Handbook for Irrigation of Wine Grapes in South Africa

8.4 DIRECT SOIL AND PLANT BASED MEASUREMENTS Water status of the soil and/or plants can be measured directly for irrigation scheduling purposes. SOIL BASED MEASUREMENTS Soil water content or soil water matric potential is usually measured to determine the soil water status. Soil water status should be monitored frequently in order to adjust the proposed irrigation volumes, if necessary. Irrigation volumes should be increased if the soil is not wetted over the root depth, or decreased if water percolates beyond the root depth. The principles, as well as possible advantages and disadvantages of the different methods are discussed below. 8.4.1.1 WETTING FRONT DETECTORS A wetting front detector (WFD) is probably the most simple, but effective, instrument to monitor whether irrigation water percolates, or not, below the root zone following irrigations (Stirzaker et al. , 2005). A funnel, which is installed just below the lower end of the root zone, catches and concentrates the water into a small reservoir through a filter at the base of the funnel (Fig. 8.17). The Styrofoam ® rod begins to float on the water in the reservoir, and consequently rises in the PVC tube, which indicates that water has passed below the root zone. Water in the reservoir can be extracted by means of a syringe via a thin plastic tube. Unfortunately, a WFD does not indicate the wetting depth if no over-irrigation occurs in the root zone. If the WFD indicates over-irrigation, volumes of subsequent irrigations must be gradually reduced until no over-irrigation is detected. Alternatively, a second WFD can be installed just above the lower end of the root zone. Irrigation volumes can then be adjusted so that only the higher WFD responds to the wetting front following irrigation. Wetting front detectors are also useful to determine the element concentration in the soil solution extracted by means of the syringe. This will enable the calculation of nutrients lost from the root zone when a vineyard is over-irrigated. Wetting front detectors are also useful to determine how efficiently salts were leached from the root zone where a leaching fraction was added to the irrigation requirement for saline or sodic soils. 8.4.1

250 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING