Water present in nature, in different places and in different forms, is called a water source. The two main kinds are underground water and surface water. A watershed and a wetland are both examples of water sources, and all three words describe parts of the same journey that water makes across the land.
Students usually meet these three words as three separate definitions to learn, and then mix them up in the examination. They are much easier to hold on to if you follow a single raindrop instead. It lands on a slope, runs down it, collects somewhere, and spreads out where the land goes flat. The slope is the watershed, the place it collects is the water source, and the flat wet ground is the wetland. One journey, three names.
Surface water and underground water are the same water
Surface water is the water you can see: rivers, streams, lakes, ponds and reservoirs. Underground water is the water held in the soil and in the cracks of rock below your feet, and it reaches us through wells, springs and boreholes. The textbook lists them as the two main kinds, which is correct, but it is easy to picture them as two separate supplies. They are not.
Rain that soaks into the ground becomes underground water. Underground water that finds its way out through a hillside becomes a spring, and the spring feeds a stream, and the stream is surface water again. It is one supply moving between two states. That is why anything which stops rain from soaking in has an effect you would not expect. When a town covers its ground with concrete and tarmac, the rain runs straight off into drains instead of sinking, so the wells in that town start to fall even though the same amount of rain is falling on it.
A watershed is defined by where water goes
A watershed means the area of land from which rainfall is collected and carried to one place. It can be as small as the slope draining into a village pond, or as large as thousands of square kilometres draining into a great river.
Now here is the part that is worth more than the definition. You cannot photograph a watershed. A forest looks like a forest and a lake looks like a lake, but a watershed has no appearance of its own, because it is not a kind of place at all. It is an answer to a question, and the question is this: if a raindrop landed here, where would it finish up? Every point of land on the Earth belongs to some watershed, including the ground your school is standing on. Nothing drains nowhere.
Jagadishpur Tal, Ghodaghodi Tal, Bishazari Tal, Gosaikunda and Rara Tal are among the important watersheds of Nepal. Look at the picture above and notice what you are really looking at. The lake is only the last few metres of the story. Everything that reaches it came off the slopes around it, so if those slopes change, the lake changes, even though nobody has touched the water.
Suppose the trees are cut on a hill two hours walk above your village. Nobody has come near your fields, your well or your stream. Yet within a few years the stream runs brown after rain, your fields flood in the monsoon, and the well drops lower each Chaitra. This looks unfair and it looks mysterious, and it is neither. The trees on that hill were slowing the rain down and holding the soil, and your village sits in the same watershed. Water carries decisions downhill. That is why protecting a watershed is work done upstream for a benefit felt downstream, and it is why it can only be done by whole communities agreeing together, never by one household alone.
A wetland is neither land nor water, and that is the point
A wetland is marshy ground that stays covered with water for twelve months of the year. The water sits close to the surface and the ground is permanently wet. Koshi Tappu is the best known wetland in Nepal. Ghodaghodi lake area, the lakes around Gosaikunda, Jagadishpur reservoir, Rara Tal, Bishazari Tal and the land around it, the Shey Phoksundo area and the lakes of the Pokhara valley are all wetlands too.
It is tempting to think of a wetland as a failed lake or as spoiled land. It is neither, and its in-between nature is exactly why it holds so much life. In nature the border between two habitats is always more crowded than either side on its own, because it holds living things from both sides plus living things that can only live on the border. A wetland has fish and frogs from the water side, grasses and insects from the land side, and wading birds that need shallow water beside dry ground and can live nowhere else. That is why a modest patch of marsh at Koshi Tappu can hold more kinds of bird than a much larger stretch of dry forest.
What a watershed and a wetland actually do for us
| The job | What that means in practice |
|---|---|
| Food | The plants and animals living there feed people and other animals. Fish from these waters are eaten in villages all around them. |
| A nursery | Fish and other water animals breed here. Shallow warm water with plenty of plants is where young fish can hide, so this is where the next generation is made. |
| A winter home | Many kinds of bird raise their chicks here, and many more use these places as a temporary home when the weather elsewhere turns harsh. |
| A store | They collect rainwater and hold it, and that stored water is then used for drinking, for irrigation and for many other purposes. |
| A balance wheel | They keep local temperature steady and keep the water cycle running, because a large wet surface evaporates steadily and cools the air above it. |
The sponge: why the same rain can cause both a flood and a drought
This is the most useful thing in the topic and it is worth reading twice. A healthy watershed with forest on it, and a wetland at the bottom of it, work together as a sponge. Rain arrives in a violent burst over a few hours. Leaves break the fall, roots hold the soil, the ground soaks water up, and the wetland fills like a shallow bowl. Then all of that water is released slowly over the following weeks and months. The river below stays full through the dry season and never rises fast enough to burst its banks.
Now take the sponge away. Cut the forest and fill in the marsh to make fields. The very same rain now hits bare ground and runs straight off it. All of it arrives in the river at once, which is a flood. And a few months later there is nothing left in store, so the river shrinks and the wells fall, which is a drought. Exactly the same rainfall, and now it produces two disasters instead of none, purely because the buffer that used to spread it out over time has gone.
Notice what this really is. It is the same idea that runs through the whole unit, appearing again in a new place. Everything here is about a rate. A wetland does not create water and it does not destroy water. All it does is change the speed at which water arrives, and changing a speed turns out to be the difference between a river that feeds a district and a river that drowns it.
Examinations often ask how the loss of life and property along riverbanks caused by floods can be reduced. The weak answer is to build a wall. The strong answer is to conserve the watershed, and now you can explain why. A wall tries to hold back water that is already moving fast and already carrying soil. Protecting the forest and the wetland upstream stops the water from arriving all at once in the first place, so the flood is smaller before it ever reaches the bank. It is always cheaper to slow water down than to stop it.
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