Lecture 6: Precipitation Averages and Interception
Key Questions1. How much and when does Whatcom County receive rain?
2. Where online can you find rainfall data for the state?
3. How is rainfall averaged over a watershed?
4. What is interception and what controls its magnitude?
5. Why does so much rain get intercepted in the PNW?
Intercepted rain on a leaf
Point Measurement of Rainfall
Accurate measurements are necessary for quantitative hydrologic analyses. Two questions arise:
1. How accurate are point measurements?
2. How accurately can point measurements be extrapolatedover an area?
City of Bellingham Rain Gauge Locations
Tipping Bucket Rain Gauge
8 inch diameter
Collects 1 mm (0.01 inch) of rain and tips, empties and send an electronic digital signal that is recorded.
Brannian Creek Rain Gauge
Variables that affect accuracy
• Wind (keep about 1 m above the ground)
• Obstacles (place in open areas away from trees and structures)
• Splashing
• Evaporation
• Annual measurement accuracy is 5-15% up to 75% for a single storm
Lake Whatcom Watershed Rain Gauge Locations
North Shore Meteorological (MET) Station
Measures rain, temperature, humidity, wind speed, wind direction, and solar radiation
Brannian Creek Rain Gauge
Geneva Rain Gauge
Bloedel Donovan Rain Gauge
North Shore Hyetograph: 2010 Water Year
Oct 1
Apr 1
Sep 30
North Shore Monthly Totals: 2010 Water Year
North Shore Meteorological (MET) Station
Measures rain, temperature, humidity, wind speed, wind direction, and solar radiation
There are 8760 hours in one year. How many hours (or what percentage) of 8760 do you think it rains in Bellingham?
In 2010, the North Shore gauge recorded rainfall 1221 hours out of 8760.
Meaning that it rained14% of the year.
Hourly rainfall frequency for the 1221 hours of recorded rainfall at the North Shore gauge in 2010.
92 % of the 1221 hours of recorded rainfall in 2010, were ≤ 0.1 inches.
What does this say about rainfall intensity in the watershed?
Cumulative Rainfall: 2010 Water Year
Oct 1
Apr 1
Sep 30
Why does it rain more in the southern part of the Lake Whatcom Watershed?
47.7
50.0 50.0
67.4
by Cory Tarilton
WWU Ski and Snowboard Club
How is the average rainfall over an area determined from point measurements?
P = g =1
G
Pg1G
47.70 + 50.02 + 49.94 + 67.36 4
P =
P = 53.76 inches
Areal Average: Arithmetic
Areal Average: Thiessen Polygons
P = 1
Area g =1
G
Ag x Pg
A1
A2
A3
A4
A1x 47.70 + A2 x 50.02 + A3x 49.94 + A4x 67.36
P = 56.28 inches
A1 + A2 + A3 + A4
P =
Statistical techniques can be used in ArcGISto estimate an areal average.
P = 55.28 inches
Areal Average: Spline Interpolation
On average the Bellingham withdrawals about 11,000,000 gallons-per-day from the lake. Assuming all the extra rainfall goes directly to the lake as runoff how many days worth of water does the difference represent (55.28 – 53.76 = 1.52 inches)?
On average the Bellingham withdrawals about 11,000,000 gallons-per-day from the lake. Assuming all the extra rainfall goes directly to the lake as runoff how many days worth of water does the difference represent (55.28 – 53.76 = 1.52 inches)?
136 days
Vegetation type and distribution affects how much precipitation hits the ground surface, and hence streamflow
Lake Whatcom WatershedLake Whatcom Watershed
Vegetation intercepts and stores precipitation (snow)
Vegetation intercepts and stores precipitation
The magnitude of interception and storage is determined by
1. Type and growth stage of the vegetation
The magnitude of interception and storage is determined by
1. Type and growth stage of the vegetation
2. Precipitation characteristics (intensity and duration)
heavy rain
light rain intermittent light rain
gross rainfall
gross rainfall
throughfall
gross rainfall
throughfallstem flow
canopy interception
through fall measurements
stem flow measurements
through fall and stem flow measurements
gross rainfall (Pg)
throughfall (Th) stem flow (St)
canopy interception (Ec)
Ec = Pg – (Th + St)
The magnitude of interception and storage is determined by
1. Type and growth stage of the vegetation
2. Precipitation characteristics (intensity and duration)
The canopy area available for precipitation interception and storage is quantified by a variable called
LEAF AREA INDEX (LIA)
LAI = the ratio of canopy area to projected ground area
LAI = 1 = 10,000 m2 per 10,000 m2
LAI = 2 = 20,000 m2 per 10,000 m2
LAI = 6 = 60,000 m2 per 10,000 m2
Regression equations are used to estimate the magnitude of throughfall (Th) and stem flow (St) that occurs in a forested watershed. The equation below is an estimate for a coastal conifer forest (units are in cm).
Th + St = Pn = 0.79∑Pg – 0.13n
Where Pn is the net precipitation, Pg is the gross precipitation (rain falling above the forest) and n is the number of storms.
Th + St = Pn = 0.79∑Pg – 0.13n
Pn is the net precipitation
The magnitude of Pn is controlled by the rainfall intensity.
Th + St = Pn = 0.79∑Pg – 0.13n
Pn is the net precipitation
Determine Pn where ∑Pg is 10 cm and n = 5
Th + St = Pn = 0.79∑Pg – 0.13n
Pn is the net precipitation
Determine Pn where ∑Pg is 10 cm and n = 5
Pn = 7.25 cm
Th + St = Pn = 0.79∑Pg – 0.13n
Pn is the net precipitation
Determine Pn where ∑Pg is 10 cm and n = 20
Th + St = Pn = 0.79∑Pg – 0.13n
Pn is the net precipitation
Determine Pn where ∑Pg is 10 cm and n = 20
Pn = 5.30 cm
Because of the high amount of low intensity rainfall events in the PNW, about 30% of the rain is intercepted.
Occult Precipitation (water)
Fog Drip
Occult Precipitation (ice)
Rhime
