Running Water

Objectives

The objectives of this unit are to introduce students to the:

pathways by which water and energy move through the atmosphere and the oceans;
the importance of these movements to weather processes and climate; and
the impacts that these movements have on people and their lives.

Background Information can be found here

Water Cycle

Water has many important properties, one the most important of which is its ability to exist as a solid, liquid, and gas at the range of temperatures and pressures found at the Earth's surface. Water can change from one state to another by the addition or subtraction of latent energy. Within each state (solid, liquid, or gas,) water can warm or cool by the addition or subtraction of sensible heat. The balance between gain and loss of water at a regional or local scale will determine climate, insofar as moisture is concerned.

Ocean Currents

Ocean currents are formed by frictional drag as winds move across the surface and by temperature- or salinity-related density differences. The major ocean currents are initiated by winds, so the global current patterns resemble the global air circulation patterns. Of course, continents and other land masses get in the way of the currents so many of them are deflected by coastlines. The net result of this is the presence of large gyres in the world's major oceans. (today's data)

Activity #1

Local Water Budget

This activity can be as short or as long as you want; but it is more realistic if extended over a year or combined with archived data. The main idea is that for every local climate, there is a balancing act between water that come into the local system and water that leave the system. If, on average over a long period of time, more water enters and remains in the local system than leaves, the climate will be humid. If more water can leave than remains, the climate will be arid. Climate is a long term average of weather conditions; if we focus in just on water and humidity, it is be easy to see that extreme variations occur in both precipitation and evapotranspiration. By graphing income (precipitation,) savings (soil moisture recharge and runoff,) and spending (evapotranspiration) month by month, we could come up with what hydrologists call a water budget climate. This graph would show at a glance which months are the wettest, which are the driest, and, more importantly, what is the balance between income, spending, and savings at any one time. In the arid western portion of the United States, water is saved in reservoirs from times of surplus (spring runoff) to be spent in times of need (high evapotranspiration periods of late summer.) In the humid eastern portion of the United States, reservoirs are build mainly to control flooding by capturing the surplus and releasing it gradually, as well as all of the other multiple uses of recreation, municipal water, and agriculture.

Steps:

1) You will need a broad, shallow metal pan (a 12" diameter, deep-dish pizza pan works great,) and information on temperature, wind speed, and wind direction. The weather information can be obtained from the internet or locally from the weather service, TV stations, or your own school's weather station. You can do this exercise for a week, a month, a year; the longer the length of time, the more information can be gathered. Questions can be adjusted for shorter lengths of time.

2) Place the metal pan at ground level in a "typical" area for your location. The pan should not be shaded more than necessary nor in the sum more than necessary. It should not be screened by vegetation, nor should it be placed on the downwind side of obstacles.

3) Fill the pan with water to the brim, and measure on a regular basis how far the water drops. Since you are interested only in how much water leaves the area via evaporation, refill the pan after each measurement.

3) Graph water loss (in centimeters) and precipitation (in centimeters) on the "Y" axis vs. time (in what ever units you have chosen) on the "X" axis. The loss is actually the potential loss that could be experienced by your area; the actual loss may be much less (but never greater!) if there is no naturally occurring water to evaporate. Your graph is a modified version of the Thornthwaite Water Budget.

Questions:

1) For the period of time you ran this experiment, is your area dry or humid?

2) If your data cover less than a year, project what you think the data would be for the rest of the year, based on internet weather and climate data for your area.

3) Relate the loss or potential loss of water from your evaporation pan to the need in your area for irrigation of lawns, parks, and crops.

4) How might your results be different in the future if a) global warming becomes a reality, or b) global cooling becomes a reality. Be sure to support each of your speculations with sound scientific principles.

Activity #2

Extreme Runoff Events

Many urbanized areas are located adjacent to streams and rivers. The location of cities and towns adjacent to waterways can be explained by historical settlement patterns: waterways provided a source of transportation, energy, a source of water for industry and irrigation, and sometimes a barrier to further travel. Rural farming and ranching activities are also located adjacent to waterways; both urban and rural areas may occasionally be affected by extreme runoff events or "flooding." In order to minimize loss of life and damage to property, flow gauges are maintained by government agencies (particularly the U.S. Geological Survey) to keep track of stream and river discharge. Students and teachers can obtain current or archived data for a stream or river near them, or an important river in another part of the nation. In this exercise, you will download and analyze peak discharge data for a river or stream of your choice, preferably one near your location.

Steps:

1) Select a stream or river recording station of your choice. Records are available for most of the United States from the United States Geological Survey. Current information on stream flows is available that is updated daily for automatic stations, less often for manual stations. Archived data is available for most stations, although the length of time represented by the data is of variable length. If data are unavailable for your local river or stream, look at the data for nearby stations or for a major river in your vicinity. A sample project for the Blackfoot River in western Montana has been worked out

2) Download the archived records of the annual peak flows (annual series) for this analysis. Some records may include all peak flows above a certain base flow - select and use only the peak flows for each year, discarding the rest. This analysis will concentrate on extreme flood events and their return intervals.

3) Open the downloaded peak flow records in a spreadsheet. Arrange the information so that you have columns for date of peak discharge and for amount of discharge.

