1 - River flow and gauges Flashcards
Precipitation trend:
- Not exactly but broadly speaking:
- Pressure > vapour pressure leads to precipitation.
- Pressure < vapour pressure leads to evaporation.
- Vapour pressure increases with temp
- Low temperature leads to more precipitation.
- High altitude corresponds to low pressure but also low temperature.
- Generally, precipitation is greater at higher altitudes.
- NB The term precipitation includes both rainfall and snowfall.
Overall – clouds (vapour) rise from sea, leading to temp decrease and precipitate at altitude
UK trend:
• Most high ground (mountainous) in West • Majority of weather from West o Most moisture from Atlantic • West = wettest – vapour stays where pressure is lowest • Rain shadow in UK
Point measurement
o Storage Rain gauges - typically daily or monthly recording - recording rain gauges - measures intensity by registering change of water level
o Or tipping bucket – two buckets on see-saw mechanism – creates voltage which produces intensity readings - continuously measures the time between 0.2 mm of rainfall – better temporal resolution
o About 400 rain gages in the UK
= how many point measurements are needed?
- UK density is 1 per 60 km2
Spatial measurement
o Weather radar or satellite –
o Transmits a pulse of electromagnetic energy which is reflected by precipitation particles.
o Intensity of echo gives information on raindrop size.
o Use empirical relationship to get rainfall rate, R = aZb where Z is reflectivity and R rainfall rate.
o There are currently 15 radars operating in the UK and Southern Ireland. Data from radars are sent to the Met. Office every 15 minutes.
Difficult to calibrate radar signal against real rainfall parameters – no intensity
o Raindrops are reflective – detects this with waves
Catchment properties
Surface-water catchments:
• River flow reacts quickly to rainfall events.
• There are periods of very low flow
Groundwater catchments:
• River flow is less “flashy”.
• Substantial summer flows.
• Most of the flow (the baseflow) is supplied by groundwater (see red line) through springs and upwelling in river channels.
Precipitation maths
Streamflow = Precipitation – Evaporation – Change in Storage
Evapotranspiration = Rainfall – Streamflow
Change in storage over decades should be negligible
Daily rainfall:
Given in mm/per day
To get rainfall into km3:
(1) divide by 1,000,000 to get into km.
(2) multiply by the area (in km2) of the catchment to get km3.
Now you have a volume of rainfall that lands in a day, which you can say has units of km3/day.
Measurement of streamflow using a velocity meter:
• Measure the flow velocity at a series of discrete points
• Ideally at 60% depth
• Record the depth of the water at the measurement points
• Flow rate Q (L3T-1) can be estimated from
o Q = ∑Bi hi vi
o Where:
N [-] is the number of increments
o Bi [L] is the breadth of increment i
h [L] is depth of increment i
vi [LT-1] is velocity inincrement i
Measurement of streamflow – Stage gauging
- Stage is the level of the river above a defined datum.
- Automatic measurement of stage allows efficient, continuous measurement of flow.
- However, the stage discharge relationship must be known.
- Often this is achieved by building a weir.
Types of evaporation
Evaporation – Evaporation of water from an open water surface.
Evapotranspiration – Includes wet canopy (top of vegetation) evaporation and transpiration
Potential Evaporation (PE) – The amount of water that would evaporate and transpire given an unlimited source of water. (what could happen)
Actual Evaporation (AE) – The actual amount of water that evaporates and transpires. (What will actually happen)
Pan evaporation:
• Involves direct measurement of water loss (change in water level) from an open water surface (pan) over a period of time
• Potential evapotranspiration, ET, is estimated from the pan evaporation, Epan, by:
• ET =KpEpan
where the lumped correction factor, Kp, accounts
• for differences between the pan and a crop, e.g.:
• Radiation balance (due to different albedos);
• Energy balance (since large amounts of energy are stored in the water);
• Numerous designs exist (U.S. Class A is shown), and the design (paint colour, shape, sunken into the ground or not, etc.) affects performance.
• In general performance is erratic, and strongly dependent on good siting and maintenance. Interference from animals and birds can be problem.
Factors affecting evaporation:
- Sunshine: The energy required to provide the latent heat necessary to change water to vapour mostly derives from solar radiation.
- Temperature: Higher temperatures control how readily water is evaporated and how much water the air can hold.
- Humidity: There is a limit to how much water a parcel of air can hold. When the air is vapour saturated evaporation will cease. (Relative humidity)
- Wind: Air parcels circulate in multi-scale Eddy patterns, replacing vapour saturated air with dryer air from above. The rate of dry air replenishment is heavily controlled by the local wind-speed.
- Soil water: Evapotranspiration is ultimately limited by the availability of water in the soil.
- Plant type: Evapotranspiration is also affected by the plant type. Various plant types transpire at different rates and/or lead to additional loss by evaporation due to various rates of rainfall interception (rainfall sometimes puddles on leaves).
Energy Balance:
Net radiation: Heat-flux mostly from solar energy
Latent heat-flux – the heat-flux required to vaporise water loss due to evaporation
Sensible heat-flux – The heat-flux that can be “sensed” above the ground. The heat you can feel
Soil heat flux – the net loss of heat into the ground. Very small when averaged over a daily period because heat gained during the day is mostly lost at night
Invoking conseration of energy
Rn = L + H + G
Evaporation by eddy flux measurement:
• Air flow can be imagined as a horizontal flow of numerous eddies
• Evapotranspiration is the net upwards flow of moisture
• Eddy flux calculations require data on vertical wind velocity and water vapour concentration (i.e humidity) samples at sub-second frequencies
• Sonic anemometers measure vertical wind velocity using time of flight of ultrasonic pulses between pairs of transducers
• Water vapour concentration is measured using infrared gas analysers
Evaporation: vertical wind velocity x water vapour concentration
Problems with Thornthwaite-Crowe?
Only works for summer months as it does not take into account reduced sunlight hours in winter
Measurment by lysimeter
Some volume of soil is isolated to provide absolute control on outflows.
• Rainfall is measured, drainage is collected, and storage can be determined by weighing the lysimeter (or by soil moisture instrumentation).
• The lysimeter should aim to reproduce natural conditions, including vegetation, siting and soil structure – shortcomings here are the main limitation of this method.
• Evaporation is then estimated by water balance method.
