More PrecipitationMore Precipitation

HydrologyHydrology

Spring 2013Spring 2013

Instructor: Eric PetersonInstructor: Eric Peterson

Storm DescriptionStorm Description

• Total amount of Precipitation

• Storm Duration

• Storm Intensity

• Storm Area (least important)

Rainfall MeasurementRainfall 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

Tipping Bucket Gage

NWS-type Calibrated Cylinder

Simple Direct-Read Calibrated

Cylinder

Practice Precipitation CalculationsPractice 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)

5 1.5

10 5.6

15 13.0

20 5.8

25 2.1

30 9.7

35 4.8

40 1.3

Determining Average Precipitation Determining Average Precipitation over an Areaover 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

Estimating Missing Precipitation DataEstimating Missing Precipitation Data

1. Station-Average Method

2. Normal-Ratio Method

3. Inverse-Distance Weighting