Farm dams have long been considered a major, yet unmanageable, component of catchment hydrology here in Victoria. Farm dams are such a ubiquitous part of the rural landscape that they are simply taken for granted. Dams used for irrigation are licensed and regulated, but Government agencies and water authorities have generally shied away from attempting to regulate farm dams for both practical and political reasons. If a landholder or farmer wants water, a dam is often their first option. And because it’s been like that for generations, there are a vast number of farm dams in Victoria’s agricultural landscape.

Several studies having investigated groundwater, wetland habitat and stream-flow in the upper Seven Creeks catchment (Car et. al. 2006, Stewardson et.al. 2009, Coates et. al. 2010) and others have broadly assessed the impact of farm dams on catchment water balance (GHD 2011). A more detailed understanding of the impact of farm dams on catchment hydrology would be useful. A step towards that goal for the Seven Creek catchment is the mapping of farm dams and estimate of storage capacity.

Figure 1. Mapped farm dams in the Seven Creeks catchment above Polly McQuinn’s Weir. Small dams (blue) = < 2 ML, Medium dams (yellow) = 2 to 5 ML, Large dams (pink) = >5 ML. See below for methodology. Google Earth satellite image copyright date: March 25, 2020

How many farm dams are there?

This study mapped more than 860 farm dams, large and small, irrigation and domestic, in the Seven Creeks catchment above Polly McQuinn’s Weir (Figure 1 & Table 1). They are located predominantly on creeks and in the wetland zone, where clay soil ensures a dam’s water holding ability. Only a small proportion are (mistakenly) constructed in friable, reddish/yellow granitic soil. The size of the icons in Figure 1 gives an exaggerated impression of their impact, but 860+ dams across 14,000 ha of farmland (the catchment area less forested, public land) runs to about one dam per 16 ha.

Dams built directly on permanent streams, rather than in the wetland headwaters of those streams, are few, numbering perhaps seven or eight and are all of Medium to Large size. The largest of these has a capacity of approximately 72 ML.

Large dams, though representing less than 4% of all mapped dams, account for more than 50% of the combined storage capacity of all 864 mapped dams. Four of the 32 Large dams (>5 ML), with combined storage capacity of 140 ML, are not filled by natural groundwater discharge, but by metered groundwater pumping for irrigation of horticulture.

Where are they?

The 811 Small dams are distributed throughout the agricultural zone in the catchment, only absent from the forested public land in the east. Of the 21 Medium dams, most are in the northern part of the catchment, in the Kithbrook and Boho South districts. Large dams occur across the catchment, are conspicuously concentrated in the Boho South district (10 of 32 dams), particularly within the small (500 ha) sub-catchment of an unnamed stream draining the eastern slopes of Boundary Hill (hereafter referred to as Boundary Hill Creek) – further discussion below.

Figure 2. The distribution of Medium dams (yellow) and Large dams (pink) in the Seven Creeks catchment above Polly McQuinn’s Weir. Note the concentration of Large dams and to a lesser extent medium dams in the Kithbrook and Boho South districts on the northern rim of the catchment. The groundwater discharge and riparian zone is indicated by the blue polygon.

How much water can they hold?

The Seven Creeks catchment above Polly McQuinn’s covers an area of about 15,300 ha and has 864 mapped dams. Most dams (94%) have less than 2 ML storage capacity. [See end of post for calculation methods].

  • The 811 mapped Small dams (<2 ML) have a combined estimated storage capacity of 333 ML
  • The 21 Medium dams (2-5 ML) have a combined estimated storage capacity of 74 ML
  • The 32 Large dams (>5 ML) have a combined estimated storage capacity of 600 ML
  • The estimated storage capacity of farm dams combined is 1006 ML
Table 1. The number of Small, Medium and Large dams in the Seven Creeks catchment above Polly McQuinn’s Weir.

Implications for stream-flow and catchment health

In upland catchments, like the Seven Creeks above Polly McQuinn’s, farm dams are thought to harvest only a very small percentage of the overall surface water flow (SKM, 2007), yet impact on stream-flow during the drier months is likely to be high because these dams can intercept runoff that would otherwise provide freshing flows in summer (when base-flow is at its lowest). The actual impact of individual dams will depend on how they are used and whether operation and construction includes diversion pipes that allow seasonal flow. It is not known how many of the Large dams have winter-fill only licenses to allow for the passage of all summer flows – presumably few. Dams supplied from reliable groundwater discharge throughout the year might have minimal impact on stream-flow (because they are generally full), whereas dams whose levels drop significantly over summer (through usage, seepage, or evaporation) can capture summer runoff events and have a proportionally greater impact.

Figure 4. The high concentration of farm dams (commercial and stock & domestic) in the catchment of Boundary Hill Creek, Boho South. Note, several of these dams are filled via pumped groundwater from metered bores. This catchment contains the largest dam (estimated volume 72 ML) in the entire catchment.

Whilst dams filled by groundwater pumping may not impact stream-flow by capturing runoff events, they likely have an indirect impact by lowering local groundwater levels and reducing discharge directly into adjacent wetlands and streams.

