1.2 Hydrology and fluvial geomorphology: discharge relationships within drainage basins

Question 1

Define discharge

Answer 1

Discharge is the volume of water that flows past a point in the river over a certain period of time

Question 2

Define rising limb

Answer 2

Rising limb is the period when discharge is rising from the start of a rainfall event until it reaches peak discharge

Question 3

Define falling limb

Answer 3

Falling limb is the period when discharge is falling

Question 4

Define lag time

Answer 4

Lag time is the time between the rainfall peak and peak discharge

Question 5

Define peak rainfall

Answer 5

Peak is when the highest amount of rainfall occurs during a rainfall event

Question 6

Define stormflow

Answer 6

Stormflow is storm runoff resulting from storm precipitation involving both surface runoff and throughflow

Question 7

Define baseflow

Answer 7

Baseflow is water that has infiltrated and percolated into the bed rock below the soil and then moves laterally under gravity or hydrostatic pressures in a downslope direction to feed springs and river channels.

Question 8

Define storm event

Answer 8

Storm event is any disturbed state of an environment or in an astronomical body’s atmosphere especially affecting its surface, and strongly implying sever weather.

Question 9

What is a storm hydrograph

Answer 9

Storm hydrograph shows how river discharge responds to a rainfall event

Question 10

What is an annual hydrograph/river regime

Answer 10

Annual hydrograph/river regime displays the pattern of seasonal variation that takes place to a river’s discharge in a typical year. The peak in summer months is explained by snow melt or a summer monsoon

Question 11

How does precipitation type impact shape of hydrographs

Answer 11

Precipitation type – the form in which precipitation is received by the drainage basin system. Rain will be available to the system very quickly, whereas snow will delay the impact on the system; but it may then have a dramatic effect on the hydrograph as it may be released quickly as meltwater.

Question 12

How does precipitation intensity impact shape of hydrographs

Answer 12

Precipitation intensity – rate at which precipitation is received at the ground surface – it is the amount of precipitation in millimetres divided by the time.

When precipitation intensity exceeds infiltration capacity of the soil or surface it lands on water will remain on the surface and overland flow/surface runoff will result.

This means that water will reach the river channel very rapidly, producing rapid rises in river channel discharge and high flood peak discharges. On a storm hydrograph this will be indicated by a short lag time, and steep rising limb.

Low-intensity rainfall is likely to infiltrate into the soil and percolate slowly into the rock, thereby increasing the time lag and reducing the peak flow.

Question 13

How does temperature, evaporation, transpiration, and evapotranspiration impact shape of hydrographs
Ignore this for now

Answer 13

a river in an equatorial climate may have a fairly constant annual pattern as it may have most of these factors constant throughout the year. A river which experiences distinct wet and dry seasons will reflect the varying input of precipitation. Drainage basins that experience freezing temperature climates will have annual hydrographs that have very marked changes as the seasons and the factors change. During the winter there may be a lack of precipitation input as precipitation falls as snow and is not available to the river. In spring and early summer there may be a sudden input of meltwater as the snow is melted by higher temperatures

Question 14

How does antecedent moisture impact shape of storm hydrograph

Answer 14

Antecedent moisture is the moisture retained in the soil before a rainfall event. This retained/residual water moisture affects the soil’s infiltration capacity. During the next rainfall event, the infiltration capacity will cause the soil to be saturated at a different rate, as the higher the level of antecedent soil moisture, the more quickly the soil becomes saturated, overland flow/surface runoff will occur.

Question 15

How does drainage basin size impact shape of hydrograph

Answer 15

the larger the size of the drainage basin the greater the amount of water is likely to be collected and released as river discharge, although this might take longer to reach the river channel and therefore have a longer lag time. If a drainage basin is very large, like the Mississippi or the Nile, a rainfall event may affect only one part of the basin.

Question 16

How does drainage basin shape impact shape of hydrograph

Answer 16

an elongated basin has relatively short lag times (ie they are said to have a flashy response), but peak discharges, although fairly low, may be sustained for a long period of time. Round basins, have longer lag times but a higher peak discharge.

Question 17

How does drainage basin density impact shape of hydrograph

Answer 17

high drainage densities mean that water will reach river and stream channels quickly, because water will have a relatively short distance to flow to a river channel, which will produce a rapid response – a flashy hydrograph – giving high, sharp flood peaks, with short lag times.

