Hi, hey everyone, welcome back to another video from Illersons Online. Okay, I'm going to be covering physical jog today, part 19 of our entire physical jog series so far. Okay, I'm going to be starting on drainage basin hydrology, right? To some of you, this is a topic which can be quite a challenge, okay, because yes, it is indeed a bit hard, okay, to actually understand sometimes, okay, but I hope that through this video and the next part as well, okay, it'll be made much easier for you to understand, all right? So...
It's going to be a long video, so just try and stay tuned, okay? We'll take a bit of breaks in between, pause if you need to absorb some stuff, okay? Alright, firstly, you have to understand what exactly the basin hydrological cycle is. Okay, the drainage basin is basically an open system.
Okay, what do I mean by open system? Okay, an open system basically means that it is emerged with the environment. So anything that's in the environment can play a part in the drainage basin hydrology. By definition over here, there is basically a clearly defined input and output beyond the confines of this drainage basin, which is going to be your precipitation. We'll learn later evapotranspiration as well.
And it is known as an area of land in which water flowing across the surface drains into a particular stream or river. Naturalized. Basically what it means is that Your drainage basin hydrology is basically an area of land, okay, whereby there's water flowing, okay, be it, okay, as a form of your stream, as a flow or as a storage.
This is basically the entire drainage basin hydrology, okay? So your main input that you would learn in this topic, okay, would be precipitation. Precipitation is basically rainfall, very, very simple, okay?
And your main outputs would be... River runoff and evapotranspiration. We'll jump into exactly what each one of these are in a bit All right, so as I have been saying, the components are basically your inputs outputs Your pathways flows and transfers as well as your storages. Essentially your pathway flows and transfers are all the same thing Usually I like to call it flows. I think it's easier to remember.
So inputs, flows, storages and output All right. So what exactly is the input? The main input of a drainage basin is basically precipitation, right? The only reason why it is only precipitation is because nothing else can produce water which will flow into your drainage basin. Water is always derived in the entire atmospheric cycle.
Your input, which is going to be precipitation, in this case in the form of rainfall, is the only way which your river channel is going to actually gain this amount of water. The water cannot just suddenly appear. It cannot appear out of thin air.
It always has to come from somewhere. In this case, it always comes from your rainfall. Okay, so that's going to be your main input.
Okay, it is basically the deposition of moisture on Earth's surface from the atmosphere. That's basically a very complex way of saying that it is rainfall. Okay, but it varies in terms of its type. Okay, which is why there could be several types of input.
Okay, right, it could be snow, it could be rain, it could be dew, or possibly condensation as well. Okay, but that one is in a very, very small amount. Okay, and then the quantity and intensity and your duration can also change. Okay, so these will all...
result in a different level of drainage basin in terms of how the drainage basin performs so are there more flows are there more storages is there lesser storage okay this is all as a result of input so you notice input over here okay when it comes to writing essays you will notice that it is basically like the main factor the biggest factor it basically affects everything in the drainage basin hydrology okay so then pathways and flows okay it is basically by definition the path in which moisture is being transported within the drainage basin so you just think of paths um or flows k as a form of transportation okay how does your water go from one place to another that is basically a flow okay so there are many different types of flows we will start off with interception loss k what is interception loss yes the name suggests interception water has to be intercepted somewhere okay so it basically refers to the holding of raindrops by plants as the water falls onto leaves and stems of vegetation cover It refers to a loss of water because it is an interception loss whereby water is being lost as a result of a form of interception. In this case, vegetation cover. Precipitation minus interception loss equals to the amount of water reaching the ground.
The amount of precipitation intercepted will depend on the leaf type. Is it a very big leaf or is it very small? If it's the smaller it is, the lesser interception the more water will reach the ground the more water goes into your drainage basin hydrology okay your wind speed is also a determining factor the intensity of precipitation all these will determine how much water is actually being intercepted and as a result being lost in the process when it hits your vegetation that is present there okay which later on affects how much water there is in your drainage basin okay now this is what starts your syllabus off okay you notice that infiltration is a huge huge huge flow um for your syllabus okay infiltration basically refers to a transfer of rainwater entering a permeable surface okay the keyword here is permeable surface okay infiltration always think of it as any any any form of water cannot even form okay any amount of water that is trying to infiltrate okay trying to attack okay trying to get in okay get into the ground okay so the ground um the amount of water which actually gets into into the ground determines how it is determined okay by how permeable the ground is okay so it contributes rainfall to your soil moisture moisture storage we'll learn this later on okay whereby the more permeable the soil the faster the infiltration rate so this one is a no-brainer right if your soil is going to be very let's say, extremely permeable, okay, because there are a lot of, let's say, a lot of holes on the surface, right, water is going to go in very fast, it's going to seep in very fast, as compared to if your ground is all concrete, okay, there is not a single crack or anything for the water to enter, the rate of infiltration is going to be much slower. Okay, so when we look at infiltration, we are always looking at infiltration rate and infiltration capacity. I think I have defined it later on.
