Farm Water Calculator: Additional information
Determining Catchment Yield for Planning Farm Dams
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- When planning a dam it is important to determine the potential yield from a catchment to ensure the dam fills on a reliable basis.
- Runoff from a catchment is dependant upon annual rainfall as well as catchment characteristics such as soil types and the type of groundcover / land usage.
- Reduction in rainfall and changes to rainfall patterns due should be considered when planning a farm dam. A small reduction in rainfall can result in a significant reduction in runoff.
Constructing farm dams can be an expensive exercise, and also a disappointing one if they fail to adequately fill on a regular basis. Careful planning can help to match the size of the dam to the potential catchment yield, or amount of rainfall likely to run off to fill it, to ensure that it has a reasonable chance of filling in most years. This also makes good financial sense, as the cost of constructing a dam is proportional to the amount of earthworks required to excavate it and / or build the embankments.
The main determinants of catchment yield are the rainfall, the area of the catchment, and catchment characteristics such as soil type and the type land use and / or ground cover. The geographic location also affects the proportion of runoff, as low rainfall areas with higher evaporation rates will generate less runoff as a proportion of total rainfall than higher rainfall areas with lower evaporation rates.
Catchment Area
The catchment area is the total area of land that contributes runoff into the dam. For relatively small catchments it may be possible to determine their area from farm plans, aerial photos of the farm or by actually measuring it out on the ground with a vehicle or global positioning system (GPS). Areas of larger catchments may be estimated from 1:25,000 topographic maps.
Annual Rainfall
To date the average annual rainfall figures from long term historical records have been used to determine the amount of runoff generated. The Bureau of Metereology (BoM) website can provide long term average annual rainfall figures, as well as rainfall for decile 1 years (the driest 10% of years) and decile 9 years (wettest 10% of years).
Queensland Department of Primary Industries in conjunction with the BoM have produced the "Rainman" program which provides analysis of long term historical rainfall and streamflow data for over 3,700 sites across Australia. Rainman can show rainfall trends and provide both annual and monthly rainfall deciles for sites.
Effects of Changes to Rainfall Patterns
Since the mid 1990's there has been a marked reduction in rainfall and runoff across Victoria and many dams once considered reliable now fail to regularly fill. While the inherent year to year variability will continue to see some years of above average rainfall, climate change predictions indicate that the decline in annual rainfall will continue, with the reduction being most pronounced in Autumn and Spring rainfall.
As an approximation, recent research indicates a 10% reduction in rainfall can result in a 30% reduction in runoff, with a consequent impact on water supplies. Consideration should be given to the possibility of reduced and less reliable runoff and the impact of this on the frequency of dam filling and its' replenishment period.
Dams have an expected life span of well beyond 50 years or more. The future effects of altered rainfall patterns due to climate change can be taken into account when determining the runoff, by:
- using a lower rainfall decile figure,
- using a percentage of long term average annual rainfall, or
- using the average annual rainfall for the past 30 years.
The Yield Coefficient or the proportion of rainfall that runs off
Only a relatively small proportion of the rainfall received in the catchment area actually runs off into watercourses or to fill dams. The majority of it infiltrates into the ground or is intercepted by vegetation. Although many factors influence this, as a rule of thumb the amount of runoff is proportional to the annual rainfall.
The graph below shows the proportion of runoff, or yield coefficient, in relation to rainfall. This is then multiplied by the catchment area and the average annual rainfall to determine the catchment yield.
- Go across the horizontal axis on the graph to find average annual rainfall
- Go up to line
- Read across to the vertical axis and determine the yield coefficient (YC)
Figure 1 Unadjusted Yield Coefficient
Calculating Catchment Yield
The catchment yield is calculated first then adjusted to take into account catchment specific characteristics
| Catchment Yield (megalitres) = | A x R x YC 10000 |
| Where | A = catchment area (hectares) R = average annual rainfall (mm) YC = yield coefficient |
Note 1 megalitre = 1,000 kilolitres = 1,000 cubic metres = 1,000,000 litres
Adjusting the yield to take into account catchment specific characteristics
The catchment yield is further refined to take into account characteristics specific to the catchment such as the the infiltration and water holding capacities of the soil, the type of groundcover and vegetation present, and where the catchment is located in regard to evaporation rates and rainfall.
| Adjusted Catchment Yield (megalitres) = SF x GF x LF | ||
|---|---|---|
| SF = Soil factors | ||
| Highly permeable soils | (sandy) | x 0.3 |
| Permeable soils | (loamy) | x 0.6 |
| Medium textured soils | (clay loam - light clay) | x 1.0 |
| Heavy textured soils | (medium - heavy clay) | x 1.3 |
| GF = Groundcover factors | ||
| Annual pasture cover | x 1.0 | |
| Perennial pasture cover | x 0.5 | |
| Timbered or forested cover | x 0.5 | |
| LF = Location factors | ||
| Catchment area is located south of the divide | x 1.1 | |
| Catchment area is located on the divide | x 1.0 | |
| Catchment area is located north of the divide | x 0.9 | |
Example
The catchment area is 35 hectare, and it is located in a 550 mm rainfall zone. The yield coefficient is 5.8 (from figure 1)
| Catchment Yield = | 35 x 550 x 5.8 10,000 |
| = 11.1 megalitres |
If it was to have a loamy soil with improved pasture, and is located south of the Great Dividing Range:
Soil factor is 0.6 loamy (permeable)
Groundcover factor is 0.5 improved pasture
Location factor is 1.1 south of divide
| Adjusted Catchment Yield | = 11.1 x 0.6 x 0.5 x 1.1 = 3.6 megalitres |
Effect of a Reduction in Rainfall on Catchment Yield
Compare the catchment yield for the same 35 ha catchment from the example above in a low rainfall year such as a decile 3 year when the rainfall is 450 mm, and applying the same soil, groundcover and location adjustment factors
| Catchment Yield = | 35 x 450 x 3.8 10,000 |
| = 6.0 megalitres |
| Adjusted Catchment Yield | = 6.0 x 0.6 x 0.5 x 1.1 = 2.0 megalitres |
So for this catchment in a low rainfall year with an 18% reduction in rainfall there is about a 44% reduction of runoff.
CONCLUSION
Careful consideration of the potential yield of a catchment can help to determine the appropriate size dam to construct to ensure that it fills on a reliable and regular basis.
In locations where the rainfall is somewhat unreliable or quite variable but the catchment is large enough, consideration may be given to constructing a larger dam that fills less regularly but once full provides a number of years of water. That is, it provides a longer replenishment period.