1. More Precipitation Hydrology Spring 2013 Instructor: Eric Peterson

3. Storm Description Total amount of Precipitation Storm Duration Storm Intensity Storm Area (least important)

4. Rainfall Measurement Measured as depth of rain per area of surface –1 in of rain would cover a given area to a depth of 1 in Typically measured ~3 ft above ground in area sheltered from wind, but unaffected by surrounding structures and vegetation Measurement Devices –Calibrated Cylinder, Tipping Bucket, Scale

5. Tipping Bucket Gage NWS-type Calibrated Cylinder Simple Direct- Read Calibrated Cylinder

6. Snowfall Measurement “Snow Pack” –Depth –Water equivalency (water depth after melting) Typically 5-20% Snow still on ground in late summer may be 60% Inventories conducted periodically –5-10 points, 50 ft apart –Depth measured and cores taken and weighed

7. Example Snow Pack Measurement Snow Depth 10 ft Snow Density 15% What is the water equivalency? –10 ft x 12 in/ft x 15% = 18 in WE

8. Practice Precipitation Calculations Distribution of rainfall during a storm on the University of Georgia Whitehall Forest in June, 1990 (Brown, 1993) Calculate total precipitation Calculate peak 5 min intensity Determine duration Calculate Average Intensity Time (min) Amount (mm) 51.5 105.6 1513.0 205.8 252.1 309.7 354.8 401.3

9. Determining Average Precipitation over an Area If rain gages are evenly distributed a simple arithmetic average may be calculated Thiessen method: essentially a weighted average based on area between gages Isohyetal method: lines of equal rainfall (isohyets) are drawn on the watershed, then a weighted average is calculated based on the area between lines

10. Estimating Missing Precipitation Data 1.Station-Average Method 2.Normal-Ratio Method 3.Inverse-Distance Weighting