Use of pyranometers for continuous estimation of ground cover fraction in orchards

Abstract

Ground cover fraction (GCF) is defined as the fraction of ground beneath the canopy covered or shaded by a crop near solar noon as observed from directly overhead. GCF is a useful variable that can be determined in a variety of experimental procedures performed at a field plot scale. GCF is usually measured in experimental field plots using ceptometers or digital imagery. The use of these techniques in the field requires the presence in situ of qualified workers and do not permit the continuous recording of GCF. Thus, only a small number of measured values of GCF are available along the season. A network of pyranometers located at the ground level and above canopy can be connected to a datalogger so a continuous series of global radiation values can be recorded for long periods of time without the presence of any staff. Continuous values of daily GCF can be worked out from those readings. This approach could be particularly useful at remote, unattended sites. Nevertheless, the feasibility of such measures must be evaluated as the main constraint is that the pyranometers must be placed nearby the plant rows to avoid possible damage by the machinery used in the farm. This work presents the daily GCF estimates from pyranometer readings (‘pyranometer‐driven’ method, GCFpyr) at two experiments: a) Experiment I, at a table grape grown under a net, from February 2007 to November 2009; b) Experiment II, at a late peach orchard, from May to September 2011. In the Experiment II measurements were taken for one full irrigated, ‘control’ tree and for one ‘deficit irrigation’ tree. The daily GCFpyr values were compared to measured values (‘reference’ method, GCFref) using either photographical techniques (table grape) or ceptometers (late peach). For computation of GCFpyr, solar radiation below and above the canopy was averaged for two time periods: a) two hours around solar noon; b) daytime period (8:00 to 18:00 Universal Time Coordinated, UTC).