SPATIAL VARIABILITY OF SOIL WATER UNDER CITRUS TREE CANOPIES IN CENTRAL FLORIDA
An essential component of the Florida Ridge citrus best management practices (BMPs) is proper management of irrigation to achieve the desired crop response and conserve water. One popular method for scheduling irrigation uses volumetric soil water monitoring sensors installed in the root zone under tree canopies. There are several dielectric devices for measuring soil moisture on the market, but many have drawbacks including disrupting the soil during installation and small sensing volumes. We hypothesized that due to the spatial limitations of existing moisture sensors and soil heterogeneity, a representative measure of soil water content under a citrus tree could not be obtained with dielectric sensors. Consequently, incorrect irrigation could cause agrochemical leaching, crop stress and ultimately yield losses, or contamination of groundwater. Intensive soil moisture readings were taken in a Candler fine sand soil under citrus tree canopies with micro-jet irrigation. Measurements were taken with a 20-cm deep Time Domain Reflectometry (TDR) moisture probe at 10-cm spacing in a 150 × 150-cm grid sampling area on the same side of the tree as the irrigation nozzle. Using the 225 moisture readings per tree, geostatistics were utilized to map the soil moisture variability under the citrus tree canopies after several types of soil moisture changing events. These included several days of drying, irrigation applications of one hour and three hours, and after rainfall. The data from root zones under the tree canopies was compared with irrigated bare soil in the same grove. Semivariogram analysis showed that the mean horizontal range of spatial dependence for soil water was 40 cm, and coefficients of variation were high (19 to 56%). Consequently, a single point measurement of soil water in the root zone under a citrus tree canopy was deemed highly unreliable and not suitable for accurate irrigation scheduling. Further research is needed to test alternative methods of measuring water status using tree canopy stress or other soil measurements.
Waldo, L.J. and Schumann, A.W. (2009). SPATIAL VARIABILITY OF SOIL WATER UNDER CITRUS TREE CANOPIES IN CENTRAL FLORIDA. Acta Hortic. 824, 147-154
irrigation, Kriging, geostatistics, time domain reflectometry