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GIS-based comparative analysis of potential soil erosion in two agricultural systems: Dry farming and center-pivot irrigation. A case study in Southern Portugal
José Rafael Marques da Silva* and Luís Leopoldo Silva**
Department of Rural Engineering
University of Évora
Apartado 94, 7002-554 Évora, Portugal
* email: jmsilva@uevora.pt
** email : llsilva@uevora.pt
Abstract
The use of low pressure center-pivot systems, with high water application rates, to irrigate soils with low permeability often leads to runoff and soil erosion problems; sometimes grater than those due to natural rainfall.
The use of GIS tools and the Soil Loss Universal Equation can allow to study these affects for any particular conditions (soil, crops, topography) aiding the choice of a system that could minimize soil erosion by water.
In this study was showed that a low-pressure center-pivot irrigation system causes much more potential erosion that the traditional dry-farming systems. And its implementation without a previous environmental impact study can hypothecate the sustainability of soils at a medium/long run.
Introduction
In order to increase productivity and profit, many farmers in Southern Portugal are changing their traditional agricultural dry-farming systems to irrigation. This change can have some environmental impacts in the region since most areas have low permeability soils and undulated lands, which presents some limitations to the proper management of some irrigation systems.
One of the first choices for farmers as been center-pivot systems, because they can be adapt to many different conditions (soils, crops, topography) and are very easy to automate. In order to reduce costs they try to irrigate great areas with only one machine, and use low-pressure systems with spray type sprinklers, which are less energy consumptive.
These low pressure systems are characterized for having great application rates that can vary from 150 to 300 mm h-1 at the end of a 400 meters lateral (Addink et al., 1983), which many times exceeds the infiltration capacity of soils leading to runoff (Beck & DeBoer, 1992) and soil erosion.
Associated with these high water application rates there is also a high water kinetic energy applied to the soil due to water droplet impact, which is directly related with soil crust formation, responsible for a significant infiltration reduction (Thompson & James, 1985), favoring runoff and erosion. Sharma et al. (1991) showed that soil detachment, the first step for crust formation, increases with the water kinetic energy.
Recent research (Ferreira, 2001) also showed that, in our conditions, erosion due to irrigation systems can be of the some or higher magnitude than erosion due to natural rainfall. Since great areas of Southern Portugal have poor soils the impact of introducing agricultural systems that could increase soil loss is something to consider. Soil losses occurring year by year have a great effect in soil productivity that can only be partially compensated with the application of more fertilizers, increasing crop costs and pollution risks.
This paper objective is to compare, for the same area, potential soil erosion from a traditional rain-fed wheat crop system and a center-pivot irrigated corn crop, using GIS tools and the Soil Loss Universal Equation.
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