An evaluation of the practical use of soil moisture sensors for irrigation scheduling : A thesis submitted in fulfilment of the requirements for the degree of Master of Applied Science in Soil Science

Abstract

Five soil moisture sensors have been evaluated for their use as practical irrigation scheduling tools. The sensors are: the neutron probe; the Time Domain Reflectometry (IDR) probe; tensiometers; 'Watermark' electrical resistance sensors; and gypsum blocks. The sensors were evaluated both by literature review and in a ten-month field trial. All the sensor types were installed in the trial plot in depthwise arrays, with threefold replication of each array, the plot was subjected to a single wetting-drying cycle by use of a mobile rainshelter and irrigation. Measurements of the electrical resistance of the Watermark sensors and gypsum blocks and of soil temperatures, were automatically recorded using a datalogger with a multiplexer. The resistance sensors were also read manually using hand-held meters supplied by the sensor manufacturers. Resistances were corrected for temperature, and in the case of the Watermarks converted to soil water suctions using the calibration built into the manufacturer's hand-held meter. After the sensor measurements were completed the site was excavated to: (i) assess the effect of soil spatial variability on sensor measurements (using bulk density and soil moisture content as indicators of variability; and (ii) assess the sensor-to-soil contact.