Part 1: Water Flashcards
Water
Explain the interactions that are involved in environmental science?
Environmental science is the study of the interactions between the Hydrosphere (water) Atmosphere (air) Lithosphere (earth) Biosphere (life)
What disciplines in science does environmental science cover?
All disciplines including physics, chemistry, biology, ecology, geography, Earth sciences, social sciences and technology
What vegetation is found in terrestrial environments where liquid water is abundant?
Forests specifically, tropical rainforests in wetter equatorial regions and deciduous forests at higher, more temperate latitudes.
What impact does reduced water availability have on the type of vegetation that can grow in an environment?
t changes it. Grasslands and tundra (a vegetation type composed of low-growing plants such as dwarf shrubs, grasses, mosses and lichens) replace forests as water availability is reduced.
Where does water exist predominantly as a solid?
Water exists predominantly as ice and snow in the polar regions (high latitudes) and at high altitudes such as mountain peaks.
What is a catchment?
A catchment is the area from which rainfall flows into a river. Any rain falling in this area that makes its way into rivers or streams is ultimately destined to flow out of the catchment at a single point – in the Teign catchment this is through the estuary into the sea. Rivers and streams do not cross the catchment boundary. It is the area of interest.
what is gradient?
Gradient is a numerical expression of steepness.
Gradient is a defined as:
gradient = height change ÷ horizontal distance.
What is the hydrosphere?
The hydrosphere includes the parts of the Earth that are mainly water, such as the oceans, ice caps, lakes and rivers
What is the water/hydrological cycle?
Water moves over, on and through the Earth in a continuous cycle driven by the Sun and gravity. This process known as the water cycle or the hydrological cycle involves water as liquid, solid (ice and snow) and gas (water vapour).
What is hydrology?
The study of water movement on and beneath the ground and the physics and chemistry of the water is called hydrology.
what are the two main types of water in the hydrological cycle?
meteoric and saline
What is meteoric water?
meteoric, which is fresh water derived by condensation from the atmosphere that accumulates as surface water (rivers and freshwater lakes) and underground water
What is saline water?
saline, which is the seawater of the oceans and many lakes.
What other type of water can be included in the hydrological cycle apart from the two main types?
Small amounts of magmatic water from the interior of the Earth are also added to the cycle by volcanic eruptions.
What is a reservoir?
All parts of the hydrosphere that store water temporarily.
what percentage of total water on earth does the ocean
as a reservoir make up?
96.5
what percentage of total water on earth do ice caps as a reservoir make up?
1.9
what percentage of total water on earth does underground water as a reservoir make up?
1.6
what percentage of total water on earth do lakes as a reservoir make up?
0.012
what percentage of total water on earth does soil moisture as a reservoir make up?
0.001
what percentage of total water on earth does the atmosphere as a reservoir make up?
0.001
what percentage of total water on earth does the rivers as a reservoir make up?
0.0001
What is the resident time of water in the ocean in the water cycle?
about 4000 years
What is the resident time of water in the ice caps in the water cycle?
about 8000 years
What is the resident time of water in the underground water in the water cycle?
from a few weeks to more than 10000 years
What is the resident time of water in the lakes in the water cycle?
a few years
What is the resident time of water in the soil moisture in the water cycle?
a few weeks to one year
What is the resident time of water in the atmosphere in the water cycle?
about 11 days
What is the resident time of water in the rivers in the water cycle?
a few weeks
What is the hydrological cycle driven by?
The suns energy and the earths gravity
What does the water cycle involve?
The water cycle involves the movement of water, in all its forms, over, on and through the soil and rocks near the surface of the Earth and in the atmosphere.
How do you calculate the residence time for water in a reservoir?
It is calculated by dividing the mass in a particular reservoir by the rate of transfer to or from the reservoir.
In what reservoir is residence time the fastest?
it is fastest in the atmosphere (about 11 days) and rivers (a few weeks).
what do you call the transfer of water from the atmosphere to the earths surface?
Water that transfers from the atmosphere to the Earth’s surface is called precipitation,
How is precipitation commonly experienced?
It is commonly experienced in the form of rain, snow or hail. Water vapour may also precipitate by condensing as dew on the ground or hoar frost on vegetation or objects.
In what forms do precipitation exist?
It exists as vapour, liquid (clouds and raindrops) or in solid form (snow and ice).
At what rate does air cool when increasing in altitude?
it cools at a rate of around 1 °C per 100 m of altitude.
Why does air expand when it increases in altitude?
because of the decrease in pressure with altitude
What is needed in the air for water droplets to form? Give some examples.
Water droplets form around small particles in the air. This happens when water vapour condenses around them. Particles such as pollen grains, fungal spores, dust and salt from sea spray.
How do the formation of clouds occur?
Water vapour condenses around small particles in the air. This condensation over a large amount results in the formation of clouds.
How big are the droplets that form clouds in diameter?
The formation of clouds composed of 0.001–0.1 mm diameter droplets.
How big are the droplets of water when they precipitate to the ground? How do they become bigger?
