AngrnAcr. A line-source sprinkler configuration provides a linearly decreasing irrigation application rate perpendicular to the sprinkler line and has been utilized to study crop response to variable irrigation amounts. The effect on measured irrigation application depths from using various types of catch-cans in those studies is not known. Derived relationships between crop yield and applied water is dependent on the accuracy of measured catch-can water volumes. The purpose of this study was to evaluate catch-can characteristic effects on measurement of sprinkler irrigation depths in a line source. This was accomplished by evaluating six types of catch-cans: (1) 83 mm diameter polypropylene separatory funnel (with evaporation-suppressing oil), (2) 82 mm diameter PVC reducer can (with evaporation-suppressing oil), (3) 151 mm diameter metal can, (4)64 x 59 mm wedge rain gauge, (5) 146 mm white plastic bucket, and (6) 100 mm diameter clear plastic funnel rain gauge. The cans were placed at five application rate conditions (2.8, 5.5, 8.7, 12.6, and 14.8 mm/h). Cumulative catch depths differed among the catch-can types. However, only the metal can and white bucket cumulative application depths at the lowest application rate were statistically different from those of the control (separatory funnel). Catch-cans with a larger diameter opening exhibited less variation in catch depths. Measured evaporation of standing water from catch-cans varied from 0.04 mm/h (funnel rain gauge) to 1.81 mm/h (separatory funnel without evaporation-suppressing oil). Water applied to a bucket's sidewall evaporated at a higher rate than standing water. Inaccuracy of application depth measurement may occur at low application rates even when catch-cans meet the ASAE Standard. The relatively good performance of the funnel rain gauge and catch-cans with evaporation-suppressing oil (and subsequently less depth than the ASAE Standard requires) suggests that it may be appropriate to re-evaluate the standard to consider such devices.
Kinetic energy of water droplets has a substantial effect on development of a soil surface seal and infiltration rate of bare soil. Methods for measuring sprinkler droplet size and velocity needed to calculate droplet kinetic energy have been developed and tested over the past 50 years, each with advantages, disadvantages, and limitations. A laser precipitation meter and photographic method were used to measure droplet size and velocity from an impact sprinkler at three pressures and one nozzle size. Significant differences in cumulative volume drop size distributions derived from the two measurement methods were found, especially at the highest operating pressure. Significant differences in droplet velocities were found between measurement methods as well. Significant differences were attributed to differences in minimum drop sizes measured; 0.5mm for the photographic method versus 0.2 mm for the laser precipitation meter. The laser precipitation meter provided smaller cumulative volume drop size distributions compared to the photographic measurement method. The laser precipitation meter tended to provide greater drop velocities which were attributed to altitude differences at experimental sites. The difference in calculated droplet kinetic energy per unit volume based on drop and size velocity data from the laser precipitation meter and the photographic method ranged from +12.5 to-28%. The laser precipitation meter generally provided a lower estimate of sprinkler kinetic energy due to the measurement of a greater proportion of smaller drop sizes. Either method can be used to obtain drop size and velocity sprinkler drops needed to calculate sprinkler kinetic energy. The laser precipitation meter requires less skill and labor to measure drop size and velocity.
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.
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