Assessing effective pasture root depth for irrigation scheduling by water balance and soil moisture monitoring

The 'effective' root depth of perennial ryegrass used for irrigation scheduling has substantial implications when estimating irrigation requirements. This study included field measurements of 20 percolation lysimeters with diameters and heights of 500 and 900 mm, respectively, installed on the Lincoln University Dairy Farm, Christchurch, New Zealand, to estimate actual evapotranspiration (ETa) based on water balance analysis for three soil depths: 500, 600 and 700 mm. Reference evapotranspiration (ETr) was estimated based on the CropWat 8 model. Perennial ryegrass height (h cm) was measured for each lysimeter. Among the three soil depths, 500 mm produced the highest regression coefficient for the crop coefficient (K-c = ETa/ETr) and h relationship. The results indicate the need to consider a 500 mm root depth to estimate irrigation requirements on this farm to achieve optimal water productivity. A noticeable fluctuation in daily soil water content for the top 500 mm soil profile, measured in time domain reflectometry probes and with an Aquaflex soil moisture sensor, also reinforces the earlier statement.