Precipitation of this water occurs when these droplets coalesce to form larger drops about 1 mm in diameter, or when ice crystals form and they fall to the Earth’s surface.
what unit is precipitation commonly measured in?
mm
What is interception?
It is when most precipitation reaches the ground, but not all of it, as some is stopped by vegetation. This process is known as interception. It is part of a subcycle of the water cycle involving precipitation, interception and evaporation back to the atmosphere, but it bypasses that part of the main cycle where water reaches the ground.
What does the loss of precipitation caused by interception of vegetation depend on?
The proportion of the precipitation that does not reach the ground, the interception loss, depends on the type of vegetation, as well as its age and density and the season.
Values of interception loss averaged over the year are higher for coniferous forest than for deciduous forest (15–35% versus 9–25%).
Why is the average value of interception loss higher for coniferous forest?
Because coniferous forest retains its leaves throughout the year and the dense canopy of fine leaves (needles) is more effective at stopping precipitation reaching the ground.
In what part of the world has the most uneven global distribution of precipitation?
Near to the equator
What type of geological landscape is precipitation greatest over?
Mountainous areas
What is evaporation?
It is the process by which water is transferred as vapour from the land or ocean to the atmosphere.
In what conditions does evaporation occur?
Evaporation from liquid water can take place at any temperature under normal atmospheric pressure. It occurs only at the surface of the liquid. THIS SHOULD NOT BE CONFUSED WITH BOILING
What happens when water boils? and what conditions does it happen in?
It should not be confused with the process of boiling, whereby the conversion of liquid water to gaseous water (steam) takes place throughout the bulk of the liquid and occurs at a fixed temperature for a given pressure.
What is the boiling point for water?
The boiling temperature of water under normal atmospheric pressure is 100°C,
What happens to the boiling temperature of water with increasing altitude and why?
It reduces with increasing altitude as the air pressure reduces. For example, the boiling point of water at the top of Mount Everest is 71 °C.
Does the rate of evaporation increase or decrease with increasing temperature?
It increases
Does the rate of evaporation increase or decrease with increasing humidity?
It decreases
What is humidity?
It is a measure of how close the air is to saturation with water vapour.
How does wind speed affect evaporation of water?
Wind speed is also important, as wind carries moist air away from the ground surface, thus reducing the local humidity and allowing more water to evaporate.
What is the rate of evaporation aided by?
One explanation for this observation is that the rate of evaporation is not limited by the availability of water and is aided by the large surface area available. The temperature of the water is also a relevant factor. Water depth can also have an influence on evaporation rates.
Explain what will happen to the eavopration rate when dry air moving over land reaches the edge of a lake?
Initially the evaporation rate will be high but as the humidity of the air rises the rate of evaporation will decrease until, with a large lake, the net evaporation rate will reach zero. This is because evaporation rates decrease with humidity.
What can we assume with the levels of humidity over oceans?
Air travelling over oceans therefore has essentially constant humidity, due to its large surface area.
What can we assume with the levels of humidity over small lakes?
for small lakes a continuous high rate of evaporation is achieved as the humid air close to the water surface quickly diffuses away as the surface area is much less.
What are the evaporation rates like in a shallow lake in summer and why?
The water temperature of shallow lakes closely parallels that of the air, so evaporation rates are much higher when the air is warm in the summer than in the winter.
What are the evaporation rates like for deep lakes throughout the seasons and why?
For large, deep lakes, water temperature lags behind air temperature (in temperate climates, the ocean is always more agreeable for bathing at the end of the summer than at the beginning). So for deep lakes, evaporation rates may fall to a minimum in the early spring and actually be higher in the winter.
Are the evaporation rates higher in fresh water or salt water?
rates of evaporation are higher for freshwater than salt (saline) water.
How much salt per kg of water does seawater contain?
Seawater contains about 35 grams of dissolved salts per kilogram of water
What percentage is the evaporation rate of sea water lower than fresh water?
evaporation rate that is about 3% less than that of pure water.
Why does seawater have a decreased evaporation rate to that compared with fresh water?
A simplistic way of rationalising this is to view the dissolved material as occupying a proportion of the surface of the seawater and thereby reducing the effective area from which water molecules can evaporate
Does water evaporate quicker from open water or land?
Evaporation from bare soil (in contrast to open water) can be limited by the supply of water and, by extension, as a result of the type of soil the water is sitting on. For example, evaporation from a saturated sandy soil can take place nearly as quickly as it can from open water, whereas the rate for a saturated clay soil is slower: between 75% and 90% of that of open water.
What can aid evaporation from land soils?
Water is evaporated from any surface covered by a film of water and from water filling the spaces between grains of soil near the surface. Evaporation rates are initially high after rainfall, but decrease with time and may reach zero if there are no mechanisms available (such as plant roots) to bring deeper-lying water to the surface. Capillary processes can bring water to the surface, and it is more effective with fine-grained soils, where the spaces between the grains are small, than it is with coarse-grained soils. However, most soil is not bare, being covered with agricultural or natural vegetation.
