Soakline: free online crop water and irrigation calculator

Soakline turns climate, crop and field data into an irrigation and drainage design you can check line by line. It computes grass reference evapotranspiration with the FAO-56 Penman-Monteith method and shows every intermediate, builds the crop coefficient curve for a season, and gives monthly crop water use, effective rainfall and the net and gross irrigation requirement with the design flow for your area.

For pressurised systems it gives the sprinkler application rate, set time and catch-can uniformity, and the drip emitter uniformity with a lateral solved emitter by emitter, including slope and friction, and a manifold. For subsurface drainage it finds the drain spacing with Hooghoudt’s equation and an exact equivalent depth, and checks the drain pipe. Everything runs in your browser, and the Verify page reruns the published worked examples it is tested against.

What Soakline can do

FAO-56 Penman-Monteith ETo for a day or a month, with every intermediate shown
Crop coefficient curve with climate adjustment and daily crop ET over a season
Monthly effective rainfall, net and gross irrigation requirement and design flow
Sprinkler application rate, set time, Christiansen CU and low-quarter DU
Drip emission uniformity, lateral solved emitter by emitter, and a manifold
Hooghoudt drain spacing with exact equivalent depth, and drain-pipe capacity
JSON projects, climate CSV import and results CSV export

How we verify Soakline

Reference checks reproduce published FAO-56, FAO irrigation manual and ILRI drainage examples and add closed-form and conservation checks. Pending sources do not count as passes. Where a reference answer exists you can run every check yourself on the Verify page, and the Soakline user guide documents every feature and limitation.

The Verify page runs the same cases as the automated test suite. Validation cases reproduce published worked examples: FAO-56 Examples 2, 3, 5, 8, 9, 10, 13, 14, 15, 17, 18, 27 and 28; the effective-rainfall and irrigation-need examples of FAO Training Manual 3; the precipitation rate, set time and F-factor table of FAO Irrigation Manual Module 8; the Hazen-Williams example, F-factor table and emission-uniformity examples of Module 9; and ILRI Publication 16 Examples 8.1 to 8.3 and Table 21.9. Verification cases check closed forms (the sum-of-squares form of the F factor, the drain spacing with the impervious layer at drain level, the inversion of Manning's equation), conservation (mass and energy along a lateral, the season water ledger) and independent code (a different series for the equivalent depth, a different solver for a single outlet).

Soakline is part of Open Tools, the free engineering tools that run in your browser.

Professional use. Results are for education and preliminary design. They must be checked by a qualified professional, and they are not a substitute for code-compliant design or licensed engineering judgement.