Lecture 3: hydrological processes Flashcards
What are the two principal foci of hydrological sciences?
- the global hydrological cycle: Transfers of water between the land ocean and atmosphere
- the land phase of the hydrological cycle: The movement of water on and under the land surface, physical and chemical interations with earth materials accompanying that movement, and the biological processes that conduct or affect that movement.
Why study hydrology?
- agriculture
- industry
- hazard risk and early warning (connection with finance)
- sustainability
- earth system science
- climate change
- scientific enquiry
What are the characteristic space and time scales in hydrology?
Soil moisture: days to months
Groundwater: months to 100s years
Runoff: Hours to months
Snow: months
What is the principle of Conservation?
inputs (I) - outputs (O) = changes in storage (S)
I-O= (triangle)S
Define the water balance
strictly refers to a control volume, but often applied to a geographic region, most commonly a large basin or a catchment/watershed
- conservation also applies to energy and momentum
What is the water budget equation?
P-ET-R= (triangle)D/dt
P= precipitation (flux)
E= evapotranspiration (flux)
R= runoff (flux)
= or Q when referring to river discharge
(trangleS/dt)= change in storage (change in state)
What are the links to the energy and carbon cycles?
- the terrestrial energy cycle relates to the balance of incoming and outgoing energy at the earths surface.
- The terrestrial carbon cycle is dominated by upake of CO2 from the atmosphere by plant photosynthesis
- These are linked to the each other and the Water cycle through ET.
what are the ET links to the terrestrial water and energy budget equations?
- what are the wnergy and water budge
Energy budget
Rn-G= H+LE
Rn: net radiation
G: Ground heat flux
LE: Latent heat flux
E: Evaporation
What are the interactions of the Carbon and Water cycles?
The two cycles interact directly where carbon is transported dissolved or suspended in running water.
- Transport of weathering products and organic matter from the continents to the oceans is an important aspect of carbon cycling which is directly linked to water flux
- tree die off due to drought in the Western US has implication for the carbon budget
- similarly the impact of changing atmospheric carbon concentrations on global climate has a profound effect on water cycling impacting terrestrial and oceanic evaporation and patterns of precipition
- the two cycles are also linked through the role of ecosystems in carbon cycling since moisture availability is a key control on plant distribution and plant life plays a key role in terrestrial carbon cycling
Processes of the Land hydrological cycle
Total terrestrial precipitation (Snowfall and rainfall)
Total terrestrial evapotranspiration
River
Subsurface runoff
Surface runoff
Soil mositure
Groundwater
Define precipitation
Refers to all liquid and frozen forms of water falling from the sky (rain, snow, hail, dew, hoar-frost) but in general only rain and snow make significant contributions to precipitation totals.
- in many parts of the world, the term rainfall can be used interchangeably with ‘precipitation’ since almost all precipitation is rainfall
- the earths atmosphere carries large volumes of water and energy around the planet as water vapour and latent heat, in response to the latitudinal imbalance in energy reciept from the sun
- at any one time, the atmosphere contains enough water vapour to produce 11mm of precipitation across the whole of the earths surface.
Why is precipitation important?
perhaps the most important flux in the hydrological cycle
- precipitation provides the majority of our water resources recharge
- it can lead to hydrological hazards:
floods: too much rain
drought: too little rain - but it is very difficult to measure, estimate and model because it is highly variable in time and space, intermittent and highly skewed in its distributions
what is needed for precipitation to occur?
you need water vapour
What is the saturation vapour (es) pressure over water?
The amount of water that the atmosphere can carry (the satuation vapour pressure es) is a function of temperature.
The saturation vapour does not increase linearly with temperature but rather exponentially through the Clausius-Clapeyron equation.
What is the Magnus-Tetens forumla- a good approximation?
es(T)=6.1094 exp[(17.625.T)/(T+243.04)].
Having water in the atmosphere is a necessary but not sufficent condition for precipitation to occur. There must also be a machanism to promote uplift, cooling and condensation.
what are the precipitation requirements and processes?
