Regulated deficit irrigation: when giving trees less water works
Precision irrigation is usually sold as “the right amount of water at the right time”. Deficit irrigation goes one step further: deliberately giving the crop less than it would use, at the moments when it hurts least.
The principle
In a widely cited review in the Journal of Experimental Botany (2007), Elías Fereres and María Auxiliadora Soriano define deficit irrigation as applying water below the crop's full evapotranspiration (ET). As water becomes scarce, they argue, the goal shifts from maximising yield per hectare to maximising production per unit of water consumed. In most cases they suggest supplying roughly 60% to 100% of full ET.
Regulated deficit irrigation (RDI) concentrates the shortfall in growth stages where the crop is least sensitive. In fruit trees and vines, the review reports that RDI has raised water productivity and farm profits in the cases studied.
How FAO models the trade-off
FAO Irrigation and Drainage Paper 66, Crop yield response to water (Steduto, Hsiao, Fereres and Raes, 2012), updated the classic Paper 33 of 1979, which related relative yield loss to relative ET deficit through a crop coefficient (ky). Paper 66 adds the AquaCrop model for herbaceous crops and a separate set of guidelines for fruit trees and vines, the crops where RDI is most used.
A field result: almonds in California
A five-year plant-based RDI trial in the northern Sacramento Valley (Stewart, Fulton, Krueger, Lampinen and Shackel, California Agriculture 65(2), 2011) withheld water around hull split, holding trees at a midday stem water potential of −14 to −18 bars. It saved about 5 inches (≈127 mm) of water a year with no significant yield loss; average kernel weight was slightly lower. The authors note the savings held even on soils with low water-holding capacity.
The trial's key choice was to steer by a plant measurement (stem water potential) rather than a calendar. Responses to deficits vary with cultivar, soil and season, which is why RDI relies on measurement.
When cutting water saves nothing
Fereres and Soriano add an important caveat. If the soil still holds enough stored water, applying less irrigation may not reduce what the crop actually transpires: the plant simply draws down the reserve, and over a season there are no net savings at basin level. Real savings appear only when consumption falls. This is also why scheduling tools such as the WiseUp strategy tool developed by the FIGARO partner CER combined soil, weather and crop data (see the project's web apps).
The same logic in a garden
The principle scales down. Established trees and shrubs tolerate short dry spells far better than lawns or vegetable beds, and deep, infrequent watering encourages deeper roots. Anyone budgeting for a property with a large garden should treat summer watering as a running cost, much like heating, rather than an afterthought.
Sources
- Fereres E., Soriano M. A. (2007), “Deficit irrigation for reducing agricultural water use”, Journal of Experimental Botany 58(2): 147–159, doi:10.1093/jxb/erl165.
- Steduto P., Hsiao T. C., Fereres E., Raes D. (2012), Crop yield response to water, FAO Irrigation and Drainage Paper 66.
- Stewart W. L., Fulton A. E., Krueger W. H., Lampinen B. D., Shackel K. A. (2011), “Regulated deficit irrigation reduces water use of almonds without affecting yield”, California Agriculture 65(2): 90–95, doi:10.3733/ca.v065n02p90.