Lecture 6: Precipitation Averages and Interception Key Questions 1. 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 extrapolated over 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.
Variables that affect accuracy Wind (keep about 1 m above the ground) Obstacles (place in open areas away from trees and structures) Brannian Creek Rain Gauge 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 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. Measures rain, temperature, humidity, wind speed, wind direction, and solar radiation
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
Western Regional Climate Center
Washington Agricultural Weather Network
WWU Ski and Snowboard Club by Cory Tarilton
How is the average rainfall over an area determined from point measurements?
Areal Average: Arithmetic P = 1 G G g =1 P g P = 47.70 + 50.02 + 49.94 + 67.36 4 P = 53.76 inches
Areal Average: Thiessen Polygons A 1 A 3 P = 1 G Area g =1 A g x P g A 2 A x 1 47.70 + A2 x 50.02 + A3 x 49.94 + A4 x 67.36 P = A 1 + A 2 + A 3 + A 4 P = 56.28 inches A 4
Statistical techniques can be used in ArcGIS to estimate an areal average.
Areal Average: Spline Interpolation P = 55.28 inches
On average the Bellingham withdrawals about 11,000,000 gallons-perday 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-perday 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 Watershed Lake 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 canopy interception stem flow throughfall
through fall measurements
stem flow measurements
through fall and stem flow measurements
E c = P g (T h + S t ) gross rainfall (P g ) canopy interception (E c ) stem flow (S t ) throughfall (T h )
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 m 2 per 10,000 m 2 LAI = 2 = 20,000 m 2 per 10,000 m 2 LAI = 6 = 60,000 m 2 per 10,000 m 2
Regression equations are used to estimate the magnitude of throughfall (T h ) and stem flow (S t ) that occurs in a forested watershed. The equation below is an estimate for a coastal conifer forest (units are in cm). T h + S t = P n = 0.79 P g 0.13n Where P n is the net precipitation, P g is the gross precipitation (rain falling above the forest) and n is the number of storms.
T h + S t = P n = 0.79 P g 0.13n P n is the net precipitation The magnitude of P n is controlled by the rainfall intensity.
T h + S t = P n = 0.79 P g 0.13n P n is the net precipitation Determine P n where P g is 10 cm and n = 5
T h + S t = P n = 0.79 P g 0.13n P n is the net precipitation Determine P n where P g is 10 cm and n = 5 P n = 7.25 cm
T h + S t = P n = 0.79 P g 0.13n P n is the net precipitation Determine P n where P g is 10 cm and n = 20
T h + S t = P n = 0.79 P g 0.13n P n is the net precipitation Determine P n where P g is 10 cm and n = 20 P n = 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