Rainfall
plant roots
capillary processes
Where does over two-thirds of total global evaporation occur? and why?
within 30° of the Equator, because of the higher temperatures in equatorial and tropical areas.
Globally where does evaporation reach its greatest values?
Evaporation reaches its greatest values not at the Equator itself, but between latitudes of 10° and 20° in both hemispheres
How does the evaporation rate near the equator affect the precipitation rate?
The strong trade winds at these latitudes carry water vapour towards the Equator, giving very high precipitation in the equatorial zone where the trade-wind systems converge.
A graph shows that evaporation from the Earth’s surface is greater in the Southern Hemisphere than in the Northern Hemisphere. Suggest an explanation for this.
The Southern Hemisphere has more ocean than the Northern Hemisphere, and evaporation is greatest from open water.
Which two environmental factors are important in determining the dependence of evaporation rates on season?
Temperature and humidity. So although precipitation averaged over England is not very seasonal (it is more so in Scotland and Wales; see Figure 1.1.6), the availability of water is due in large part to the seasonality of evaporation.
How much energy is required to convert 1 kg of liquid to the gas phase?
What does this represent?
To convert one kilogram of liquid water at 100 °C to the gas phase at the same temperature requires an energy input of about 2.4 × 106
This represents the latent heat of vaporisation of water.
What is the albedo?
The proportion of that radiation that is reflected back into space, the albedo
What is the radiant energy from the sun used for?
some is used to heat the soil, vegetation or surface water (G)
some is used for heating the air above the land surface (H)
a tiny proportion goes towards the growth of plants (and so is ignored in the context of this discussion).
What does the amount of radiantion reaching the surface of the earth depend on?
The amount of radiation reaching the surface of the Earth at a particular point also varies with latitude, season, cloud cover and time of day.
Please look at the following flash card to compare the albedo values of different surfaces.
Albedo value ranges for different surfaces. Surface Albedo Forest (deciduous) 0.16–0.22 Forest (coniferous) 0.05–0.14 Grass and grain crops 0.12–0.28 Water 0.08–0.14 Snow 0.35–0.85 Soil and rock 0.05–0.45
What is transpiration?
This is the process by which plants draw up water from the soil (through the xylem) and transfer it to their leaves, from which it evaporates through pores (stomata) in the leaf system. Vegetation increases the amount of water returned to the air, not only by interception and then evaporation, but also by transpiration
What is evapotranspiration?
Over land areas it is also difficult to separate the effects of evaporation and transpiration, so the two are usually combined into one parameter, called evapotranspiration. Evaporation and transpiration are parts of the water cycle that are difficult to quantify, as it is hard to measure the transfer to water vapour directly.
What is potential evapotranspiration?
The calculation of the maximum value of evaporation for a saturated surface and transpiration using parameters such as humidity, temperature and wind speed.
Why is the actual value of evapotranspiration always less than the potential evapotranspiration?
It is the maximum possible evapotranspiration that could take place given an unlimited supply of moisture. Because most land surfaces are neither open water nor saturated, and are partly or wholly covered in vegetation, actual values of evapotranspiration are always less than potential evapotranspiration.
How do seasons affect evapotranspiration?
Evapotranspiration is usually greater than precipitation in the summer months, and less than precipitation in winter
What is hydrologically effective precipitation?
The precipitation that is not intercepted, evaporated or transpired back to the atmosphere either soaks into the ground or becomes surface flow. A rough indication of the quantity of water available from underground or from rivers in any area is given by the excess of precipitation over actual evapotranspiration. This is called the hydrologically effective precipitation.
Can you give some reasons as to why it might be difficult to measure evapotranspiration from vegetation covered ground?
Estimating evapotranspiration from vegetation-covered ground is particularly difficult. The type and size of vegetation, along with soil water levels, density of ground cover, energy available, air humidity and wind speed all affect evapotranspiration and can vary in importance over short periods of time.
What does the difference between the potential and actual rates of evapotranspiration depend on?
The difference between the potential and actual rates of evapotranspiration depends on a number of factors, including soil moisture content, vegetation type, rainfall and air temperature. For example, a dry wind on a sunny day blowing over an arid area would result in very little actual evapotranspiration, but the potential evapotranspiration would be very high.
What is the most widely adopted model for estimating potential evapotranspiration?
The most widely adopted model for estimating potential evapotranspiration is the Penman–Monteith model.
What does the Penman–Monteith model represent?
It represented evaporation from an open water surface as a function of both the ‘drying power’ of the air (i.e. its humidity, and the wind speed) and the net radiation energy available for evaporation, and was related to a vegetation-covered surface by use of a coefficient, f. (The value of f for grass was deemed to vary from 0.8 in summer to 0.6 in winter.)
What is the missing factor in the Penman–Monteith model with evapotranspiration?
The missing factor was the resistance to evaporation from the vegetation.
What are two principal resistances from vegetation that discourage evaporation?
aerodynamic resistance and stomatal resistance
What is aerodynamic resistance?
It controls the rate at which water vapour can move away from the leaf canopy into the atmosphere. It is a principal resistance from vegetation that discourage evaporation.
