Crop yield response to water - Cra

Box 1 Simulating deficit irrigation in cotton productionBackgroundCotton is grown in many water limited regions where deficit irrigation may be practisedeither as a necessity driven by lack of water or for economic reasons (costs of water and/orenergy for pumping). The United States southern high plains region is exemplary of bothlimited water and high pumping costs. AquaCrop simulations were carried out for a Texaslocation at 35°11′ N, 102°6′ W, 1170 m elevation above sea level. The slowly permeablesoil is a Pullman silty clay loam with a strong argillic horizon containing approximately50 percent clay above a wavy boundary of a calcic horizon at 0.1 to 0.14 m depth. The soilwater-holding capacity is about 200 mm to 1.5-m depth (Tolk and Howell 2001). Meanannual precipitation is 490 mm, 65 percent of which falls during the growing season (May-August). ET o greatly exceeds precipitation in all months.AquaCrop simulationSimulations were performed for cotton sown in rows on raised beds and with the furrowsdiked to store irrigation and precipitation. Irrigation was either Full (FI), indicating thatsoil water was replenished to replace that lost to ET, or one half of that (Deficit, DI).Irrigation scheduling was performed assuming a lateral-move sprinkler irrigation systemthat applies ~25 mm per irrigation. The sowing rate was at 21 seeds/m 2 . Late in theseason, FI was reduced relative to the crop-water requirement (ET c demand), so as toenhance crop maturation.Reference evapotranspiration for input into AquaCrop was calculated using the FAOET o Calc computer programme (FAO, 2009) and weather data measured at a weatherstation close to the cotton field. AquaCrop field management parameters were set so thatno runoff occurred (due to the furrow dikes), and soil fertility was non-limiting. Five soildepths were considered, with initial water contents of 23, 33, 34, 30, and 27 vol percentat depths of 0.10, 0.29, 0.45, 0.66, and 1.00 m, respectively, as measured in the field. Thecrop calendar was set as 10 days from sowing to emergence, 94 days from sowing tomaximum root depth, 121 days from sowing to start of senescence, 140 days from sowingto maturity, 60 days from sowing to flowering, and 71 days the duration of flowering.ResultsSimulated yields were in the range of 3.3 to 3.6 tonne/ha, (equivalent to 1.3 to 1.4 tonne/haof lint) and were comparable to values reported in the region. Deficit irrigation (DI) seedlintyields were ~95 percent of full irrigation (FI) yields. The water productivity of DI cottonwas ~10 percent greater than that of FI (both in the range of 0.49 to 0.54 kg/m 3 (seed pluslint), or 0.19 to 0.21 kg lint/m 3 ). Crop ET was about 15 percent greater for FI than for DI,both in the range of 625 to 720 mm, which matches well observed values in several regions.However, DI received 240 mm of irrigation, only 43 percent of the FI amount.Conclusions and recommendationsFarmers in the region pump from a water table about 90 m below ground and, givenrising fuel costs, the energy savings of DI were more than US$250/ha. At cotton pricesranging from US$0.4 to 0.8/kg, the loss in production associated with DI represents onlyUS$100 to 200/ha, giving the economic edge to deficit irrigation.AQUACROP APPLICATIONS 57