Hydrology part 1

Question 1

What is a drainage basin?

Answer 1

A drainage is natural system, it is the area that is drained by a river and any tributaries.

Question 2

What happens if the surface dips below the water table?

Answer 2

If the surface dips below the water table, ground water will fill the surface space to become surface water.

Question 3

What happens if the surface water dries up?

Answer 3

If the the surface water dries up, ground water will continue to replenish the area that is under the water table.

Question 4

What happens if the water table rises?

Answer 4

If the water table rises, surface water levels will also rise.

Question 5

How can groundwater levels be depleted?

Answer 5

Groundwater levels are depleted by:

-Human extraction

-Ground water replenishing surface levels.

Question 6

Describe two ways groundwater can be recharged.

Answer 6

Precipitation infiltrates the ground and percolates until it reaches groundwater. When precipitation is higher, and evaporation is lower, such as in winter, groundwater levels will be recharged.

Surface water bodies (such as lakes and river) seeping into groundwater stores.

Question 7

Explain how the type of precipitation affects the amount of groundwater storage. Use an example.

Answer 7

In storm events, large amounts of rainfall quickly saturates the ground to its field capacity increasing overland flow (saturation excess overland flow). Storm events are therefore less effective at recharging. Therefore, storm events are less effective recharging water stores than prolonged rainfall.

Question 8

Explain using data how type of precipitation affects the amount of groundwater recharge.

Answer 8

-If 20mm of rain fell over the course of 24 hours, this would infiltrate the soil and percolate into the rocks and increasing the groundwater stores, with low overland flow.

-However, if 20mm of rain fell over the course of an hour, there would be less water infiltrating the soil and percolating into the rocks, reducing of groundwater stores, but increasing overland flow (saturation excess overland flow).

Question 9

Describe two ways springs can form.

Answer 9

-Permeable rock meeting impermeable rock, causing infiltrated water to build up and eventually escape as a spring.

-Where the water table meets on the surface (especially on a hill), causing the discharge of groundwater.

Question 10

How can more precipitation affect the variation of a hydrograph.

Answer 10

Overall, more precipitation will lead to a flashier storm hydrograph, and will also create higher levels of discharge on an annual hydrograph.

Question 11

Why do storm events influence hydrographs?

Answer 11

Storm events influence hydrographs due to fast and heavy rainfall falling in short periods of time. When above average amounts of precipitation fall in short periods of time, there is not enough time for water to infiltrate (infiltration excess overland flow), and instead flows into the channel as overland flow. Bankfull discharge is reached quickly and any discharge above that line on the hydrograph flows outside the river channel.

Question 12

How may snow affect lag time?

Answer 12

If precipitation falls as snow it can be stored as surface water for longer. Therefore, it would take longer to reach the channel, affecting the lag time.

Question 13

How do higher temperatures influence storm hydrographs?

Answer 13

When temperatures are higher, water particles have more energy, meaning they are more likely to evaporate.

Therefore, in warmer climates, water stored in soil, bodies of water, on trees evaporates. This means more water can be stored in them before they reach their capacity.

Also, the soil is drier meaning it has more cracks and pores. This means that more of it can infiltrate the or ground or be stores, meaning less water overall reaches the river. Therefore, decreasing overall discharge.

Question 14

What is Antecedent moisture?

Answer 14

Antecedent moisture is the pre-existing level of moisture within soil before precipitation.

Question 15

How do soils with with a high antecedent moisture influence storm hydrographs?

Answer 15

Highly saturated soils (soils with a high antecedent moisture) have a lot of water already stored, meaning that the soil will become saturated much sooner., and water will flow as overland flow instead. Thus, there will be more water flowing as channel flow rather than throughflow or infiltration. Thus, increasing river discharge.

Question 16

How do soils with a low antecedent moisture influence storm hydrographs?

Answer 16

Soils with a low antecedent moisture may become too dry for water for deteriorate. This means more water flows as overland flow and thus there will more river discharge.

Question 17

What does higher river discharge cause?

Answer 17

Higher river discharge leads to a flashier hydrograph (a hydrograph with higher discharge as well as a quicker lag time).

Question 18

How can Spring and summer influence storm hydrographs?

Answer 18

Spring and summer usually has more vegetation growth. This means that there is more interception by vegetation and in turn more transpiration. Therefore, there is a lower antecdent moisture so more water potentially more water can infiltrate the ground and be stored. This will lead to less overland flow, thereby reducing channel flow.

Precipitation is usually lower in many regions, therefore leading to less saturated soil (more infiltration etc.)

Question 19

How does the size of a drainage basin influence storm hydrographs?

Answer 19

In larger drainage basins, overland flow has more distance to cover before it reaches a river, potentially leading to a longer lag time. Furthermore, large basins cover more area, therefore there is potentially more water flowing into the river and it tributaries, leading to a higher peak discharge.

Question 20

How does the shape of a drainage basin influence storm hydrographs?