A ‘rough’ surface will cause much more turbulence in moving air than a smooth surface. In other words, wind blowing over a freshly mown lawn will be less effective in causing evaporation than wind of the same speed blowing across a forest with trees of different heights. This effect is the aerodynamic resistance
What is stomatal resistance?
Stomatal resistance, which controls transpiration. It is a principal resistance from vegetation that discourage evaporation.
In stomatal resistance, water vapour diffuses through the stomata into the atmosphere partly in response to meteorological variables (available energy, humidity and wind speed) and partly depending on the vegetation type (the size and number of stomata). The stomatal resistance is of major importance in the evaporative process because it is usually an order of magnitude greater than aerodynamic resistance
What does aerodynamic resistance of evaporation depend solely on?
The effect is the aerodynamic resistance depends solely on the physical properties of the vegetation cover. A ‘rough’ surface will cause much more turbulence in moving air than a smooth surface. In other words, wind blowing over a freshly mown lawn will be less effective in causing evaporation than wind of the same speed blowing across a forest with trees of different heights. This effect is the aerodynamic resistance
What type of resistance of discouraging evaporation is greater in magnitude?
The stomatal resistance is of major importance in the evaporative process because it is usually an order of magnitude greater than aerodynamic resistanc
What meteorological variables cause a response to water vapour diffusing through the stomata of vegetation?
In stomatal resistance, water vapour diffuses through the stomata into the atmosphere partly in response to meteorological variables (available energy, humidity and wind speed)
What two factors does stomatal resistance depend on?
The meteorological variables and the size and number of vegetation.
What is the Aerodynamic resistance/s m−1 for open water, grass, arable, heather and forest?
Open water 125 s m−1 Grass 50–70 s m−1 Arable 30–60 s m−1 Heather 20–80 s m−1 Forest 5–10 s m−1
What is the Stomatal resistance/s m−1 for open water, grass, arable, heather and forest?
Open water 0 s m−1 Grass 100–400 s m−1 Arable 100–500 s m−1 Heather 200–600 s m−1 Forest 200–700 s m−1
What is the Canopy resistance/s m−1 for open water, grass, arable, heather and forest?
Open water 0 s m−1 Grass 40–70 s m−1 Arable 50–100 s m−1 Heather 60–100 s m−1 Forest 80–150 s m−1
How does the Penmen- Monteith model express evaporation?
the Penman–Monteith model expresses evapotranspiration in terms of humidity, available radiant energy, and vegetation resistance.
What is transpiration?
Transpiration is the process by which plants draw water from the soil, transfer it to their leaves and it then evaporates. Evaporation and transpiration can be combined into one parameter, evapotranspiration.
What is potential evapotranspiration?
A maximum theoretical value for evapotranspiration, called potential evapotranspiration, can be calculated from meteorological parameters for any area using the Penman-Monteith equation.
What sources add water to a lake?
water can be added to the lake from several sources: precipitation on to its surface, streams that flow into it, groundwater that seeps in, and overland flow from nearby land surfaces
what sources does water leave a lake?
Water also leaves the lake through evaporation, transpiration by emergent aquatic vegetation, outlet streams, and groundwater seepage from the lake bottom.
what will happen if the total inflow to the lake is greater than the total outflow?
If, over a given period of time, the total inflow to the lake is greater than the total outflow, then the lake level will rise as more water accumulates
What will happen If the outflow exceeds the inflow over a given period?
If the outflow exceeds the inflow over a given period, then the lake level will fall
what is the water balance equation?
inflow = outflow + change in storage
list the inputs and outputs of water to the drainage basin?
Precipitation adds water to the drainage basin. Water is removed via evaporation and as streamflow.
What is the equation for the water balance calculation?
P − (Q + E) = ΔG + ΔS
where P is precipitation, Q is streamflow, E is evapotranspiration, and ΔG and ΔS are, respectively, changes in groundwater and soil water storage. This
What is an ecosystem comprised of?
An ecosystem is comprised of living and non-living components (including water) found in a particular area. These components interact in different ways
An ecosystem may be a forest, grassland or lake or a mixture of these. Often it includes humans, either directly or indirectly via their activities. While ecosystems may be affected by humans, they also provide essential services for humans, such as the prevention of soil erosion on hillsides by trees and the capture of unwanted organic matter by reed beds.
What 4 categories does The UK National Ecosystem Assessment (UK NEA) place ecosystems services in to?
It places ecosystem services into four categories:
cultural
provisioning
regulating
supporting (basic infrastructure for life).
What are provisioning services in regards to the UK national ecosystem?
Provisioning services: The products obtained from ecosystems.
For example,
food
fibre
fresh water
genetic resources
What are regulating services in regards to the UK national ecosystem?
Regulating services: The benefits obtained from the regulation of ecosystem processes.
For example,
climate regulation hazard regulation noise regulation pollination disease and pest regulation regulation of water, air and soil quality
What are supporting services in regards to the UK national ecosystem?
Supporting services: Ecosystem services that are necessary for the production of all other ecosystem services.
For example,
soil formation
nutrient cycling
water cycling
primary production
What are cultural services in regards to the UK national ecosystem?
