Consumptive water use and crop coefficients of irrigated sunflower

In semi-arid environments, the use of irrigation is necessary for sunflower production to reach its maximum potential. The aim of this study was to quantify the consumptive water use and crop coefficients of irrigated sunflower (Helianthus annuus L.) without soil water limitations during two growing seasons. The experimental work was conducted in the lysimeter facilities located in Albacete (Central Spain). A weighing lysimeter with an overall resolution of 250 g was used to measure the daily sunflower evapotranspiration throughout the growing season under sprinkler irrigation. The lysimeter container was 2.3 m x 2.7 m x 1.7 m deep, with an approximate total weight of 14.5 Mg. Daily ET (c) values were calculated as the difference between lysimeter mass losses and lysimeter mass gains divided by the lysimeter area. In the lysimeter, sprinkler irrigation was applied to replace cumulative ET (c), thus maintaining non-limiting soil water conditions. Seasonal lysimeter ET (c) was 619 mm in 2009 and 576 mm in 2011. The higher ET (c) value in 2009 was due to earlier planting and a longer growing season with the maximum cover coinciding with the maximum ET (o) period. For the two study years, maximum average K (c) values reached values of approximately 1.10 and 1.20, respectively, during mid-season stage and coincided with maximum ground cover values of 75 and 88 %, respectively. The dual crop coefficient approach was used to separate crop transpiration (K (cb)) from soil evaporation (K (e)). As the crop canopy expanded, K (cb) values increased while the K (e) values decreased. The seasonal evaporation component was estimated to be about 25 % of ET (c). Linear relationships were found between the lysimeter K (cb) and the canopy ground cover (f (c)) for the each season, and a single relationship that related K (cb) to growing degree-days was established allowing extrapolation of our results to other environments.