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

67 23). Offspring sex for each initial nest date was calculated by summing the offspring sex estimated for each of the two nesting bouts. The sex ratio of the offspring varies dramatically with date of first nesting (Figure 35) reaching almost 100% males for turtles that lay their first nest mid-year. The actual period of first nesting extended from August 17 to September 12 (Figure 35), a period on the range of dates where temperature change would exert its greatest effect on offspring sex ratios. Nevertheless, water temperature change with change in flow was modest, in the order of only 1oC across the range of flow scenarios examined in this study. A 1oC change in temperature could be expected to result in a ca 7-day shift in timing of nesting. An increase of 1oC in water temperatures would be expected to shift sex ratios in favor of males from 50% to 58%, however the change is much lower than that caused by natural weather variations and shifts in nesting time in response to other influential factors (35 days). Figure 35. Percent males produced in two nestings per year, 40 days apart, plotted against the time of first nesting. The actual dates of first nesting are given by the period between the two vertical lines. Note that temperatures outside our direct measurement period (May-December) are estimated from regressions against air temperature, and so extend into the wet season, albeit unrealistically. Synopsis and Recommendations Table 1 is an integrated analysis of the impact of flow reduction on the life history of the pignosed turtle. It shows the outcome for each of several flows, which we have classified as boom or bust. Table 1. Impact of flow alteration on the life history of the pig-nosed turtle. Flow categories are based on the flow levels used in our modelling, chosen to represent dry season flow. All but zero flow occur naturally. The impact of flow reduction of 3-12 cumecs are shown. Frequency of Occurrence (%)1Flow (cumecs) Outcome Natural - 3 cumecs -6 cumecs -9 cumecs -12 cumecs