Lecture Panel 4 Flashcards
What does a non recording gauge do?
gives total rainfall since last check - basically a bucket with a funnel on top
What does a recording gauge do? How does it work?
gives a steady record of rainfall
-inside there is a float attached to a pen, and the height of water is recorded over time on a chart attached to a slowly rotating drum
How does a tipping bucket gauge work? Drawbacks? Good for what?
rain is channelled through a funnel into one of two small containers in a see-saw configuration. When one side fills up, it tips and an electrical signal is recorded.
- not enough rain might fall to tip mechanism, or intensity could be so high that constant tipping makes measurement inaccurate
- good for getting an idea of intensity
How do optical gauges work?
- measurement of precipitation is proportional to disturbance to a beam of infrared light from a diode detected by a sensor
- can measure rain or snow
How does rain angle affect gauge measurement?
effective aperture becomes smaller as angle of incident rain becomes steeper
-diameter must not be less than 30mm
What is average precipitation measurement error?
5-10% for rain, and errors in snowfall measurement are greater
How could terrain affect airflow around your gauge?
- the gauge itself deflects air
- if there is no local topography, gauge deflects air up and horizontally
- if there is a near (but not too close hill) we can get upward deflection over gauge
- if there is a hill very close, wind can blow over hill and into gauge
Two factors that affect measurement accuracy
- precipitation type (eg. snow, hail)
- extreme events
What can be used to measure snow-water equivalent?
heated tipping bucket
- but heating leads to evaporation
- in high snowfall rate melting is too slow and gauge overflows
Result of gauge wind effect
speeds up wind in a small area over gauge top, deflects rain away from gauge
How can we combat gauge wind effects?
Use a wind shield - eg an Alter Shield, or Nipher shield
What is the point of the Nipher shield?
to prevent vertical wind eddies in the vicinity of the gauge mouth
What is the best way to get rid of wind effects?
Double fence shield - takes large area
What increases gauge error in windy conditions? Decreases?
- Less error with big drops and gauge near ground, shield helps
- More error with small drops and elevated/exposed gauge
What is the rule of thumb for measurement error in wind?
1% loss for every 1mph increase in wind speed
-2.2% for every 1 m/s increase
What two factors are associated with gauge measurement uncertainty in windy areas?
drop size and gauge location
What do we need to know to correct gauge measurement error?
Wind speed and drop size
How can we measure drop size?
disdrometer, or optical sensors
General rule for gauge placement around obstacles?
stay away by 2x the object’s height
What do tall objects cause in the wind?
turbulence induced downward motion
What are snow pillows and how do they work?
device for measuring snowpack
- measures water equivalent of the snow pack based on hydrostatic pressure created by overlying snow.
- pillow contains antifreeze liquid.
What is occult precipitation?
fog drip - accounts for 20% of precipitation at high elevations in Northern New England
What is considered trace precipitation?
less than 0.25mm - in places like Alaska 10% of total is “trace”
What affects precipitation bias?
- increases at higher elevation and higher latitude
- areal averaging must account for orography in mountainous terrain
What are gauge standards in Canada, USA, UK, and WMO?
Canada - diameter = 91mm, height = 305mm, no wind shield
USA - diameter = 203mm, height = 787mm, no wind shield
UK - diameter = 127mm, height = 300mm, no wind shield
WMO - diameter = 127mm, height = 1000mm, yes wind shield
small mountainous island with irregular precipitation gauge density
140-300 km sq/gauge
temperate and tropical mountains gauge density
300-1000 km sq/ gauge
flat areas in temperate, tropical, and mediterranean regions gauge density
1000-2500 km sq/gauge
gauge density for arid and polar regions
5000-20000
what should be done with suspect gauge data
Quality check and flag - do not delete
Where does the global gauge network cluster?
wealthy, densely populated areas
Why is gauge data important?
- key input into any hydrological analysis
- base data for many hydrological models
- 5 - 25% error in precipitation is not trivial
What can accurate gauge measurements help us predict?
temperature and observed runoff - prediction accuracy increases in areas with greater gauge density
-for runoff estimation, need 30-50 gauges per 10^6km