Condensation occurs when the temperature is <= the dewpoint temperature Td
- without small particles in the air (e.g. dust, soot, sea salt, clay, sulfate, phytoplankton) to act cloud condensation nuclei (CNN), the air can become supersaturated
- the number of nuclei in the air range between around 100-1000 per cm3
- typical CNN=0.2um
- typical cloud droplets are 20-100um
- rain drops are around 2000um-2mm
- downward velocity must exceed uplift velocity for precipitation to fall. Uplift velocities in convective storms can be large.
what is precipitation produced by?
- cooling of air to the dew-point temperature
- condensation on nuclei to form cloud droplets or ice crystals
- growth of droplets or crystals into raindrops or snowfalkes
- importation of water vapor to maintain this process
Cooling of sir due to verticle uplift is the main process for producing precipitation and this occurs under different meterological conditions in which the cooling is rapid enough:
- fronts and extra tropical cyclones
- ITCZ
- tropical cyclones
- convective precipitation
- orographic precipitation
what is ET?
is the sum of evaporation and transpiration
define evaporation
Sum of evaporation from open water surface (e.g. lakes), soils, snowpack (sublimation) and vegetation canopy (interception)
define transpiration
process by which water contained in plant tissues is lost to the atmosphere by the process of evaporation
what are the global evapotranspiration statistics?
it is the majority of the terrestrial water budget- about 2/3 of land precipitation is evaporated
- about 42% is transpired from plants (25-64%)
- 3% is open water evaporation (lakes, wetlands, reservoirs)
- the remainder is interception loss with some bare soil evaporation
ET>R on most continents
The partioning of ET into its components (transpiration, interception, soil evaporation, etc) is poorly understood
why is evapotranspiration important?
it provides the link with energy and carbon cycles and links to the atmosphere
It is essential for the growth of natural and agricultural systems
Over the long term the difference between P and E is the available water resources for human use and management
It is important for understanding water resources and management e.g. for irrigation needs, reservoir evaporation
it provides mechanisms for precipitation recycling and can connect regions
it does and will play a mjor role in climate change impacts and feedbacks
What are the ET processes at the leaf scale?
Key to evapotranspiration (and terrestrial carbon cycling) are the processes of
photosynthesis and respiration.
• Photosynthesis is the process of the production of carbohydrate molecules from carbon
dioxide and water using energy from light. Plants fix gaseous carbon dioxide from the
atmosphere into solid form in their tissues.
• CO2 is released to the atmosphere by living things through the process of respiration. Life
derives energy from the combination of sugars and oxygen and CO2 is a by-product of this
reaction
• Leaves absorb solar
radiation, exchange
energy with their
environment through
radiation, sensible heat
loss and transpiration
• Leaves lose water and
uptake carbon dioxide
through their stomates
• Numerous plant
adaptations exist to
minimize water loss and
maximize carbon
uptake
ET processes at the plant/stand/catchment scale
Leaf scale processes contribute to canopy scale ET (e.g. the whole tree)
- Leaf absorption contributes to canopy scale absorption
- Leaf scale vapor exchange contributes to canopy scale exchanges
• This is further modified by the arrangement of canopy elements
- Leaf clustering, canopy leaf area, etc
- Canopy geometry (height, shape, volume) and spacing between plants
• This then scales up to a stand of vegetation (e.g. forest)
• Then to a catchment/watershed that contains multiple land covers (forest, grasslands, etc)
each of which have their own characteristics that modify ET (e.g. water efficient users) –
important for land use change
what factors affect ET?
- wind
- solar radiation
- temperature
- humidty
- soil moisture availability (when this increases so does ET)
- ET will happen at the potential rate (PET) until soil moisture depletes below a certain level, and then declines until moisture is too low to be accessed.
define: potential evaporation (PET)
The environmental or atmospheric demand for evapotranspiration over a large
area of uniform vegetation given an unlimited supply of water and no resistance
to evaporation.
define evapotranspiration (ET0)
The environmental demand for evapotranspiration but for a short green reference crop (grass), completely shading the ground, or uniform height and with adequate water status in the soil profile. Sometimes incorrectly reffered to as potential ET (PET). Used extensively in water rsources to calculate water demand (e.g. for irrigation).
