Measurement on physical parameters of raindrop energy
© Guo et al.; licensee Springer 2013
Published: 11 December 2013
Rainfall erosivity factor (R) is one of the most commonly used factors in soil erosion models. While rainfall energy (E) is the most elementary physical parameter to predict R. Based on comparative analysis of previous soil erosion models and rainfall erosivity factor measuring methods, integrated application of modern photogrammetric techniques, image analytic methods and automatic control theories, this paper provided a new method based on image analytic to calculate the rainfall energy and R factor, which obtains raindrop's volume and velocity by means of modern photogrammetric technique. Results show that this method can improve both efficiency and accuracy of rainfall energy calculation and other rainfall physical parameters measurement.
Soil erosion models research is a front field of soil erosion subject, frequently, rainfall factors are indispensable element at any model, R is one of the most frequently used factor. After comparing with various models, a conclusion was found that rainfall energy, rainfall intensity (I) and rainfall precipitation (P) are the most widely used rainfall parameters. Among these parameters, I and P can be acquired through observational datas from weather stations, while there are not yet an observing method to obtain rainfall energy, which is usually calculated through statistic models. Fortunately, with the development of modern high technology, especially the development of modern photogrammetric technique, image analytic method and automatic control theory, physical parameters of rainfall could be measured directly, and it is of great valuable to calculate the rainfall based on these physical parameters, which make it possible to obtain rainfall energy by measuring instead of statistic models. No doubt, it would be of great significance to soil erosion subject's experimental techniques innovation.
Presentation of question
Calculation of rainfall energy
Where E is the total energy of a rainfall (MJ/hm2), and I30 the maximum 30-min intensity (mm/h).
As the USLE has widely been used for predicting soil loss in countries all over the world, the formula (1) is generally recognized as one of the most classic calculation methods to predict R .
Where e is the energy of one unit rainfall (J), i is rainfall intensity of differential time (mm/min), t is differential time and T is the total time of one rain.
Where e is the energy of per unit rainfall (J/m2 mm) and I the intensity of per unit rainfall (mm/h).
Through analyzing nature raindrops' characters, the calculation formula of Northwest, Northeast and Southern of China were established respectively by Zhongshan Jiang , Suyuan Liu , Fujian Zhou  to calculate the rainfall energy, which is express as, e=a+blogi or e=aib, where e is the energy of per unit rainfall(J/m2 mm), i is the rainfall intensity, a and b is calculating coefficients.
As a result, how to determine the rainfall energy (e) and intensity (I) of one unit period is the key to measure and calculate the energy. Between these two parameters, I is a common rainfall character value which can be obtained by measuring rainfall precipitation and rainfall duration, while the measurement and calculation of energy is relatively difficult.
The traditional ways to measure raindrop's quantity and down velocity
One way to measure rain drop's quantity is called Stain method (or filter paper method) . The precise process are as follows: firstly, a piece of filter was spread with water-solubled dyestuff, which doesn't show color when dry but a permanent rough circular stain will emerge when it was wetted by raindrops. Then, every stain's diameter was measured, raindrop's diameter can be determined according to a relationship between diameter of drops' and stain's. At last, their quantity can be calculated according to their volume if they are regard as sphere. The velocity of raindrops is distinguished by two situations , when a drop's diameter < 1.9 mm, velocity is calculated according to amendatory Sha Yuqing formula, when a drop's diameter ≥ 1.9 mm, calculated by amendatory Newton formula. An individual drop's energy can be computed based on moving object's energy formula when a drop's quantity and velocity are known, the unit area's rainfall energy of an individual rainfall is the sum of every single raindrop's energy.
From these above current classic rainfall energy calculation, it can be concluded that the R factor of an individual rain is determined by energy and intensity, intensity can be computed through measured data by self-restrainting hyetometers, while energy is usually calculated by regression models, which were obtained through typical measurements among rainfall energy, rainfall intensity and rainfall quantity.
Obviously, obtain rainfall physical parameters through stain method, calculate rainfall energy through a regression mode is unscientific when adopted in large region, because this method tends to neglect spatial and temporal variation of rain, will unavoidably reduce the measurement accuracy. Moreover, stain method's accuracy and efficiency are limited because it depends a lot on human measurement. As a result, there is still an urgent demand to develope an instrument which can measure rainfall physical parameters timely, thus creating a method to calculate rainfall energy rapidly in soil erosion subject.
