquin1, Yolanda Zuasti2, Jorge Delgado3, Raquel Gastesi4, Javier Casalí1, Mikel Goñi1, Antonio Hayas5, Adela P. Galvín6, and Adolfo Peña5
Eduardo Luquin et al.
  • 1IS-FOOD, Dept. of Engineering, Public University of Navarre, Pamplona, Spain
  • 2Instrumental Tracasa, Sarriguren, Spain
  • 3Dept. of Cartographic, Geodetic and Photogrammetric Engineering, University of Jaén, Jaén, Spain
  • 4Dept. Territorial Information Systems, Tracasa, Sarriguren, Spain
  • 5Dept. of Project Engineering, University of Córdoba, Córdoba, Spain
  • 6Area of Construction Engineering, University of Córdoba, Córdoba, Spain

The identification of areas susceptible to gully formation is an objective that has important consequences for erosion control. It allows for the optimization of resources by focusing on prevention and control efforts on the most susceptible areas, avoiding the frequent evolution of ephemeral to permanent gullies. The issue is of great interest in Spanish olive groves, many of which are affected by serious problems of gully erosion.

Gullies are formed in the swales, which allows the use of topography-based tools to predict their location.

The Compound Topographic Index (CTI) proposed by Thorne et al. (1986) is calculated for each pixel as an estimate of the flow capacity to cause erosion, as it includes the product of the pixel draining area and its slope. Its application requires the identification of a critical value of the CTI (CTIc), above which the potential areas of gully occurrence will be located. Using historical orthophotos, the gullies observed were digitized for 2011 in the experimental areas called Morente (11 km2 of traditional olive groves on degraded and poor vertisols) and Matasanos (6 km2 of intensive olive groves also on vertisols) and nearby area, with cereal crops.

The objectives of this work are: to identify CTIc values corresponding to cultivated areas in Cordoba, mainly olive groves; to develop and evaluate an application that allows a user without great technical skills to obtain the CTI; to evaluate the capacity of this CTIc to reproduce gullies observed in nearby areas or in different time periods (2005) to establish cause-effect relationships between changes in landuse in this type of phenomenon, using the aforementioned tool.

Part of the digitized gullies, representative of olive grove areas, were used to obtain the CTIc of each gully, by modifying it until the best reproduction of the gullies observed was achieved, then their average value was taken as CTIc. To calculate the CTI, a 5m resolution DEM was used, obtained from LiDAR PNOA 2014.

In the framework of the Innolivar project, a desktop GIS application has been developed in a free software environment such as QGIS, which allows the calculation of the CTI. The APET tool (AGNPS Potential Ephemeral Gully Evaluation Tool) recently implemented has helped in the development of this application.

The CTI calculation by the application, after the determination of the CTIc threshold, serves to identify critical areas from a DEM, which is free and available in many countries. A first qualitative evaluation by visual verification indicates a very good characterization of the gullies. Subsequently, the goodness of fit of the gully position between the digitized gullies and the app-calculated gullies according to the CTIc is evaluated quantitatively by obtaining a binary confusion matrix by lengths. In the Morente area, an error of omission of 29% and of commission of 16% was obtained.

It can be concluded that the application generated that allows the application of the CTI methodology makes identification of areas susceptible to gully formation possible in an efficient and relatively simple manner, helping to achieve a more sustainable agriculture.

How to cite: Luquin, E., Zuasti, Y., Delgado, J., Gastesi, R., Casalí, J., Goñi, M., Hayas, A., Galvín, A. P., and Peña, A.: Evaluation of the Compound Topographic Index (CTI) for the location of gullies in cultivated areas of Córdoba (Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9591,, 2021.