Alternative Methods for Determining Crop Water Status for Irrigation of Citrus Groves

Due to the increasing demand for water in Florida, the supply of water available for irrigation is decreasing and irrigation costs are rising. Based on previous findings, soil sensors have been found to be inaccurate in small numbers and unable to accurately measure the soil moisture status under trees for scheduling citrus irrigation. As a result, irrigation managers must come up with a method of scheduling irrigation that is based on plant-specific needs in order to apply only the necessary amount of water for the tree to remain productive while avoiding excess. Several methods for measuring tree canopy stress as an indicator of plant water status were tested both in a controlled greenhouse environment and in the field. These methods include infrared radiometers for thermal infrared canopy temperature measurements, multispectral cameras for water stress index using reflectance, and the GreenSeeker (R) sensor for measurements of canopy normalized difference vegetation index (NDVI) also from reflectance measurements as an indicator of plant water status. All of these measurements were plotted against stem water potential (SWP) as a reference of plant water stress. Results showed that the thermal infrared radiometer is capable of predicting water stress using a crop water stress index (CWSI) derived from canopy minus air temperature and vapor pressure deficit in a greenhouse setting, while field measurements were found to be less accurate due to instrument sensitivities to wind and low solar radiation levels during cloudy periods. The multispectral camera was able to accurately predict plant water status (R-2 = 0.90***) using a ratio of reflectance at the 840-nm and 670-nm wavelengths. Similar results were found using the commercially available GreenSeeker (R) for NDVI; however, the regression analysis showed that while significant, it was less accurate than the multispectral camera (R-2 = 0.31***).