The principle of rainfall energy calculation formula derivation
Theoretical basis for rainfall energy calculation
Where E is individual rain drop's energy (J), m is its quality (kg) and v the velocity (m/s).
Derivation for rainfall energy calculation formula
Where e1 is an individual rain drop's energy (J/ an individual rain drop), m the quantity, d the density (kg/m3), b the bulk (m3) and v the final velocity of a drop (m/s).
Where e2 is the total energy of view field of measurement (J/m2), b is the volume of the drops (m3), v the final velocity (m/s) and n the quantities of raindrops.
Where e3 is the total energy of a individual view field of measurement (J/m2), e21, e22, e23, ..., e2n is the rainfall energy (J/mm) from the first drop to the n-th drop, t1, t2, t3,..., tn (s) is the interval time of adjacent two rainfall stages.
Where e4 is the energy of a unit area's rainfall (J/m2), K is the convert coefficient, p are the Places of points, and e3 (J/m2) is same as above.
Above are about formulas to calculate the energy of a unit area, the results show that, in the given boundary condition, a unit area's energy can be calculated through science calculation as long as raindrops' bulk (b) and final velocity (v) are known. As a result, how to measure these two physical parameters is the key to calculate rainfall energy.
Measurement of raindrops' physical parameters
Choice Digital Single Lens Reflex (DSLR) with high definition and high shutter speed
Obtain raindrops images as they fall is the key to measure their bulk (b) and final velocity. Through market research and screening, we know that the exposure velocity of DSLR has reached to 1/10000s~1/300s, it is satisfied to take raindrops' images. Experiments have proved that if an object's diameter is 3 mm with speed of 2 m/s, use DSLR with 1/10000s' exposure velocity to take its static images is appropriate, the error of rain's volume can be controlled under ±7%, when use the DSLR with 1/300s exposure velocity to take rain's tailing images, raindrops' velocity error can be controlled under ±17%. Thus, use DSLR to take raindrops' images as they fall, combine with computer images analysis properties and rainfall process automatic control, raindrops physical parameters can be obtained accurately.
Manufacture raindrops sampler
Raindrops sampler is used to collect drops, which are placed in rainfall field, 0.5~1.0m higher than ground when sampling so that raindrops can through sampler continuously and naturally under the precondition of nature rainfall process is not be disturbed. Structural diagram of raindrops sampler is as follows:
Software development of raindrop images analysis and calculation
Two kinds of images of view field of sampler in the same time can be obtained by different DSLR, one of them are taken by DSLR with 1/10000s exposure velocity, raindrops in these images can be considered as static, raindrops distribution and every drops' geometrical size can be obtained by analysing of these images. The other images are taken by DSLR with 1/300s exposure velocity, drops in these images have tail, final velocity of raindrops' can be calculated according to the length of tail and its development time.
Conclusions and discussions
It is very hard to develop a sampler which can represent raindrops real physical parameters because of the variability of natural rainfall "pattern". How to adjust sample channel width in different rain intensity so that there are no overlapping rainfall projection in the view field of sampler's profile will directly affect accuracy of raindrops physical parameters measurement and calculation. To resolve this problem, not only a lot of experiments are required to determine appropriate sample channel width in different rain styles, but also the adjustability is demanded in the progress of sampler integration.
The above descriptions are about measurement progression of a measuring point, However, a certain region's individual rainfall physical parameters and characters need multi-point repeat measurements to determine measure points and its distribution in the uneven rainfall fields.
To a region rain measurement, its rainfall physical parameters and characters can be measured through successive measurements through above method, however, this involve a tremendous amount of work. So, how to establish relationships between typical measurements and previous data such as rainfall intensity, precipitation and so on, then calculating rainfall physical parameters and characters according to these established relationships is a great issue need deepen study in future.
This article was funded by the Chinese Academy of Sciences (CAS) scientific research equipment projet: "Soil erosion process observing devices research (YZ201163). The publication costs for this article were funded by Scientific & Technical Development Inc.
This article has been published as part of SpringerPlus Volume 2 Supplement 1, 2013: Proceedings of the 2010 International Conference on Combating Land Degradation in Agricultural Areas (ICCLD'10). The full contents of the supplement are available online at http://www.springerplus.com/supplements/2/S1.
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