REMOTE-SENSING OF CROP COEFFICIENTS FOR IMPROVING THE IRRIGATION SCHEDULING OF CORN

Improved irrigation water management requires accurate scheduling of irrigations which in turn requires an accurate calculation of daily crop evapotranspiration (E(t)). Previous work by Neale et al. (1989) and Bausch (1993) have indicated that the reflectance-based crop coefficient (K(er)) for corn responded to crop growth anomalies and should improve irrigation scheduling. Thus, the purpose of this study was to develop a new procedure for using the K(cr) in irrigation scheduling and present results of simulations comparing different basal crop coefficient (K(eb)) curves for corn to evaluate their effects on estimated crop E(t). Irrigation scheduling simulations were performed using SCHED, the USDA-ARS Irrigation Scheduling Program, and three K(eb) curves (the one in SCHED, Wright's (1982) tabular data, and the K(cr)-based K(cb)). Simulated crop water use using the K(cb) curve in SCHED was considerably greater during vegetative growth (60 to 100 mm) than simulated crop water use using Wright's K(cb) or the K(cr) derived K(cb) curves for three growing seasons. Crop water use between the K(cr)- based K(cb) and Wright's K(cb) were different by approximately 20 mm each growing season. Crop water use was less in 1990 and 1992 for the K(cr) derived curve and greater for 1991; crop development was directly responsible for the differences. Although the differences between the Wright and K(cr) basal crop curves were minimal, irrigations with the K(cr)-based K(cb) were more appropriately timed. Irrigations that are correctly timed minimize overirrigation as well as underirrigation.