Optimal spacing of soil conservation barriers : example of rock bunds in Burkina Faso

Though construction methods vary widely, use of physical or biological barriers to conserve soil and water is common throughout the world. Rock or earthen bunds are common physical barriers. Strips of perennial grass, shrubs or trees serve as biological barriers. Often these barriers are arranged on a slope in roughly parallel contour bands. The spacing between barriers has important economic consequences, because distance from the barrier may create patterns of soil fertility and water availability that influence crop yields and because the spacing determines land available for cropping. The objective of this study was to develop a method for determining the optimal economic spacing of conservation barriers and apply that method to spacing of rock bunds in Burkina Faso. The steps in the optimization method include estimating a continuous yield response to distance between barriers, developing a mathematical expression to describe how costs change as spacing is altered, and optimizing using calculus. The method is general and can be applied to determining spacing of any conservation technique that is applied in bands. For example, this method could be adapted to spacing of grass strips, hedges, windbreaks, or terraces. This analysis suggests that the economically optimal spacing of rock bunds on the Central Plateau of Burkina Faso depends on the type of construction, materials transport cost, and how labor is organized