Rainfall Energy Research Articles

The Effect of Urbanization on Kinetic Energy Distributions in Small Watersheds

Surface runoff and the suspended and dissolved solid loads carried from two similar watersheds are expressed in kinetic energies for a variety of rainfall events. One watershed is rural, the other urbanized, so comparison of their responses is used to determine the effect of urbanization on energy distributions. In the rural watershed, an average of only 0.053% of the gravitational potential energy of rainfall is converted to kinetic energy of runoff. The conversion in the urban watershed is 7.8 % on the average, meaning substantially more of the rainfall's energy is transformed into a form which can perform erosion within the watershed. Transport of material in suspended or dissolved form from the urban watershed is tremendously higher as a consequence. In addition, urbanization changes the erosion pattern from one where dissolved loads predominate over suspended loads to one where both loads are of equivalent magnitude. Kinetic energies are also considered as functions of the recurrence interval of the events, called energy spectra. In the rural watershed for the range of runoff events monitored, significantly more kinetic energy is present in the outflow during baseflow conditions than in that from surface runoff events. Baseflow kinetic energies are 100-1,000 times larger than their runoff counterparts. After urbanization, however, the energy spectra for runoff and suspended and dissolved loads are all dominated by surface runoff. Surface runoff has from 4-200 times more kinetic energy than baseflow. Within the range of event recurrence intervals monitored, 0.02-0.15 years, dominant energies in the urban watershed generally occur at a recurrence interval between 0.07 and 0.08 years.

Field studies of rainsplash erosion

AbstractStudies on sandy soils of the Cottenham Series in mid‐Bedfordshire confirm in the field the relationships between splash erosion, rainfall energy and ground slope obtained in the laboratory experiments of other workers. Only 0·06 per cent of the rainfall energy contributes to splash erosion and rates are low, attaining a maximum of 0·082 kg m−2 y−1 on a slope of 11°. The major role of splash action is in the detachment of soil particles prior to their removal by overland flow.

Erosion Processes on Steep Granitic Road Fills in Central Idaho

AbstractA set of thirty 20.2‐m2 (1/200‐acre) erosion plots was used to study erosion occurring on steep road fills constructed with granitic soil materials in the Idaho Batholith. Erosion data were collected for a 3½‐year period from 1969 through 1972. Erosion on bare control plots averaged 3.4 metric tons/km2 per day for water years 1970 through 1972. Erosion was reduced an average of 44% and 95% by tree planting and straw mulching, respectively. Daily erosion rates were consistently higher during summer periods than during snowfree winter periods, presumably because of greater rainfall energy during the summer. Dry creep accounted for at least 15% of the total annual erosion for the years sampled and was as high as 40% in 1971. The erodibility index was a poor predictor of erosion for rain periods. Observations in the area suggest that wind was an important erosion factor on the steep slope studied. The median particle size of eroded materials (D50) tended to decrease throughout the summer and fall until mid‐October when it abruptly increased. Soil crusting during the summer and soil freezing and thawing in the fall may have caused these seasonal trends.

Parameters for estimating annual runoff and soil loss from agricultural lands in Rhodesia

Cumulative values of rainfall energy, momentum, and precipitation depth were investigated as predictors of annual soil loss and runoff from selected arable and grazing land field trial plots. Energy parameters were the most accurate predictors of soil loss from bare soils, explaining 96.4% of the variation in results from a clay loam and 80.0% from a sandy soil. However, little accuracy was lost by adopting the more practical precipitation depth parameters. There was little to choose between momentum, energy, and precipitation depth parameters as predictors of soil loss from the vegetated plots or as predictors of runoff from both bare and vegetated conditions. Consequently, precipitation depth parameters were selected as being the most practical for soil loss and runoff estimation under the conditions investigated. Percentage vegetal cover was shown to be an additional important variable on grassland.

Soil‐Water Pressure During Intermittent Simulated Rain Application Measured with a New Rapid‐Response Tensiometric Technique

AbstractA method is described for measuring instantaneous changes in soil‐water pressure in soil beds during intermittent application of simulated rainfall. Differences in soil‐water pressure do occur in soil beds, depending on nozzle off time. Intermittent off periods between nozzle on times affected soil‐water pressure which influenced the accumulated rainfall energy required to initiate runoff.

