Introduction
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.
Features of Water on Land
As described in the previous lesson, water that falls as rain on land can take a number of different pathways. Let's go into more detail now about water's presence on land. Basically, you need to study a number of definitions.
When rain falls, it can immediately run over the land and down into a brook or stream; this is one type of runoff. In addition, groundwater can seep out of the ground, say at the sides of a valley, and join brooks, streams, and rivers as runoff. So runoff does not have to come immediately from water that runs over the land to a drainage channel. Brooks, creeks, streams, and rivers all are different scales of drainage channels. Together, these drainage channels, along with ponds and lakes, form what is known as surface water, or water at the land's surface.
As we have seen in the previous lesson, a great deal of water is underground, in the soil and in shallow and deep groundwater. The water that falls as rain and does not run off immediately infiltrates the soil. This is called infiltration. There are two different places that the infiltration can go. First, it can become part of the soil's water, which is called capillary water, because it resides in the fine networks of air passages (capillaries) within the soil. This water adheres to particles in the soil, and is the major source of water for plants. Plants' roots feed on the capillary water of the soil.
The second place that the infiltrated water can go is downward, pulled by gravity toward deeper levels of the soil and eventually into the cracks and passageways in rocks beneath the soil. This is called gravitational water, because the water is pulled downward by gravity. Eventually, it will stop when it reaches a layer of rock that has no cracks or porosity. Such a layer is impermeable. However, sometimes the rock is quite porous; in fact, there might be underground layers of gravel or sand that can hold a lot of groundwater. The groundwater also flows, at a limited rate. This is why someone who lives in the country can keep pumping water from their groundwater supply—the ground water at that spot keeps refilling the well. Of course, one cannot pump too much too quickly, because the well has only a certain recharge rate.
The underground water usually has an upper most level, which is called the water table. A well must reach below the water table, obviously, to obtain water. When there are large, well-developed layers of porous material or rock that contain large amounts of water, the layers are referred to as aquifers. The U.S. Midwest has a giant aquifer called the Ogallala aquifer, which extends over many states and upon which millions of people depend and will continue to depend until it is depleted.
Groundwater, as mentioned, can emerge from the ground, out of cracks or from the porous layer in which it flows. If it emerges as a definite flow, the spot is called a spring. Springs are good to know about when hiking. If the water emerges from the ground as a general area of moisture, the spot is called a seep.
Fresh water on land is not salty. Usually, fresh water is defined as water that contains less than one-tenth of 1% salt. Contrast that to the ocean's salt water, which is about 3% salt. In places where the land meet the ocean, estuaries can form, which are bays or swampy areas that receive fresh water flows from a river that mixes with the ocean water. Thus, in estuaries, the water's salt is in between that of fresh water and that of the ocean. Such estuarine water is brackish. More inland, wetlands such as swamps can be completely filled with fresh water. These are inland wet-lands and are crucial to biodiversity as well as natural water purification.
Rivers Erode the Continents
Rivers carry more than water to the ocean. They carry sediment. If you've ever seen the Mississippi in its lower reaches, you see why they call it the big muddy. It's full of suspended sediment. The sediment consists of small particles of clay and small amounts of organic materials, but other bits of rock are there as well. This suspended material, carried along by the water because it is so small, is called suspended solids. Let's look at the world's top ten rivers again, ranked according to their discharge rate, as before, but now add the numbers for the suspended solids they carry to the ocean each year.
We see some amazing facts in this table. The Ganges/Brahmaputra, number 3 in terms of discharge and equal to only about 25% of the Amazon's flow, has the most suspended solids. Why is that? Though the Amazon drains the eastern side of the Andes mountains, for much of the Amazon's flow, the land is flat and does not add much to the suspended solid load. The Ganges/Brahmaputra drains the Himalayas and drains overall more mountains, which supply that pair of rivers (which meet before they drain together into the Indian Ocean) with lots of sediment. That's why many sedimentologists go to India and Bangladesh to study the tremendous sediment load carried by the Indian rivers.
And look at the tiny load of sediment carried by river number 2, the Congo. Unlike the Amazon, the Congo drains almost no mountains, just highlands in central Africa. The big sediment load of the Mississippi derives partially from the fact that so much of the Mississippi's drainage basin is in agriculture, and often, a lot of erosion comes from land that is cultivated.
What is the total sediment load carried by the world's top ten rivers? If we add up the loads in Table 12.2, we arrive at the number 3,928 million tons of sediment per year. In the practice questions that follow, you will be asked about how much this is per person per year, from these ten rivers.
In addition to the sediment that you can see with your eyes, rivers carry dissolved material. For example, the ocean's water is 3% salt, yet you do not see it (though you can taste it). Where is the salt? It exists in the ocean water as dissolved ions, invisible to the eye because these ions exist as molecules, not particles, in the water. All rivers carry both solid sediment and dissolved loads.
The difference between solid sediments and dissolved loads carried by rivers reflects a basic distinction that geologists make between two types of weathering processes. Weathering processes wear down rocks. Weathering processes slowly destroy mountain ranges. These processes turned what had been an ancient mountain range as mighty as the Himalayas into today's Appalachian Mountains in the eastern United States. As noted, weathering processes come in two types.
The first type of weathering is physical weathering. In this process, wind, water, and ice can erode the rock, breaking it down into smaller and smaller particles. You can see some of the results as small grains of minerals in the soil. Sediments suspended in water, for the most part, derive from physical weathering.
The second type of weathering is called chemical weathering. In chemical weathering, the rocks and mountains are not broken down; they are dissolved. Before human industry began adding acids to the atmosphere, rain was mildly acidic, from the natural chemicals in the air. Soil water, with these mild acids and other acids added by soil life, dissolves certain rocky materials, turning the minerals into ions of various kinds. For example, chemical weathering creates calcium and phosphorus ions in the water. These ions can be used as nutrients by plants when the plants take up the soil water by their roots. And of course, these ions can flow into the rivers and on to the ocean.
Different elements have different propensities for being dissolved by chemical weathering. Iron, for example, does not dissolve easily. About 99.8% of the iron that is in rivers is in the form of suspended solids. In contrast, calcium dissolves quite readily. Only 40% of the calcium in rivers worldwide is in the form of suspended solids. That means that 60% of the calcium in rivers consists of dissolved ions from chemical weathering.
Globally, the mass of solid sediments carried by rivers is more than the dissolved load. This is because certain abundant elements, such as silicon and aluminum, travel mostly as suspended solids in the rivers.
Practice Problems of this concept can be found at: Water on the Land Practice Questions
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From Earth Science Success in 20 Minutes A Day. Copyright © 2005 by LearningExpress, LLC. All Rights Reserved.
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