Alice Springs town basin, review 2003
Read, R. E.
E-Publications; E-Books; PublicationNT; Report ; no. 42/2003
Made available via the Publications (Legal Deposit) Act 2004 (NT).
Department of Infrastructure, Planning and Environment
Report ; no. 42/2003
Check within Publication or with content Publisher.
G- 2 From this a first estimate was obtained of the relation between evapotranspiration and measured evaporation. Using this volume change corrected for evaporation was plotted against rainfall for each period, and a relation obtained between rainfall and diffuse recharge. The volume changes for each period were then corrected for rainfall using the above relation and plotted against measured evaporation (Fig. 2). The relation obtained from this graph was used for a second plot of change in storage corrected for pumping and evaporation against rainfall (Fig. 3). From this the following estimates were obtained: E = 0.0687 V Where E is evapotranspiration in ML as above V is measured pan evaporation at Alice Springs Airport in mm. Since average annual V is 2374 mm/year, average E is 163 ML/year. Rd = 2.23 G 12.4 Where Rd is diffuse recharge in ML, always > 0. G is rainfall at Anzac in mm. The 5 mm threshold value seems too low, and is a result of the two month period. More realistically the threshold would be about 10 mm for a single event. The coefficient is also high. Since the Town Basin has an area of 7.2 km2 it implies that 30 % of rain over the threshold becomes recharge. However when the nature of the town is considered it is possible. Much of the town area is hard surfaces, which produce a lot of run-off. Most of this run-off goes into the Todd, and becomes recharge. Drains from outside the 7.2 km2 considered as Town Basin also flow to the Todd. Of the water that does not run to the Todd much is concentrated on areas such as lawns which are already well watered. Since periods of significant rainfall and no river flow are rare this equation must be considered very approximate, and extrapolating into wetter months is risky. This is confirmed by the contours of rise in water level from September to October 1997 (App. D, Fig. 21) which is the high value in Figure G- 3. It can be seen that most of the water level rise is due to a recharge mound near where the Sadadeen Drain empties into the Todd. Since some of this is not truly diffuse recharge it will be referred to as local recharge to distinguish it from recharge due to flow in the Todd above Anzac. Applying this equation to 62 years of rainfall data from Alice Springs Airport gives an average local recharge of 538 ML/year. This calculation is not meaningful, since at times of river flow the contribution from the storm drains will make no difference to total recharge. Averaging local estimated local recharge for monitoring periods with no flow gave 112 ML/year over the six years. This would be an underestimate as there would be events with significant rainfall and no river flow in the periods for which river flow occurred. A reasonable guess for local recharge is 200 ML/year. (continued on p. G-4)