The role of rainfall impact in soil detachment and transport

The relative importance of raindrop impact and flowing water to the erosion process was determined in a laboratory study using three soils, loam, silt loam, and sandy loam, prepared with preformed rills and subjected to simulated rainfall. The source and the mode of transport were determined for all soil detached and transported from the plot under conditions of both normal rainfall energy and greatly reduced rainfall energy. Decreasing rainfall impact energy by 89% without reducing rainfall intensity decreased soil losses by 90% or more, and it was thus indicated that the impact energy of raindrops is the major force initiating soil detachpent. For all three soils, 80–85% of the soil loss originating in the area between the rills was transported to a rill before leaving the plot, and it was thus indicated that the transport of detached particles from the plot was accomplished mainly by rill flow.

Examining the Process of Soil Detachment from Clods Exposed to Wind-Driven Simulated Rainfall

S detachment from clods exposed to wind-driven rain is much greater than detachment caused by similar rainfall intensities without wind (6)*. Seemingly, changes in drop size or in the profile drag exerted by the wind account for the difference, but, until recently, insufficient data were available to support these hypotheses. One recent study revealed that waterdrops have less vector velocity when falling through a wind tunnel than when falling in still air (2) . The present study explains further the process involved in soil detachment under wind-driven rainfall. Ellison (3, 4 ) , who was concerned with the total water-erosion process, reported that raindrop splash was the principal agent of soil detachment. Bennett et al (1) later modified this by saying raindrop splash was one of several important factors in the soil-erosion processes. He stated that drop impact mixed the soil with the surface water, thus contributing muddy water to the runoff. The effects of drop impact were observed in this study and compared with observations of Ellison and Bennett. Soil loss resulting from rainfall in still air is closely related to the kinetic energy of the rain (10). The product of rainfall energy and intensity commonly is used to relate simulated rainstorms to natural rainstorms. If wind accompanying a rainstorm were to increase soil loss only by increasing the rainfall energy, this same rainfall index could be used to relate simulated windrainstorms to natural wind-rainstorms, if rainfall energy were determined in wind. But if wind contributes to soil loss in ways other than changing the rainfall energy — and this study indicates that it does — then the wind forces on the soil also must be considered. This paper reports the results of the study in which wind shear stress and

Influence of Rainfall Energy on Soil Loss and Infiltration Rates: II. Effect of Clod Size Distribution

AbstractVarious clod size distributions were subjected to drop impact from a water drop applicator that delivered only 5‐mm drops. Clod beds were exposed to drops in 30‐ by 45‐cm pans. Point of “failure” of the clod bed was the accumulated energy when runoff began. Increasing the maximum clod size from 20 to 40 mm did not delay the time for runoff to begin on four silt loam to silty clay textured Iowa soil samples taken from continuous‐corn plots. Removing the < 2‐mm clods from the < 40‐mm sample had little effect on the time to runoff on continuous‐corn soils, but did increase the time to initial runoff on the silt loam soil from meadow. Removing the fraction < 8 mm from the < 40‐mm sample significantly increased the rainfall energy required to initiate runoff in all instances tested. Removing the fraction < 30 mm from the < 40‐mm sample nearly doubled the energy to initial runoff. However, clod beds of this 30‐ to 40‐mm size range from continuous corn did not withstand 0.18 joules cm‐2 of rainfall energy—the energy that occurs in western Iowa during the critical erosion period. Clod beds of the 30‐ to 40‐mm size range from continuous meadow, in contrast to clod beds from corn land, did withstand 0.18 joules cm‐2 of rainfall energy.

INFILTRATION RATE AS RELATED TO RAINFALL ENERGY

Infiltration rate with respect to rainfall energy was determined on a Sceptre clay, Haverhill loam and Hatton fine sandy loam. The infiltration rate of saturated Sceptre clay dropped from 24.0 to 0.6 cm per hour with an application of 2.0 cm of simulated rain. The infiltration rate of saturated Haverhill loam dropped from 4.0 to 0.6, and that of Hatton sandy loam from 14.5 to 9.5 cm per hour. Raindrop energy was from a 4.6-mm diameter drop tailing through a 6.55-m height.To maintain a high level of infiltration, surface soil must be protected from rainfall energy.

