Lecture Panel 4

Question 1

What does a non recording gauge do?

Answer 1

gives total rainfall since last check - basically a bucket with a funnel on top

Question 2

What does a recording gauge do? How does it work?

Answer 2

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

Question 3

How does a tipping bucket gauge work? Drawbacks? Good for what?

Answer 3

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

Question 4

How do optical gauges work?

Answer 4

• 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

Question 5

How does rain angle affect gauge measurement?

Answer 5

effective aperture becomes smaller as angle of incident rain becomes steeper
-diameter must not be less than 30mm

Question 6

What is average precipitation measurement error?

Answer 6

5-10% for rain, and errors in snowfall measurement are greater

Question 7

How could terrain affect airflow around your gauge?

Answer 7

• 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

Question 8

Two factors that affect measurement accuracy

Answer 8

• precipitation type (eg. snow, hail)

- extreme events

Question 9

What can be used to measure snow-water equivalent?

Answer 9

heated tipping bucket

• but heating leads to evaporation
• in high snowfall rate melting is too slow and gauge overflows

Question 10

Result of gauge wind effect

Answer 10

speeds up wind in a small area over gauge top, deflects rain away from gauge

Question 11

How can we combat gauge wind effects?

Answer 11

Use a wind shield - eg an Alter Shield, or Nipher shield

Question 12

What is the point of the Nipher shield?

Answer 12

to prevent vertical wind eddies in the vicinity of the gauge mouth

Question 13

What is the best way to get rid of wind effects?

Answer 13

Double fence shield - takes large area

Question 14

What increases gauge error in windy conditions? Decreases?

Answer 14

• Less error with big drops and gauge near ground, shield helps
• More error with small drops and elevated/exposed gauge

Question 15

What is the rule of thumb for measurement error in wind?

Answer 15

1% loss for every 1mph increase in wind speed

-2.2% for every 1 m/s increase

Question 16

What two factors are associated with gauge measurement uncertainty in windy areas?

Answer 16

drop size and gauge location

Question 17

What do we need to know to correct gauge measurement error?

Answer 17

Wind speed and drop size

Question 18

How can we measure drop size?

Answer 18

disdrometer, or optical sensors

Question 19

General rule for gauge placement around obstacles?

Answer 19

stay away by 2x the object’s height

Question 20

What do tall objects cause in the wind?

Answer 20

turbulence induced downward motion

Question 21

What are snow pillows and how do they work?

Answer 21

device for measuring snowpack

• measures water equivalent of the snow pack based on hydrostatic pressure created by overlying snow.
• pillow contains antifreeze liquid.

Question 22

What is occult precipitation?

Answer 22

fog drip - accounts for 20% of precipitation at high elevations in Northern New England

Question 23

What is considered trace precipitation?

Answer 23

less than 0.25mm - in places like Alaska 10% of total is “trace”

Question 24

What affects precipitation bias?

Answer 24

• increases at higher elevation and higher latitude

- areal averaging must account for orography in mountainous terrain

Question 25

What are gauge standards in Canada, USA, UK, and WMO?

Answer 25

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

Question 26

small mountainous island with irregular precipitation gauge density

Answer 26

140-300 km sq/gauge

Question 27

temperate and tropical mountains gauge density

Answer 27

300-1000 km sq/ gauge

Question 28

flat areas in temperate, tropical, and mediterranean regions gauge density

Answer 28

1000-2500 km sq/gauge

Question 29

gauge density for arid and polar regions

Answer 29

5000-20000

Question 30

what should be done with suspect gauge data

Answer 30

Quality check and flag - do not delete

Question 31

Where does the global gauge network cluster?

Answer 31

wealthy, densely populated areas

Question 32

Why is gauge data important?

Answer 32

• key input into any hydrological analysis
• base data for many hydrological models
• 5 - 25% error in precipitation is not trivial

Question 33

What can accurate gauge measurements help us predict?

Answer 33

temperature and observed runoff - prediction accuracy increases in areas with greater gauge density
-for runoff estimation, need 30-50 gauges per 10^6km