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Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 12/3/2009 Publication Date: 3/19/2010 Citation: King, B.A., Bjorneberg, D.L. 2010. Characterizing droplet kinetic energy applied by moving spray-plate center pivot irrigation sprinklers. Transactions of the ASABE. 53(1):137-145. Interpretive Summary: The kinetic energy of discrete drops impacting a bare soil surface is generally observed to lead to a drastic reduction in water infiltration rate due to soil surface seal formation. Under center pivot sprinkler irrigation, kinetic energy transferred to the soil prior to crop canopy development can have a substantial effect on seasonal runoff and soil erosion. In the design of center pivot irrigation systems, sprinklers with minimum applied kinetic energy could potentially minimize seasonal runoff and erosion hazard. Droplet kinetic energy from irrigation sprinklers has traditionally been characterized using area weighted kinetic energy per unit drop volume. This method of characterization heavily weights the largest drops which travel the farthest from the sprinkler and have the largest kinetic energy. The results of this study show that this characterization is not correlated to actual kinetic energy transferred to the soil by the center pivot sprinklers. Characterization of sprinklers according to specific power which is the rate at which droplet kinetic energy is applied to the soil may be a better parameter to characterize center pivot irrigation sprinklers in regards to runoff and soil erosion potential. Specific power could potentially be used to identify sprinklers which will minimize runoff and soil erosion in the design of center pivot sprinkler irrigation systems. Specific power could also be used to design sprinklers which minimize kinetic energy applied to the soil under center pivot sprinkler irrigation. Technical Abstract: The kinetic energy of discrete drops impacting a bare soil surface is generally observed to lead to a drastic reduction in water infiltration rate due to soil surface seal formation. Under center pivot sprinkler irrigation, kinetic energy transferred to the soil prior to crop canopy development can have a substantial effect on seasonal runoff and soil erosion. In the design of center pivot irrigation systems, selection of sprinklers with minimum applied kinetic energy could potentially minimize seasonal runoff and erosion hazard. Size and velocity of drops from five common center pivot sprinklers with flow rates of approximately 43 L/min were measured using a laser in the laboratory. The data were used to evaluate various approaches to characterize kinetic energy transferred to the soil by each of the five sprinklers on a center pivot irrigation system lateral with 2.5 m spacing between sprinklers. Specific power, which represents the rate kinetic energy per unit area is transferred to the soil as a function of distance from a sprinkler and analogous to a sprinkler radial water application rate distribution, was used to estimate actual kinetic energy transferred to the soil by overlapping specific power profiles of sprinklers equally spaced along a center pivot lateral. Kinetic energy of irrigation sprinklers has traditionally been characterized using area weighted kinetic energy per unit drop volume. This method of characterization heavily weights the largest drops which travel the farthest from the sprinkler and have the largest kinetic energy. This characterization was not correlated to actual kinetic energy transferred to the soil by the sprinklers. Sprinkler kinetic energy per unit volume of sprinkler discharge was also used to characterize sprinkler kinetic energy calculated but was not correlated to actual kinetic energy transferred to the soil by the sprinklers. However, kinetic energy per unit volume of sprinkler discharge was found to be more representative than kinetic energy per unit drop volume. Measured runoff and sediment yield of the sprinklers from a previous study were compared to time averaged specific power. Runoff and erosion appeared to be more dependent upon sprinkler type than time averaged specific power. The sprinklers with the lowest runoff and sediment yield had the lowest time averaged specific power. However, there was a substantial increase in runoff and sediment yield with little associated increase in time averaged specific power applied for some sprinklers. Visually, the functional difference between sprinklers was the manner in which water drops were distributed over the wetted area with respect to time. Sprinklers that visually appeared to distribute water drops more evenly over the wetted area with respect to time had the highest runoff and sediment yield, and sprinklers that had well defined rotating streams of water drops had the lowest runoff and sediment yield, largely independent of time-averaged specific power applied to the soil. |
