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In solid set sprinkler irrigation systems, many factors have influence on water partitioning efficiency and uniformity. Drop size formation in the irrigation process is affected by some of these factors. Soil crustiness and damage to bare soil structure are problems associated with large drops. This study shows the results of measured drop size distributions in medium-sized sprinklers. An optical spectropluviometer (OSP) with infrared beam was used to measure drop size distributions. A variety of factors were checked: sprinkler type, nozzles, jet straightening vane and working pressure. The results showed that working pressure has the most influence on drop sizes; under low-pressure conditions, drops with a diameter of about 9 mm can be formed. The influence of the jet straightening vane and the types of nozzles and sprinklers has also been studied. A good similitude is obtained between the medium drop diameters measured and those simulated with the SIRIAS ballistic simulation model.