What are PET and ET0 a reflection of?
Reflection of the energy available to evaporate water, the ability of the atmosphere to accept water vapour and of the wind available to transport the water vapour from the ground up into the lower atmosphere.
define actual ET
the actual ET that occurs given the atmospheric demand but which may be less than potential/reference because of soil moisture limitation. ET is said to equal reference evapotranspiration or PET when there is ample water.
How would you calculate the reference ET0 for a crop
ET0= function(radiation, temperature, humidity, wind)
Calculate the crop water requirements- and therefore irrigation needs
ET crop (mm/day)= Kc (per month)x(Eto(mm/day)
Et crop (mm/month)= 30xET crop (mm/day)
Irrigation needs (mm/month)=ET crop (mm/month)-P eff(mm/month) P eff<p></p>
explain soil moisture
76% of land precipitation globally enters the soil
- what in the soil provides:
> all the water for natural and cultivated agriculture
>almost all the water that enters groundater reservoirs
- soil moisture is highly variabile in space and this variability is controlled to first order by different drivers depending on the scale.
Define soil moisture
Volumetric water content (theta) also called water content or soil moisture content
O=Vw/Vs
Vw- volume of water
Vs- volume of soil
O ranges from>0 to porosity
The total amount of water stored in any layer of soil (soil water storage) is usually expressed as depth (mm) which is:
SM(mm)=O x depth of soil layer
Saturation or wetness is the proportion of the soil pores that contains water (0-1)
describe the movement of water in the unsaturated zone
Precipitation can enter the soil through the process of infiltration and then can move through the unsaturated zone to the saturated zone (GW)
• If the pores of the soil near the surface are
not completely filled with water then some
rainfall and snowmelt enters the soil via the
process of infiltration
• The pores of soil in the unsaturated zone
contain air and water, and pressure forces
less than atmospheric pressure
• Gravity and pressure gradients due to
spatial variations in water content cause
water to flow downslope or downward to the
saturated zone (groundwater)
• The water table is the fluctuating upper
boundary of the saturated zone, at which
pressure is equal to atmospheric
• Water moving down into the saturated zone
is groundwater recharge
• Soil pores are completely filled with water in
the saturated zone
What are the groundwater processes and concepts?
- groundwater is water under positive pressure (>atmopsheric pressure) due to the weight of water above it
- the water table is the fluctuating upper boundary of the groundwater zone at which pressure = atmospheric pressure
Recharge. Most water enters the groundwater reservoir when inflitrated water arrives at the water table as recharge
Discharge. Under natural conditions, groundwater eventually discharges into rivers, lakes or directly to the oceans and also through capillary rise to evaporation.
Water supply. It is crucially important as a source of rivers and lakes as a direct source for water supply.
GW constitutes about 30% of the world fresh water and 99% of the total liquid freshwater
Residence times are relatively slow- on average about 235 years globally for moderate to large scale regional systems but can range from a few to 1000 years.
Acts as a water filter by removing particulates and contaminants
Ground water processes: processes that produce runoff/flow
- overland flow (surface flow)
- Infiltration excess (Hortonian) overland flow – important in arid regions (high intensity
rainfall) , frozen regions, man-made surfaces
-Saturation (Dunne) overland flow – one important process in humid regions
- Subsurface flow
- Flow in the unsaturated zone (matrix or macropore) – interflow between surface and water table
-Flow in the saturated zone (near-stream GW or perched GW) – slow, damped response
Describe streamflow processes
Streams are the routes by which almost all the runoff
on the continents is returned to the oceans to
complete the hydrological cycle. There are three
important aspects to this:
- Water Resources. The flow in stream constitutes
the sustainable water resources available for
human use and management and for in-stream
ecosystems - Flood Risk and Forecasting. Flood risk
predictions are estimates of the probabilities of
floods of various magnitudes. Flood forecasts are
estimates of the actual magnitudes of flooding
produced by rain or snowmelt events that is
occurring or is forecast to occur - Water Quality (temperature, dissolved
constituents, and particulates) affects the
suitability of water for use by humans and natural
organisms, is strongly influenced by the
physical/chemical and biological processes that
occur as water moves over the land and through
streams.