Low drainage densities mean that water will reach river and stream channels more slowly, because water will have a relatively long distance to flow to a river channel, which will produce a much slower – a delayed hydrograph – giving longer lag times and lower flood peaks.

Drainage density is found by measuring the total length of river and stream channels in a drainage basin and dividing it by the total area of the drainage basin.

Question 18

How do the slopes in a drainage basin impact shape of hydrograph

Answer 18

steep slopes will encourage greater overland flow/surface runoff – producing flashy hydrographs as the water will be moving quickly down the slope with little time to infiltrate the soil, while on more gentle slopes the water will have more time to infiltrate the soil and there will be more areas of surface storage producing longer lag times and delayed hydrographs.

Question 19

How do the porosity and permeability of soils impact shape of hydrograph

Answer 19

Infiltration is the actual entry of water of water into the surface of the soil, whereas percolation is the downward movement of infiltrated water through the pores and spaces of soil once the water has actually entered the soil or surface. A clay soil may have water quickly building up on the surface which will then start to flow over the surface as overland flow (quick flow). This will reach the river channel quickly and so a hydrograph will have a high peak discharge, with short lag time and steep rising and falling limbs – a flashy hydrograph.

In contrast to a clay soil, sandy or loamy soil has a high infiltration capacity as it has large pore spaces. There will be less surface runoff/overland flow and more throughflow and baseflow which are slower. As a result the hydrograph will have a lower peak discharge, longer lag time and more gentle rising and falling limbs – a delayed hydrograph.

Also, a thin soil will not have the storage capacity of a deeper soil; this could mean that its storage capacity is reached quickly, possibly leading to increased overland flow.

Question 20

How does rock type impact shape of hydrograph

Answer 20

permeable and porous rocks (eg limestone and chalk) store more precipitation and release it more gradually/slowly as baseflow/groundwater flow, producing a delayed hydrograph with a lower flood peak discharge and long lag times.

Impermeable or non-porous rocks (eg sedimentary clay, igneous granite and metamorphic schists) do not absorb as much water and so have more water running overland as surface runoff – called quick flow and so have a more rapid response producing a flashy hydrograph with high flood peak discharges, with steeper rising and recession limbs and short lag times.

Question 21

How does vegetation type impact shape of hydrograph

Answer 21

Dense vegetation will encourage both greater interception and infiltration which means that it will slow down the arrival of water into the river channel, producing lower peak discharges, flatter rising and recession limbs and longer lag time – a delayed – hydrograph.

Sparse/thin vegetation cover has opposite effects – both less interception and infiltration which will mean that it will speed up the arrival of water into the river channel, producing higher peak discharges, steeper rising and recession limbs and shorter lag time – a flashy hydrograph.

Question 22

How does land use affect shape of hydrograph

Answer 22

o The abstraction (removal) and storage of water by humans can have a major effect on the flows and storages within a drainage basin. When water is taken from river channels it will affect river channel discharge, while groundwater abstraction will lower water tables and reduce baseflow and the amount released as channel flow.

o The replacement of a natural woodland by permanent pasture will lead to a reduction in interception and hence interception storage (on leaves, stems, trunks, etc.). A natural woodland will slow the flow of water to the soil surface and will reduce the amount of surface runoff/overland flow (quickflow). Infiltration rates are normally greater under woodland and the water will make its way to the river by throughflow and baseflow/groundwater flow rather than overland flow. If permanent pasture – grassland – is ploughed up and used for growing crops – called arable farming – for a short time there will be no vegetation cover and even less interception until the crop grows and interception increases.

o The impermeable surfaces of concrete, roofs, and sealed roads found in urban areas will intercept most rainwater and prevent infiltration. This may lead to rapid surface runoff/overland flow either on the surface or by storm drains. Water may reach river channels in minutes rather than hours and days and in large amounts. This will produce very steep rising and falling limbs on the hydrograph, a very short lag time and a high flood peak. Afforestation will have the opposite effect – by decreasing the amount of surface runoff/overland flow through increased interception and so lengthen lag times, decreases peak discharges and make the rising and recession limbs more gentle – producing delayed hydrographs in river channels that in their natural state had flashy hydrographs.