Influence of Rainfall Energy on Soil Loss and Infiltration Rates: I. Effect over a Range of Texture

AbstractSoil and water losses were determined from sieved air‐dry samples of five Iowa soils varying in texture using a laboratory rain simulator with a 3‐m. drop fall, a 4.85 to 5.00 mm. drop diameter, and an intensity ranging from 3.43 to 6.78 cm. per hour. The soil samples were contained in a pan 15.2 cm. deep, 30.5 cm. wide and 45.7 cm. long and tilted to a 9% slope. Runs were 90 minutes in length. The infiltration rate was the most important factor influencing total soil loss. Relative total soil losses for 90 minutes of run at high intensity were silty clay > silty clay loam > silt > loam > fine sand. At the low intensity, the positions of silty clay and silty clay loam and of silt and loam were reversed. Total soil loss varied with intensity, but infiltration was essentially constant over the entire range except on the fine sand. With equal water loss the order of erodibility was find sand > silty clay > silty clay loam > silt > loam. Total kinetic energy required to initiate runoff was constant over a range of time and intensity.

Effects of Rainfall Energy on the Permeability of Soils

AbstractThe effects of simulated rainfall on the air and water permeabilities of sand and several disturbed soil samples were investigated. Various rainfall intensities were provided by a rainfall simulator. Soil was packed in a cylindrical infiltrometer, to which was connected eight manometers. Permeability of the soil to water was measured during and after the application of rainfall.It was found that a rainfall of 30 minutes duration at an intensity of 2.8 inches per hour caused maximum change in water permeability of the soils. Water permeability of quartz sand was not changed by rainfall. Water permeability of soils was changed; (a) only in top 1.5 cm. by 1.6 inches per hour rainfall, (b) greatly in top 1.5 cm. and somewhat less in second 1.5 cm. layer by 2.8 and 4.0 inches per hour rainfall. In saturated soil columns, water permeability was not governed entirely by the permeability of the least permeable layer. The thickness of the dispersed and compacted soil layer appeared to be about 1 mm. Below it a less compacted layer, about 3 cm. deep, resulted.

The Effects of Rainfall Intensity on Soil Structure and Migration of Colloidal Materials in Soils

AbstractThis paper presents the effects of rainfall energy on the clay content, specific surface, aggregate stability, and organic matter content of the several layers of three soil types.After 30 minutes of simulated rainfall at an intensity of 1.6 inches per hour the magnitudes of the above properties of the first layer (0 to 1.5 cm.) decreased considerably, while these properties of the sublayers (1.5 to 12.0 cm.) did not change. After 30 minutes of rainfall at 2.8 and 4.0 inches per hour, the decrease in the magnitudes of these soil properties in the first layer was greater than that occurring under the 1.6 inches per hour rainfall. Thirty minutes of rainfall with intensities of 2.8 and 4.0 inches per hour changed the physical properties of the second layer (1.5 to 3.0 cm.) somewhat, although not of the same magnitude as they were changed in the top layer. The effects of 2.8 and 4.0 inches per hour rainfall were about equal in magnitude, while the effects of 1.6 inches per hour were lower.Clays labeled with Rb86 migrated in soil profiles during the application of rainfall. About 1% of the clay migrated to a depth of 3 cm. At about 7.5 cm. the radioactivity of the soil was equal to the background.

Discussion of paper by Frank J. Dragoun, ‘Rainfall energy as related to sediment yield’

It is interesting to compare the mechanics of removal of soil from the land surface, treated by the author, with that of transportation of sediment in a river. Although these two processes are related fundamentally, they differ in several important features.

Rainfall energy as related to sediment yield

Two mixed-cover agricultural watersheds yielded data for an evaluation of the relationship of the kinetic energy of a rainstorm to suspended sediment yields. The relation is developed on the hypothesis that the cultivated acres are the only source of sediment. This statistical analysis is extended to cover the interaction effects of rainfall energy as a function of factors such as rainfall intensity and antecedent moisture. The results are examined and compared with the relationship of other factors of rainfall and watershed characteristics as a means of estimating sediment yields.

