Canal/Ditch Seepage Loss Calculator
Estimates how much water an unlined delivery ditch or canal loses over a season to seepage and bank vegetation — the question a ditch rider or water-district manager asks when deciding whether lining a reach of canal would pay for itself.
Calculator
Read this off Figure 5.8 (USDA-SCS, 1985) for your soil/lining type — e.g. roughly 1.4 ft³/ft²/d for a loam soil. This tool doesn't embed that figure since it's a graphical range, not a table, but typical unlined-canal rates run from under 0.5 (fine-textured, high-fines soils) to over 3 ft³/ft²/d (coarse, gravelly soils).
Typical range is 0.5-1.0% loss per mile for ditch-side vegetation.
About this calculator
Seepage loss depends on the wetted area of the channel (its length times the wetted perimeter of its cross-section) and a soil-dependent seepage rate. Vegetative loss is estimated separately as a percentage of the flow, per mile of channel.
Seepage loss = (wetted perimeter × length) × seepage rate × days
Vegetative loss = (% per mile) × Q × (length / 5280 ft) × days
This tool carries full precision through the flow-rate unit conversion (Q in cfs → ac-in/hr), rather than the textbook's convenient approximation of 1 cfs ≈ 1 ac-in/hr, so results may differ from the book by roughly 1%.