Cultural services: The non-material benefits people obtain from ecosystems.
For example, through
spiritual or religious enrichment
cultural heritage
recreation and tourism
aesthetic experience
What is a pore within a rock?
Spaces of all shapes and sizes within the rocks
Does sedimentary rocks have pores?
In sediments, and consequently sedimentary rocks , there are often pores between the grains which can be filled with water. There may also be spaces between rock beds (along bedding planes) or along joints (vertical breaks), or fractures, which can also contain water.
Where does most underground water exist? for example in huge caverns of infiltrated between something else?
huge quantities of water exist underground nonetheless – within rocks. This is because many rocks contain pores (spaces that come in all shapes and sizes).
What is infiltration?
The movement of water through the ground surface into the soil and rock beneath is called infiltration
What is permeability?
Permeability is a measure of the ease with which water can move through a substance: the greater the permeability, the easier the infiltration.
What does the rate of infiltration depend on?
The rate at which infiltration can take place depends on, among other things, the permeability of the soil or rock
The total amount of infiltration also depends on the time available for water to seep into the ground. Heavy rainfall usually results in rapid runoff, and relatively little infiltration into the ground.
How will the following have an effect on the total amount of infiltration, and why?
Dense vegetation.
Dense vegetation increases interception, which reduces infiltration, although the effect will be offset to some extent by the dense vegetation reducing the rate of runoff and thus increasing the time for infiltration, and therefore the amount of it.
How will the following have an effect on the total amount of infiltration, and why?
Steeply sloping land.
Water rapidly runs off steeply sloping land surfaces, so there is little time for significant infiltration to occur.
How will the following have an effect on the total amount of infiltration, and why?
.Roads and buildings.
Tarmac, concrete and roofing surfaces do not allow water to pass through them, so roads and buildings promote overland flow and reduce infiltration.
How will the following have an effect on the total amount of infiltration, and why?
Water cannot pass through frozen subsoil, so it will reduce infiltration.
There are two distinct zones containing water beneath the ground surface, what are they?
the unsaturated zone and the saturated zone
What does the unsaturated zone beneath ground surface consist of?
The unsaturated zone has mainly air-filled pores, with water held by surface tension in a film around the soil or rock particles.
What does the saturated zone beneath ground surface consist of?
the saturated zone beneath all the pores are filled with water.
What is the water table and where does it sit?
The boundary surface between the unsaturated zone and the saturated zone is the water table, which is the level of water in a well (strictly, in a well that just penetrates to the water table).
In which zone is underground water kept?
Water below the water table, in the saturated zone, is groundwater.
What is the capillary fringe?
Just above the water table there is a zone called the capillary fringe, in which water has not yet reached the water table
What is capillary retention?
capillary retention, a process whereby water clings to the walls of narrow openings.
What does the thickness of the unsaturated zone depend on?
The thickness of the unsaturated zone depends mainly on the climate (particularly the precipitation), but also on the topography. In arid and mountainous regions this zone may be hundreds of metres thick, whereas in areas of high rainfall it may be only a few metres thick.
Where would a saturated zone reach the surface?
Beneath swamps, lakes or rivers, the saturated zone reaches to the surface
How far can the saturated zone extend? in which way does water predominantly move?
The saturated zone extends downwards as far as the permeability of the rock will allow – a few tens of metres in some places, a kilometre or more in others. Water movement in the saturated zone is predominantly sideways (unlike in the unsaturated zone, where it is mainly downwards).
How is porosity of a rock signified?
In percentage
What does the percentage of porosity of a rock represent?
It represents pore space volume divided by the total volume of the rock
How do you calculate porosity of a rock?
pore volume/total volume x 100% = porosity in %
What are the principal factors that control porosity?
The principal factors that control porosity are:
grain size and shape
the degree of sorting (a well-sorted sediment has a narrow range of grain size)
the extent to which cement occupies the pore spaces of grains
the amount of fracturing.
What is secondary porosity?
The porosity of rocks may be increased by processes that occur after the rocks have formed. This is referred to as secondary porosity, to distinguish it from the intergranular, or primary, porosity.
Name two types of secondary porosity?
solution porosity
fracture porosity
What is solution porosity?
One type of secondary porosity is solution porosity, which develops where part of a rock has been dissolved, leaving open spaces (Figure 1.2.7e). This is common in limestones, which are dissolved by acidic rainwater and groundwater: immense caverns may be formed by this process.
How is fracture porosity caused?
Through cracks in rocks
Generally are consolidated rocks more porous than unconsolidated rocks?
Consolidated (compacted and/or cemented) sedimentary rocks, and igneous and metamorphic rocks, are usually less porous than unconsolidated sediments
In broad terms, how does porosity vary with the grain size of (a) unconsolidated sediments and (b) consolidated sediments?
The porosity will vary with grain size in the following ways:
a. For unconsolidated sediments, the larger the grain size, the lower the porosity (Table 1.2.1).
b. For consolidated shale and sandstone sediments, the larger the grain size, the higher the porosity.
