River Energy + Water Budget & Water Security Flashcards
The following is the water budget equation:
ΔS = P – E – T ± SRO ± GF
What does it mean?
ΔS = net change in storage
P = precipitation
E = evaporation
T = transpiration
SRO = surface runoff GF groundwater flow
What is porosity?
• amount of empty space within a given material.
• In a soil or rock the porosity (empty space) exists between the grains of minerals.
• The amount of water a material can hold is directly related to the porosity since water will try and fill the empty spaces in a material
What is permeability?
• how connected pore spaces are to one another.
• If the material has high permeability than pore spaces are connected to one another allowing water to flow from one to another.
• However, if there is low permeability then the pore spaces are isolated and water is trapped within them
Gradient of land from upper to lower course?
Decreases; Steep → Gentle
River speed from upper to lower course?
Increases
River depth from upper to lower course?
Increases; Shallow → Deep
Width of river channel from upper to lower course?
Increases; Narrow → Wide
Roughness of channel (presence of obstructions) from upper to lower course?
Decreases; Rough → Smooth
Factors affecting river energy? (2 main points)
Velocity and Volume
Velocity → The speed of the river. It means how fast or slow water is flowing, which is influenced by the amount of friction the water encountered in the river channel.
Volume → the amount of water flowing in a river.
Factors affecting volume of river? (4 main points)
- Size of Drainage Basin
- Presence of Vegetative Cover
- Permeability of Rocks
- Climate (precipitation, temperature)
Factors affecting velocity of river? (3 main points)
- Channel Roughness
- Channel Shape
- Gradient of River
What happens when velocity/volume increases/decreases?
When Volume ↑, Energy ↑
When Velocity ↑, Energy ↑
When Volume ↓, Energy ↓
When Velocity ↓, Energy ↓
- Size of Drainage Basin
• Basically, when size of drainage basin ↑/↓, the number of tributaries ↑/↓, volume of flow ↑/↓, so river energy ↑/↓.
1. Size of Drainage Basin (affects volume)
• Drainage basin is an area of land drained by a river and its tributaries. The two key defining characteristics of a drainage basin is the drainage divide that traces the perimeter of the basin, and the river or drainage network that conveys the surface water out of the basin.
• A larger drainage basin will have a greater volume of water. The greater number of tributaries bringing water to the river results in a higher surface runoff and subsequently a higher volume of flow.
• The larger the drainage basin, the greater number of tributaries, subsequently the higher the volume of flow.
• Basically, when size of drainage basin ↑/↓, the number of tributaries ↑/↓, volume of flow ↑/↓, so river energy ↑/↓.
- more tributaries are found at the lower course of the river (so volume at lower course > upper course).
- Presence of Vegetative Cover
• Basically, when vegetation is dense/sparse, rate of interception↑/↓, and rate of infiltration ↑/↓, so there is lower/higher surface runoff, and volume of water ↓/↑, so river energy ↓/↑.
- Presence of Vegetative Cover (affects volume)
• The presence of vegetation affects the rate at which water is intercepted and is able to infiltrate the ground.
• An area covered with vegetation will have higher rates of interception and infiltration, and lower surface runoff.
- Less surface runoff into a river will lead to lower volume of water. (Figure A)
• On the other hand, an area with no or little vegetation will have no interception and little infiltration into the ground.
- This will result in more surface runoff and higher volume of water. (Figure B)
• Basically, when vegetation is dense/sparse, rate of interception↑/↓, and rate of infiltration ↑/↓, so there is lower/higher surface runoff, and volume of water ↓/↑, so river energy ↓/↑.
- Permeability of Rocks
• Basically, as permeability of rocks ↓/↑, the rate of infiltration ↓/↑, which causes high/low surface runoff, so volume of flow ↑/↓, thus river energy ↑/↓.
- Permeability of Rocks (affects volume)
• Rocks which allow water to pass either via a network of pore spaces between particles or along bedding planes, cracks, and fissures are permeable.
• All rocks are permeable but not to the same degree.
- Permeability depends on the grain size of the rock and the size of pore spaces between the grains.
→ Permeable rocks like sandstone, chalk and limestone which have coarse grain size and large pore spaces allow water to infiltrate easily. Conversely, rocks like shale, clay and granite which have tightly packed small grains do not allow water to pass through.
• When water flows through an area of permeable rock, more water infiltrates the ground. As a result, there is low surface runoff and this leads to a lower volume of flow.
• The opposite is true when a river flows through an area of impermeable rocks, little water infiltrates into the ground which means that there is high surface runoff and volume of flow is higher.
• Basically, as permeability of rocks ↓/↑, the rate of infiltration ↓/↑, which causes high/low surface runoff, so volume of flow ↑/↓, thus river energy ↑/↓.
- Climate
Factors affecting climate: precipitation and temperature
• Basically,
During wet/dry season, the rate of precipitation ↑/↓, so volume of water in river ↑/↓, so river energy ↑/↓.
When duration and intensity of rainfall ↑/↓, volume of water in river ↑/↓, so river energy ↑/↓.
When temperature ↑/↓, rate of evaporation and transpiration ↑/↓, so volume of water in river ↑/↓, thus river energy ↑/↓.
- Climate - Precipitation and Temperature (affects volume)
• The amount of precipitation and temperature affects the volume of river.
• During dry season (↓ precipitation), a river channel may be almost empty therefore the volume is low.
• During the wet season (↑ precipitation), the volume of water will be high and water may overflow the channel and flood the surrounding land.
• The intensity and duration of rainfall affects the amount of rainfall.
