Intercepting activities

There are many activities which can cause a reduction in catchment yield or recharge to aquifers. The National Water Initiative is principally interested in the impact of three main activities which are known to significantly impact on catchment yields, all of which have either been activities not requiring water licenses, or have only recently been identified (and now requiring a license) as having significant impacts on catchments. The three are listed in Clause 55 of the NWI and include: farm dams, overland flows (e.g. flood harvesting) and large scale plantation forestry. These activities are briefly discussed in this section including discussions on the following:

• Forestry
• Bushfire impacts on catchment yields
• Farm dams
• Floodplain harvesting

More detailed discussion of these intercepting activities is provided in the Water Availability Level 2 Report – National Perspective, available from the Publications page.

Forestry

New forests (either commercial plantations or environmental plantings / areas of native revegetation) have been shown to have significant impact on catchment yields. This is due to the high water requirements for growth of new forests in the first 4-10 years after planting as well as the higher interception and transpiration rates of forest over grassland.

In South Australia legislation exists for forestry activities to be licensed for the water that they intercept and use, and for the groundwater that they extract in the root zone. However, the legislation is likely to be implemented over a 20 year timeframe. Other states have not legislated to license forestry activities in terms of the actual water use by the trees.

The impacts of forestry on catchments is becoming more intensely documented and managed in recent year. Studies into water use by blue gum and pine plantations by CSIRO in south east South Australia have indicated that plantations will use all available water up to the potential evapotranspiration limit. This means that all the runoff and recharge will be intercepted and used by the plantation. Along with the interception of runoff and recharge in the catchment, the plantations may also extract (from the root zone) up to 500 mm/yr from shallow water tables (Benyon and Doody, 2004).

Bushfires

Within managed forests, the impact of regeneration on water yields is likely to be balanced over the entire catchment as a result of the cyclic nature of logging. However in areas impacted by large bushfires the impact on yield through regrowth can be significant. Impacts are greatest where there is a high fire severity with a high proportion of tree death and the forest lost contained a high proportion of mature trees.

Some recent studies in Victoria also indicate that regrowth of forests after bushfires can cause similar reductions in catchment yields. Studies of the 2003 ACT bushfires, suggest that forest recovery will impact on catchment yields for 130 years after the event. These modelling predictions showed that the maximum impact may be as high as a 15% reduction in streamflow, with flows returning to pre-2003 levels after about 80 years.

Predicted ACT severe bushfire yield reduction relationship used for estimating the impact of the 2003 bushfires (Source: ACTEW, 2006)

The major fires which occurred in south east Australia in January and February 2003 burnt out a total of 13,900 km² of largely native forest in the south-eastern uplands. Follow-up studies of the impact of these fires suggest that the severity of the burn was highly variable as a result of differing tree species and period since previous burn. Hence in some places water yield was already impacted. Overall the prediction of yield reduction for the Murray-Darling Basin ranged from nothing through to about 120 GL/year.

A parallel impact to water yield from bushfire is on water quality and river and wetland health. Significant loads of charcoal, sediment, nutrients and organic matter can be carried into waterways as a result of rainfall events following bushfires. Again the severity of the impact is dependent on the nature of the fire, the topography and the size of the rain event.

Farm dams

Farm dams are used to harvest or store water for onsite irrigation
Image by Phillip Jordan, sourced from SKM

Farm dams, defined as small on or off-stream storages, play an important role in Australian agriculture as they capture runoff that can be used during dry periods of low to no rainfall. Small dams may store just a few megalitres for stock and domestic use, while larger dams may be used more intensely for irrigation purposes. Water use from farm dams is included in the self-extraction volumes in the AWR 2005 water balances.

In highland areas farm dams generally intercept ephemeral (non-permanent) streams which are predominantly groundwater sourced. These groundwater ‘springs’ take waters that would otherwise feed into the creeks and river tributaries, thereby, reducing surface water flows in the catchment downstream in the same way as on-stream storages.

