Unsaturated Infiltration Experiment at the Maricopa Environmental Monitoring Site

Unsaturated infiltration experiments were conducted at the Maricopa Environmental Monitoring Site. Water was uniformly applied to a 50 x 50 meter area using a drip irrigation system at a controlled rate of 2.66 cm/day creating unsaturated flow conditions. The purpose of these experiments is to collect high-resolution field data using reliable monitoring techniques, which can be used for conceptual model testing. Water movement was monitored at nine locations with neutron probe access tubes, deep tensiometers and suction lysimeters down to a depth of 14 meters. Wetting front arrival times at a given location and depth were almost identical whether recorded with tensiometers or the neutron probe. However, there was significant variation between wetting front arrival times at the nine monitored locations across the 50x 50 m infiltration area: from 5 to 13 days at 5 m depth and from 16 to 31.5 days at 12 m depth. The coefficient of variation of wetting front arrival times decreased with soil depth, most likely due to layering. In-situ field neutron probe calibration yielded different calibration parameters for each soil layer. This multi-layer calibration reduced the error in water balance from 50 % (using a single calibration curve) down to 15 % at all monitored locations. It also caused water content in coarse sand layers to drop by almost 8 % relative to that obtained with a single calibration curve. It was found that collecting data more frequently, using fewer but more reliable field monitoring devices, yield more continuous records and more accurate as well as meaningful hydraulic parameter estimates.