What are the stream flow processes in catchments?
• A catchment is the
basic unit of streamflow
processes
• Runoff generated on
the land and in soils
drains via the
catchment stream network.
• The catchment is the area that topographically
contributes all the water that passes through a
specified cross section of a stream
• Therefore, generally everything upstream that is
higher will flow to this point
• Also called a drainage basin or river basin or
watershed
• The stream network can be complex and can be
divided into into sub-catchments
Describe streamflow regimes: specific factors that dominate the streamflow of a catchment or regime called streamflow regimes
At the catchment level, streamflow is controlled by a wide variety of processes and
combinations of these, depending on climate, surface characteristics and flow paths, and
underlying geology: surface run-off or overland flow; summer evapotranspiration; wintertime
snow accumulation and the timing and speed of spring melt; and groundwater.
Combinations of specific factors that dominate the streamflow of a catchment or region are
called streamflow regimes
Figure left shows daily flows for neighbouring catchments in the Thames basin. Although they experience
almost identical climatic conditions, the flow regime for the Lambourn, which is sustained by outflows from
the underlying chalk aquifer, is markedly more stable than that for the Ock which drains a largely
impermeable clay catchment – allowing a more immediate response to rainfall events
use the streamflow regimes of san marcos and augusta creek as examples
The San Marcos in Texas is fed by the Edwards aquifer,
and exhibits a steady baseflow, upon which streamflow
peaks are superimposed.
The Augusta Creek in Missouri is typical of a small river
in a temperate climate, showing large variability from
day to day and month to month.
The Colorado is mainly fed by snowmelt and so
is dominated by a large peak when the region
warms in late spring.
Cave Creek in Arizona is in a dry climate, where
baseflow is zero and streamflow is flashy after
infrequent short but intense storms.
How do you connect hydrological processes over time?
it is a filtering process
precipitation
v
runoff :Surface runoff
v
Moisture index
v
baseflow
what is the water balance of a watershed?
Inputs (i), outputs (o) and storage (S)
I
precipitation
groundwater in (Gin)
O
evaportranpiration (et)
groundwater out (gout)
river discharge (Q)
storage (S)
in groundwater, rivers and lakes
what can we usually measure?
P: rain gauges Q: stream gauges ET: hard to get Gin: hard to get Gout: hard to get S: often hard
change in storage=ET + Q, or ET=P-Q
What are the dimensions and units of the water budget?
volume= m3/ km3
density p = often assumed to be constant (1000kgm-3) hence water mass M=pV
(this means that conservative of mass equals conservation of volume)
I and O often expressed as rates of fluxed, or volume/time (m3 s-1)
I O and S can also be expressed as a change in water depth (m) averages over the catchment or region/country.
Simply divide by the
area of the catchment (m3 s-1/ m2 = m s-1). In this case, instead of discharge Q
(m3 s-1) we speak of runoff R (m s-1)
Other typically used units are mm/day or mm/year.
Connecting the Global system to processes at local, catchment, basin, region and continent scales
- aggregation and interaction of processes
2. global changes transferring to local impacts
What is the global and continental water budgets
Globally: all ET eventually becomes P and vice versa
P = ET, 952 mm/year = 2.6 mm/day
However, also globally:
• Oceans: ET (85%) > P (77%)
• Land: P (15%) > ET (23%)
Therefore, there is a net transfer of water from the ocean
to the land, and this ends up as runoff, R
assuming no net change in land water storage
Therefore moisture flux from oceans to land (M) = runoff from land (R)
Considering all land masses together:
P = ET + R (over the long term)
750 mm 408 270
100% 64 36
What are some important hydrological concepts?
residence time (Tr) Tr= S/I or S/O (assumes that I or O, that is a steady state) units: S=m3 (km3), I or O= M3/s or km3/yr, therefore Tr=1/s-1=s (or years)
runoff ratio (R/P): fraction of precipitation that becomes runoff. this can be estimated from long-term annual means of P and R.
