Recap: a hydrograph is a graph that shows the discharge of a point in the river over a period of time, usually to show the change in discharge caused by rainfall. Discharge is just the amount of water that flows through that certain point and is measured in cumecs - cubic meters per second. The significance about hydrographs is that we can find out about the patterns in a certain drainage basin, how much water passes through a point and how long it takes for the water to get there, and from there we can implement protective measures to ward against floods and things like that.
Since all that water has to come from somewhere, let's look back to the hydrological cycle.
Overland flow and through flow all contribute to the amount of water in the drainage basin. The amount of overland flow/surface runoff is dependent on rainwater intensity and the infiltration capacity of the soil. If it's been raining heavily for a while and the soil is already saturated, then it stands to reason that there would be more overland flow. Throughflow is the water that flows horizontally in the soil (it's different from groundwater flow, that is in, well, groundwater but this is in the soil).The factors that would affect the amount of water would be:
- Urbanisation In this case, we're assuming urbanisation doesn't include drains. (But if there were, it would lead to a longer lag time, because the water takes a longer time to get to the river, or if the drains lead to reservoirs or water storage areas, then a lower peak). Urbanisation would basically entail a concrete jungle, and concrete has really poor permeability. As a result, there would be a lot more water flowing on the surface.
- Reforestation Reforestation would mean more trees, and more interception (and also more evapotranspiration). So there would be less water reaching the surface, and less overland flow. And of course, it would offset some of the carbon in the atmosphere, and become a carbon sink.
- Deforestation The lack of trees would conversely mean less interception, and more water would reach the surface.
- Permeability of soil Very permeable soil means that it is more likely to absorb water and not so permeable soil means that more water would "pass" over it rather than being absorbed. This is assuming that infiltration capacity is constant, because the soil could be very permeable but cannot absorb a lot of water.
- (If you can think of some more factors, leave it in the comments :>)
But I think the thing to note here is that, a lot of human activity is affecting not only the hydrological cycle, but also the discharge in the rivers. In reality, a lot of the floods and water-related disasters could be due us, our need to build buildings, cut down trees and so on. So we should balance our activity with our environmental conscience - because the environment can only take so much and bring the conditions back to equilibrium.
Hi Yi Shien! I just want to add a bit on your last point.
ReplyDeleteFirstly, I agree that human activities partly cause floods and water-related disasters because there are also other factors such as geographical location. For instances, the tsunamis Japan had to suffer is not occurs due to urbanization or deforestation. In fact, Japan does not know how to prevent tsunami from occurring because it is nature and unfortunately, Japan lies on its pathway. That's also the reason why Japan's policies are more about "Living with Tsunami" rather than "Preventing Tsunamis".
Secondly, canal system also affects hydrograph and hydrological cycle in one country because it allows water to discharge and hence, prevents flood. One good example for this would be Singapore. Besides the fact that Singapore is at a nice geographical location that natural disaster could hardly harm it, Singapore also has a well-operated canal system which acts like a water collection point. That helps water to be discharged faster and leave no room for flood to occur. Compared it to Vietnam where the canal system is not that well-operated. After a rain, Vietnam can be flooded with water at waist level. It is because the canal system is not sufficient and good enough to discharge the amount of water (even though the amount of raining water in Vietnam is comparable to that in Vietnam). That's also the reason why the Vietnamese find it a bit surprising when one all the Singapore's newspaper display on their first page that Orchard Road is flooded with water at knee-level only (haha). They have been used to what happened in Vietnam.
Thirdly, despite urbanization, Singapore can still maintain its good environment because Singapore has wisely used the space on land to balance between urbanized buildings and vegetation. In fact, one can easily find trees anywhere in Singapore. That's the reason why Singapore is known as one of the greenest countries in the world.
Hope these help a bit. :)
Posted by Quynh
Hey the Other Factor that can affect the hydro-graphs are:-
ReplyDelete1. Area of the drainage basin
2. Slope of the drainage basin
3. Types of soil and rocks in the area
4. Shape of the drainage basin
5. Dams on the river
6. The drainage density i.e no.of tributaries draining the area