What is the difference between porosity and permeability?
Porosity is a measure of how much water can be stored in a rock, whereas permeability is a measure of the properties of a rock which determine how easily water and other fluids can flow through it. Permeability depends on the extent to which pores are interconnected.
What does permeability depend on?
Permeability depends on the extent to which pores are interconnected.
The next card will show a summary of porosity and permeability
The rate at which water infiltrates into the ground depends on the permeability of the rocks and the state of the ground surface. Below the ground surface is an unsaturated zone which has air in the pore spaces, and a saturated zone which has all the pores filled with water. The water table is the boundary between the unsaturated zone and the saturated zone, and is the level at which water stands in wells. Water below the water table is called groundwater. The water table follows the topography of the ground surface but with gentler gradients.
Porosity is a measure of how much water a rock can store. The permeability of a rock is a measure of the properties of the rock which determine how easily water can flow through it. The porosity and permeability are generally greater in unconsolidated sedimentary rocks, particularly sands and gravels, than in consolidated sedimentary, igneous or metamorphic rocks. Both porosity and permeability can be increased by processes that occur after the formation of the rock, such as solution or fracturing. These are called secondary porosity and secondary permeability.
What is the head of water?
The difference in energy between two points that are l metres apart horizontally on a sloping water table is determined by the difference in height (h) between them. This height is called the head of water.
What is the slope of a water table known as?
The slope of the water table is called the hydraulic gradient and is defined as h/l. T
What is Darcys law?
the relationship between the speed of flow and the hydraulic gradient which is now known as Darcy’s law.
Aside from permeability of rock what what does hydraulic conductivity also depend on?
The hydraulic conductivity also depends on the density and viscosity of the fluid, so it will vary accordingly. When the fluid is water, the most important factor that affects the hydraulic conductivity is temperature. For example, an increase in water temperature from 5 °C to about 30 °C will double the hydraulic conductivity and, from Darcy’s law, will therefore double the speed at which the groundwater flows.
What two categories can rocks be divided in to in regards to their hydraulic conductivity?
Rocks can be divided into two broad categories – permeable and impermeable – on the basis of their hydraulic conductivity.
For a rock to be regarded as permeable what should their hydraulic conductivity be like?
Rocks with hydraulic conductivities of 1 m per day or more are regarded as permeable.
For a rock to be regarded as impermeable what should their hydraulic conductivity be like?
Rocks with hydraulic conductivities of less than 10−3 m per day are usually regarded as impermeable.
What limitations does darcys law have in regards to the properties of a rock?
Darcy’s law assumes that the rock unit is homogeneous; that is, the properties are the same at all locations. For a sedimentary rock, this would indicate that the grain size distribution, porosity and degree of cementation vary only within small limits. A rock would have the same amount of cracks at all locations. In reality, rocks tend to be heterogeneous, having different properties in different locations. For example, the formation of cracks is typically concentrated along preferred fractures or bedding planes.
What limitations does darcys law have in regards to the direction of flow of water?
At any point in the rock, the permeability will usually also vary with direction. For example, it is easier for water to flow between grains in the horizontal direction (parallel to the bedding) in the rock shown in Figure 1.2.11 than in the vertical direction. Equality of properties in all directions at a point is isotropy; variations in properties with direction is anisotropy. Most sedimentary rocks are anisotropic with respect to permeability because they contain grains that are not spherical but elongated in one direction or shortened in another (Figure 1.2.11). Fluids can pass more easily parallel to the long axis than perpendicular to it. Thus this texture causes permeability anisotropy, even though the rock may be homogeneous.
Darcy’s law takes no account of tortuosity – the fact that water must flow around the grains that make up the rock. This means that the actual flow of a water particle through a given length, l, will be longer than l because the fluid has to travel around the grains of the rock
Darcy’s law applies only to very slowly moving groundwater. In slowly moving fluids, fluid flow is laminar and the molecules of water follow smooth lines, called streamlines (Figure 1.2.13). As the speed of the fluid increases, flow becomes turbulent and the water molecules no longer move along parallel streamlines. Turbulence can modify the relationship between hydraulic head and fluid speed, so Darcy’s law breaks down.
What is a drawdown in regards to pumping water?
The difference in height between the water table before pumping and the level of water in the well during pumping is called the drawdown.
What is a sale intrusion?
Seawater intrusion into wells can become a problem where large amounts of groundwater are extracted near a coast, so that saline groundwater moves inland. This is called a saline intrusion.
The next card will give a brief summary on groundwater,
Groundwater will flow in response to differences in elevation and pressure. Darcy’s law relates the rate of the groundwater movement (Q) to the hydraulic conductivity (K), the cross-sectional area (A) and the hydraulic gradient or slope of the water table (h/l):
Equation label: (Eqn 1.2.2)
The hydraulic conductivity depends on the permeability of the rock and the properties of the water. Water generally flows in the direction of the hydraulic gradient and the slope of the water table.
Pumping water from wells or boreholes lowers the water level in the surrounding area. Water flows into the borehole, and this creates a cone of depression around it. The difference in height between the water table before pumping and the water level in the well during pumping is called the drawdown.
