Grades 6–8 · Systems
How do farmers work out when crops need more water?
A field does not need the same amount of irrigation every day. Roots take up water from soil, and leaves release much of it as vapor through transpiration. Water also evaporates from the soil. Together these losses are called evapotranspiration.
In the diagramKeep track of water around the rootsFictional daily water-depth account · all values in mmRainIrrigationETStart: 40+ rain 5+ irrigation 10− ET 8− drainage 0Storage: 47 mm= 47 mmAssumed capacity: 60 mm; excess drains away.A water account, not a real field’s watering schedule.
Farmers can measure soil moisture and keep a water account. Rain and irrigation add water; evapotranspiration removes it. Water may also run off or drain below the roots. Crop type, growth stage, root depth and soil water-holding capacity all affect how the account is interpreted.
The fictional bucket begins with 40 millimeters of stored water. Adding 5 millimeters of rain and 10 of irrigation, then subtracting 8 of evapotranspiration, leaves 47 millimeters. If additions would raise storage above the assumed 60-millimeter capacity, the model sends the excess to drainage. These are depths of water spread over the field, not depths of soil.
A real irrigation choice also considers weather forecasts, equipment capacity and how much depletion the crop can tolerate. The picture explains why measurements and changing conditions matter; its invented numbers do not prescribe a watering schedule.
About this explanation
Starting knowledge: Add and subtract quantities and recognize that roots use water stored in soil.
Sources
- University of Minnesota Extension — Evapotranspiration-based irrigation scheduling or water-balance method
- University of Minnesota Extension — Irrigation management