Crop Coefficients Specific to Phenological Stages for Evapotranspiration-Based Irrigation Management of Onion and Spinach

Weighing lysimeters are used to measure crop water use during the growing season. By relating the water use of a specific crop to a well-watered reference crop such as grass, crop coefficients (K_C) can be developed to assist in predicting crop needs using meteorological data available from weather stations. This research was conducted to determine growth-stage-specific K_C and crop water use for onions (Allium cepa L.) and spinach (Spinacia oleracea L.) grown under South Texas conditions. Seven lysimeters, consisting of undisturbed 1.5 x 2.0 x 2.2-m deep soil monoliths, comprise the Texas AgriLife Research - Uvalde lysimeter facility. Six lysimeters, weighing about 14 Mg, have been placed each in the middle of a 1-ha field beneath a linear low-energy precision application irrigation system. A seventh lysimeter was established to measure reference grass reference evapotranspiration. Daily water use for onion and spinach was measured at 5-min intervals. Crop water requirements, K_C determination, and comparison with  existing Food and Agricultural Organization (FAO) K_C values were determined over a 2-year period for each crop. The K_C values determined over the growing seasons varied from 0.2 to 1.3 for onion and 0.2 to 1.5 for spinach, with some of the values in agreement with those from FAO. It is assumed that the application of growth-stage-specific K_C will assist in irrigation management and provide precise water applications for a region of interest.