Irrigation & Soil Water Balance: The Soil as a Bank Account

Soil water as a bank account

The simplest useful model of root-zone soil water is bookkeeping: an account with deposits (rainfall, irrigation) and withdrawals (evapotranspiration, drainage, runoff), tracked day by day. It is a crude simplification of real unsaturated flow physics, but it is the model that most irrigation scheduling in the world actually runs on, because it needs only a rain gauge, a weather station and a soil map.

SW(t) = SW(t-1) + Rain + Irrigation − ETc − Drainage − Runoff

SW      = root-zone soil water content
ETc     = crop evapotranspiration for that day
Drainage = water lost below the root zone once SW exceeds field capacity

Field capacity and wilting point

Two thresholds bound the useful range. Field capacity is the water content remaining once gravity has drained the large, air-filling pores, typically one to three days after saturation — the practical ceiling, because anything wetter drains away regardless of what the plant wants. Permanent wilting point is the water content at which the remaining water is held so tightly by capillary and adsorptive forces that roots can no longer extract it fast enough to avoid permanent wilting. The difference between the two is the total available water (TAW) — the whole reservoir a crop can theoretically draw down.

Readily available water and the depletion trigger

Crops do not use TAW evenly. As soil dries past roughly the top half of the available range, the plant's own resistance to water uptake rises and it begins closing stomata to protect itself, cutting photosynthesis well before visible wilting. That threshold — usually 40 to 60% of TAW depending on species — defines the readily available water (RAW), and it is RAW, not TAW, that a well-run scheduler tries never to fully deplete. Irrigating only when the plant is already wilting means yield has already been lost; irrigating the moment RAW is crossed keeps the crop in its comfort zone.

Evapotranspiration: ETo, Kc and ETc

Water leaves the root zone mainly as evapotranspiration — the combined loss from soil evaporation and plant transpiration. The FAO-56 method (Allen et al., 1998) splits this into two steps. First, the Penman-Monteith equation computes a reference evapotranspiration ETo for a standardised short grass surface, purely from weather data: net radiation, temperature, humidity and wind. Second, a crop coefficient Kc rescales ETo for the actual crop and its growth stage:

ETc = Kc × ETo

Kc_ini  ≈ 0.3–0.5   sparse canopy, mostly bare-soil evaporation
Kc_mid  ≈ 1.0–1.2   full canopy, peak water demand
Kc_end  ≈ 0.6–0.9   senescing canopy, reduced transpiration

Kc traces a curve over the season that rises through establishment, plateaus through the mid-season and declines through ripening — which is why a fixed daily irrigation volume is almost always wrong; the crop's own demand moves by a factor of two or three across its life cycle.

Why counter-flow scheduling beats fixed intervals

A calendar-based schedule (irrigate every N days) ignores weather entirely and either wastes water in a cool, cloudy week or under-waters in a heatwave. A balance-based scheduler recomputes SW every day from the actual ETc and rainfall, and triggers an irrigation event only when SW crosses the RAW threshold — refilling back to field capacity, never beyond, so no water is wasted as drainage. That is the logic this simulation runs: a running account, a demand curve that moves with the crop stage, and a hard rule for when to write a deposit.

Frequently asked questions

Try it live

Everything above runs in your browser — open Irrigation & Soil Water Balance and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.