Rain
Can you measure the speed and force of raindrops?
What is the effect on soil, with and without ground cover?
Could you simulate the effect of rain?
Introduction: (Initial Observation)
Meteorology is the science that studies atmospheric phenomena and weather conditions. We want to be able to predict weather and it’s effects on our environment. Conditions such as rain, snow, flood and tornado are specially important because of their potential hazard and economic effects.
Rain is among the weather conditions that cause erosion and may lead to flood, landslide and avalanche. Although it is now easy to predict rain based on the type of cloud and other weather conditions, it is not clear how can we predict the speed and the force of rain. Speed, force, volume and duration of rain are information that we need in order to predict if a rain may cause flood, avalanche and landslide.
In this project we will try to determine the speed and force of raindrops. We do this because speed and force of rain drops are two factors that affect the rate of erosion. We will then simulate the effect of rain on soil, with and without grass and vegetation.
Information Gathering:
Find out about the clouds and rain. Read books, magazines or ask professionals who might know in order to learn about the effect of rain on erosion of soil. Keep track of where you got your information from.
Soil Erosion – the battle to keep civilization from destroying themselves begins with saving the soil. To read about how some civilizations have not been able to do this go to Erosion of Civilizations
You do not miss the soil until it is gone.
Problems caused by soil Erosion:
- Loss of valuable topsoil. Agriculture land costs an average of $1000/acre for the upper 4 feet of soil. However, topsoil is considered to be ten times more valuable than the subsoil. When soil is removed from a field it includes: the soil particles, nutrients, water and water holding capacity.
- Damage due to deposition of soil from up-slope by burying more valuable land with less valuable soil.
- Damage to fields because gully erosion is reducing the field size and taking land out of production.
- Pollution due to off site or Non-Point Pollution. It is estimated that non-point pollution causes 3 to 18 Billion dollars per year or $50.00 per acre if one-half the farmland is causing the pollution .
Non-Point Pollution=sediment, nutrients, and pesticides. - Erosion causes a steady but slow productivity decline. For example: if a soil lost 2.5 inches it would have a 5-15% decline in productivity and if the soil lost five inches, the decline in productivity would be 10-35% .
Causes of Soil Erosion:
Impact of RAIN DROPS
Raindrops fall at 20 m.p.h. Raindrops cause surface soil pores to fill with soil particles and thus reduce infiltration. Particles are separated due to beating of rain drops. After the surface pores are filled, surface flow begins due to a lowering of infiltration rates
Erosion Types:
Sheet Erosion – thin film of water over the entire field moving down-slope Another view of sheet erosion
Rill Erosion – collection of sheet erosion water into channels (rills) that erode the bottom and side of the rill.
Gully erosion – increasing size of rills eventually lead to a gully or a channel too large for crossing by farm equipment.
Another view of a large gully. Large Gully
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to study the effects of rain on the erosion of the soil with and without vegetation.
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Variables that may affect the soil erosion by rain are the speed of raindrops, the size of raindrops, total amount of rain (how many inches each time or each day) and the rate of precipitation (how many inches per hour). The slope of the land and the soil composition may also affect the rate of erosion. I want to study on the effect of raindrops size and speed and soil composition on the erosion of the soil, so following are may variables for my research.
Independent variables are: The speed of raindrops, the size of raindrops, the soil composition.
Dependent variable is: The rate of soil erosion.
Controlled variables are: The slope of the land, total amount of rain.
In each experiment I will test the effect of one variable only. So the two other independent variable will also become controlled. For example when I want to test the effect of the rain speed, I will control and make sure that the size of raindrops and the soil composition will remain constant among my test samples.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
I think the rate of soil erosion increases by the speed of the rain and the size of raindrops. I also think the soil that is not covered by vegetation will be eroded more than the soil covered by vegetation.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1:
In this experiment I test the effect of the speed of raindrops on the soil erosion.