Use of the Rainulator for Runoff Plot Research

AbstractThe portable rainfall simulator, known as the rainulator, which has been recently developed at Lafayette, Indiana, was designed for use as a research tool to aid in more rapidly evaluating factors that influence erosion, runoff, and infiltration. Intensities of 2½ or 5 inches per hour with near the kinetic energy of natural rainfall at these intensities are produced. Other characteristics which are desirable for runoff and erosion research on rectangular field plots are embodied in the design. The number and length of plots to which simulated rainfall may be applied simultaneously may be varied by varying the number of units used.The rainulator will be used primarily for obtaining comparative evaluations of various topographic, soil, water, crop, and management factors under field plot conditions. Numerous factors are well suited to study by this method. The types of comparisons and evaluations which are and are not well adapted to study on rectangular field plots in general and specifically with the rainulator are also discussed.

A Rainfall Erosion Index for a Universal Soil‐Loss Equation

AbstractExtensive regression analyses of basic soil‐loss data were designed to determine the best indicator of the capacity of a storm to erode soil. The rainstorm characteristic found to be outstanding as such an indicator is the variable whose value is the product of the rainfall energy and maximum 30‐minute intensity of the storm (designated as EI). This variable explained from 72 to 97% of the variation in individual‐storm erosion from tilled continuous fallow on 6 soils. Seasonal rainfall erosion index values computed by adding the EI values of storms > 0.5 inch explained as high as 94% of the yearly deviation in total soil loss from fallow during the summer season. Tested against data from plots in continuous row crop for 10 or more years at each of four widely separated locations, the summed EI values explained from 72 to 85% of the yearly variation in soil loss within corresponding cover periods. Expected annual values of the index and seasonal distribution of the erosion potential may be readily computed from local rainfall records. This has been done for 60 locations in the 31 Eastern States.About 8,000 plot‐years of basic erosion data are being analyzed to evaluate factors for a universal soil‐loss equation for which the erosion potential of expected local rainfall serves as the base. Tests show that estimates of average erosion losses computed in this manner are sufficiently accurate to serve as sound bases for conservation farm planning.

Rainfall energy and its relationship to soil loss

A relatively simple procedure is presented for computation of kinetic energy of a rainstorm from information on a recording‐raingage chart. An equation is developed describing rainfall energy as a function of rainfall intensity. The effects of rainfall energy and its interaction with other variables are evaluated in multiple regression analyses based on data representing four soil types. Application of this information to separate the effects of rainfall from those of physical and management characteristics in plot data is discussed briefly.

Studies on the soil-erosive force of precipitation

In generally, rainfall intensity was denoted by only masses. In present paper, we coscidered the relation between kinetic energies and rainfall intensities.1. For instance, the kinetic energy of a raindrop which is maximum among all we observed is 4.6×104 ergs, and on moderate storms, summation of energies amounts to about 106erg/cm2.2. The kinetic energy of rainfall increases according to a power function of the intensity, i. e. we obtainE=75.9I1.20whereE: energy of rainfall erg/cm2 min.I: intensity of rainfall. mg/cm2 min.As a factor of soil erosion, it is more rational to consider the kinetic energy of rainfall instead of amount only.

Size of raindrops and their striking force at the soil surface in a red pine plantation

It is usually assumed that a forest canopy breaks the force with which rainfall strikes the soil, thus protecting it from erosion. To show that this is frequently untrue and that the protective value of the forest to the soil must, therefore, be due to lesser vegetation or to the layers of unincorporated organic material lying on the soil surface is the purpose of this paper.Measurements were made of raindrop size beneath a red pine canopy and in an adjacent open field near New Haven, Connecticut during several storms in 1946 and 1947. By reference to published tables of velocities of falling water drops, the kinetic energy at the soil surface of the rain under the pine canopy and in an open field was determined. It was found that at rates of precipitation less than two inches per hour the kinetic energy of rainfall beneath the red pine canopy exceeded that of rainfall in the open.