Environmental Flashcards
The water cycle?
Atmosphere ->
Precipitation & snow ->
Surface runoff, infiltration ->
Ground water flow -> ocean
Evaporation
River discharge?
The flux of water through the river cross section of a point along a river
(L^3 T^-1) and (LT^-1) for runoff)
Hydrograph?
Plot of discharge over time
Q= vA (river)
River flux = average velocity * area of cross section
River velocity meters? (3)
- valeport propeller meters
- acoustic Doppler velocity profilers (ADVP) larger rivers
- measure river height (stage) and use stage discharge graph to predict discharge
Weir?
A rise in a channel bed which creates sub- critical upstream flow and super critical downstream flow with the critical section at the weir
Where can you derive a reliable theoretical stage- discharge relationship?
Weirs
How do you predict an end to a storm?
Identify the start of the stormflow by finding the inflexion point in the hydrograph. This often coincides with the start of the precipitation event and the start of the stormflow.
Then times the lag time (peak storm flow- peak precipitation) * 4(N) and then add this to the end of precipitation.
How can you get base flow from a hydrograph with stormflow?
You can remove stormflow by using a straight line
Catchment area?
The drainage area contributing to flow at a point on a river
Precipitation? (2/4)
- rainfall
- snow
- sleet
- hail
Hyetograph?
Plot of rainfall over time
ΔS = P - E - Q - R
Change in internal catchment storage = precipitation - evaporation - river runoff - groundwater recharge
Humidity?
The amount of water vapour in the atmosphere at a given point
Absolute humidity (ρw) vapour pressure (e) [mb]?
The mass of water vapour per unit volume of air [g/ m^3]
Dew point (Td)
The point where air parcels have cooled down through condensation enough to become saturated
r= (ed/ ea) * 100
relative humidity = parcel’s vapour pressure / the saturation vapour pressure at the same temperature * 100
What happens to the amount of water in air as temperature increases?
It increases too
What impact does the pressure gradient have as air makes its way up the atmosphere?
It will expand and cool potentially generating precipitation
What are four mechanisms that make vertical air movement?
- convection
- orographic ascent
- shear ascent
- frontal ascent
Convection?
Localised heating at surface produces buoyant air parcels
Orographic ascent?
Air forced to flow over an obstacle
Shear ascent?
Differing wind velocities with height induce atmospheric turbulence in all directions, including vertical ascent
Frontal ascent?
The meeting of air masses of different origins and properties results in the cooler ( more dense) air undercutting the warmer (less dense) air. This leading to widespread ascent, which, in turn, can also give rise to localised connective ascent
Three ways of measuring precipitation?
- manual rain gauges
- automatic rain gauges
- tipping bucket rain gauges
Three sources of errors for rain gauges?
- deflection of air by rain gauge ( horizontal flow)
- upward deflection over gauge (turbulent flow)
- downward deflection of air over gauge (turbulent flow)
How does a remote sensing weather radar work?
A narrow beam of microwave electromagnetic energy undergoes Rayleigh scattering when the beam encounters water droplets. A certain proportion of radiation is back scattered and can be measured
Spatial variability of precipitation*
Precipitation is often measured at a point scale. To calculate and analyse the water balance of a catchment, we need the flux over the basin area A. Therefore, it is often necessary to interpolate precipitation to obtain a spatial average. Many methods exist. But note that:
- the network density affects spatial accuracy
- the required distribution of stations is related to data requirements and physical features
- the quality of interpolation depends on rainfall type, typography, time scale
Isohyets?
Lines of equal precipitation across the catchment drawn by a skilled analyst
λE=Rn +C +V + G - Ps
λ= latent heat of vaporisation of water
E= evaporation
Rn= net radiation
C= sensible heat transfer
V= change in energy storage
G= energy exchange with the ground
Ps= energy consumed by photosynthesis
β= C/ λE
Bowen’s ratio = sensible heat transfer/ latent heat of vaporisation of water * evaporation
Factors of evaporation?
Meteorological:
- solar radiation
- relative humidity
- wind speed
Physical:
- salinity (sea water evaporates less than fresh water)
- water depth
- size of the water surface
Water cycle in plants?
Precipitation is collected
->
- Interception loss
-stemflow
- through full
Factors affecting interception loss?
- interception storage capacity of the vegetation cover
- type and morphology of the vegetation cover
- velocity of evaporation
- duration and intensity of the precipitation event
- precipitation event frequency
Evapotranspitation (Et)?
The combined process of direct evaporation and transpiration as plants enhance evaporation through root water uptake and transpiration
Factors of transpiration?
- number, size and distribution of stomata
- the thickness and permeability of the epidermis
- the area of internal surfaces exposed to intercellular spaces
- the arrangement of the vascular tissue
- solar radiation, stomata close at night, which reduces transpiration
Et0 =
(0.408Δ(Rn-G) + γ(900/(T+273)) u2 (es-ea)
———————————————————
Δ+ γ(1+0.34u2)
Et0 = initial evapotranspiration
Rn= net radiation at the crop surface
G= soil heat flux density
T= mean daily air temperature
u2= wind speed at 2m height
es= saturation vapour
ea= actual vapour pressure
Δ= slope vapour pressure curve
γ= psychrometric constant
Ep= Kc Et0
Ep= potential ET
Kc= crop or vegetation coefficient
Et0 = reference ET
Et = Kc Ks Et0
Et= actual ET
Kc= crop or vegetation coefficient
Ks = water stress coefficient
Et0 = reference ET
How is stem flow measured?
Using simple setups of water collection combined with a water flow meter
How is through fall measured?
With rain gauges installed under the canopy. Because of the spatial heterogeneity of the canopy, several rain gauges are required to get a representative sample
How can Et be measured?
With lysimeter experiements. A lysimeter contains a known volume of soil including a vegetation layer. The total mass of the soil volume is frequently weighed. By closing the water balance, the evaportranspiration flux can be obtained. This needs careful monitoring of precipitation and other changes in mass. Given the complexity of lysimeter experiments, set values are typically calculated using hydrometeorological measurements combined with empirical crop characteristics.
