How to Determine the Size of Irrigation Ditches

Observe the general shape of the irrigation ditch when it doesn’t have water in it. Some ditches have a trapezoid shape while others are more elliptical. The shape of the ditch determines the equation to determine the ditch cross-section area.

Measure the width of the ditch bottom for a trapezoid ditch. Find the width of the ditch at the water surface and the depth of the water. Make the measurements in feet. Add the width of ditch bottom to the width at the water surface. Divide this number by 2 and multiply the result by the water height. This gives the area in square feet.

Determine the width of the ditch at the water surface and the depth of the water for an elliptical-shaped ditch. Multiply the width of the ditch by the water depth. Divide this number by 4 and multiply by PI (approximately 3.14). This equation gives the area in square feet.

Select a 50 to 100 foot straight section of the ditch. To determine the rate of water flow in gallons per minute, drop an apple, orange, tennis ball or a similar object into the middle of the irrigation ditch. Determine the time it takes for the object to travel the distance. Divide the length of the ditch section by the average time in seconds. This gives the surface velocity in feet per second. To obtain the average stream velocity, multiply the surface velocity by 0.8 for a smooth lined ditch or 0.6 for a rough surfaced ditch. Multiply the cross-section area of the ditch by the average stream velocity to determine the rate of flow in cubic feet per second. Multiply this number by 450 to determine the gallons per minute of water flow.

Obtain a current or flow meter. Lower the meter into the flowing water in the ditch. A flow meter is relatively expensive but results in a more accurate determination of the gallons per minute of water carried by the ditch. Use the flow meter to determine the velocity in feet per second. Multiply the velocity in feet per second by the ditch area. Convert this number to gallons per minute by multiplying by 450.