ET₀ and evapotranspiration: scheduling irrigation from weather data

Full article available in Uzbek and Russian.

ET₀ (reference evapotranspiration) measures water loss from a hypothetical reference grass surface, driven purely by weather — temperature, humidity, wind, and solar radiation. The FAO-56 Penman-Monteith equation is the global standard for computing it:

ET0 = [0.408Δ(Rn−G) + γ(900/(T+273))u2(es−ea)] / [Δ+γ(1+0.34u2)]

Each term maps to a real driver: Δ (temperature sensitivity of vapor pressure), Rn (net radiation), G (soil heat flux), γ (psychrometric constant), T and u2 (2m air temperature and wind speed), and es−ea (vapor pressure deficit — how "thirsty" the air is).

Actual crop demand is ETc = ET0 × Kc, where Kc shifts across growth stages — low at establishment (Kc_ini ≈ 0.35), peaking at full canopy (Kc_mid ≈ 1.15-1.20 for cotton), and declining at maturity (Kc_end ≈ 0.5-0.6).

A simple water balance predicts irrigation timing: track cumulative ETc minus effective rainfall since the last irrigation, and irrigate when it approaches the readily available water (RAW) threshold. Example: at 6.9 mm/day depletion and a 35 mm RAW threshold, irrigation is due on day 6 — calculated in advance from the weather forecast, before any visible stress.

This makes weather-based ET₀ scheduling predictive, complementing soil moisture sensors, which are confirmatory. Together they give the most reliable irrigation timing.

Figures are general and illustrative — verify against official local agromet and irrigation authority data.