There is usually saline groundwater under the land at a coast, with a wedge of denser saline groundwater under the fresh groundwater. The depth to the saline groundwater depends on the height of the water table above sea level and the densities of the fresh and saline water.
what is a catchment?
the area from which rainfall flows into a rive
What is the basic water balance equation
The basic equation is:
inflow = outflow + change in storage
Any difference between the total inflow and total outflow is reflected in the water storage in the catchment.
The water balance equation may be expanded and expressed as:
P = Q + E + ΔG + ΔS
where P is precipitation, Q is total runoff (at catchment outlet), E is evapotranspiration, ΔG is change in groundwater storage and ΔS is change in soil water storage.
What is often combined into a single storage term and how is this determined?
Water is held in the pore spaces of both soils and rocks and sometimes in surface pools. To calculate a water balance (or a water budget), it is necessary to consider the change in the amount of water being stored within the area of study. When input and output fluxes can be directly measured, the storage component – which is often more difficult to measure directly – is generally calculated by difference. (This is often combined into a single storage term that includes groundwater and soil storage.)
What will happen to rainfalling in a catchment that is in excess of evapotranspiration loss and beyond the ability of the ground to store it?
Rain falling on a defined catchment that is in excess of the evapotranspiration loss, and beyond the ability of the ground to store it, will flow under gravity, ultimately to the main river of the catchment. If the runoff does not flow toward the river then, by definition, the rain that gave rise to that river did not fall in that catchment.
What is channel precipitation?
Probably the most obvious route is for rain that falls directly into drainage channels. This water is known as channel precipitation (Qp) and simply flows along the drainage system, ultimately to the catchment outlet.
What is overland flow?
Water also reaches the drainage channels by flowing over the land, the appropriately termed overland flow (Qo). Depending on the nature of the ground, the slope and the vegetation cover, this may be as a thin layer of flowing water or, more commonly, as tiny trickles or rivulets.
What is groundwater flow?
Some of the water that penetrates the ground surface percolates through to where the ground is saturated and flows through this region as groundwater flow (Qg).
What is throughflow?
The remaining water that penetrates the ground surface flows through the upper unsaturated layers as throughflow (Qt).
What units is runoff usually expressed as?
Runoff (which is also often referred to as streamflow or discharge) is usually expressed as volume per unit of time. Commonly used units are cubic metres per second (m3 s−1), also known as cumecs. Using these units, the runoff (Q) at any point along the course of a river is given by the product of its cross-sectional area, A (m2), and average speed, v (m s−1):
Runoff may also be expressed as a depth equivalent over a catchment: millimetres per day or month or year. This is a particularly useful unit for comparing precipitation, evapotranspiration and runoff rates, since precipitation and evapotranspiration are invariably expressed in this way.
What factors affect run off?
The relative importance of the different components of total runoff is a function of topography, soils, geology and vegetation. It also changes with time. Precipitation is not constant: the processes of the water balance are not operating in a steady-state situation. It is the changes that occur over time which make the study of hydrology difficult (and interesting). Variations in the components of runoff occur during an individual rainstorm, over different parts of the catchment, over seasons and even over years.
What is base flow?
Water which takes a longer route and contributes to the flow of rivers in non-rainy periods is known as baseflow. This is often a combination of delayed throughflow (Qdt) and groundwater flow (Qg).
What is quickflow?
This quickflow is usually a combination of overland flow (Qo) and quick throughflow (Qqt
What does the contribution of baseflow vary greatly with?
The contribution of baseflow to river flow varies greatly with the geology, topography and season. For rivers with a catchment of permeable rocks, there may be no water from overland flow in the rivers. All of the river flow in this case will be baseflow. In Britain, baseflow usually forms a higher proportion of the total flow in summer than in winter. This is because evapotranspiration is higher in summer and overland flow is therefore lower, whereas groundwater is released to rivers as baseflow more consistently throughout the year.
What is the infiltration capacity of the soil?
The factor that determines whether rain falling on the ground penetrates the ground or not is the infiltration capacity of the soil. This is the maximum rate at which rain can be absorbed by a particular piece of ground.
Explain the basic idea of the Horton hypothesis?
Rain falling on the ground surface is divided initially into two components:
one part penetrates the surface and becomes throughflow or, if it penetrates more deeply into a saturated region, groundwater flow
the other part is overland flow which does not penetrate the ground.
The factor that determines whether rain falling on the ground penetrates the ground or not is the infiltration capacity of the soil. This is the maximum rate at which rain can be absorbed by a particular piece of ground.
How can the infiltration capacity vary with different types of rain?
With light rain, all water falling on the surface infiltrates, and there is no overland flow.
Moderate rain results in no overland flow until the (initially high) infiltration capacity of the soil falls – only then will there be overland flow.
High-intensity rain rapidly results in a precipitation excess – there is overland flow for almost the whole of the duration of the storm. Precipitation reaches the ground faster than it can be absorbed by the ground.
What is saturation overland flow?
