Comparing Reflected PAR and NIR in Pistachio Orchards with Different Canopy Structure and Row Conditions Using Commercial and Custom-Built PAR Sensors and Pyranometers and the ACASA Model

Two pruning methods and two row cover treatments were implemented in two pistachio orchards in Southern California throughout the growing season of 2022. The goal of this research was to monitor shortwave solar radiation intercepted by the different canopy structures, in the context of differing interrow treatments, and compare them to the modeled radiation module output from the Advanced Canopy Atmosphere Soil Algorithm (ACASA). Some pistachio blocks had their canopies pruned to have flat tops while the other canopies were pruned to have pyramid tops, and some interrows had grass sown as the cover crop while other interrows were fallow and weed-free. One hypothesis was that the radiation reflected from the interrow treatments could differ, causing altered overall canopy intercepted shortwave radiation, and potentially resulting in enhanced fPAR. The cover crop rows were mowed at the end of spring, leaving behind dried, very reflective grass residue. Incoming shortwave radiation and photosynthetically active radiation (PAR) were measured at the top of each of the orchards, and shortwave radiation and PAR that were reflected from the row floor were also measured in each of the treatments. The PAR and near-infrared radiation (NIR) that were reflected to the underside of the canopies were compared across treatments to determine if certain treatments could potentially result in more PAR (increased fPAR) or NIR being available to the canopies. Preliminary measurement results indicate that there was nearly double the amount of reflected NIR in cover crop blocks compared to blocks without cover crops, regardless of canopy pruning treatments, while reflected PAR was only marginally greater in cover crop blocks compared to those without cover crops. Field measurements were consistent with the ACASA model’s radiation output. This project also involved designing, constructing, and deploying custom-made, inexpensive pyranometers and PAR sensors, which were calibrated with equivalent Campbell Scientific, Inc. sensors, to allow better sampling of the heterogenous canopy radiation environment in an increased number of sites.