Evaporation in Armidale

Image:  Evaporation pan at the East Armidale weather station. The pan is mounted on a
wooden platform which allows air movement below it. The plastic measuring cylinder is also shown.
The small metal cylinder within the pan is the stilling well containing a fixed point. A measured
amount of water is added or removed daily so it is covering the fixed point.


Large amounts of water can be lost through evaporation from water storages on farms, particularly during hot dry summers, and during times of low rainfall the amount of water loss through evaporation can be critical for the survival of pastures, crops and livestock.
Evaporation readings have been taken at the East Armidale weather station since 1983 and this data has allowed a profile to be prepared for this area.
A standard Class A pan is used, 120.6 cm in diameter and 25 cm deep set on a wooden platform. A wire mesh bird guard is fitted to the top.

Evaporation is a product of air temperature and humidity, temperature of the water, amount of solar radiation, and the speed of wind blowing across the pan. At 9am daily a measured quantity of water is added to the pan to replace the amount that has evaporated since 9am the previous day. If it has rained then water may need to be removed, so the rainfall amount, as recorded in the nearby rain gauge, is taken into account in the evaporation calculation.

Pan evaporation has been found to be generally higher than in nearby dams, ponds or lakes, the reason being the sides of the pan are exposed to the sun, and the temperature over the pan is higher than the temperature over the dam or lake. To counter this the pan coefficient (Kp) is used to calculate approximate evaporation from nearby larger bodies of water.
The value of the coefficient is determined by the general location and exposure of the pan, and type of surrounding vegetation.
The following formula is then used:
Lake evaporation = Kp x pan evaporation

The pan coefficient for the East Armidale weather station is 0.7. Therefore the formula becomes:
Lake evaporation = 0.7 x pan evaporation.


Daily evaporation readings can be found in the Armidale monthly data files
Today’s evaporation reading can be found on the Bureau of Meteorology’s daily observations page after about 10.30am here.

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