Canal Seepage Loss & Water That Never Arrives

Seepage is the water that soaks out of a canal through its wetted bed and sides into the surrounding soil as the water flows. In unlined earthen canals it can be the single biggest loss between the source and the field — water that is pumped or released but never reaches a crop, lowering the overall efficiency of the irrigation system.

Loss = wetted perimeter × length × seepage rate. The wetted perimeter is the length of bed and sides that the water actually touches in cross-section, the length is how far the canal runs, and the seepage rate is the depth of water that soaks away per unit area per day. Multiply them to get the volume lost per day.

It is the part of the canal's cross-section in contact with water — the bottom plus the two wetted side slopes — for a given flow depth. Seepage happens across this whole surface, so a wide, shallow channel with a large wetted perimeter loses more than a deep, narrow one carrying the same flow.

Seepage rate depends on soil and lining. Coarse sand and gravel beds can lose well over 0.3 m³ per m² per day, loams much less, and a good concrete or HDPE lining only a small fraction of that. Local soil tests give the best value; the tool lets you enter the rate that matches your channel.

Conveyance efficiency is the share of water entering the canal that actually reaches the field — the rest is lost mostly to seepage and some evaporation. A high loss percentage means low conveyance efficiency, so more water must be released at the head to deliver the same amount to the crop.

Lining with brick, concrete or HDPE — or compacting and sealing the soil — cuts the seepage rate sharply, often by 70–90%. Enter the lined seepage rate to compare against the unlined channel and see the daily water saved; that saving is what justifies the lining cost over the system's life.

The loss percentage compares the seepage to the flow being carried, so it tells you how efficient the channel is regardless of size. A large canal can lose a big volume yet still be efficient, while a small leaky channel may lose a huge share of its flow. Aim to bring the percentage down.

Yes, all else equal — seepage scales with the wetted area, which grows with length. That is why long earthen feeder canals are prime candidates for lining, and why delivering water in pipes over long distances often beats open channels where seepage soils and distances are large.

Evaporation also removes canal water, but in most earthen channels seepage dominates the loss by a wide margin. This tool focuses on seepage because that is where lining and compaction make the biggest difference; add a small allowance for evaporation in hot, slow-moving canals.

They are a sound planning estimate. Real seepage varies with soil moisture, water depth, flow and the canal's condition, and it falls as the bed silts up over a season. Use field-measured seepage rates where you can, and treat the output as a basis for comparing lining options and sizing losses.