The concentration of storage and groundwater pumping in the small Boundary Hill Creek catchment at Boho south, suggests dam impact on stream-flow might be higher in this small catchment than elsewhere in the study area. This 500 ha catchment has 33 mapped farm dams with a total storage capacity of 240 ML, most of which (89%) is held in five Large dams (>5 ML). Just three dams account for 73% of the 240 ML of storage in the catchment (two of which are filled from groundwater pumping).

Summary

Demand for reliable, good quality water is high and will only increase as rainfall patterns change and the catchment becomes drier (CSIRO climate modelling for Strathbogie). The impact of farm dams on stream-flow is more complex than simply measuring their combined storage capacity and determining where they occur in the landscape, which has been the intent of this exercise. The sheer number of farm dams on the Strathbogie Tableland, along with the amount of water they divert away from surface flow, suggests they may already be having a significant impact. Proper impact assessment requires knowledge of how dams are constructed and used, within a water balance framework (eg SKM 2007). There is no such analysis for the Seven Creeks catchment above Polly McQuinn’s Weir, though with the passing of every summer and the increasing number of farm dams, the need for such a study grows.

Methods for calculating farm dam storage

Mapping farm dams

Google Earth satellite imagery from 2011 to 2018, taken at different times of the year, was used to identify dams in the landscape and determine surface area at maximum fill level. Whilst the focus was on medium to larger farm dams, eg > 0.5 ML, the mapping reliably identified even small dams with a surface area of about 100 m2. I suspect mapping did not reliably capture dams of surface are <100 m2.

These images show how surface area was calculated, in order to estimate dam volume.

Estimating farm dam storage capacity

Farm dam storage capacity was calculated using the method of Sinclair Knight Merz (2004). This method calculates farm dam storage using the equation:

Volume (ML) = 0.000145 x SA1.314 (where SA= surface area of the dam when full, in m2).

The number of dams with calculated storage capacity greater than 2 ML (SA>1414 m2) was relatively small, so the volume of each of those dams was calculated individually; these dams were further divided into two categories – greater or less than 5 ML. The mapped farm dams were grouped into three categories: ‘Small’= less than 2 ML; ‘Medium’ = between 2 and 5 ML; ‘Large’ = greater than 5 ML.

The large number of dams with calculated capacity less than 2 ML (SA<1414 m2, based on the above calculation) precluded individual calculation. Instead, the Mean and Median storage capacity of a sample of Small dams was calculated and used to estimate storage capacity of all Small dams in the study area. Volumes of all dams, including the 19 mapped Small dams, in the catchment of Boundary Hill Creek were calculated using the above equation (Table 2). The Mean (0.56 ML) and Median (0.41 ML) differed enough to suggest the data may be slightly skewed in the sample, so the Median was used to represent all dams in this catchment and also across the study area. Hence, the combined volume of all 811 Small dams across the study area was calculated to be 332.5 ML (Table 1, above).

Important note

Digging a dam on your property may require a variety of permissions prior to construction. The bigger the proposed dam, the more likely it is to be subject to regulation. Dams in wetlands and waterways should be avoided where possible and may also require special permission. Anyone preparing to construct a dam should read this brochure and contact Goulburn Murray Water.

References

  • Carr, G.W., Moysey, E. D., Mathews, S., Frood, D., White, M., Griffioen, P., Morgan, J. and Rosengren, N. (2006) The location of Spring Soak and Peatland wetlands in the Goulburn Broken Catchment – Wetland Implemetation Plan Stage. A report to the Goulburn Broken Catchment Management Authority.
  • Coates, F., Tolsma, A., Cutler, S. and Fletcher, M. (2010). The floristic values of wetlands in the Highlands and Strathbogie Ranges. Arthur Rylah Institute for Environmental Research. Department of Sustainability and Environment, Heidelberg, Victoria.
  • GHD (2011). Upper Goulburn groundwater resource appraisal; management zones and water balances (Report prepared for Goulburn Murray Water).
  • Goulburn Murray Water (2013). Strathbogie Groundwater Management Area, Local Management Plan.
  • Sinclair Knight Merz (2004). Estimating available water in catchments using sustainable diversion limits – farm dam demand factors (Victoria; Report prepared for the Department of Sustainability and Environment).
  • Sinclair Knight Merz (2007). Addition of small catchment dams to the Goulburn-Broken catchments of the monthly and daily Goulburn simulation models. Report prepared for the Department of Sustainability and Environment November 2007. SKM Ref: 5_GB_Final.doc in SKM (2004) – above.
  • Stewardson, M., Western, A., and E. Wallis (2009). The hydrology of wetlands in the Strathbogie Ranges. Department of Civil and Environmental Engineering, The University of Melbourne. Report prepared for the Goulburn Broken Catchment Management Authority

The Bogies and Beyond Groundwater Monitoring Project is supported by the Victorian Government and the Goulburn Broken Catchment Management Authority.

For more information contact Bertram Lobert bertram.lobert@activ8.net.au 5790 8606, 0409 433 276 or go to: https://strathbogierangesnatureview.wordpress.com/?s=groundwater