Measuring the Momentum of Throughfall Drops and Raindrops
The methods previously used to determine the momentum and kinetic energy of throughfall drops in the field do not account for the drop's shape at impact or for the variations of the drop's velocity caused by chaotic air currents. The drop's shape at impact is critical because it influences the drop's measurable momentum, kinetic energy, impact force, and the amount of soil that can be displaced by the falling water drop. Since the momentum and kinetic energy of raindrops and throughfall drops are used as indices of soil particle displacement the most accurate momentum and kinetic energydata would be required to produce the most accurate estimate of soil particle displacement. The purpose of this project was to develop and utilize a simple digital electronic instrument which could be used in the field to directly measure the momentum of throughfall drops and raindrops. The instrument consisted of a voltage amplifier, voltage comparator, reference voltage, digital counting circuit and a digital display. The instrument was activated by an impact sensor constructed from an 8.0 ohm, 5.0 inch diameter audio speaker. A falling water drop's momentum was quantified as the amount of time (milliseconds) that the speaker's amplified voltage exceeded the reference voltage. The instrument displayed the drop's momentum as a function of time which was recorded and later converted into units of momentum (g*m/s) by an empirically determined calibration curve. The instrument was utilized in Benton County, Tennessee by measuring raindrop momenta during a single storm event and the throughfall drops produced by commercially planted loblolly pine trees and indigenous deciduous trees. The substitution of the loblolly pine for the deciduous trees represents a human induced change of the vegetation which could have an impact on the drainage basin's erosional characteristics. The purpose of the investigation was to determine if there were any differences between the sample means. The sample having the highest mean momentum would have the greatest potential to dislocate soil particles and cause subsequent soil erosion. The analysis of the sample data revealed that both of the throughfall drop momentum sample means (evergreen: 0.602 g*m/s, deciduous: 0.355 g*m/s) were greater than the raindrop momentum sample mean, 0.02 8 g*m/s. The differences between the sample means suggested that the throughfall drops have a greater potential to displace soil particles and cause subsequent soil erosion than the raindrops that produced the throughfall drops. Also, the evergreen throughfall drop momentum sample had the highest mean of the three samples, indicating that the evergreen throughfall drops had the greatest potential to displace soil particles. The nonparametric Wilcoxon rank sum test was used to determine if the samples' respective population means were equal. The results of the Wilcoxon test indicated that both of the throughfall drop momentum population means were greater than the raindrop momentum population mean, thus reaffirming the results of the sample data analysis. However, the test indicated that the throughfall drop momentum population means were equal, suggesting that the evergreen and deciduous throughfall drops have equal potential to dislocate soil particles and cause subsequent soil erosion.