Climate Change Lesson 11 : Water Balance and the Great Lakes
Water balance is an accounting of all water volumes that enter and leave a 3-dimensional space over a specified period of time. The water balance of a place, whether it is an agricultural field, watershed, or continent, can be determined by calculating the input, output, and storage changes of water at the Earth’s surface.
Specially designed climagraphs show water balance by indicating water surplus and deficit. Surplus is shown by all the bars which extend above the temperature line; a water deficit occurs when the precipitation bar is below the temperature line.
Factoring in predicted changes in temperature and precipitation can show changes in water balance. The level of water in a lake depends on the balance between four things: how much rain and snow fall directly into the lake; how much water evaporates from the surface of the lake; how much water comes into the lake from rivers and groundwater springs; and how much water goes out of the lake in rivers or groundwater seepage.
The Great Lakes are a globally unique resource – there is nothing like them anywhere else in the world. The total area of the lakes is a bit less than 100,000 square miles, but they hold enough water to cover the entire United States to a depth of nearly ten feet.
Rain and snow and all of the rivers and creeks from about 300,000 square miles (total area of the Great Lakes basin) eventually empty into the Great Lakes.
This basin is only one half of one percent of the total area in the world, but has 20% of the fresh water in the world. In other words, each square mile in the Great Lakes area has 40 times as much water as the average square mile in the world. Four of the Great Lakes are at nearly the same elevation, and the Welland Canal connects the last lake in the system, Lake Ontario. Thirty million people live around these lakes.
The Great Lakes are likely to change if climate changes. According to the latest prediction models the temperature around the Great Lakes may go up by 2 or 3 degrees in the next 100 years. Rainfall is expected to increase in winter and decrease in summer, which may cause lake levels to go up in winter and down in summer. Ice cover has decreased significantly. Many climate models, combining temperature and precipitation for all seasons of the year, predict that the average water level may go down about one foot. Some models predict a drop of nearly 6 feet. A few models say the water level might actually go up.