Since the input of energy into the earth system is greatest in the equatorial regions, more water is evaporated and more energy is stored (as latent in water vapor and sensible heat in ocean water) in this region than any other. Water vapor and its stored energy can then be re-distributed around the globe via the global circulation system.
The oceans acts as a giant storehouse for water - any water that leaves the surface of the ocean by evaporation must return to the ocean via precipitation and runoff from land masses. The water cycle traces the pathways that water (and its included heat energy) takes as it moves from the ocean to the atmosphere to the land and back to the ocean.
At any point in time, the largest proportion of the earth's water is stored in the ocean, and lesser amounts are in transit through the system in the atmosphere, streams, rivers, and lakes. Additional amounts of water are in storage (for variable lengths of time) as ground water, snow and ice fields, and glaciers. (insert link here to percentages or include table?) These amounts of water and the balancing that continually occurs between categories is collectively known as the water budget. Taken over a long period of time, the water budget is more or less in balance, with constant amounts in the ocean, in transit through the system, and in storage. During periods of climatic change, the proportions allocated between ocean, transit, and storage will change. Since the total amount of water on the earth is fixed, increasing the amount in one category will result in a decrease in one or more of the other categories. If the global average temperature were to increase, snow and ice fields, and glaciers would decrease in volume, while ocean volumes would increase.
All forms of all water in weather processes ultimately has its origin in the water cycle, through evaporation from the ocean's surface. Complications to the cycle include evaporation from damp or wet land, transpiration from plants, evaporation from falling precipitation, precipitation that falls directly back to the ocean's surface, and so on. The balance between gain and loss of water at a regional or local scale will determine climate, insofar as moisture is concerned. While the earth as a whole can be considered to be a more or less closed system, a smaller regional or local area cannot be so considered. A very useful factor, used in both weather and climate studies, is evapotranspiration. Evapotranspiration is a combination of evaporation and transpiration from plants. As a practical matter, it is difficult to separate out the effects of transpiration from evaporation, so the two are combined as evapotranspiration. Both are functions of temperature, humidity, and wind. Any area for which adequate records exist can be analyzed for the balance between loss and gain of water and a water budget climate can be determined.
If more precipitation falls in an area than is lost by evapotranspiration, the area is humid; if more is lost by evapotranspiration than falls as precipitation, the area is arid. As an example, areas in the zone of westerly winds such as northwest coastal Washington are very humid because the wind blowing in from the ocean brings a tremendous amount of moisture. The uplift caused by the coastal landforms results in cooling of the air and subsequent precipitation. There is not enough solar energy at this latitude to evapotranspire all of the moisture that falls, so the area runs a constant surplus of moisture and is considered humid. On the other hand, those areas in the vicinity of the subtropical highs such as the southwestern United States-northern Mexico experience sinking, high pressure air and diverging wind. This keeps moisture-laden air away from the land which, coupled with adiabatic heating and high evapotranspiration rates, result in aridity.
Water has a very high specific heat compared to land, 1 kilocalorie/kilogram/degree centigrade change in temperature. Since atmosphere is heated from the bottom by radiation from the earth's surface, currents are therefor a powerful force for heating and cooling of the overlying atmosphere. The tremendous storehouse of specific heat energy in the Gulf Stream, for example, is passed on to the atmosphere as both sensible heat and latent heat (along with water vapor) as the current flows northeast. By the same token, the cold California Current can remove heat energy out of the overlying atmosphere, resulting in cooling and condensation.
One of the most renown currents is the El Nino. This phenomena is actually part of a larger system, known as the El Nino - Southern Oscillation, or ENSO; Some scientists would also add La Nina. The southeasterly and northeasterly trade winds converge at the equator, driving the northern and southern equatorial currents. Between these two currents is the equatorial counter current, allowing a return flow to the east, towards the South American coast. The southern oscillation refers to the see-saw movement of the low pressure at the equator. The equator is essentially a low pressure zone, but along this low pressure zone are areas that are relatively lower. When the low pressure is located in the western Pacific, the trade winds are relatively strong, the north and south equatorial currents are strong, and warm water accumulates in the western Pacific, bringing this area abundant rain. The warm counter current is relatively weak, but strengthens around the Christmas season, hence the name El Nino, the Child or Christ Child. As the equatorial low pressure zone moves east, the trade winds weaken and the warm counter current strengthens, and El Nino conditions (warm current off of the coast of South America) strengthen. As warm water piles up against the coast of south America, the upwelling of cold, nutrient-rich water is prevented. As the equatorial low moves back towards the west, conditions return back to "normal."
An El Nino event refers to a stronger than "normal" or "average" warm counter-current and the accompanying rainy conditions that arrive at the coast of South America. An El Nino Current shows up nearly every year, but only the stronger than average are referred to as the El Nino - Southern Oscillation (ENSO) event. When the Trade winds strengthen and the up-welling cold (Humbolt) current becomes stronger than average, the event is referred to as La Nina (the Female Child)
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.
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).
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?
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.