Make a rain simulator and test environment:
Place a tray on a 20º slope and hang a bucket or metal can above the tray. The can or bucket must have holes at the bottom for water to exit in the form of rain. Make the holes about an inch apart and insert a short string or tooth pick in each hole.
The purpose of a string or tooth pick is to prevent a continuous flow of water to come out. This simulates the rain drops. A knot on string can hold it in place.
The bucket will be your rain simulator. Test it in advance to see what level of water in the bucket gives you the best rate of rainfall.
Changing the height of bucket results a change on the speed of raindrops at the soil level.
Procedure:
Fill up the tray with your soil sample. place the simulator bucket about two feet above the tray. Prepare one gallon water and gradually add it to your rain simulator bucket. Gather the run-off water in another container such as a clear jar.
Repeat this experiment with rain simulator bucket mounted 6 to 8 feet above the tray. Gather the runoff water in another clear jar.
If you can have two identical setup, such as two trays and two rain simulator buckets, you may perform the two experiments at the same time.
Wait until the run off waters settle and visually compare the level of soil in these two jars. More sediments and soil in the jar represents more soil erosion.
You may optionally filter the runoff water and measure the weight of dry soil in run-off water as a more accurate way of comparing the erosion.
Does the speed of raindrops result a higher rate of erosion?
Experiment 2:
In this experiment I test the effect of the size of raindrops on the soil erosion.
Procedure:
Fill up the tray with your soil sample. place the simulator bucket about two feet above the tray. Prepare one gallon water and gradually add it to your rain simulator bucket. Gather the run-off water in a clear jar.
Repeat this experiment with larger raindrops and gather the runoff water in another clear jar. In order to get larger raindrops you may keep the level of water in rain simulator bucket higher. This will cause more water to come out of the holes and the size of raindrops will be bigger. So you may try to add all the water into the rain simulator bucket quickly.
If you can have two identical setup, such as two trays and two rain simulator buckets, you may perform the two experiments at the same time.
Wait until the run off waters settle and visually compare the level of soil in these two jars. More sediments and soil in the jar represents more soil erosion.
You may optionally filter the runoff water and measure the weight of dry soil in run-off water as a more accurate way of comparing the erosion.
Does the size of raindrops result a higher rate of erosion?
Experiment 3:
In this experiment I test to see how do plants and organic mater in soil affect the rate of erosion. I will test the effect of rain on:
- plain soil
- soil mixed with organic mater
- soil covered with plants.
Procedure:
Fill up one tray with the soil sample and call it plain soil.
Mix some of your soil sample with organic mater such as leaves, broken tree branches, wood chips and plant root. Use this mix to fill up the second tray and call it soil with organic mater.
Fill up a third tray with soil sample and cover it with plants and broken branches or wood chips. Name it covered soil.
Install the rain simulator bucket about 3 feet above the first tray. Prepare one gallon water and gradually add it to the rain simulator bucket. Gather the runoff water in a clear jar.
Repeat this experiment with the other two soil compositions and gather the runoff water in two other clear jars.
If you can have three identical experiment setup, such as three trays and three rain simulator buckets, you may perform the three experiments at the same time.
Wait until the run off waters settle and visually compare the level of soil in the three jars. More sediments and soil in the jar represents more soil erosion.
You may optionally filter the runoff water and measure the weight of dry soil in run-off water as a more accurate way of comparing the erosion.
Do plants and organic mater affect the rate of erosion?
Experiment 4:
What is the speed of raindrops?
The speed of raindrops can be both measured and calculated. There are possibly hundreds of different methods that we can use to measure the speed of raindrops. Some methods require design and construction of a special device. Some others may be performed using what we have at home. For example every moving object creates a trace on photography film. The length of the trace depends on the speed of the object and it’s distance from the camera. So in a dark rainy night we may use a light to illuminate the rain and take a picture of the falling raindrops. The rest is some calculations.
Speed of rain can also be calculated.
