Collector design for measuring high intensity time variant sprinkler application rates

Peak water application rate in relation to soil water infiltration rate and soil surface storage capacity is important in the design of center pivot sprinkler irrigation systems for efficient irrigation and soil erosion control. Measurement of application rates of center pivot irrigation systems has traditionally used tipping bucket rain gauges. Calculation of application rate from tipping bucket rain gauge measurements restricts computed application rate to a discrete multiple of the rain gauge resolution and time interval. This limits the resolution of application rate measurement, especially for time intervals less than 15 min. A collector was designed to measure time variant high-intensity sprinkler application rates under field conditions with greater resolution than a tipping bucket rain gauge. The collector funneled water into a 50-mm (2-in.) diameter tube providing a depth multiplication factor of 18.26:1. The depth of water in the tube was measured with a low pressure piezo-resistive pressure sensor connected to a differential amplifier circuit. Combination of the depth multiplication factor of the collector and differential amplifier circuit provided a collector resolution of 1.4 mm/mV (0.055 in./mV). A data logger was used to record water depth in the collector tube during an irrigation event. A digital differentiating filter was designed and used to reduce the effect of random electrical noise in the sensor output on calculated application rate. The collector was tested in the laboratory and under field conditions simulating center pivot sprinkler irrigation. For a range in application rates from 15 to 200 mm/h (0.7 to 8 in./h) and application depths from 20 to 35 mm (0.8 to 1.4 in.) in the laboratory, the maximum collector error was 2.1 mm/h (0.08 in./h). Collector-measured application rate patterns under field conditions were well-correlated to simulated application rate patterns using radial application rate profiles for the sprinklers tested. Collector-measured peak application rates were not significantly different from those predicted by the Kincaid (2005) model. The collector functioned as designed in field tests and provided an effective and efficient means of measuring high-intensity application rates from center pivot irrigation systems under field conditions.