Operator`s manual

57
Manual
Annex 8: Sample Calculation of Open Water Evaporation Using Priestley-Taylor
1. Introduction
This document presents a sample calculation of open water evaporation of a large water body in an arid zone using
Priestley-Taylor (PT). The sole aim of this exercise is to demonstrate the methodology. The values of the variables
used in the calculation are fictitious and do not represent actual measurements. The evaporation calculated in this
document, therefore, does not reflect an actual situation.
2. The Energy Balance Method
The energy balance method is based on an accounting of all energy fluxes at the water surface. This method is
represented by the following general equation:
LE = K + L – G – H + A
w
- ES
In which:
LE = energy flux associated with evaporation
K = net short-wave radiation input
L = net long-wave radiation input
G = net output through conduction to the ground
H = net output through sensible heat exchange with the atmosphere
A
w
= net input associated with the inflow and outflow from the lake
ES = change in energy storage in the lake
Evaporation is always accompanied by a net energy loss of the water body, as the movement of a molecule from a
water body to the air requires the breakage of the hydrogen bonds, which only the highest ‘charged’ molecules can
accomplish. The energy associated with this ‘escape’ is referred to as: ‘Latent Heat of Vaporization’. In most cases,
solar radiation provides the majority of positive energy flux into the water body, and is therefore the dominant forcing
for evaporation. However, at times, the heat stored in a water body can contribute significantly to the evaporation.
Calculating changes in energy storage in a large lake is a complicated undertaking. By choosing a time step for
which the energy storage is likely to be constant, for example one year, it is possible to minimize these difficulties.
As the aim of this document is to demonstrate the methodology, the sample calculation takes into account changes
in energy storage on a daily basis.
3. Priestley-Taylor Equation
3.1 General
The Priestley-Taylor (PT) method was developed in 1972. It is an exponent of the Energy Balance Approach and
neglects all components other than the radiation, the heat fluxes, and the change in energy storage. This is a justified
approach for a large lake, with minimal variation in: (a) daily lake level, and (b) daily lake bottom temperature.
Priestley-Taylor is expressed by:
LE = x / ( + g) x (R
n
- ES)
In which:
LE = energy lost in the evaporation process
= Priestley-Taylor coefficient
Annexes