4) Sort the data in descending order, so that the largest annual peak flow is first and the smallest is last on your list.

5) Insert a column numbered from 1 to however many years of record you have downloaded. This column ranks the annual peak flows from the largest (ranked 1) to the smallest. Each annual peak flow now has an "order number."

6) Hydrologists have developed a relationship between the rank of an annual peak flood, the number of years of record available, and the return period* of a particular peak flood:

R = (n+1)/m

where "R" is the return period or recurrence interval in years, "n" is the number of years of record, and "m" is the rank of the peak flow or the order number ("m" equals 1 for the largest annual flow, and "m" equals "n" for the smallest annual flow).

Insert another column in your spreadsheet that calculates "R," the return period for each annual peak flow using this formula.

*It is important to remember that the return period is not a guarantee that a peak flow of a particular size will occur every so many years, but a statement about the probability of a peak flow of a certain size occurring in any one year. As an example, a peak flow with a return period of 100 years DOES NOT occur every 100 years, but it does have 1 out of 100 chances of occurring in any one year. Similarly, the 50 year peak flow has 2 chances out of 100 of occurring in any one year, a 20 year peak flow has 5 chances out of 100, and so on.

7) Graph the Return Period ("X" axis) Vs Peak Discharge in CFS ("Y" axis). The resulting curve (for most data sets) is logarithmic on Cartesian coordinates; for this reason many hydrologists will plot this type of data using a logarithmic scale on the "X" axis, resulting in nearly a straight line plot. Create two graphs, one using Cartesian coordinates, and one using a logarithmic scale on the "X" axis.

8) Prepare a new graph by re-sorting the data by date, from oldest to most recent and plotting year ("X") vs. discharge ("Y" axis).

Questions:

1) Predict the discharge for the 10 , 20 , 50 , 100 , 500, and 1000 year events. Do this graphically by examining the semilog plot, and algebraically by calculating a regression curve using the spreadsheet data. Discuss the differences, if any, between your two sets of predictions; include in your discussion your thoughts on which method (graphical or mathematical) you think would be most accurate. Compare the current year's peak discharge with your historical date; what is the return period of this year's highest flow? Discuss why it is possible to predict the value of the 1000 year event or even the 100 year event with only 50 years worth of data; how accurate is your prediction of the 1000 year event likely to be?

2) Make a list of all possible reasons you can think of for recording this type of data and doing this type of analysis. Examine your list carefully and eliminate all those that are frivolous, repetative, or simplistic. From the remaining reasons, determine which is of the greatest concern for your particular geographic location and write a report discussing your findings and how they may address these concerns.

3) City and county planners, insurance companies, government agencies, and individuals pay a great deal of attention to the "100 year flood event." Discuss why this particular return period is most often used. Contact your city/county planning office(s) for information concerning "100 year flood plain" restrictions. Contact local insurance companies for information on the cost of flood insurance inside and outside the 100 year flood plain. Discuss the interactions between hydrology, economics, and people in your local area.

4) Examine your graph of year vs. peak discharge. The discharge at any particular point along a river reflects conditions (land use patterns, such as agriculture, mining, urbanization, timbering; geological/geographical factors such as soil, vegetation, weather, climate) upstream from that point. Can you discern any trends in your data such as increasing or decreasing runoff? Test this by calculating (with your spreadsheet or a graphing calculator) a correlation coefficient between "year" and "discharge." You can also break the data up into segments of several years of data if you think you see shorter term trends. Keeping in mind the definition of climate as "a long term (100 years +/-) average of weather conditions," is there any indication from your data and analyses that your local climate is changing? If your data shows a distinct trend (confirmed by correlation calculations) one way or another, can you eliminate other possible factors such as changing land use patterns, a change in the location of the recording gauge or construction/destruction of water storage facilities? Discuss the implications of your results to the current debate in scientific and popular literature about "global climate change."

5) The discharge of a river or stream is essentially a function of how fast water is flowing through a cross-sectional area of the channel in a set length of time, most commonly written as:

Discharge = Width X Depth X Velocity (cubic feet/second = Feet X Feet X Feet/Second)

If you make the assumption that the discharge value at a recording station is valid for the adjacent length of the river channel (a good assumption,) you can make predictions for the depth of water (called stage or flood stage) to be expected at various points along the channel.

For example, a recording station is located at the point where a river enters the city limits would give you information on the discharge. downstream the river valley may narrow or widen. Using the value of discharge and topographic map cross-sections, it is possible to predict how high and how wide a particular discharge would flood. You may have to make some simplifying assumptions about velocity in order to do this analysis. Determine the area that would be affected by the 100 year flood at your location, and compare it with the information provided by your local city/county planning office. Remember that although urban area floods may affect more people and result in more dollars worth of damage, flooding of farm and ranch land in rural setting can be just as devastating.

It is important to remember that the return period is not a guarantee that a peak flow of a particular size will occur every so many years, but a statement about the probability of a peak flow of a certain size occurring in any one year. As an example, a peak flow with a return period of 100 years DOES NOT occur every 100 years, but it does have 1 out of 100 chances of occurring in any one year. Similarly, the 50 year peak flow has 2 chances out of 100 of occurring in any one year, a 20 year peak flow has 5 chances out of 100, and so on.