Question 23

How does temperature affect where water is stored and how it moves

Answer 23

Important impact on stores and flows but more indirectly as affects evapotranspiration rates
Higher temps = more potential evapotranspiration (EVT) output as more energy to convert water to vapour; this will reduce surface storage. Especially in low lying areas where water table is close to surface and this may even lead to loss of soil moisture and groundwater stores.

• In arid areas, Evapotranspiration due to high temps accounts for 100% of water output
• Said that with every 1 ºC increase in temperature, the atmosphere is able to hold 7% more moisture so EVT is increasing, reducing surface stores especially
• Higher temps – plants open stomata to release more vapour so EVT increased
• Warmer air can hold more moisture so greater potential EVT possible ie the water loss that would occur if there was an unlimited supply of water in the soil for use by the vegetation. Eg actual evapotranspiration rate in Egypt is less than 250mm because that is around the annual rainfall. If the rainfall was 2000mm, there would be enough heat to evaporate that amount of water so the potentail evapotranspiration rate is actually 2000mm.
• Increased EVT will lead to more rainfall so greater inputs into the system

Question 24

How does precipitation affect where water is stored and how it moves

Answer 24

Most important impact on stores and flows

Seasonal variations in soil moisture budgets affect the amount of water in stores and flows in the drainage basin system – precipitation determines the field capacity of the soil ie how much moisture it can store before overland flow and or saturation

In temperate areas, during the winter with high rainfall and lower temperatures leading to low rates of evapotranspiration, there is a soil moisture surplus as the field capacity is reached and exceeded – this will provide plenty of soil moisture for transpiration by plants, overland flow as the ground is saturated and more water available for percolation to the groundwater store and groundwater recharge

In summer months when potential evapotranspiration exceeds precipitation, water will be held in the soil as it is not saturated, it will be taken up by plants and so river discharge levels will go down or even dry up in arid areas

Antecedent rainfall is important as it determines whether the soil storage is full and so will determine if there is overland flow

Rainfall intensity is important – even if the field capacity has not been reached, intense rainfall may exceed the infiltration capacity ie maximum rate of infiltration and so lead to Hortonian Overland Flow

Raindrop size is also important – larger raindrops are not infiltrated as quickly and will increase the likelihood of Hortonian Overland Flow

Question 25

How does humidity and wind speed affect where water is stored and how it moves

Answer 25

Humidity:
More humidity = less potential EVT as the air can hold less water. Eventually reaches saturation.

Wind speed:
Higher wind speed = more EVT (clears humidity produced by transpiration + increases EV)

*Potential Evapotranspiration (potential EVT) = water loss that would occur if there were an unlimited supply of water in the soil for use by vegetation
*e.g. Egypt – actual EVT = <250mm; potential EVT = 2000mm+ (due to high temps)

Question 26

How does gradient affect where water is stored and how it moves

Answer 26

Steeper slopes results in less infiltration and more and faster surface runoff. This will lead to higher river discharge

Question 27

How does vegetation affect where water is stored and how it moves

Answer 27

Higher rates and losses to evapotranspiration flows /outputs for vegetated areas

Vegetation increases interception and infiltration and transpiration for vegetated areas. It slows down the speed at which rain reaches the surface so there is greater infiltration (infiltration capacity increased)

EG on bare soils where rainsplash may occur the soil crumb structure may be destroyed– this lowers the infiltration capacity to 10mm/hr whereas on vegetated soils it can reach 50-100mm/hr

Interception varies with different types of vegetation – grasses with small surface areas will expereince less interception loss compared with deciduous woodland. This will affect the flow of stemflow which will be more in densely forested areas.

Coniferous trees also intercept more than deciduous trees in winter but the reverse is true in summer so seasonality is important

In agricultural settings, crop density is important – bare soils between rows of crops will see a reduced infiltration rate

Planting crops/afforesting = long term greater interception + infiltration. But in short term heavy machinery used to plant the crops may reduce porosity by compacting the soil, so greater overland flow.

