A landscape with a slope of over 10% will require special planning to avoid unwanted runoff. A way around this is to create flat terraces held in place by retaining walls. Low precipitation devices set at the correct angles will be needed to irrigate a sloped area that has not had any structural changes.
Attach a string to either a stake or a screwdriver. At the highest point of ground plant the stake with the string (or screwdriver) into the soil. If you have an assistant they can walk to the bottom of the slope with the remainder of the string. Make sure the string is held taught and is level by using a line level. The leftover string will fall to towards the ground, attach a weight to the string to form a solid 90° angle. Measure the length of string from the high point to the assistant's hands. Then measure the height from the string in the assistant's hands to the ground. To find the percentage of slope simply divide the rise (the measured height) by the run (the measured length) and multiply by 100. If the measurement is less than 10% there is no need to show it on your design plan. If it is over 10% you can show this on your design plan by shading it in with diagonal lines. Large sloped areas can be measured by picking an smaller (more manageable) area by eye that has the same general angle and using the same methods as shown to determine the slope.
Determine the Percentage of Slope
Divide the measure of height by the measure of the length and multiply by 100.
Example above: 2 ÷ 14 x 100 = 14.3%