Maybe? Okay, maybe not. Okay, but essentially, infiltration rate is how fast infiltration happens. Okay, infiltration capacity is how much can be infiltrated at a certain amount of time.
Okay, so you notice one is time, one is amount, okay? Alright, so the factors affecting infiltration, you need to know this. Okay, basically, firstly, rainfall intensity. So the higher the intensity of rainfall, okay, the surface will definitely reach infiltration capacity faster, which means that it will reach the maximum amount to which water can be infiltrated.
and as a result there will be less infiltration. On the other hand, another factor would be a vegetation cover. By a higher density of vegetation cover, it will slow down rainfall because of interception loss.
This can help infiltration by slowing the infiltration rate. Because lesser water touches the ground, lesser water can go in within that period of time. So that will slow down your infiltration rate. A layer of hummus can also hold rainwater.
Hummus is basically rather impermeable. Hence, it can aid in slowing down the infiltration as well. It would basically cause water to just flow on top before seeping in.
It will take a much longer time. For those who don't know what harmus is, it is basically decomposed matter. So the type of soil will also matter.
Your porosity versus permeability. This is where the concepts that come into play. Porosity is basically the amount or how large the pore spaces are within the soil or within the rocks. So basically the greater the amount of pore spaces, they are basically kind of like, you think of it as bubbles, air bubbles within the soil. The larger and the more amount of pore spaces there are, these bubbles are being able to fill up with water, more water.
But if there are lesser pore spaces, then there's lesser bubbles which can actually hold any water that is being infiltrated. Permeability is different. It's on the surface, how much water can actually come in in the first place. If the layer is not even permeable, water will take a long time and it may not be able to come in as fast as well. So an impermeable layer of soil will result in lesser infiltration.
For example, concrete, there are no holes on top, so it's very hard for water to actually seep in. It is an extremely impermeable surface. Or on the other hand, for example, soil or clay, which has a lot of pore spaces, can help to increase infiltration.
Okay, so more pore spaces means that more water can come in. Okay, your infiltration capacity, okay, will not be hit as early. Okay, as a result, the soil can actually hold more water.
This will encourage infiltration to occur. Hence, there will be more water which can actually seep downwards into the soil. Okay, the next flows that we have, okay, will be percolation.
Very important as well. Okay, percolation is basically slower than infiltration. So it's kind of like the next step. Okay, firstly, I have got...
infiltration then as I go down into the soil I'll have percolation P okay so basically it contributes to groundwater storage we'll learn this later on okay so it is basically the flow of rainwater which has been filtered downwards in the subsurface soil through the joints and pore spaces of the soil so this is basically what happens after the water has actually infiltrated okay think of it like like a war zone once the water has come in Then it goes in further to wipe out the rest of the enemies. That is percolation. So it goes further behind the first line of defense.
First line of defense is basically infiltration. So once it has passed this level of infiltration, it will move on to the groundwater storage area, the base flows, all those kind of stuff at the bottom, in the form of a flow called percolate. That is basically what they call the moving water in that area.
So the rate of percolation will definitely slow down as it gets deeper because the layers of rocks and soil get more compact. So your pore spaces are actually reduced. This part I've gone through before in soil profile. Go and check my previous videos on soil profile.
I cover exactly all your different horizons. Water will play a part in those as well. That is why percolation slows down because your rocks become very compact.
The factors affecting percolation are very simple. Firstly, the volume of pore spaces in the soil and the frequency of joints. Essentially, like I've said, the air bubbles.
Are there still a lot of pore spaces left? Basically, the more pore spaces there are, the more water can actually be held. And more cracks and fissures also mean that the water can pass through different layers very easily and very fast. Okay, so likewise, if there are more pore spaces, it means that the soil can actually hold more water.
