Lecture 4-Precipitation 2

Question 1

Precipitation measurements

Answer 1

• accurate measurement of precipitation is an essential foundation for quantitative hydrological analysis
• precipitation is difficult to measure accurately
• temporal/spatial variation larger than other hydrological system components (not precipitation all day every day, rain distribution is uneven so the location we take a measurement from makes a difference)

Question 2

Precipitation: what we need to measure

Answer 2

• amount
• duration
• intensity = amount/duration

Question 3

Accuracy issues

Answer 3

• how accurate are point measurements?

- position, types of measurement, frequency of measuring

Question 4

Methods of measurement

Answer 4

• non-recording storage gauge (home gardens)
• tipping bucked rain gauge
• optical rain gauge
• rapid response drop counting rain gauge
• weighing bucket (snow)

Question 5

Tipping bucket

Answer 5

• ‘seesaw’ calibrated to a certain amount (0.5mm), counts number of ticks back and forth for each 0.5mm
• good when recording every few minutes
• also good for medium amount of rainfall (otherwise could loose small amounts, or may not keep up with large events)
• not good with snow/frosts

Question 6

Knowing measurement method used

Answer 6

• need to know so can understand what errors would be likely

- and for comparing to historic data if methods have changed over time

Question 7

Point measurement issues:

Answer 7

• wind turbulence (rain not making into bucket)
• undercatch
• steep terrain (wind blowing down hill, not representative measurement depending on location)
• forest canopy (trees impeding precip)
• extreme events (measurement method can’t keep up)

Question 8

How to address measurement issues:

1. gauge design

Answer 8

-orifice should be at least 30mm diameter

resolution = 0.1mm for tipping bucket

Question 9

How to address measurement issues:

2. wind effects vs. runoff

Answer 9

• typically 2m above ground to avoid capturing runoff

- or set level with ground to avoid wind

Question 10

How to address measurement issues:

3. evaporation

Answer 10

• closed vessel

- non-volatile immiscible oil or anti-freeze (cold environments)

Question 11

How to address measurement issues:

4. placement

Answer 11

-away from obstructions (trees)

Question 12

How to address measurement issues:

5. representative locations / missing data

Answer 12

• selecting the most representative station locations
• times when station is not working and data is missing, think about how your station is related to others to fill in gaps

Question 13

Rainfall statistics

Answer 13

• average rainfall over an area
• frequency of intensities
• frequency of intense storms
• calculate probable max precipitation

Question 14

Areal measurement

Answer 14

• array of rain gauges

- remote sensing (rain radar)

Question 15

Catchment mean areal rainfall accuracy

Answer 15

• number of gauges
• time interval
• size of area

Question 16

Catchment mean areal rainfall techniques

Answer 16

• arithmetic average
• create polygons (Theissen)
• create contours (Isohyetal)

Question 17

Arithmetic average method

Answer 17

-average from table values

Question 18

Theissen polygons (nearest neighbour) method

Answer 18

• finish polygons
• count squares (weighting)
• multiply weight by each gauge catch
• total = average areal rainfall

Question 19

Isohyetal method

Answer 19

• count squares in each elevation band (weighting)
• multiply by band midpoint
• total = average areal rainfall

Question 20

Theissen polygon / nearest neighbour concept

Answer 20

Gives weights based on how much area each station represents, larger area will have more weight
-issue: sharp steps, based on area which might not be the best means of weighting importance