Greater flows through soil with vegetation as old root systems act as pipes and percolines and provide pathways in soil for movement down/across which encourages throughflow and percolation so soil and groundwater stores will increase

Broadleaved beech tree = holds more water so greater potential for higher rates of transpiration. A single large oak tree can transpire 40,000 gallons of water vapour in a year
Saguaro cacti – specially adapted to retain moisture so reduces transpiration rates

Question 28

How does rock type and structure impact where water is stored and how it moves

Answer 28

There are numerous stores of water within the drainage basin including soil water, surface water and groundwater. The permeability and porosity of rocks and soils is key in influencing stores of water.

Porosity is the ratio of the of the amount of empty space to the amount of solid space in a material.

Primary permeability is a measure of the ability of a liquid to flow through a material and secondary permeability is the ability of water to flow through bedding planes and joints in a rock.

Clay soils have a relatively high porosity (40-70%) and can absorb a lot of water. However, it has very low permeability. Clay particles are positively charged so it tends to flocculate and stick together when water enters it. This means that it absorbs water well but then becomes impermeable, thus preventing water to flow through it rapidly. Clay tends to allow water to infiltrate at around 0-4 mm/hour. This means that surface storage tends to be quite high as it cannot infiltrate very fast, which will result in greater surface runoff and higher channel storage. Soil storage will be higher as it is slow to percolate downwards which will mean that groundwater stores will be lower.

On the other hand, sandy soils, which have a high porosity and permeability. allow water to pass through very quickly as there are larger spaces between the particles. Sandy soils tend to allow water to infiltrate through it at around 3-12 mm per hour. Therefore, as sandy soils allow water to absorb and pass through quickly, surface storage is lower as it infiltrates fast and soil storage is lower as it percolates rapidly too. This means that groundwater stores will be higher.

The primary and secondary permeability and porosity of rocks influences stores of water greatly. Rocks that have low permeability (e.g. granite) will not allow much water through them. If the rock is towards the surface this will result in greater surface storage. However, some rocks (e.g. granite) have an extensive joint network which creates percolines, and can allow water through it, to an extent. If impermeable rock is underlying porous soils, then it will result in more soil storage as. Rocks that have higher permeability and porosity (e.g. limestone and sandstone) will allow water to pass through them faster, thus reducing surface storage, overland flow and channel storage, and will increase groundwater storage.

Soil structure is also important- importance of soil horizons and add in podzols with impermeable hard iron pans in the B horizon which impedes percolation, soil structure eg comparing platy peds v crumb structure and also the juxtaposition of rocks eg impermeable ones overlying permeable ones which created perched water tables / aquifers which increase storage above and may increase surface storage if they intersect with the surface and create a spring.

Question 29

How does soil type impact where water is stored and how it moves

Answer 29

Porosity and permeability - In coarse textured soils water is stored in large pore spaces whereas in fine textured soils eg clay, water is held in pore spaces of varying size and very high suctions and can have porosity of 50% However because these pores in clay are not connected, permeability is limited which affects infiltration rates

Infiltration rates of 0-4mm/h are common clays, whereas 3-12mm are common sands

Soil structure: the way in which the particles which make up a soil cling together. A group of particles sticking together is called an aggregate or a ped. These can be of different shapes, such as platy or blocky.

The shape and alignment of these peds determines the size and number of pore spaces through which water, air and roots can penetrate and so will determine flows of water

Those with a granular and crumb structure (eg loam soil with balanced composition of sand, silt and clay) are permeable… whereas blocky (eg clay) or platy peds with their layered-plate overlapping like structure are impermeable (resists infiltration and percolation)

Soil Horizons: a layer within a soil profile, with distinctive colour, texture, depth and chemical composition.

Throughflow speeds up in soils with natural pipes and percolines which are created by soil horizons and old root networks as water concentrates and flows between soil horizons

However, in Podzols (type of soil found in upland areas or where there is coniferous forest), there is a hard iron pan in the B Horizon (second layer down in the profile of the soil) . This is a hard continuous layer of deposited iron, which often creates an impermeable layer and causes waterlogging above it so reducing throughflow and percolation of water deeper into the soil.

