Use of Crop Canopy Size to Estimate Crop Coefficient for Vegetable Crops

Planting time, plant density, variety, and cultural practices vary widely for horticultural crops. It is difficult to estimate a basal crop coefficient that can incorporate these variations. Canopy cover, as an indicator of intercepted sunlight, is related to crop water use. We used a weighing lysimeter to measure daily crop water use and a multi-spectral camera to measure canopy cover for two vegetable crops — head lettuce and bell pepper, and related canopy cover to basal crop coefficient. The ratio of crop coefficient to canopy cover declined from about 4 with small canopy cover (0.1) to about 1.3 for a mature crop with canopy cover of 0.9. The relationship was similar for these two crops. Because light interception other than at mid-day will depend on the canopy structure, adjustment may be needed for canopy structure. A generalized canopy cover:basal crop coefficient relationship would allow weather-based irrigation scheduling for a wide range of horticultural crops based on canopy measurements, and possibly based on remotely-sensed vegetation indices.