Using infrared thermometry to improve irrigation scheduling on variable soils

An experiment was conducted to assess a modified Priestley and Taylor calculation (developed in a companion paper) that utilises surface temperature (T-s) to estimate evapotranspiration (ETMPT) and to assess the benefits of variable rate irrigation (VRI) based on ETMPT measurements. A ryegrass seed field with Low, Mid and High available water capacity soil zones was used. Four replicates of two irrigation treatments were established in each zone; uniform rate irrigation (URI) where the same amount of irrigation was applied to all three zones based on neutrone probe measurements; and VRI where ETMPT was measured and different amounts of irrigation applied to each zone to replace this. T-S was measured continiously in each plot using stationary infrared radiometers, normalised difference vegetation index measured twice weekly with a hand-held instrument and meteorological variables were recorded in the field. Neutron probe access tubes were installed to 1.6m depth in all plots and monitored weekly to provide alternative evapotranspiration estimates (ETNP). The ETMPT decreased from 333mm in the high zone to 282 mm in the low zone. This in combination with faster canopy senescence and lower seed yields showed the low zone encountered water stress even though irrigated weekly. There was good agreement between ETMPT and ETNP in the VRI treatments and the High zone of the URI treatment. The higher values of ETNP in the Low and Med zones of the URI treatment were attributed to drainage. The URI treatment had 254.2 mm of irrigation applied to all zones during the season whereas VRI treatment had 183.4, 210.2 and 220.2 mm applied to the Low, Mid and High Zones, respectively. There, were no differences in ryegrass seed yield between the two irrigaiton treatments and less drainage from the VRI treatments. From this we conclude that VRI based on ETMPT is effective and field application of ETMPT is worth further development.