Submitted to: Irrigation Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/1/2000
Publication Date: 9/1/2000
Citation: Fares, A., Alva, A.K. 2000. Evaluation of capacitance probes for optimal irrigation of citrus through soil moisture monitoring in a entisol profile. Irrigation Science. Interpretive Summary: This paper describes the importance of continuous monitoring of soil moisture to improve irrigation scheduling, and a new instrument that enables carrying out the above objective in a convenient user-friendly manner. Enviroscan RT5 is a capacitance probe that adopts the principles of dielectric constant which varies widely between the dry soil, air and water. The soil moisture content is a function of different proportions o the soil, air and water. Thus, the response based on dielectric constant represents the changes in the soil moisture content. The soil moisture content measured by using the capacitance probes calibrated very well with that measured by the gravimetric soil moisture content. The latter required destructive soil sampling which is tedious and altered the soil condition considerably. Installation of the capacitance probe both within and below the rooting depth provides the soil moisture data for soil water mass balance calculations which provides an estimate of crop evapotranspiration and water leaching below the root zone. The former is an important data for determining the crop water need.
Technical Abstract: Continuous monitoring of soil moisture content within and below the rooting zone can facilitate optimal irrigation scheduling aimed at minimizing both the effects of water stress on the plants, and also the leaching of water below the root zone, which can have adverse environmental effects. The use of Sentek capacitance probes (EnviroSCAN RT5) in scheduling citrus irrigation was evaluated using 3-year-old Hamlin orange trees [Citrus sinensis (L.) Osb.] on Swingle citrumelo rootstock [Citrus paradisi Macf. X Poncirus trifoliata (L.) Raf.] grown in a Candler fine sand (hyper thermic, uncoated, Typic Quartzipsamments). Available soil moisture calculated according to capacitance probe readings of soil moisture agreed well with that calculated using soil water release curves determined in the laboratory. A utility program was developed to process the data collected by the capacitance probe into a spreadsheet format. Processed data were used to calculate soil water storage within and below the citrus root zone at desired time intervals. Irrigation set points (i.e., full point equivalent to maximum desirable water storage and refill points I and II) were defined based on field capacity determined both in the field and in the laboratory and permanent wilting point. It was possible to maintain the water content in the root zone between the full and refill points I and II during most of the growing season. Although soil water content in the root zone exceeded the full point during periods of high irrigation, it drained rapidly within 24 - 48 h after the end of such irrigation events Using soil moisture depletion in the root zone during periods of low water application to estimate citrus evapotranspiration (ET), the calculated daily average ET during 10-day period in November was 1.33 mm day-1