Water on the Land Study Guide
Rain that falls on land is vital for human life and the lives of all other organisms. This water seeps into the soil, then into groundwater, and some of it eventually flows into rivers. These rivers carry sediments from land to the ocean, thus eroding the continents. Here we look at the main features of water on the land.
Rainfall and Rivers
For anyone who has traveled, the different kinds of vegetation and animals that live in a particular region can be striking. The amount of rainfall received in a region plays a large role in determining the kinds of plant and animal life in a particular region. Regions are even named or categorized by the amount of rainfall received and type of vegetation growing.
Deserts, for example, are created when a region receives less than 10 inches of rainfall per year. Rain-forests, in contrast, have more than 80 inches of rain a year, and some very rainy places can have a couple hundred inches of rain per year, more than half an inch per day, on average. The global average rainfall is about 30 inches of rain annually.
What determines how much rain falls in each region? The details are as complex as the landscape itself, but we can discern a couple basic large-scale patterns. To see these patterns, we need to revisit some features of the wind patterns: the Hadley cells and air circulation over mountains.
Recall that the Hadley cells create warm, rising air currents in the tropical regions. This ascent takes the air up into the troposphere and cools it—perfect conditions for clouds and rain. Indeed, the tropics tend to be very rainy. The high levels of sunlight that create the tropical temperatures also evaporate lots of water from the oceans in the tropics. Winds can bring this moisture-laden air over the continents. But that alone is not enough to make heavy rainfall. This is where the ascending currents of the Hadley cells come into play, for they ensure that the moisture-laden air drops water in those very tropics. Indeed, the great rainforests of the world are located in the tropics: South America, Africa, and Indonesia.
After the air currents of the Hadley cells have lost most of their water vapor to rain, they move north and south in the upper troposphere and then descend (as you recall) around 30° north and south. These descending air currents are dry; their water was lost in the tropics. Descending dry air means little rain, perfect conditions for deserts. And in fact, we find some of the major deserts of the world at about 30° north and south latitudes. Examples include the famous Sahara Desert of Africa. Other examples are the deserts in the Southern Hemisphere, specifically South Africa, most of Australia, and parts of Chile. The Sonoran Desert of Arizona and Mexico is one more example in the Northern Hemisphere.
Another kind of air circulation we discussed was the lifting of wind over mountains. This also has a large impact on rainfall. Consider the westerlies that blow winds from across the Pacific Ocean eastward into Washington, Oregon, and California. These winds carry moisture that they received via evaporation from the Pacific Ocean. They then reach the mountains of those western states, the Sierras and Cascades. The winds lift up, the air cools, the water vapor condenses into clouds, and it rains.
The rain in these mountain systems occurs on their windward sides, the sides that first receive the wind, because that's where the uplift takes place. Once the air crosses over the top of the mountain, it is dry. On the other side of the mountain range, the leeward side, the air contains little moisture. The eastern side of the mountain ranges of Washington, Oregon, and California are very dry, almost deserts in fact. These dry sides of mountain ranges are called the rain shadow.
Rainfall largely determines the flow of rivers. Another determining factor is the total area of drainage of a river, which is known as the watershed. The watershed is the area that potentially collects the rain that feeds into a flow of water on whatever scale—creek, brook, stream, or river. The term watershed is the same as drainage area. Let's look at the ten largest rivers of the world, in terms of their discharges into the ocean, or in other words, their flow rates into the ocean.
You might think: Why isn't the Nile on this list? The Nile, in fact, is the world's longest river. However, its drainage area is not so large because of another factor—that much of the drainage area of the Nile is desert. So you can see the impact that rainfall has on the discharge of rivers.
Let's look at the table in more detail, for patterns. Although the rivers are ranked according to discharge, from the Amazon at number 1 to the Mekong at number 10, the ranking is not the same for drainage area. True, the Amazon and Congo, which are ranked number 1 and 2 for discharge, are also number 1 and 2 for drainage area. But after that, the ranking in terms of drainage area breaks down.
Compare the Mississippi and the Congo, for example. Their drainage areas are almost the same, and yet the Congo's discharge is more than double that of the Mississippi. That is because the Congo is tropical and sits under the ascending rain-creating branch of the tropical Hadley cell. The same reasoning is true when comparing the Lena (primarily in Russia) with the Mekong in Southeast Asia. For the same drainage area, the Mekong has three times the discharge rate as does the Lena.
The flow from the Amazon is truly amazing: 220,000 cubic meters per second. Let's calculate how much water that is per person per day, globally. There are 86,400 seconds in a day. So in a day, the flow of the Amazon is 18.9 billion cubic meters. Today, the world has about 6.4 billion people. Dividing the flow of the Amazon by the number of people on Earth, we can compute that 3 cubic meters per person per day flow out the mouth of the Amazon. At about 250 gallons per cubic meter, that's nearly 750 gallons of water per person per day flowing from the Amazon River.
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