Irrigation Water Amount per Crop

Crop irrigation water demand: how to size daily and seasonal needs

What drives the irrigation requirement is crop evapotranspiration (ETc). It comes from reference evapotranspiration (ETo, around 5 mm/day in a Brazilian summer) multiplied by a crop coefficient (Kc) that shifts with growth stage. Maize peaks at 5–8 mm/day during tasselling, soybean draws 4–7 mm/day at pod-filling, and coffee sits at 3–5 mm/day. Add up the daily demand over the whole cycle and you get the seasonal depth: maize 500–700 mm, soybean 450–650 mm, coffee 1,000–1,300 mm/year. Worked example: maize at 7 mm/day across 10 hectares needs 7 × 10,000 × 10 = 700,000 L/day, which is 700 m³/day. The depth you actually apply then gets adjusted for effective rainfall, soil texture (sandy vs. clay), and how efficient the system is (drip vs. pivot).

Applications

Sizing centre-pivot, drip and sprinkler systems, scheduling with EMBRAPA SIMA and SISDA tools, applying for rural water-use permits (outorga) at ANA and state environment agencies, budgeting energy costs on irrigation pumps, running deficit-irrigation strategies in coffee, and lining everything up with weather forecasts and reservoir levels.

FAQ

How is ETc calculated in the field? You use ETc = ETo × Kc. ETo comes off weather stations via Penman–Monteith FAO 56, and Kc comes from tabulated curves for each crop and phenological stage.

Drip or pivot, which uses less water? Drip hits 90–95 % application efficiency against 75–85 % for pivots. So drip is the choice for perennial high-value crops, while pivots still rule on grains.

Why is coffee total demand so high? Coffee is perennial, so it transpires all year and still needs water through flowering and bean filling. That pushes its annual total well past what an annual grain crop ever uses.

What about regulated deficit irrigation? With conilon and arabica, applying controlled water stress during certain phases can cut 20–30 % of the water and cost you little or no yield, provided you manage it with soil-moisture sensors.