A low run off ration: water loss from ET is big
A high runoff ratio: water loss from ET is small
PET: ET that would occur if there was an unlimited supply of water at
the surface. Also know as atmospheric demand because it depends on the atmospheric
conditions (e.g. how much energy there is to evaporate the water and how much water the
atmosphere can accept)
Aridity Index: P/PET: indicator of the degree of dryness of the climate at a given location. The ratio of supply to potential atmospheric demand.
Classification Aridity Index Global land area
Hyperarid AI < 0.05 7.5%
Arid 0.05 < AI < 0.20 12.1%
Semi-arid 0.20 < AI < 0.50 17.7%
Dry subhumid 0.50 < AI < 0.65 9.9%
What is the global pattern of the water budget?
The spatial variation of water, from local to global scales, is driven by the patterns of climate
and the underlying land-surface characteristics of elevation, slope, vegetation, land use and
water bodies. Some of these factors are not mutually exclusive, such as climate and
vegetation, whereas others, such as land use, may be independent of climate in the case of
irrigated agricultural land. At large scales, the climate is the dominant driver of the spatial
variation in the water budget.
Global pattern of the water budget- what drives the basic distribution of climate?
• The basic distribution of climate is a result of the general circulation of the atmosphere, which represents the major flows of water and energy around the world. • The circulation is a result of the differential heating of the Earth by the sun, the rotation and tilt of the Earth and the distribution of the land masses. • Over the year, the global incoming radiation from the sun (solar radiation) is balanced by the outgoing energy emitted by the Earth (terrestrial radiation). • However, the sun’s energy is most intense in the tropics and lowest at the poles (because of the angle of incidence of the sun’s rays), where outgoing radiation is greater than incoming solar radiation. • This creates a surplus of energy near the equator and a deficit of energy near the poles. The climate system tries to maintain a balance and so the surplus of energy is dissipated towards the poles by various atmospheric and ocean flows.
what is the circulation between the equator and the subtropics called?
Hadley cell- first proposed by english meteroligist george Hadley.
remind yourself of this diagram.
The intense solar radiation in the tropics
causes the heating and rising of moist
air, which cools and condenses to form a
region of intense clouds and precipitation
(ITCZ).
The rising moist air in the ITCZ spreads
polewards when it hits the stable air of
the stratosphere
At 30o N and S latitudes, air descends to
form a band of high pressure called the
subtropical high. The descending branch
of the circulation is relatively dry.
Part of the subtropical dry surface air travels back to
the lower pressure at the equator to replace the moist
rising tropical air and complete the circulation.
Beyond the subtropics, other general criculation cells exist in the mid-latitudes and polar regions. The average atmopsheric flows can therefore be described by a three-cell circulation model (Hadley, Ferrel and Polar cells).
- At the boundaries of these cells, jet
streams will form, which are narrow, fastmoving
air currents near the tropopause,
which flow westwards. Warm air moving
polewards from the subtropical high meets
with cold polar easterly air. - Jet streams are not stationary in latitude
but meander greatly, as well as moving
north and south
-The meander of the polar jet stream has a
large impact on mid-latitude weather
systems and thus drought.
-In reality, the general circulation of the
atmosphere is far more complex and
variable than that predicted by the threecell
model
What are the main drivers of precipitation?
The ITCZ is the pronouced band of high precipitation- moisture evaporated from the tropical oceans converges in the ITCZ
- the peaks in precipitation coincident with the mid-latitude zone of rising air are produced mainly by extratropical cyclonic storms that tend to develop along the polar front.
other factors: topography, air temperature, frontal activity, wind directions relative to moisture sources- all make the global distribution more complex
Remind yourself of the monsoon!
