Territory Stories

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



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


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


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




Daly River


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.


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




Environmental Flows; Modelling; Biota

Publisher name

Northern Territory Government

Place of publication



Final Report


57/2002; National River health program


75 pages ; 30 cm

File type



Attribution International 4.0 (CC BY 4.0)

Copyright owner

Northern Territory Government



Parent handle


Citation address


Page content

15 reaching maturity, a substantial residual yolk body is internalised; the embryos then enter aestivation within the egg (Webb et al. 1986). The trigger for hatching is inundation by flooding or the first torrential rains of the wet season. Embryos have sufficient resources to carry them through about 60 days at 30oC. This species has temperature-dependent sex determination (Webb et al. 1986; Georges 1992). Although pig-nose turtles nest in the dry-season, dates vary dramatically in response to weather and ambient temperatures. This in turn determines timing of maturity of the eggs in relation to the early wet season cues for hatching. Too early and the eggs perish in the ground. Too late and they are prematurely washed away by rising waters. Furthermore, the primary determinant of offspring sex is date of laying, so altering water temperatures (but not temperatures of the adjacent ground) will have profound effects on offspring sex ratios, and subsequently population numbers. Pig-nose turtles undergo extensive movements during the dry-season. This is because of the limited and unpredictable distribution of nesting sandbanks in a system that is dramatically remodelled each wet season. Access to suitable banks requires free movement during the dry, a life history requirement likely to be concordant with a range of other aquatic species. The complex interaction of this species reproductive biology with water temperature, connectivity, and flood pulses (see below) makes it a sensitive indicator species for assessing the impact of modified environmental flows. As a species particularly sensitive to environmental perturbations, protecting its interests will likely bring attendant benefits for a wide range of other species whose requirements are less stringent. It can therefore be regarded as an umbrella species. Adverse impact on this species, and those fish and other aquatic life with concordant requirements, would be regarded as major degradation. This project draws substantially on the base of ecological knowledge on the species, arising from recent studies funded by the Australian Research Council, namely o River banks are extensively remodeled each year, and dry-season movements are a critical element in location of suitable nesting areas; o Timing of nesting depends upon water temperatures, and this has an over-riding influence on offspring sex ratios alteration of flows, with concomitant alteration of water but not ground temperatures will have an impact on hatchling sex ratios that can be estimated. o Nesting is close to the water (0.3-2 m) and premature flooding causes egg mortality. Timing of nesting in relation to early flood pulses is a critical consideration for egg survivorship. o Home ranges and extent of movement in relation to nesting banks is known. o Hatching is stimulated by torrential rains and/or flooding. Mismatch in the timing of egg development with respect to early wet season rains and flood pulses leads to high mortality. Again, timing of nesting in relation to early flood pulses is a critical consideration for survivorship. o Data available on the biology of the species are sufficient for quantitative modeling together with that on the environmental flows.

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