Territory Stories

Modelling dry season flows and predicting the impact of water extraction of flagship species

Details:

Title

Modelling dry season flows and predicting the impact of water extraction of flagship species

Creator

Georges, Aurthur; Webster, Ian; Guarino, Fiorenzo; Jolly, Peter; Thoms, Martin; Doody, Sean; CRC for Freshwater Ecology (Australia); University of Canberra. Applied Ecology Research Group

Collection

E-Publications; E-Books; PublicationNT; 57/2002; National River health program

Date

2002-11-20

Location

Daly River

Abstract

The aim of this project is to contribute to recommendations on environmental flows to ensure that they are consistent with maintaining the biota of the Daly River, given competing demands of agriculture, recreation and tourism, conservation and Aboriginal culture. Our focus is on flow, connectivity and water temperatures.

Notes

Made available by via Publications (Legal Deposit) Act 2004 (NT); Submitted to the Northern Territory. Department of Infrastructure Planning and Environment

Table of contents

1. Project Details -- 2. Executive Summary -- 3. Interpretation of the Brief -- 4. Variation of the Brief -- 5. Background -- 6. The Daly Drainage -- 7. The Pig-nosed turtle -- 8. Analysis of Historical Flow Data -- 9. Analysis of Contemporary Flow Data -- 10. Modelling Flow Reduction -- 11. Water Temperature Versus Flow -- 12. Impact on Flagship Species -- 13. References

Language

English

Subject

Environmental Flows; Modelling; Biota

Publisher name

Northern Territory Government

Place of publication

Palmerston

Edition

Final Report

Series

57/2002; National River health program

Format

75 pages ; 30 cm

File type

application/pdf

Use

Attribution International 4.0 (CC BY 4.0)

Copyright owner

Northern Territory Government

License

https://creativecommons.org/licenses/by/4.0/

Parent handle

https://hdl.handle.net/10070/885434

Citation address

https://hdl.handle.net/10070/885435

Page content

9 In terms of population dynamics, a boom period occurs when conditions are such that reproductive output not only ensures that current population levels are sustained, but is also sufficient to fully offset low recruitment that that would have otherwise resulted in population decline in preceding "bust" periods. Boom years may well be infrequent, and it is to be expected that the bust years would numerically dominate the boom years. Changing the frequency of boom relative to bust periods through flow alteration is likely to have substantial long-term impact on the population levels sustained locally. It is clearly evident that all flow categories, regardless of their adverse impact on the turtles, have a finite probability of occurring even under a natural flow regime. For example, flows as low as 2 cumecs (0-3.4 category) can be expected to occur naturally in 14% of years. The issue for flow allocations becomes not one of what reduction leads to an unsatisfactory outcome in a given year, but rather what reduction leads to an unacceptable increase in the frequency of unsatisfactory years (bust years). This analysis clearly shows the substantial impact of a fixed flow reduction as small as 3 cumecs. Catastrophic years, where water is extracted in excess of natural flow (mining) will occur in 11.3 % of years. The boom years, important for sustaining turtle populations, will reduce in frequency from 1 in every 5 to 1 in 10 years. This will have a serious impact on the turtle populations. The lesson here is that a flexible allocation regime is necessary, whereby the cap on flow reduction is defined to be sensitive to the magnitude of the dry season flow in any given year. For example, a 3 cumecs reduction in flow from a 13 cumecs dry season flow will have relatively little impact on turtle life history, whereas the same reduction from a base of 4.8 cumecs, which occurs in 40% of years naturally, will have a major impact. A 3 cumecs reduction in any of the 14% of years with already extremely low flows will be catastrophic, reducing flows and water levels to values not experienced in the years of our historical data set. Under a flexible allocation regime that meets the needs of environmental flows, assurance of water supply will not be possible. Indeed, one of the major conclusions to come from this analysis is that water should not be drawn directly from the river or from groundwater close to the river if we wish to meet the dual objective of providing adequate environmental flows and assurance of water supply for agriculture. The only satisfactory option is to rely upon the buffering characteristics of groundwater supply taken some distance from the river. The buffering comes from the fact that such water would be derived from the cumulative effects of recharge from successive wet seasons, and so an averaging would occur across boom and bust years. This would buffer the effects of water extraction in any one year, and allow for fixed allocations. Recommendations The dry-season flows represent only a very small proportion of the annual flow. Catastrophic draw-down of dry season environmental flow can occur with no change or only a small change in percentage terms in total annual flow. Consequently, allocations of environmental flow defined in terms of total annual flow or median annual flow are unlikely to be effective in protecting the key elements of dry-season flow. Any water allocation policy and limits would need to include both surface water and groundwater, because of the close relationship between the surface flows of the Daly River proper and the groundwater systems upon which it depends.


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