Any mass is attracted to the Earth by the pull of gravity. Gravity accelerates all objects towards the ground at a specific rate. Without any other forces present, the speed of an object in free fall will increase the farther or longer it falls. However, air friction or air resistance also exerts a force on an object (raindrop) that opposes the weight force of gravity. The air resistance and weight force on the droplet couple together to determine the terminal velocity for a given object.
In general the air resistance on an object depends upon several variables. First, it depends upon the shape of the object. Its shape determines the object’s drag coefficient: the more aerodynamic the shape, the less drag. Second, it depends upon the size of the object; specifically the cross-sectional area presented to the airflow (perpendicular to the direction of travel). And lastly, it depends upon the speed of the object. At low speeds the object’s resistance is directly proportional to speed, and at higher speeds the object’s resistance is proportional to its speed squared. Most objects falling through the air would be considered to be moving at a higher speed, even though that speed might not be great compared to some velocities.
The speed at which an object falls increases until the upward force of air resistance equals the downward force of gravity, at which time the object reaches the terminal velocity. We know raindrops come in different sizes, so we need to consider an average size. Let us consider the average raindrop to have a radius of about 0.2 cm and a mass of about 0.034 grams. Aerodynamic engineers would give the rather round shape of a raindrop a drag coefficient of about 0.5. When all the parameters are considered the terminal velocity of a typical raindrop is calculated to be about 9 meters per second or 20 mph. A smaller raindrop of radius 0.15 cm has a terminal velocity of about 7 meters per second or 16 mph. In general, depending upon their size, raindrops fall between 15 and 25 miles per hour no matter how high they are when they begin their descent.
Simple procedure:
Use a dropper to drop water from a height of 10 feet on an empty, upside down aluminum pan or metal can. Use a chronometer to measure the time that it takes for a drop of water to descend 10 feet. chronometer is a portable timekeeper, with a heavy compensation balance, and usually beating half seconds; — intended to keep time with great accuracy. You use a metal pan so you can hear the sound of raindrop touching the ground.
Repeat this about five times and take the average. Divide the travel distance (10 feet) by the number of seconds to find out the speed of raindrop based on feet per second. You may optionally convert the units to meter per second or miles per hour. Note that since this water drop is released from a low elevation, it’s speed is lower than a raindrop coming from higher elevations, however this can give you an estimate.
Calculate the Impact Force:
The impact force of a moving object can be calculated using the formula F = m*V.
In other words the impact Force = mass X Velocity. In this formula if you write the mass in kilogram and the velocity in Meters per Second, the force will be calculated in Newton. For example if an object with the mass of 100 kg hits the ground at the speed of 2 meters per second, then:
Impact force = 100*2 = 200 [N]
A rain drop with mass of 0.034 grams that hits the ground at the speed of 9 meters per second, has an Impact force of: (0.034 gram is 0.000034 kilogram)
Impact force = 0.000034 * 9 =0.000306 N
Materials and Equipment:
List of material may vary based on your final experiment design. Following is a sample:
Materials:
- Wood blocks
- Three shallow pans
- soil
- Fixture for hanging rain simulator
- Bucket or can with holes at the bottom.
- three clear jars
- Cotton string
- Wood chips, broken plants or plant parts.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
No calculations are required for this project unless you want to calculate the speed or rain.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Soil erosion begins with the first raindrop. The collision of the raindrop with the aggregates at the soil surface starts the erosion process. Raindrops travel at about 20 mph. When they hit the soil, the aggregates (soil particles tied together) are broken apart, and reduced to the sand, silt, and clay particles that make up the soil. At first, water moves down into the rest of the soil, carrying these finer particles along. But soon the fine soil particles begin to clog up the pore space, and water infiltration rates (water movement into soil) are reduced.
Smaller particles, especially the silt and clay fractions, are suspended in the water creating puddles on the soil surface. When the infiltration rate is less than the rainfall rate, runoff begins to occur. Crop residue on the soil surface plays an important role in protecting the soil from erosion, and in maintaining higher infiltration rates. For soil conservation purposes, a minimum of 30% of the soil surface should be covered with crop residue. From a soil erosion point of view, more residue is better.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