…
What are the factors driving ET distribution?
similar to the pattern of radiation abalnce reflecting the impotance of energy to supply, the latent heat that goes with the phase change from liquid to vapor.
- maximum in the tropical rain forests of south america, africa and southest asia (wet tropics) where there is plenty of available water.
The lowest areas are in the desert regions- e.g. sahara, antartica, arctic of N america and norther Eurasia.
Why is ET water or energy limited?
On land ET is controlled by the atmospheric demand (PET) and the availability of water.
thus ET on land may be either energy limited- where potential evaportranspiration is less than precipitation (humid regions)
or water limited where potential evapotranspiration exceeds precipitation (dry regions).
What is precipitation recycling and moisture sources?
The fraction of precipitation that falls into a region that is due to water that evaporated from within that region- the remaining precipitation may be sources from the ocean or other land outisde the region
- moisture for precipitation may be sourced from ocean, surounding land or locally
What is precipitation recycle scale dependency?
- recycling depends on the area you are looking at
- at global scale all precpiptation is recycled (P=ET)
- at point scale, all precipitation is sources from somewhere else.
really good map on blackboard- high values indicated locations from where the evaporated moisture will fall again as precipitation over continents
why is precipitation recycling important?
recycling is important for water resources because:
- a substantial amount of moisture that becomes precipitation is recycled from downwind
- several factors can alter the availability of moisture
>droughts downwind
>changes in land cover (e.g. forest versus crops)
> irrigation increases ET
> climate change is changing circulation patterns and impacts could be amplified via recycling.
What is the example: irrigation can cause precipitation downwind?
Irrigation in the Midwestern US may be inducing rainfall locally and downwind
- model simulations with and without irrigated land shows precipitation induced.
- change in storage and change of state)
desrcibe the global distribution of runoff/streamflow
- averaged over the long-term, R=P-ET
- runoff is highest where P is high, or ET is limited by available energy
- The highest rates are 3000mm/yr on the east coast of Bay of Bengal
- and about 1000mm/yr over the whole of the amazon bazin
describe cold season processes- snow and snowmelt
- Snow occurs predominently on the northern continents and at high elevations
- hydrological and water resources importance of snowpack sotorage and snow melt
- more than one-sixth of the earths population relying on glaciers and seasonal snow packs for their water supply
- more than 50% of runoff is derived from snowmelt in much of the N hemisphere
- for mountain regions this may be as much as 85% of runoff
- in regions with significant seasonal snowpack (e.g. wetsern US, Euro Alps, scandanavia) the amount and timing of snowmelt is very important for runoff and river flows, and therefore water resources (and for ecosystems and floods) and groundwater recharge
- good diagram on bb
Describe changes in land water storage
For water resources, we are interested in the seasonal cycle of storage changes- how much water is available throughout the year
storage= soil moisture groundwater, wetalnds, lakes, reservoirs, snow, glacier, ice caps, permafrost
What are the implications of spatial and temporal variability of the water budget on water resources?
regional examples:
- snowpack in Sierra Nevada mountains of california
- West African Monsoon and the Niger river
- Water supply from glaciers in the Indus rivers
What are the challneges for water resources in the Niger River Basin?
- low average rainfall
- strong dry/wet season
- High rainfall inter-annual variability
- High water losses along the river
- Also: socio-political factors
Why is the dependency on western US water resources on snowpack in the Sierra Nevada Mountains bad?
31 year average= glacier retreating
What is the glacier/snowmelt dependence in the himilayas?
The rivers that drain these mountains influence the lives of about 40 per cent of the world’s
population. The rivers provide household water, food, fisheries, power, jobs and are at the heart
of cultural traditions.
NMI= ratio of glacier/snowmelt resources to water resources generated downstream-
order of rivers
Indus Brahmaputra Ganges Yangzte Yellow
