HOMS COMPONENT

COMPUTING SHALLOW LAKE EVAPORATION USING CLASS A PAN DATA

1.   Purpose and objectives

     To calculate daily evaporation loss from shallow lakes, ponds, and reservoirs using data from a nearby standard Class A Evaporation Pan.

2.   Description

     The method is based on the equation derived by Kohler, Nordenson and Fox (1955) used to correct the evaporation loss from a Class A evaporation pan for heat advection through the pan sides and bottom. The resulting corrected "lake evaporation" values are applicable to nearby small water 
bodies with negligible heat storage.

3.   Input

     Daily measurements from a standard Class A evaporation pan installation including: pan water loss or gain, pan water temperature, screen air temperature, wind run at 15 cm above the pan rim, and the depth of rainfall, if any. The station elevation is also required.

4.   Output

     Estimate of daily evaporation loss for small water bodies.

5.   Operational requirements and restrictions

     Applicable only to small water bodies with negligible heat storage (typically with area 5 km2 and depth 10 m). The accuracy of the estimates increases with the accumulation period, i.e. monthly estimates based on an accumulation of daily values are more reliable than the individual daily values.

6.   Form of presentation

     The procedure is given in the form of a simple algorithm which can be programmed into a computer or programmable calculator.

7.   Operational experience

     The procedure has been applied in Canada since 1970.

8.   Originator and technical support

     Hydrometeorology and Marine Division
     Atmospheric Environment Service
     Environment Canada.

9.   Availability

     From the HOMS National Reference Centre for Canada.

10.  Conditions on use

     Free of charge; acknowledgement of source is expected.

Last updated: 14 DEC 1999)
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