Answer 20

In circular drainage basins, different starting points of overland flow are likely to be to similar distances apart, meaning they are more likely to join the river at a similar time. This leads to a large quantity of water concentrating into an area in a short space of time, therefore increasing discharge as well as shortening lag time.

In contrast, in an oval drainage basin of a similar area, the starting points of overland flow will be different distances apart, meaning they will arrive at different times. Due to this, the lag time will be longer.

Question 21

What is drainage density?

Answer 21

The drainage density is the total length of all rivers and streams in a drainage basin divided by total area of the drainage basin.

Question 22

What is drainage density an indicator of?

Answer 22

Drainage density is an indicator of how well the drainage basin is drained.

Question 23

How does a drainage basin with a high drainage density influence storm hydrographs?

Answer 23

In a drainage basin with a high drainage density, there are more streams and rivers carrying water, meaning the peak discharge is high and it is reached quickly, meaning there is a short lag time. As well as this, the amount of rivers mean the water is drained quickly, leading to steep falling limb.

Question 24

How does a drainage basin with low drainage density influence storm hydrographs?

Answer 24

In areas of low stream density, overland flow is more likely to infiltrate rather than make its way into channels, therefore lengthening the lag time and lowering the peak discharge, as there is less water draining into the river.

Question 25

How does porosity influence storm hydrographs?

Use examples

Answer 25

The porosity of clay soils is higher than that of sandy soil (overall there is more space between each soil particle). In periods of lighter rain, (when there is more infiltrate time for water to clay soil), overland flow may be higher in areas with sandy soils as there are less empty spaces within the soil, leading to higher river discharge.

Question 26

How does rock type influence storm hydrographs?

Answer 26

Permeable rocks such as sandstone (a sedimentary rock) infiltrate water. However, impermeable rocks such as slate (a type of metamorphic rock) do not infiltrate water as there are is no space for the water to seep into. Therefore, drainage basins with permeable rock will have less overland flow, leading to less discharge. Furthermore, water stored in soil on top of permeable rocks can percolate into rocks below, making the soil less saturated and therefore more capable of holding water.

Question 27

How does topography influence storm hydrographs?

Answer 27

A rough, jagged topography reduced velocity of water, as the flow of water is interrupted with bumps, rocks, debris etc.

In contrast, a smoother topography allows water to move quickly. Therefore, in areas with jagged topography, peak discharge is usually lower due to slower overland flow. Thus, longer lag time.

Question 28

How does relief influence storm hydrographs?

Answer 28

A steeper slope allows water to travel faster as the force of gravity is stronger. As well as this, water therefore has less time to infiltrate, and more of it flows as overland flow. This makes the lag time quicker and the peak discharge higher (a flashier hydrograph).

Question 29

How vegetation influence storm hydrographs?

Answer 29

Tree and plants roots create passages for water to infiltrate into the lower layers of the soil, meaning less water as overland flow and therefore less water goes into the river channel. Furthermore, vegetation decreases soil moisture content as vegetation takes up water and releases it into the atmosphere through transpiration. The process is more significant in forested areas, rather than grassy fields.

Question 30

How does deforestation influence storm hydrographs?

Answer 30

-There is less interception by trees so overland flow increases.

-The soil is no longer held together by roots, so soil water storage decreases.

-There are fewer plants so less transpiration.

-Overall, this leads to flashier hydrograph, with a shorter lag time and higher peak discharge.

Question 31

How does livestock influence storm hydrographs?

Answer 31

-Livestock can trample the ground reducing infiltration as the pore spaces are lost.

Question 32

How does ploughing influence storm hydrographs?

Answer 32

-Ploughing increases infiltration by creating a looser soil, which decreases overland flow. However, digging drainage ditches (often seen around field edges) increases surface run off stream flow.

Question 33

How does hillside terracing influence storm hydrographs?

Answer 33

-Hillside terracing (for rice padi fields) increases surface water storage and therefore decreases run off.

Question 34

How does irrigation influence storm hydrographs?

Answer 34

Irrigation can lead to ground water depletion, therefore affecting the water table and therefore allowing more water to enter channels before the river is in flood.

Question 35

How does urbanisation influence storm hydrographs?

Answer 35

Roads and buildings have impermeable surfaces, reducing infiltration and increasing overland flow. This reduces lag time and increases peak discharge.

Question 36

Define erosion

Answer 36

Erosion is the process of the wearing away of soil and rock. In a river channel, both the force of water as well as fragments of rock cause the river bank to wear away in certain areas over time.

Question 37

What is corrasion?

Answer 37

This is the process of rocks scraping and grinding (also known as abrasion) along the river channel, causing soil and rock to wear away. This usually causes the river bed to deepen and widen, and is especially prominent when a river is flowing with high velocity as it enough energy to carry larger rocks.

Question 38

What is solution?

