Crop Coefficients for Microsprinkler-Irrigated, Clean-Cultivated, Mature Citrus in an Arid Climate

Crop evapotranspiration $\left({\mathrm{ET}}_c\right)$ was measured over a clean-cultivated, mature navel orange orchard with microsprinkler irrigation located near Lindsay, California. Hourly mean latent heat flux density was determined as the residual of the energy balance equation with measured net radiation, soil heat flux density and sensible heat flux density estimated using the surface renewal method. The ${\mathrm{ET}}_c$ was compared with ${\mathrm{ET}}_o$ calculated using hourly weather data and the ASCE-EWRI Penman-Monteith equation. Following pruning and topping of the trees in the spring of 2001, the $K_{\mathrm{co}}$ values slowly increased as the canopy developed in the following season. An average $K_{\mathrm{co}}=0.82$ was observed. In the following year, the mean summertime value increased to about $K_{\mathrm{co}}=0.95$ , and in 2003 and 2004, the summertime value averaged near $K_{\mathrm{co}}=1.00$ , which is somewhat higher than observed for drip irrigated trees in southwestern Arizona and considerably higher than reported in the widely used Food and Agricultural Organization of the United Nations publications that were based on infrequent surface irrigation.