In many regions, small farm dams, particularly those for stock and domestic purposes, do not require a licence (see table below for definitions and licensing requirements across Australia). Most jurisdictions hold data on the volume of farm dams that are licensed, but have no data on the number and volumes of unlicensed farm dams. In addition, there is very little information available nationally about usage from farm dams of any type or size. In recent years as water has become scarcer, there has been an increase in the proportion of farm dams used for irrigation purposes. In south-eastern Australia there is a trend towards constructing larger farm dams, some impounding hundreds of megalitres of water, to irrigate high value horticultural crops.

Victoria has developed a relatively comprehensive database on farm dams using topographic maps, aerial photography and estimates of farm dam capacity based primarily on surface area.

Estimated capacity of farm dams in Victoria in 2004-05 (Source: Victorian State Water Report 2004-05)

While in most cases the impact of an individual farm dam is relatively small, the cumulative impact of the large number of farm dams that exist in many areas of Australia on streamflows can be significant. In particular farm dams normally have the greatest impact on low flows and can cause the delay of the wet season or prolong dry periods. This occurs because the first significant rain after an extended dry period will fill farm dams before reaching streams downstream hence delaying and reducing the magnitude of flows downstream.

A South Australian study, demonstrated that farm dams in the Onkaparinga River catchment capture 5% of median annual flow, but 16% of low flow (10^th percentile) across the catchment. The study also showed that the impact in some tributaries of the Onkaparinga River was much higher, with median flows reduced by 20% and low flows (10^th percentile) reduced by 99% in the Mitchell Creek catchment due to farm dams.

Studies conducted for the Murray-Darling Basin Commission provide an estimate of decreased annual streamflow by 0.66 ML per ML of farm dam storage and importantly provide estimates of combined hillside farm dam storage capacity in the Murray-Darling Basin of around 2,200 GL. This produces an estimated streamflow reduction of 1,900 GL/year. There is evidence that the growth in farm dam numbers tends to be strongest during periods of drought and that in the last ten years the number of farm dams in the Basin has increased by 37% with an associated increase in storage capacity of 48%. For further discussion of the impacts of farm dams and other intercepting activities in the Murray-Darling Basin, see Risks to the Shared Water Resources of the Murray-Darling Basin.  

In summary, the impact of farm dams is regarded as significant as left unchecked they will certainly increase and diminish water availability to existing and future downstream users, and the environment. Limiting future farm dam numbers will depend primarily on regulation.

Farm dam definitions and licensing requirements across Australia

Floodplain harvesting

Floodplain harvesting can be defined as the collection, extraction or impoundment of water as it flows across floodplains, where the floodplain is the area that is inundated with water in a 1 in 100 year flood event. Floodplain flows come from either local runoff that has not yet entered the main channel of a river or from water that has overflowed from the main channel of a river during a flood event up to 1 in 100 year magnitude.

Floodplain harvesting includes the capture of floodplain flows using pumps and/or other purpose built equipment or structures as well as the capture that occurs incidentally through existing infrastructure for other purposes (e.g. flood levees, irrigation channels). The filling of natural wetlands, billabongs or other depressions within the floodplain is not considered to be floodplain harvesting, provided that the water remains for environmental purposes and is not extracted from these sources.

Harvesting water from floodplains reduces the amount of water reaching or returning to rivers and hence decreases the amount of water available to meet downstream river health, wetland and floodplain needs and the water supply entitlements of other users. Flood flows are an extremely important part of the flow regime of most Australian rivers as it is only at time of flood when the floodplains, wetlands and the river channel are all connected.

Floodplain harvesting is still largely unregulated in Australia and in areas where floodplain harvesting occurs at high levels (Queensland and northern New South Wales) it is difficult to make accurate plans for equitable water sharing and demand management without accounting for floodplain harvesting.

More detailed discussion of these intercepting activities is provided in the Water Availability Level 2 Report – National Perspective, available from the Publications page.

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