Evapotranspiration and responses to irrigation of broccoli

Abstract

In cold, semi-arid areas, the options for crop diversification are limited by climate and by the water supply available. Growing irrigated crops outside the main season is not easy, because of climatic and market constraints. We carried out an experiment in Albacete, Central Spain, to measure the water use (evapotranspiration, ET) of broccoli ( Brassica oleracea L. var . italica Plenck) planted in late summer and harvested at the end of fall. A weighing lysimeter was used to measure the seasonal ET under sprinkler irrigation. Consumptive use reached 359 mm for a period of 109 days after transplanting. The crop coefficient ( K c) for broccoli was obtained and compared to the standard recommendations for normal planting dates. Dual crop coefficient computations of the lysimeter ET data indicated that evaporation represented 31% of seasonal ET. An analysis of the variation in daily K c values at a time of full cover suggested that the use of a grass lysimeter as a reference ET (ET o) was superior to using the ASCE Penman–Monteith (ASCE PM) equation at hourly time steps, which in turn caused less variability in K c than when using the FAO-56 Penman–Monteith (FAO-56 PM) equation at daily time steps for the ET o calculation. An additional experiment aimed at evaluating the yield response to applied irrigation water by the drip method (seven treatments, from 59 to 108% of ET c) generated a production function that gave maximum yields of near 12 t ha −1 at an irrigation level of 345 mm, and a water use efficiency of 3.37 kg m −3. It is concluded that growing broccoli in the fall season is a viable alternative for crop diversification, as the lower yields obtained here may be more than compensated for by the higher produce prices in autumn, at a time of the year where irrigation water demand for other crops is very low.