Net Irrigation Requirement

Irrigation only has to supply what the rain did not. Subtracting one from the other is the easiest arithmetic on this page — a crop that used 145 mm in a month during which 42 mm of rain was effective leaves $145 - 42 = 103$ mm for the irrigator — and the entire difficulty lies in the word effective.

Effective rainfall is the part of the rain that actually reached the root zone and stayed there. Rain that ran off the surface did not count. Rain that fell on a full profile and drained past the roots did not count. Rain intercepted by the canopy and evaporated off the leaves before reaching the ground did not count, and on a closed canopy that can be several millimetres of every shower. A 40 mm downpour on a soil already at field capacity may contribute almost nothing, while 15 mm of gentle rain on a half-empty profile contributes nearly all of itself. Total rainfall from a gauge is therefore the wrong number to put into this equation, and using it is the mistake this formula exists to prevent.

There are standard ways of estimating the effective fraction, from crude rules — discount small showers below about 5 mm entirely, cap any single event at what the root zone can still absorb — to the USDA Soil Conservation Service monthly method, which fits an empirical curve to monthly rainfall and monthly crop use. All of them are approximations, and the honest alternative is a daily soil water balance that tracks the profile explicitly and lets the arithmetic decide how much of each event the soil could hold.

Two consequences follow. When effective rainfall exceeds crop use over a period, the net requirement is zero rather than negative: surplus rain does not bank forward beyond what the root zone can hold, and the excess drains away, carrying salts and nitrate with it. And this is the net requirement, meaning water that must arrive at the root zone. Converting it to the depth that must leave the pump means dividing by the application efficiency, and on saline soils adding a leaching fraction on top — deliberate over-irrigation whose whole purpose is to push accumulated salt below the roots.