Answer 38

This is process of water dissolving parts of rock/soil that makes up the river channel. Solution is the most prominent when the river channel is made from water soluble rocks, such as chalk, gypsum, or limestone. Water can slowly wear away at the river channel over time through this process.

Question 39

What is hydraulic action?

Answer 39

Hydraulic action is the sheer force of water. It can cause rocks to fragment off when the channel is hit.

Question 40

What is cavitation?

Answer 40

Cavitation is where water forces its way into small cracks, causing the air in these spaces to be compressed and put under pressure, widening cracks and fragmenting the rocks.

Question 41

What are the main points of the Hjulstörm curve?

Answer 41

-The graph is logarithmic

-The critical erosion velocity curve is the minimum velocity needed for the material to be tranposrted and then eroded. The velocity must reach this line for the material to be transported initially, but once the velocity exceeds this line, there is a likelihood that the particle will be eroded.

Question 42

What does the critical erosion velocity curve show?

Also, why do silts and clay not follow the general trend of the CEV?

Answer 42

The critical erosion velocity curve show the relationship between the material size and velocity needed to transport and erode it. In general, the larger the material size, the more energy needed to transport the material, therefore the higher the velocity needed.

This is not true for clay and silts, because these bond together. This means that it is harder to break the particles up to transport or erode them.

Question 43

What is the settling velocity curve?

Answer 43

The settling velocity curve is the point at which the material no longer has the energy required to be transported, meaning it is deposited. If the velocity falls below this line there is a likelihood that the particles will be deposited. The large the material, the higher the velocity needed to keep the particles transporting.

Question 44

Why is there no settling velocity for clays and silts?

Answer 44

Once clays and slits are transported they will continue be transported, and will not be deposited.

Question 45

Describe drainage basin characteristics in tropical rainforests.

Answer 45

-On average tropical rainforests receive 2000-10000mm of precipitation per year. Also, the soil in tropical rainforests have a high porosity (54-68%). This means that there will high infiltration rates and thus low surface run off.

-Furthermore, rain is intercepted and stored by the the rainforest canopy. Therefore, there are high rates of evapotranspiration. The water vapour cools and condenses to form clouds.

Question 46

Describe drainage basin characteristics in arid semi-arid areas.

Answer 46

Arid climates have less than 350mm of precipitation per year. In semi-arid areas there is an average of 350-500mm of precipitation. In almost all arid and semi-arid areas almost all water is transpired/evaporated (there is excessive heat as temperatures in the hottest months range from 29 and 35 °C).

-Drainage in arid environments frequently exhibit low infiltration capacities and soils with no moisture. Therefore, surface run off is prevalent in this climate. Furthermore, streamflow is often intermittent because of lower precipitation and deep groundwater flow levels.

Question 47

Describe drainage basin characteristics in tundra climates.

Answer 47

Tundra winters have mean temperatures below 0°C for 6 to 10 months of the year. This creates a layer of permanently frozen ground below the surface (permafrost). This means that infiltration will not occur as the ground is frozen and thus impermeable.

Question 48

What is infiltration-excess overland flow?

Answer 48

Infiltration-excess overland flow occurs when the rain is falling more quickly than the infiltration rate.

Question 49

What is saturation-excess overland flow?

Answer 49

Saturation-excess overland flow occurs when soil spaces are so full of water than more rain can be absorbed.

Question 50

What is laminar flow?

Answer 50

Laminar flow is the flow of water in parallel streams. There are no cross currents of swirls. It is one directional orderly flow.

Question 51

What is turbulent flow?

Answer 51

Turbulent flow is disorderly flow. It changes in in velocity (speed and direction). It is caused by friction, especially when river channels are irregular. Eddies (swirling reverse currents) are common in this flow.

Question 52

What is Helicoidal flow?

Answer 52

Helicoidal flow is the corkscrew movement of water. It occurs in bends in the river channel called meanders. It is responsible for the erosion and deposition in a meander.

Question 53

What is the thalweg?

Answer 53

The thalweg is the fastest flow (maximum velocity) within a river. This is also the deepest point within the river channel (as erosion is the greatest when stream flow is fastest).

Question 54

Describe the characteristics of straight channels.

Answer 54

-Straight channels are mainly in the upper course f the river.

-It is a single, straight channel.

-Vertical erosion is prevalent

-Thalweg moves from side to side.

Question 55

Describe the characteristics of a meandering channel.

Answer 55

-Meandering channels occurs in the middle and lower course of the river

-They consist of a series of bends and curves from side to side

-Deposition occurs on the inside of the bend. Erosion occurs on the outside of the bend.

Question 56

Describe the characteristics of braided channels,

Answer 56

-Braided channels have large deposits of sediments within the channel, causing multiple channels separated by islands of sediment.

-Braided channels occur where discharge fluctuates.

-Deposition occurs when the river loses its energy (river velocity decreases and friction increases).

-Braided channels do not usually occur in the upper course.