Soils, Crops and Fertilizer Use 
Chapter 2: Troubleshooting soil physical problems 

A field's slope has a marked influence on the amount of water runoff and soil erosion caused by flowing water. Slope is usually measured in terms of percent. A 10 percent slope has 10 meters vertical drop per 100 meters horizontal distance (or 10 ft. per 100 ft.). A 100 percent slope equals 45 degrees. Soil conservation measures become necessary on land with as little slope as 12 percent.
You can measure slope with a homemade device. This is covered in Chapter 3 on soil conservation.