Cotton irrigation norms: calculating by hydromodule zone

Full article available in Uzbek and Russian.

Irrigated land in Uzbekistan is classified into hydromodule zones based on soil texture, groundwater depth and groundwater mineralisation. The zone — not the crop alone — determines the irrigation regime, which is why universal per-hectare figures are misleading.

The single-application irrigation norm is calculated as m = 100 × H × γ × (β₁ − β₂), where H is the effective root-zone depth (0.5–1.0 m depending on growth stage), γ is bulk density, β₁ is field capacity and β₂ is measured pre-irrigation moisture. The result is a net norm; dividing by canal efficiency gives the gross volume actually delivered — a 20–30% difference that is frequently overlooked.

The weakest input in this calculation is β₂. Determining it by auger sampling takes days and yields a single point. Soil sensors measuring at 10 and 20 cm every 15 minutes replace the estimate with a measurement, and allow verification that water actually reached the root zone after irrigation.