Irrigation by Rainfall

Supplemental Irrigation Based on Rainfall (mm)

To size supplemental irrigation you look at the water balance between what the crop loses to evapotranspiration and what the rain actually delivers: deficit = ETc − P, with ETc = Kc · ETo under the FAO 56 method. One handy thing about the units is that 1 mm on the pluviometer means 1 L/m², so the irrigation depth you need in mm is the same number as the deficit in liters per square meter.

You only irrigate when P < ETc. Once you turn that net figure into a gross volume, system efficiency comes into play. Drip irrigation reaches up to 90%, microsprinklers sit around 85%, and a conventional sprinkler runs closer to 75%. Regional Kc values and ETo references come from Brazil’s ANA (Agência Nacional de Águas) Plano Diretor de Irrigação.

Applications

Irrigation engineers, farm managers and extension agents reach for it to plan a week's water budget, check that pivot or drip programs are set right, work out water-use efficiency figures, and show they are within the water grants (outorga) issued by state environmental agencies.

FAQ

How do I convert mm of irrigation to liters? Take the mm and multiply by the irrigated area in m². So a 10 mm application over 1 ha (10 000 m²) comes to 100 000 L, or 100 m³.

Should I count all rainfall as effective? No. Heavy rain (>30 mm/h) runs off, and only 50–70% of it really soaks in. For a balance you can trust, apply an effective-rainfall coefficient from the USDA SCS method.

Where do I get reliable ETo data? A few sources give reference evapotranspiration from the Penman–Monteith FAO 56 equation: INMET stations, Agritempo (Embrapa), and Xavier’s daily gridded dataset for Brazil.