Large weighing lysimeters for water use and deep percolation studies

Two weighing lysimeters were installed at the University of Arizona's Karsten Center for Turfgrass Research, The goals of the facility are to measure evapotranspiration (ET) and to perform water use and deep percolation experiments in a deep soil profile under both natural precipitation and irrigation conditions, Each lysimeter is 4.0 m deep, 2.5 m in diameter, and equipped with 96 sampling ports for soil solution samplers, tensiometers, time domain reflectometry (TDR) probes, and thermocouples, The weighing scales have a capacity of 45 Mg and can detect a 200-g mass change, equivalent to +/- 0.04 mm of water on the surface. The lysimeters were filled with Vinton fine sand (sandy, mixed thermic Typic Torrifluvent). Monitoring the unvegetated lysimeters for 18 weeks indicated that about 66% of all precipitation percolated into the soil profile, In June 1994, the lysimeter surfaces and surrounding area were planted with `Tifway' bermudagrass (Cynodon dactylon x transvaalensis var. Tifway), and irrigation began. Three phases of turf water use were identified during the first 155 days after planting-establishment, full cover and overseeding. The percentage of water that percolated into the soil profile deeper than the root zone was 35.6%, 10.1%, and 48.3%, respectively, showing the effectiveness of active root water uptake in reducing deep percolation, The TDR, neutron probe and tensiometer methods provided very similar indications of the depth of the wetting front. Differences in depth estimates could be caused by differences in sampling volume among the three methods, The TDR and neutron probe provided estimates of soil water storage that usually were within 10% of one another, and consistent with changes in lysimeter mass.