Effective management of irrigation water for carrot under constant and optimized regulated deficit irrigation in Brazil

The optimized regulated deficit irrigation (ORDI) determines the level of deficit (ratio between actual and maximum evapotranspiration “ETa/ETm”) to be applied at each development stage of a crop in order to reach the maximum yield for a certain global deficit at the end of the cropping period. One of the weaknesses of this methodology is its inability to forecast the effect of deficit irrigation on the quality of the harvest, which may affect the profitability of the crop. A research conducted in Seropédica (Brazil) during 2013 and 2014, aimed to assess the effect of deficit irrigation on the yield, quality, water productivity, and profitability of a carrot crop “Brasilia cv.”. Ten strategies of irrigation management were applied to the crop: full irrigation, 90, 80, 70, and 60% of ETm using constant deficit irrigation (CDI) (the ETa/ETm ratio was the same at all the development stages), 90, 80, 70, and 60% of ETm using ORDI, and rainfed. The experimental design was composed of random blocks equipped with a drip irrigation system. The moment and the amount of water applied to each treatment were determined by the soil water balance module of the MOPECO model, and corroborated by the monitoring of soil moisture samples. “Full irrigation” treatments achieved the highest yield, quality, and profitability per unit of the cropped area, while ORDI increased the water productivity and reached the highest profitability per unit of volume of irrigation water supplied to the crop (average up to 12.26 € m−3 for the “0.6 ORDI” treatment vs. 8.38 € m−3 for the “full irrigation”). CDI and ORDI treatments reached similar total yields. However, the marketable yields were 26.6% average higher for ORDI, advising against using the CDI strategy. Consequently, in areas where water is not scarce, the use of regulated deficit irrigation strategies is not justified. Nevertheless, in areas where water is the limiting factor, but not the irrigable land, ORDI may increase the total gross margin of a farm by supplying the same volume of irrigation water to a greater area.