In the semi-arid savannas where rainfall is limited and highly unpredictable, retaining more water in the soil can make the difference between a poor and a good harvest. You can slow down the runoff water by establishing contour bunds. Contour bunds are permanent ridges of earth that follow positions located at the same altitude. For over 20 years, farmers in Burkina Faso and Mali have made contour bunds in their fields on rolling land with less than a 5% slope.