- High rainfall of long duration will result in high volume of flow, compared to low rainfall for a brief period.
• Temperature also plays a part in influencing the volume of rivers.
- When the temperature is high, there is more evaporation and transpiration, so volume of water would decrease.
• Basically,
During wet/dry season, the rate of precipitation ↑/↓, so volume of water in river ↑/↓, so river energy ↑/↓.
When duration and intensity of rainfall ↑/↓, volume of water in river ↑/↓, so river energy ↑/↓.
When temperature ↑/↓, rate of evaporation and transpiration ↑/↓, so volume of water in river ↑/↓, thus river energy ↑/↓.
- Channel Roughness
Basically, when protruding rocks on the river banks and the presence of boulders on river bed are present/absent, the amount of resistance and friction between the river flow and the river channel ↑/↓, so the channel roughness ↑/↓. Since there is more/less friction to be overcome, hence velocity ↓/↑, so energy or river ↓/↑.
- Channel Roughness (affects Velocity)
• Channel roughness is a measure of the resistivity offered by the material constituting stream.
• Roughness results in more friction between the river flow and channel.
• The rougher the river channel, the slower is the river flow.
Flow of Water in Rough and Smooth Channels
• Channel roughness is influenced by protruding rocks on the river banks and the presence of boulders on the river bed.
• Roughness is increased with the presence of boulders on the river bed and protruding rocks on the river banks.
• River flow encounters more resistance and friction in a rough channel.
• Friction needs to be overcome and this slows down river flow in a rough channel.
• A river flows faster in a channel made of smooth silt and clay.
• Basically, when protruding rocks on the river banks and the presence of boulders on river bed are present/absent, the amount of resistance and friction between the river flow and the river channel ↑/↓, so the channel roughness ↑/↓. Since there is more/less friction to be overcome, hence velocity ↓/↑, so energy or river ↓/↑.
- Channel Shape
• Basically, when channel shape is flat and wide/deep and narrow, the wetted perimeter is larger/smaller as it has a larger/smaller surface area that is in contact with water. Since the wetter perimeter is large/small, it has to overcome greater/lesser friction so the flow of the river would be slower/faster, hence velocity ↓/↑, so energy of river ↓/↑.
- Channel Shape (affects Velocity)
• The river has to overcome friction with the banks and bed of the channel.
• The shape of the channel or its cross section affects the wetted perimeter.
- The wetted perimeter refers to the extent to which water is in contact with its channel.
• The greater the wetted perimeter, the greater the friction between the water and the banks and the bed of the channel, and the slower the flow of river.
• Flat, wide channels (like river A) have a larger wetted perimeter as it has a larger surface area that is in contact with water. The greater the wetted perimeter, the greater the friction it has to overcome and the slower the flow and vice versa for deep, narrow channels (like river B).
• Basically, when channel shape is flat and wide/deep and narrow, the wetted perimeter is larger/smaller as it has a larger/smaller surface area that is in contact with water. Since the wetter perimeter is large/small, it has to overcome greater/lesser friction so the flow of the river would be slower/faster, hence velocity ↓/↑, so energy of river ↓/↑.
- Gradient of River
• Basically, when the angle of inclination is high/low, the gradient would be steeper/gentler, hence the velocity of the river↑/↓, so the energy of the river ↑/↓.
- Gradient of River (affects Velocity)
• The gradient of a slope depends on the angle of inclination or the rate of rise or fall. A steep slope has a sharp inclination and a rapid rate of change compared to a gentle slope.
• Gradient decreases downstream; the river usually have a steep gradient near its source and a gentle gradient towards its mouth.
• A river flowing down a steep slope or gradient has a higher velocity than one which flows down a gentler gradient.
- For example, the speed of flow in a river that plunges down a steep slope in the form of a waterfall is much higher than the speed of flow in a river that winds its way down a gentle slope
• Basically, when the angle of inclination is high/low, the gradient would be steeper/gentler, hence the velocity of the river↑/↓, so the energy of the river ↑/↓.
• When size of drainage basin ↑/↓, ?
• When size of drainage basin ↑/↓, the number of tributaries ↑/↓, volume of flow ↑/↓, so river energy ↑/↓.
• When vegetation is dense/sparse, ?
• When vegetation is dense/sparse, rate of interception↑/↓, and rate of infiltration ↑/↓, so there is lower/higher surface runoff, and volume of water ↓/↑, so river energy ↓/↑.
• As permeability of rocks ↓/↑, ?
• As permeability of rocks ↓/↑, the rate of infiltration ↓/↑, which causes high/low surface runoff, so volume of flow ↑/↓, thus river energy ↑/↓.
• During wet/dry season, the rate of precipitation ?/?, so ?
• During wet/dry season, the rate of precipitation ↑/↓, so volume of water in river ↑/↓, so river energy ↑/↓.
• When duration and intensity of rainfall ↑/↓, ?
• When duration and intensity of rainfall ↑/↓, volume of water in river ↑/↓, so river energy ↑/↓.
• When temperature ↑/↓, ?
• When temperature ↑/↓, rate of evaporation and transpiration ↑/↓, so volume of water in river ↑/↓, thus river energy ↑/↓.
• When protruding rocks on the river banks and the presence of boulders on river bed are present/absent, the amount of ?
• When protruding rocks on the river banks and the presence of boulders on river bed are present/absent, the amount of resistance and friction between the river flow and the river channel ↑/↓, so the channel roughness ↑/↓. Since there is more/less friction to be overcome, hence velocity ↓/↑, so energy or river ↓/↑.
