Decision Support for Farm Dams in W.A.

By Keirnan Fowler, Environmental Engineer, SKM

The south west of Western Australia (WA) has a large number of private earth dams, or farm dams, capturing water for stock, domestic or commercial use. These are generally located on-stream and a series of these dams can occupy most of a watercourse’s length. Individual storages are usually small, relative to the average (natural) inflow at the site. However, taken together across a catchment, the combined storage can often be large relative to the entire catchment (natural) streamflow. These distributed storages support a range of high-value agricultural activities such as viticulture and fruit and vegetable cultivation. This style of water resource development is termed ‘self supplied’ irrigation in WA.

But for planners, agriculturalists and water authorities the management of these dams raises key questions including how much water such dams intercept and when; and what is the reliability of irrigation water supplied from the dams to agricultural land. The questions become particularly vital when it comes to assessing how proposed new dams will affect reliability of supply to other water users downstream.

The distributed nature of the farm dams present particular difficulties to hydrologists. Ideally, the individual characteristics of each dam would be taken into account in simulation models. This includes data such as dam volume, surface area, inflows and water use. In reality, such detailed data is beyond the scope of most projects because there may be hundreds or even thousands of farm dams in view.

This issue is a hot topic in WA, where the Department of Water (DoW) has identified the need for tools capable of simulating the impact of farm dams on rivers and the reliability of the water they supply. Such tools need to provide analysis of both current levels of development and possible future scenarios.

Addressing this need demands a decision support tool which can go beyond simply quantifying diversions to represent the reliability of water supply. Results are needed at a range of scales, including the scale of the individual user. This unique challenge requires a new set of flexible tools, coupled with a good understanding of the modelling approaches and uncertainties.

To address the need, Sinclair Knight Merz (SKM) and DoW, with the support of the National Water Commission (NWC), are developing new functionality for eWater’s integrated river basin-scale modelling system, Source.  The functionality is designed to assist the Western Australian government in farm dam management, with the potential to help stakeholders understand the hydrology and reliability of farm dams.

The new functionality will allow reporting on spatial scales varying from the catchment scale down to individual dams. Proposed new dams can be assessed for their effect on the reliability of supply to other water users downstream.

Farm Dam by Jesse Barrett SKM

Outcomes for river managers

The tool will assist with key questions for planning and policy such as:
•    How reliable is the water supplied by farm dams? For commercial farm dams, reliability of water supply may be quantified by the proportion of years that dam owners have sufficient water for their crops.   Years when the farm dams are full and overflowing represent a maximum in available water, whereas other years when the dams do not fill may cause water shortages. This year-by-year variation in storage volume is a key reporting element for the new functionality.
•    Does the reliability vary among users? That is, do some users enjoy high reliability while other nearby users experience lower reliability? This assessment is only possible if farm dams are represented individually in a model.
•    Is the reliability likely to change? If more development occurs, this may alter the reliability of supply for existing users. In Source, various levels of development can be represented by creating a number of scenarios with increased numbers or volumes of farm dams. A further driver for change may be changes in climate. Possible climate change may be represented in the Source model via changes in timeseries inputs such as streamflows, rainfall or evaporation. The model is run over a user-defined modelling period (currently up to 35 years).
•    How much water is intercepted by farm dams, and when? This question has been the focus of many farm dam studies in the past, and the new functionality retains the ability to report farm dam interception in a variety of ways. In WA, this question is particularly important to assess the impact of farm dams on delivery of environmental flows.

Source: key benefits

Although other models exist that can simulate interception of river flows by farm dams, Source presents a number of advantages over alternative software, including:
•    Streamlined process for building catchment models: as described above, Source has facilitated a streamlined process for building hydrological models on the basis of spatial data. When using alternative software, the spatial and hydrological modelling components are often separate tasks requiring manual integration.
•    Integrated catchment modelling: farm dams can potentially be modelled in conjunction with other river model nodes such as water supply reservoirs. Also, the generation of runoff can be linked to rainfall runoff models, which is not possible in some alternative software.
•    Improved reporting of results: the Results Manager of Source is ideally suited to this application. The number of potential outputs is very large, with dozens of reporting options for each farm dam. Source also allows custom outputs to be developed that can be made specific to the WA policy and planning context.
•    Flexible framework: being based on the TIME framework, Source lends itself to further customisation to meet future hydrological modelling challenges, many of which may be currently unforeseen.  In producing the custom functionality for this tool, SKM has found that many of the ‘building blocks’ of the tool already existed as elements in TIME and that the writing of plugins was largely an exercise in linking these elements together into a customized process.

About the project

The project is being conducted by Sinclair Knight Merz (SKM) and Western Australia’s Department of Water (DoW) with the support of the National Water Commission (NWC). The new functionality is being produced with the assistance of CSIRO and is due for completion by November 2011, and will be rolled out to a number of catchments across WA’s Warren, Donnelly and Whicher regions by February 2012.

For more information, contact Robert Donohue at DoW (08 6364 6822 / Robert.DONOHUE@water.wa.gov.au)or Keirnan Fowler at SKM (03 8668 3450 / kfowler@globalskm.com).

 

Streamlined processes for building models

The essence of the new approach is that each farm dam is accessible and reportable as a node in the schematic editor of Source.

Although this leads to highly complex models, often with hundreds of nodes, the customisable nature of Source permits automation of many key tasks.
 
The new functionality will be contained within a set of Source ‘plugins’ and is suited to models where farm dams are the focus of study. This complements the existing farm dams filter model which is a faster and less detailed solution suited to more general river models. Similar modelling principles underlie both methods.

The new functionality allows a streamlined process for building farm dam models. In a process similar to the ‘Geographic Wizard’, Source can use spatial data about dam location and catchment topography to automatically create a node-link network. The assignment of node inputs is semi-automated using a ‘master table’ where the properties of many of the farm dams can be assigned at once. Each farm dam is represented as a custom ‘farm dam node’ developed specifically for the decision support tool.

Image by Lisa Walpole SKM