Jude Dominic
Yesterday's lesson was mainly about hydrographs. A hydrograph shows the discharge of a river over a period of time. Storm hydrographs show the river's response to rainfall events. This is an example of a hydrograph:
The x-axis shows the precipitation as well as runoff discharge, which is measured in cumecs(cubic metres per second) and the y-axis shows time in hours. The green bars refer to the amount of rainfall. The curve shows the discharge through storm flow(throughflow, overland flow etc.), which is indicated by the blue part of the curve, and baseflow, which is indicated by the red part of the curve. The lag time is the time between the peak rainfall and peak discharge. The curve can vary due to different factors. If there is very heavy rain and the soil is saturated or frozen, there will be heavy overland flow and the curve will be very steep. However, if rain is light and there are a lot of trees and other vegetation( which results in more interception and obstructs overland flow), the curve will be less steep.
Hydrographs are highly useful to people managing drainage basins. The discharge rate of a river can sometimes be so high that the river floods its banks. Follow this link to see an animation of a flood(go to the bottom of the page and play the animation): http://www.geology.sdsu.edu/classes/geol351/hydrographs.htm. Human activities can also change a hydrograph. For example, there will be a generally low discharge rate in an area with much vegetation. However, if people clear the vegetation to gain land for other purposes, the discharge rate will become higher and flooding may occur.
We were asked to do the questions on page 231 of our notes as homework, so I may as well post up the answers here(although I'm not sure if they're correct. And I'm not going to give you the hydrographs here, because I can't draw hydrographs well on my pc).
2a) Lag time is the amount of time between peak rainfall and peak discharge.
2b) If lag time is short, a large amount of water will flow out in a short time, so there is a greater chance of flooding. If it is long, the water will drain out at a lower rate and there is a reduced risk of flooding.
3(i) a small drainage basin with very steep slopes: I won't give the graph but will describe it. The curve is a very steep one with a short lag time as the discharge rate is very high because of a steep slope.
3(ii) a linear shaped drainage basin: This graph has a gentle curve and long lag time. This is because the water discharges out slower due to the absence of a slope.
3(iii) a drainage basin that has recently been deforested: The curve is steep and the lag time is short. This is because there is less interception and overland flow because of the absence of trees.
3(iv) a long period of mainly light rain in summer: The curve is gentle and the lag time is long. This is because of less rain and because of more evaporation because of the summer sun. The tree canopy will also be at its highest at this time of year, so there is more interception.
3(v) a heavy rainstorm falling on thick snow in march: The curve is steep and the lag time is short. This is because the soil is frozen and so there is very heavy overland flow.
5. Hydrographs are useful to people for predicting floods and finding out how human activities like deforestation and urbanisation can affect the discharge rate of a drainage basin.
posted by 08ip01 at
3:52 PM
7 Comments:
There are other factors that affect the hydrograph as well:
- Shape of the drainage basin
- Steepness of the drainage basin
- Size of drainage basin
- etc (:D)
(Vu Ngoc Quang)
There are 5 factors affecting the shape of the hydrograph actually:
- Characteristic of the drainage basin: Shape, size, gradients, etc.
- Present condition of the drainage basin: If the drainage basin is saturated, if the drainage basin is forested, etc. (I believe u guys should be able to interpret what would happen in those cases).
- Types and amount of precipitation: What if there is only snow, will there be any discharge whatsoever? No, right? For the same amount of precipitation, if the rain fall is concentrated in a few hours, then the spongy and soft surface soil is possibly washed off or compacted together which greatly reduces infiltration capacity of the soil, hence higher run off and higher peak discharge with steeper rising limb of the hydrograph.
- Human activities such as damming or channeling also affect the hydrograph.
- And finally, may be the most important factor I think, is the position where the measurement for the hydrograph is taken.
The previous comment was posted by me, Hazel.
Yup indeed, there are many factors that affect the hydrograph. The surroundings is an important factor, whether the area is a rural or urban area creates a gentler and steeper graph respectively, due to the less impermeable surface and the variation of surface types. Vegetation types (Deforestation and amount of interception) and seasons also affects the hydrograph. Very dry weather creates a crust on the river bed and wet winters create increase in dishcarge.
Nice post Jude. Now i realise how many factors lead to the shape of the hydro graph. Eg. Environment, Vegetation, Weather. Yea!
Hazel is absolutely right. Just to expand on what she had said, the conditions of the drainage basin like the type of soil can largely affect the hydrograph. If the soil present is porous, then its infiltration capacity will be higher and the hydrograph will have a lower peak. And vice versa.
For urban areas, some might think that it has a less efficient discharge because of the concrete ground as compared to rural areas, which boasts natural patches of grass.
However, because of mankind's great planning, efficient drainage systems are constructed to swiftly clear our rainfall. But sometimes, when the monsoon goes out of hand because of the drastic climatic conditions caused by humankind, our roads do get flooded and disrupt businesses.
Prevention is better than cure. I believe we should do everything in moderation so as to decelerate the harmful consequence we are subjecting Earth to. Also, some irresponsible people like to litter, hence choking up drains. T_T
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