The amount of water lost due to evaporation from the surface of a farm dam can be a significant proportion of the total water stored in the dam, and in cases such as shallow dams in low rainfall zones, the total annual evaporative loss could be up to 50% of the dams total storage volume.
The evaporative loss from an open water body, such as a dam, is influenced mainly by the surface area of the water, as well as the shape and depth of the dam, the surrounding topography and vegetation, and its location. While accurately measuring or quantifying the evaporative loss for an individual dam is very difficult, as well as being expensive and impractical, it can be estimated as a proportion of the total evaporation from a Class A pan, which are used by the Bureau of Meteorology.
Across Victoria the total annual Class A pan evaporation (Epan) varies from around 1200mm in Gippsland to about 2000mm per annum in the northwest, with approximately 70 to 75% of the annual evaporation occurring over the period October to March. A generally accepted factor of 0.67 (or 67% of the Class A pan) is applied to the Class A pan evaporation figure to estimate the evaporative loss from the dam (Egross). The evaporative loss can be partly offset by rain falling directly onto the dam, the difference being the net evaporative loss (Enet).
The evaporative losses and rainfall are initially measured in millimetres. To calculate the total evaporative loss from a dam (Edam), the net evaporative loss is multiplied by the surface area of the water in the dam.
The net evaporative loss per annum from a dam is also influenced by how frequently the dam fills and overflows, and the period of time for which water is stored in the dam. When determining the amount lost from evaporation, or gained through additional rainfall onto the surface, consideration needs to be given to what period (months) to take into account.
In high rainfall areas where a dam reliably fills and overflows in most years only the rainfall and evaporation over the summer and autumn may need to be accounted for, while it can be neglected for the winter and spring period.
In low rainfall zones a dam may not completely fill every year, only filling in exceptionally wet or above average rainfall years. In most other years, say 4 out 5 years, the dam may only be partly full, and would be losing water due to evaporation over the entire 12 months, although it would also be benefiting from all rainfall directly onto it during this time.
While Class A pan evaporation data relating to specific sites is available, the Bureau of Meteorology website provides maps of annual and monthly Class A pan evaporation across Victoria that should suffice. The maps can be found at:
E gross (mm) = 0.67 x Epan
| Where | |
| E gross (mm) | Loss from a water body (ie dam) due to evaporation |
| E pan (mm) | Evaporation from a Class A Open pan for the period of interest |
| 0.67 | is a conversion factor |
The net evaporative loss from the farm dam takes into account the amount lost due to evaporation and the amount gained from rainfall direct onto the dam surface.
E net (mm) = E gross - R
| where | |
| E net (mm) | Net Losses from surface of dam (evaporation - rainfall onto dam surface) |
| R (mm) | Rainfall (over the period of interest) |
Total loss from the dam is calculated by multiplying the net evaporative loss by the surface area,
| E dam (kilolitres) = | E net (mm) x A top 1000 |
| where | |
| E dam (kilolitres) | Total Losses from dam due to evaporation |
| A top (square metres) | Surface area of dam when full |
| Note | 1 kilolitre = 1000 litres = 0.001 megalitres |
| To convert | kilolitres to litres, multiply Etotal (kilolitres) by 1000 megalitres, divide Etotal (kilolitres) by 1000 |
Example
Note - The evaporative loss calculated here uses the area of water surface when the dam is full. As it is in the high rainfall zone, only the evaporative loss and rainfall over the period November to April are taken into account.
| Rainfall | 250 mm | |
| Class A Pan Evaporation | 900 mm | |
| Surface area of the dam | 1200 square metres | |
| E gross (mm) | = 0.67 x 900 = 603 mm |
|
| E net | = 603 - 250 = 350mm |
|
| E total = | 353 x 1200 1000 |
|
| = 424 kilolitres | (424000 litres or 0.424 megalitres) | |
The amount of water lost from a farm dam can be a significant proportion of the total amount of water stored, particularly for smaller, shallower dams. Reducing the total surface area of the water by having fewer larger, deeper dams rather than many smaller shallower dams reduces the evaporative loss. Shallow water heats up more rapidly than deeper water, so shallow dams are also likely to have a greater rate of evaporative loss than deeper dams.
There are a variety of products available to reduce evaporative losses from farm dams such as covers that are placed over the dam or additives that create a film on the water surface. However the economics and feasibility of using these should be carefully considered.
When planning new farm dams, evaporative losses can be minimised by reducing the surface area and increasing the depth where possible. In times of drought or water shortages, losses can be reduced from existing dams by maintaining fewer key dams rather than numerous smaller, shallower dams. Moving water to key dams reduces the total surface area and increases the depth of water stored, thereby reducing the amount of water lost to evaporation.