Precipitation Faculty of Applied Engineering and Urban Planning

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CE 374K Hydrology Lecture 5: Precipitation

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Presentation transcript:

Precipitation Faculty of Applied Engineering and Urban Planning Civil Engineering Department Hydrology Precipitation Atmospheric Moisture Lecture 3 Week 3 2nd Semester 2008/2009

Causes and Mechanisms of Precipitation Formation Condensation occurs as a result of cooling of the air to a temperature below the saturation point for water vapor. It can be caused by: Dynamic or adiabatic cooling. Mixing of air masses having different temperatures. Contact cooling Cooling by radiation

Causes and Mechanisms of Precipitation Formation Source of moisture Lifting mechanism (orographic or heating) Phase change from vapor to water - Energy Small nuclei or dust for droplet formation Droplets must grow as they fall to earth

Causes and Mechanisms of Precipitation Formation

Causes and Mechanisms of Precipitation Formation Condensation nuclei must exist for the formation of cloud droplets Which can be artificially supplied to clouds to induce precipitation under certain conditions

Rainfall Measurement

Rainfall Measurement

A network of five to ten gages per 100 mi2 is usually required. Tipping bucket interpreted for total volume and intensity variations Hyeterograph is a plot of rainfall intensity with time (in/hr or mm/hr)

Point Precipitation In order to predict the rainfall at another point, or to supply missing data for a particular station one may use one of two methods: –Station Average Method –Normal Ratio Method –National Weather Service Method

Areal Precipitation For most hydrologic analyses, it is important to know the aerial distribution of precipitation. (ADP) Usually average depths for representative portions of the watershed are determined and used for this purpose (ADP).

Areal Precipitation For multiple stations, the average depth of precipitation over the watershed is determined using the information from all stations: Arithmetic Method Thiessen Polygon Method Isohyetal Method Distance Method

Thiessen Polygon

Assignment No. 1 Read Section 1.3 from text book Prob. 1.3, 1.4, 1.5, 1.6, 1.8, 1.15 Submitted on 22-3-2009 Prepare a 1-page paper on the use of NEXRAD rainfall for hydrologic prediction Submitted not later than 24-10-2009

Check up this paper: http://hydrology.neng.usu.edu/giswr/archive00/termpapers/ghabayen.htm