Soil moisture storage along with infiltration, percolation and groundwater recharge will all be affected by seasonality in temperate areas

Question 30

How do dams and reservoirs impact where water is stored and how it moves

Answer 30

Increase in evaporation due to construction of large dams and holding reservoirs to increase storage of water

Eg Lake Nasser storage reservoir behind the Aswan dam loses up to a third of its water due to evaporation

Decrease in river discharge if water retained by dam – this controls flow and reduces risk of flooding so reduces potential increase in surface storage

Increased percolation as water stored percolates and increases groundwater store and flow as it seeps out of reservoir ·

The impact of this human modification on stores and flows in drainage basins is increasing; number of large dams (more than 15m high) that are being built is increasing rapidly and reaching a level of almost two completed every day. There are over 48000 large dams. With increasing industrialisation and demand for water rising with the rise of the middle classes, the demand for dams is only set to increase.

Whilst water loss by evaporation may be reduced by using chemical sprays on the water, by building sand-fill dams and by covering the reservoirs with plastic, as the climate warms and rates of evaporation increase with warm air able to hold more moisture, this issue can only get worse.

Question 31

How does water abstraction impact where water is stored and how it moves

Answer 31

Impacts:
Groundwater refers to subsurface water that is stored under the surface in rocks. It accounts for 97% of all freshwater

Abstraction from groundwater supplies lowers the water table which in turn may lower the base flow in rivers, even causing them to dry up. In coastal areas, it can lead to salt water intrusion of groundwater aquifers affecting fresh water stores

Over-abstraction is common in areas where the demand for water exceeds the amount available during a certain period. This is especially in areas with low rainfall and high population density and in areas where there is intensive agricultural or industrial activity.

Saline intrusion is widespread in the Mediterranean coastlines of Spain, Turkey and Italy where the demand from tourists is a major cause of over-abstraction

In Malta, groundwater can no longer be used for domestic consumption or irrigation because it has been contaminated by saline intrusion.

Irrigation is the main cause of groundwater overexploitation in Italy where overexploitation of the Po River in the region of Milan aquifer has led to a 25m decrease in groundwater levels over the last 80 years

In the High Plains of Texas in the 1930s, the groundwater system was stable and in a state of dynamic equilibrium with long-term recharge (refilling of water pores where the water has dried up or been extracted by human activity) equal to long-term discharge. However, groundwater is now being used at a rapid rate to supply centre-pivot irrigation systems. In under 50 years the groundwater level has declined 30-50m in a large area to the north of Lubbock, Texas.

Evaluation: Groundwater abstraction is essential in some parts of the world and can be managed:

Artificial groundwater recharge is possible as a means of storing water – this stops streams drying up in dry periods and the bore holes can used to extract water when it is in short supply – this is used in southern England

In areas where water supply is low, abstraction for irrigation allows farmers to grow crops. It is also used to allow countries to industrialise.

Also possible to recharge groundwater sources provided sufficient water is available – in permeable areas, water spreading is possible (a form of infiltration and seepage)

In impermeable areas pumping water into deep pits or wells is possible eg used extensively on heavily settled plains in Israel in order to replenish reservoirs and overcome problems of saltwater intrusion

However:
In Europe, groundwater is the main source of fresh water and as 97% of freshwater is stored in groundwater supplies, the scale of this problem is serious.

Ground water abstraction is prevalent across the world, regardless of climate – from Denmark to Saudi Arabia, from Australia to the High Plains of Texas in the US – the scale of the problem is significant.

Solutions are only possible if the country has sufficient water and technology. As climate changes and rainfall patterns shift, this will become increasing problematic and whilst some soil moisture may be recycled by evaporation into atmospheric moisture within a matter of days or weeks, groundwater may not be recycled for as long as 20,000 years – this is a very long term problem. Where recharge is not taking place, groundwater is considered a non-renewable resource.

Question 32

How does urbanisation impact where water is stored and how it flows

Answer 32

Urbanisation is the increase in the proportion of the population living in urban areas. It is usually accompanied by urban sprawl as the % of land area covered by urban areas increases.

Impact:
Removal of trees and vegetation to facilitate urban growth drastically reduces evapotranspiration and interception whilst increasing surface run off but reducing surface storage (short term storage through throughfall and stem flow).

During the initial construction of the urban settlements, decreased infiltration, throughflow, percolation and lowered groundwater levels are common as surface gets paved over with impermeable concrete and the ground is compacted reducing porosity.

As groundwater levels subside, base flows in rivers (the water that is fed into the river by groundwater seepage) also decreases and storm flows (excess from throughflow and overland flow) increase.

As continued and complete urbanisation continues, there is decreased porosity and permeability. Discharges in rivers can reach higher peaks after rainfall events and water travels more quickly to the river as overland flow.

