|Fares, A - UNIVERSITY OF FLORIDA|
Submitted to: Dahlia Greidinger International Symposium Nutrient Management under Salinit
Publication Type: Proceedings
Publication Acceptance Date: December 1, 1998
Publication Date: May 1, 1999
Citation: ALVA, A.K., FARES, A. PRECISION SCHEDULING OF IRRIGATION IN SANDY SOILS USING CAPACITANCE PROBES. DAHLIA GREIDINGER INTERNATIONAL SYMPOSIUM NUTRIENT MANAGEMENT UNDER SALINITY, p. 267-279. 1999. Interpretive Summary: Continuous measurement of soil water content is the key for improved scheduling of irrigation for maximizing water uptake efficiency with minimum leaching losses below the rootzone depth. Capacitance probes which work on the principle of dieletric constant were used for continuous monitoring of soil moisture content in sandy soils (>95% sand) in Florida citrus groves. The soil water content measured by the probes showed a goo agreement with that by the conventional gravimetric technique. The paper describes the procedure to install the probes, download the data, calculate soil water mass balance components, developing set points for scheduling irrigation. The response of these probes to changes in soil moisture content is quite rapid and reproducible. The capacitance probe data at various depths show clear indication of soil moisture extraction pattern by the trees, thus, provides guidelines on the depth of rooting under different production conditions. With the knowledge of the full point and refill points, irrigation scheduling can be done accurately to minimize any moisture stress within the rootzone, with minimum leaching below the rootzone.
Technical Abstract: Recent advances in microelectronics have made the technique of measuring the dielectric constant of the soil-water-air more efficient as an alternative means of measuring the in-situ soil water content. As the dielectric of water (80) is large compared to that of the soil matrix (<10) and air (1), any change in water content will strongly influence the dielectric value of the soil-water-air mixture that is measured by the capacitance probe. The Sentek EnviroSCAN probe, a new capacitance probe technique, has been tested and used to schedule irrigation for several annual and perennial crops. Our research on the use of this technique as an aid to improve irrigation efficiency of citrus, grown on deep sandy soils (>95% sand) with low water holding capacity (7.5% volumetric water content) has demonstrated remarkable improvements in irrigation scheduling to minimize the effects of soil moisture stress and also leaching of water below the root zone. A computer program has been developed to transfer the capacitance probe data into a spreadsheet and calculate total water content for a given depth profile for any desired time interval. A soil water mass balance program was developed that uses the soil water content, determined by the capacitance probe, and irrigation and rainfall, to calculate evapotranspiration and redistribution of water below the root zone. Using the capacitance probe data, it was possible to calculate irrigation efficiency, and canopy and effective rainfall. This device allowed us to determine the effects of several rainfall and irrigation events on water content within and below the root zone throughout the entire growing season.