Step 3. Estimating lot area run-off
This worksheet estimates the amount of run-off from livestock pens.

Example Information:

• 25yr/24hr storm = 2.2 inches precipitation (see Montana precipitation map)
• Lot area = 4.36 acres
• Unpaved animal feeding areas hold about the first 0.5 inches of precipitation. Thus storms of less than 0.5, which is the majority of the storms in this area, produce no runoff.

Equation:

RO = [(P - 0.5) ÷ 12 in./ft] x LA

Variables:

RO	= lot runoff, ac-ft
P	= Precipitation, inches
LA	= lot area, acres

RO = [(2.2 – 0.5) ÷ 12] x 4.36 ac = 0.62 ac-ft