Instrumented rainfall- and groundwater-protected irrigation shelters were used to establish relationships (daily crop factors) between pan evaporation and daily water use for several vegetables. Use of these daily crop factors (water use/pan evaporation) and pan evaporation data for scheduling irrigations are described. Snap bean (Phaseolus vulgaris L.) is used to illustrate irrigation scheduling by this method. A table of the model output with columnar headings of age, root depth, date, pan evaporation, crop factor, daily water use, cumulative water use, allowable water use, rainfall, and irrigation is presented. When irrigation was applied according to the model, soil water tension was held below 25 db at 6-inch (15-cm) soil depth. With varying irrigation rates under a line-source irrigation system, marketable pod yields were maximized at 100% of the model rate. Marketable yields of summer squash also were maximized when irrigation was applied at 100% of the model rate. Marketable yields of sweetpotato were not affected by irrigation rates ranging from 1% to 177% of the model rate.
Measurements of daily, 3-day, and 6-day cumulative pan evaporation using a #2 wash tub or a modified steel drum and a ruler provided an accurate, easy, and inexpensive way to schedule irrigation. Pan factors for these containers, which were covered with a 5-cm-mesh wire under humid climatic conditions, were 1.0 and 1.1, respectively.