Automated irrigation of apple trees based on measurements of light interception by the canopy

In most horticultural crops, irrigation requirements can vary depending on the interaction between meteorological conditions and size and structure of the canopy. Lysimeter studies have shown a strong relationship between transpiration and the amount of radiation intercepted by the canopy (RI), suggesting the opportunity to design irrigation strategies based on the measurement of RI. Here we present a system for continuous measurement of radiation interception by tree canopies and its direct use for automated irrigation. The measuring setup consisted of a pyranometer and a sensing bar containing photodiodes installed above the canopy and an arrangement of equivalent bars deployed below the canopy. The irrigation principle consisted of applying an amount of water each day proportional to the amount of solar radiation intercepted by the canopy during the previous day. In order to deal with rain events and conditions of low transpiration activity, the method was complemented by soil water measurements. Under unattended conditions, RI-based irrigation was able to keep tree water status in an acceptable range, supplying a total amount of water similar to that of a water balance approach with the advantage of autonomous operation over the season. However, RI-based irrigation slightly overestimated irrigation requirements in spring while it underestimated it in late summer. This bias was mainly due to a lag between the annual pattern of incoming radiation and that of reference evapotranspiration. This might be corrected by additionally considering air temperature and/or vapour pressure deficit in the estimation of the required irrigation volume.