Storm drains and channel improvements may reduce surface storage as water is channelled away quickly rapidly over impermeable surfaces into drains and gutters. This reduces overland flow in the longer term but leads to problems downstream with increased discharges in the river along its course. The reduced surface storage will also decrease evapotranspiration rates

Evaporation may actually increase as warmer temperatures caused by anthropogenic (human caused heat eg air con, central heating, industry, vars etc) and higher specific heat capacity of building materials in urban areas actually increase air temperatures (urban heat island effect) · Urbanisation can also impact on groundwater abstraction as water is taken from aquifers for an increasing population with increasing demands eg Jakarta, Indonesia

Evaluation:
Urbanisation is on the increase – the UN says that by 2050 it will have reached 68% of the world’s population. 90% of this increase will take place in Asia and Africa and will impact stores and flows in areas where water is often scarce which may lead to even more groundwater abstraction.

Question 33

How does deforestation impact where water is stored and how it moves

Answer 33

Removal of trees and vegetation to facilitate urban growth and farming drastically reduces evapotranspiration and interception whilst increasing surface run off but reducing surface storage (short term through throughfall and stem flow) and reduces the time lag so that channel storage in rivers is much higher leading to risk of flooding.

Infiltration rates are reduced – infiltration is up to 5 times greater under forest compared with grassland. This is because the forest trees channel water down their roots and stems. With deforestation there is reduced interception – infiltration capacity on bare soils is 0-10mm/hr v 50-100mm/hr on vegetated soils as vegetation otherwise produces better drainage through roots acting as pipes and percolines and better soil fertility encouraging earth worms etc which aerate soil and increase porosity. Bare soils also lead to rainsplash which may create an impermeable crust on the surface. Machinery used to deforest also increases soil compaction due to machinery so porosity is decreased, reducing infiltration and leading to greater overland flow

Population growth in Nepal, in the Himalayas, has led to forests being cut down to provide fuelwood and terraced fields for agriculture. Overgrazing of the deforested land has led to over-grazing and washing of sediment overland with increased run off and this has caused river channels to lower their channel capacity. The local rivers are tributaries of the Ganges which have seen increase in flood risk further downstream in Bangladesh as a result of deforestation. Illegal logging by the Taliban in the KPK valley in Pakistan was also partially blamed for the 2010 floods since interception and infiltration were reduced, overland flow sped up transfers to rivers whilst sediment washed off slopes and logs stored in the rivers, reduced the channel capacity increasing the risk of overtopping and floods.

The province of Yunnan in China saw mass deforestation in the 1960s because of the need for land to produce food for a growing population. This led to an increase in flooding of the Yangtze due to higher peak discharges in 1998 which saw the government introduce an afforestation programme although the positive impacts on stores and flows was longer term

Evaluation: Scale of the problem: According to Global Forest watch there was a 9.7% decrease in tree cover from 2000 to 2019 · Some countries are more affected than others

Question 34

How does afforestation impact where water is stored and how it flows

Answer 34

Afforestation is believed to have the opposite effect as deforestation with increased evapotranspiration and interception and so on.

Evaluation: The province of Yunnan in China saw mass deforestation in the 1960s because of the need for land to produce food for a growing population. This led to an increase in flooding of the Yangtze due to higher peak discharges in 1998 which saw the government introduce an afforestation programme although the positive impacts on stores and flows was longer term

Evaluation – however …
The impact of trees on interception and overland flow and peak discharges will vary with type eg coniferous trees intercept more than deciduous ones in the winter as they lose their leaves and less in the summer.

Following afforestation. Sediment yields in the River Severn catchment were seen to have increased by 4 times indicating greater overland flow.

Believed that afforestation in the shorter term has negative impacts because new sapling trees leave little ground cover of vegetation and bare soil is left for tractor access routes and fire and wind breaks.

Bare earth leads to rainsplash and low infiltration rates of only 0-10mm/hour whereas on vegetated ground, infiltration rates can reach 50-100 mm/hour

But….
Despite the short term negative impacts on stores and flows, after only 5 years, the amount of erosion and sedimentation in the Severn Valley catchment had declined suggesting that in the longer term, afforestation will indeed have the opposite effects of deforestation .