Hence, there's more room for percolation to take place. Okay, the next flow we have is going to be true flow. True flow is basically the horizontal flow of water in soil moisture storage.
It is used as excess flow of percolation. This one is not very important. Just understand that true flow occurs within the soil moisture storage.
Okay? Moving on, we have got base flow. Ignore the typo that I made up there. It's not true flow.
We're looking at base flow now. Base flow is basically anything that occurs within the groundwater storage. It's all the way at the bottom, the rock bottom.
Basically, groundwater flow or base flow is the horizontal flow of water in the groundwater storage. So it's all the way at the bottom of the ground and it goes towards the river channel. It can only go vertically. sorry not vertically you can only go horizontally okay towards the river channel so think of it this way this is the top of the soy okay i have got another level over here and then i have got the rock bottom this is the rock bottom down here okay basically true flow is all of the water that goes within this layer here so it goes towards the river channel okay and then up here is basically Okay, so true flow is up here and then down here is going to be base flow.
Okay, anything within here is your soil moisture storage. Anything within down here is your groundwater storage. So look at it this way. It's a very, very simple cross diagram to it.
Okay, it's extremely slow due to high saturation because of the amount of rocks there are. It's very, very compact down there. Okay, overland flow. One of the more important flows you have to understand. It is the fastest flow out of all of the flows.
Killed by a portion of water does not manage to infiltrate the soil. Okay, so any water that is basically left on the surface that cannot infiltrate the soil, because of, let's say, maximum infiltration capacity was reached, or the rainfall intensity is too high, infiltration rate has slowed down to a minimum, water basically cannot infiltrate. It will basically become this other flow called overland flow.
So there are two types of overland flow. There is your Hortonian overland flow as well as the saturated overland flow. So HOF, your Hortonian overland flow, is basically when rainfall intensity exceeds the rate of infiltration.
okay understand this first i will go through it after this okay so if occurs when soy is at its water holding capacity so when the land is already saturated and water just simply cannot infiltrate okay then that is where swf will occur all right i'll go through one by one hf firstly occurs after infiltration okay so this is basically like what i just did just now it is post infiltration session okay when infiltration is really awkward but infiltration capacity was maxed out no more water can infiltrate it becomes hof Okay, so when conditions impede infiltration and channeling water to overland flow, occurs when PPT, precipitation, exceeds infiltration capacity or rate of infiltration when the ground is frozen or on steep slopes. Okay, so essentially, when water is unable to infiltrate for, let's say, certain physical conditions, such as steep slopes, you know, water starts to flow down the slope, that is HOF. Or either that, if your infiltration capacity has been reached, it also becomes HOF when there is excess rainfall.
Okay. SOF occurs when the ground is saturated and rainwater cannot infiltrate at all and it flows on the surface. So it's different. This means that basically infiltration cannot even take place to begin with.
So it may develop when rainfall is heavy and takes place over a few days resulting in an increase of height in water table or high soil antecedent moisture. What is soil antecedent moisture? I think I'll go through later on.
But for those who do not know, soil antecedent moisture is basically when the rain... Previously, let's say the day before, here and there is still excess water in the soil. which means that there are no pore spaces for any water to infiltrate when the next rainfall comes hence this can cause your soil to become saturated saturated basically means it is extremely dense very wet okay can cause it to be saturated and hence water is unable to infiltrate and this results in your saturated overland flow okay if you didn't catch that go and replay it one more time getting that causal links i just said is very very important it's crucial Okay, so just take note that HOF always occurs before SOF, and the soil must be near saturation before SOF can occur.
So like I said just now, it must be very, very dense, packed full of water before it can occur. Okay, so this is just a pictorial illustration, okay, HOF versus SOF. Okay, so basically HOF, as seen on the left-hand side, okay, is when infiltration exceeds overland flow, okay, and hence the water can... can start to to run off the surface so you notice that down below okay there are actually still some pore spaces so some water is infiltrating but because of the high rainfall intensity there's so much water on the surface that the infiltration rate becomes very very slow okay the water is unable to infiltrate that fast and possibly at the surface the capacity has really been reached so it takes a very long time to trickle down the soil okay and as a result there's some water which is left on top and this becomes your hof On the other hand, your SOF is a bit different.