Where the water table is near the surface, the additional water from heavy rain causes the water table to rise. If it rises to the ground surface, any additional water will not penetrate the saturated ground (Figure 1.3.6). The infiltration capacity of the soil is zero and all precipitation falling on the surface is excess precipitation, translating as overland flow. This type of overland flow is referred to as saturation overland flow.
What is the idea called the variable source area concept?
Hewlett also saw that areas of a catchment which produced overland flow would vary in time and space. For instance, the areas next to rivers would be where the water table was nearest the surface and these would produce overland flow the quickest. The saturated zones of a catchment would vary over time, expanding in a storm and contracting after the rain stopped. This idea is called the variable sources area concept.
What is a hydrograph?
A hydrograph is a plot of river discharge at a particular cross-section of river against time, and can be used to predict peak river flows.
What is lag time of a hydrograph?
The lag time is the time between the maximum rainfall rate and the time of peak discharge.
What is lag time dependent on?
Lag time is dependent on a number of factors, including the slope of the land, intensity and duration of the rain, underlying soil and rock, and land use. Trees and vegetation tend to increase lag time by intercepting rain and delaying its route to the ground. If trees are removed and bare is soil exposed, or even if light crops are planted, water can run over the land more quickly with a shortening of lag time. The same effect is observed when hard, impermeable surfaces such as roads and buildings replace vegetation.
What are the four main routes of precipitation to travel through a catchment area?
The four main routes for precipitation to travel through a catchment are: channel precipitation, Qp; overland flow, Qo; groundwater flow, Qg; and throughflow, Qt.
What factors determine the relative importance of the routes of precipitation through a catchment area?
The factors determining the relative importance of the routes include: the nature of the ground surface, vegetation, and underlying soil and rock; topography; the duration of a rainstorm; and the existing water content of the ground.
What are hydrographs useful for understanding?
Hydrographs are useful for understanding river discharge patterns and predicting peak river flows. They can be used to link rainstorm and river flow rate with time and to estimate flood frequencies from discharge data.
What does the shape of a short period hydrograph depend on?
The shape of a short-period hydrograph (the record for a few days) depends on the size, shape, geology, vegetation and land use of the river catchment.
What does the shape of a long period hydrograph depend on?
The shape of a long-period hydrograph (e.g. for a year) depends primarily on the type of climate in the river catchment.
A river flooding is the result of what?
River flooding is the result of a complex series of factors including precipitation, infiltration rates and routes of water flow. Flood alleviation will depend on individual river characteristics.
What factors affect the lag time of a storm?
Factors affecting the lag time of a storm include the following.
Rainfall amount and intensity: heavy rain will not sink into the ground. Instead it will become overland flow or runoff and quickly reach the river.
Antecedent rainfall: rainfall that has already happened can lead to saturated ground. Further rain will then flow as surface runoff towards the river.
Land use: vegetation intercepts and delays the rain reaching the ground. Bare soil, rock and urban tarmac and concrete allow little infiltration, leading to rapid runoff and reducing the lag time.
Slope angle: steep slopes can lead to rapid runoff with water reaching the river quickly, whereas gentler slopes delay water reaching the river and mean that there is a greater chance of it infiltrating into the soil.
Rock and soil type: impermeable soils and rocks will not allow rainwater to sink down and so will speed up runoff. Permeable soils and rocks will allow infiltration and percolation of water into the bedrock, in turn slowing the delivery of the water to the river.
Temperatures: these affect the form of precipitation and the amount of evapotranspiration. If temperatures are below freezing, precipitation will be in the form of snow, which acts as a solid water store on land. If temperatures rise rapidly, melting snow can reach the river quickly in a rapid ‘flush’ of meltwater. High temperatures increase evapotranspiration from surfaces and plants.
Water management: dams and reservoirs can be used to regulate water flow and even out river discharges. River channelisation can speed water flow through a river.
Where do valley springs develop?
Valley springs develop in valleys where the ground surface intersects the water table
Where do stratum springs develop?
Stratum springs form where the downwards flow of groundwater is prevented by an underlying impermeable layer of rock. This can result in a line of springs emerging at the boundary between the two layers
Where do solution channels typically occur?
solution channel springs typically occur in limestone districts where groundwater has created caves and channels by dissolving calcium carbonate along bedding planes and fractures. The water is then returned to the surface where impermeable strata prevent further downwards migration.
What is the gradient of a river?
(the ratio of the drop in elevation divided by the distance over which the drop occurs)
What is a graded stream?
All streams, from small rills to large rivers, exhibit the general shape of an upwardly concave curve, known as a graded stream in a graph
What factors can affect flow rate of a river?
One of the factors that affects the flow type is the velocity of the water, v.
another factor affecting how water flows is the depth of the water, d, as the water encounters the friction of the riverbed
What flow type of dominant in streams?
True laminar flow is seldom significant in streams. Turbulent flow is the dominant type of flow.
What is laminar flow?
laminar flow is when water occurs at a low flow speed
What is turbulent flow of water?
higher speed gives turbulent flow.
What are isovel lines?
Points of the same water speed can be connected by lines known as isovel lines.
