Use of crop canopy cover to estimate lettuce Etc in a greenhouse

Abstract

FAO PM Reference evapotranspiration, as described by Allen et al. (1998) is used extensively to calculate crop water needs, in the form of Crop evapotranspiration, ETc. The main challenge to its everyday use is the estimation of the correct crop coefficient (Kc). Although this can be done using crop growth stages, the farmer is left with a lot of guesswork. The objective of this work was to estimate the possibility of using canopy cover (Cc) measured by a standard camera to establish the Kc values of lettuce crop grown in a greenhouse. In a greenhouse in southern Europe, 0.3m diameter weighing mini-lysimeter were planted with lettuce. The Kc was calculated as a ratio of daily crop water use from the lysimeter and grass reference evapotranspiration (ETo). Canopy cover ratio (%Cc) was determined using the area of green in the lysimeter that was visible in the images. The results were plotted as a relation between Kc and Cc and an exponential approximation was made.The results were very positive showing a significant correlation between the calculated Kc and %Cc in the mini-lysimeters. The methodology proved to be highly effective in the early stages of the crop development, until the crop completely covers the soil. After that, the %Cc continued to increase until harvest, while the Kc indicator did not change.It can be concluded that this simple methodology can be used successfully to obtain good estimates of ETc, and thus optimize the water use in greenhouses.