Oops. Okay, SOF is a bit different. You notice that the soil is really all filled with water. Okay, it's all filled with water.
So the water is unable to even enter. Okay, hence it starts to fall along the side. Here on top on the surface as a form of your saturated overland flow instead.
Okay, so that was basically flows and your input that I've just covered. Okay, I know it has been a bit to absorb. So just take a quick break before we move on to stores and output.
all right and then after that um we should be nearing the end already okay so take a quick break okay one two three four five seconds all right let's move on okay so stores basically interception and biological water storage is the first one we're going to be covering okay basically interception and biological water storage essentially is very simple okay like it states okay biological is basically when there is vegetation cover on the earth surface which holds a certain amount of precipitation. So it can only be removed via evaporation. So this is basically like just now the first thing we covered, interception loss.
When there's interception loss, it basically means that water has touched the vegetation cover and it has fallen to the ground. So there's some water which has been lost. But at the same time, you realize that then where did the water go? The water is actually stored on the vegetation itself.
So that is basically interception storage. Simple, right? Okay, soil moisture storage.
It is important. So, moisture storage refers to the storage of moisture in the soil. Okay, so this occurs basically after infiltration and before percolation.
Okay, so you think of it as this. This is the surface over here. And then, this is where...
percolation starts to occur so the red arrow is percolation okay basically soil moisture storage is this whole region in here this is soil moisture storage so it's between the surface as well as the um your your groundwater storage the middle part is your soil moisture storage okay so this is your soil moisture storage and the bottom down here is your groundwater storage we'll learn this later on okay so it refers to the areas above water table where pore pieces are not saturated with water filled with both water and air okay this is just by the book definition all right next groundwater storage like i've just said here refers to the storage of water in the subsurface zone that is fully saturated below the water table okay so the upper surface of the water of the saturated zone okay is called the water table water table is basically an imaginary line okay where the pore spaces are completely filled with water and it separates the zone of aeration versus zone of saturation okay essentially what it means is when you have a plot of of of soil okay let's say there is water that covers all the way down here this is basically the water table it is basically an imaginary line okay because up here is all air this part here is all air up here and down here is all water so the line that crosses in between over here that is the water table simple Okay, channel storage refers to the river channel. Okay, it is a physical confine of a river. So this is where water from your groundwater storage, your subsurface, your soil moisture storage, it is where all your flows and everything travels towards the channel itself.
Okay, so it receives water indirectly through the flows from other stores and directly from precipitation. Okay, let's say if it rains and it falls into the river, that will basically help to increase the level of water in the river channel as well. Okay, then we move on to outputs.
Okay, there are basically two types of outputs. I won't go through it because it's so simple to understand. Then, okay, in fact, I'll cover more of evapotranspiration in the next video. Okay, stay tuned for that.
But firstly, we've got river runoff. Basically, the water that is flowing from river channel towards the sea. So it is basically lost from the drainage basin system.
So this goes out into the huge open sea that is away from this drainage basin. That is basically a river runoff. Okay, and next I've got evapotranspiration, which is basically water lost due to evaporation and transpiration. Okay, so when the water evaporates due to hot weather, that results in a loss of water as well. Okay, so lastly, exam requirements.
Okay, basically, you just need to understand the entire drainage basin hydrology in terms of your inputs, outputs, flows, and storages. Okay, be able to explain and discuss the function of the different parts of the drainage basin hydrology and how they all work cohesively to form one system. Alright, very, very simple. Okay, it tends to come up for essay questions and possibly...
you need to identify a bit of it in your drq your data response or case id question all right so drainage basin hydrology is actually very simple okay you're going to be looking more at your other things okay such as um essentially your factors okay that affect the drainage basin hydrology so for this part of video just understand what the flows your storage as your input output is here and then the next part we'll cover more on the factors as well as your water balance um actually the factors i've covered most of it in this video already so just go and look through the entire video again if need be okay and um that should actually help you out for your drainage basin hydrology all right so if you did enjoy this video okay be sure to give it a thumbs up okay um really does help me out a lot as well as the leaf um i mean be sure to subscribe okay if you actually enjoyed the content okay stay tuned for more i've got more content coming out this year um a lot more job a lot more econs possibly some gp as well so just stay tuned for that if not i'll see you guys in the next one bye