Abstraction and climate impacts on the Bonriki freshwater lens, Tarawa, Kiribati. An optimization approach to maximize the resource during droughts

Transcription

1 Abstraction and climate impacts on the Bonriki freshwater lens, Tarawa, Kiribati An optimization approach to maximize the resource during droughts Presenter: Sandra Galvis-Rodriguez Authors: Sandra Galvis-Rodriguez 1,2, Amandine Bosserelle 1, Peter Sinclair 1, Vincent Post 2, Adrian Werner 2 1 Secretariat of the Pacific Community - Geoscience Division, Suva, Fiji 2 Flinders University, Adelaide, Australia

2 CAIA Project Objectives Study the impacts of different climate and abstraction scenarios on freshwater lenses in atolls Develop practical, technical and management options, to improve water security and resilience of fresh groundwater resources Study Cases Bonriki, Tarawa, Kiribati Vaitupu, Tuvalu

3 Case Study I: Bonriki Island Credit: Marie Bourrel Bonriki water reserve supplies 67% of the households in South Tarawa South Tarawa: 56,000 inhabitants Surface area: km² Extensively used aquifer in the Pacific

4 Bonriki Water Reserve Buota Island Quantity Average annual rainfall since 2004: 1,740 mm/yr Estimated mean annual net recharge: 947 mm/yr (1,992 m 3 /d) Bonriki Village Average abstraction rate since 2004: 1,480 m 3 /d Infiltration galleries and monitoring network Design Yield: 1,660 m 3 /d Salinity Maximum EC recorded at the Trunk Main: 1,200 μs/cm with up to 3,800 μs/cm at individual galleries EC water quality threshold at the trunk main

5 3D Numerical Groundwater Model Model Domain Cross Section 3D Modelling Simulation Bonriki Lens 5 % Seawater 20 % Seawater 50% Seawater

6 Long term simulation / Calibration Period: 01/1997 to 06/2014 Modelled VS. Observed EC at each gallery and at the trunk main Observed Modelled PUB water treatment plant

7 Application of the model Management of the Lens Different climate/rainfall scenarios Verify necessity to reduce design abstraction (1,660 m 3 /d) during drought EC must stay below Define which galleries shall be reduced (sensitivity analysis)

8 yr projection 2015 Wet Climate and Abstraction Scenarios Base Case Scenario 3-yr Drought 3-yr Drought ,500 μs/cm yr projection 30 years of historical rainfall ( ) Projected period: yr Drought (similar to ) Abstraction - 1,660 m 3 /d

9 Sensitivity Analysis Which galleries produced higher increments of salinity at the trunk main? One simulation per gallery (total of 21 simulations): Increment of abstraction at the gallery - 18 m 3 /d (1,678 m 3 /d) Resulting increment of salinity at the trunk main Sensitivity = ΔEC/ΔAbstraction

10 Management of the lens Management Option yr projection 3 year drought Similar to event Management rules: Reduce abstraction of the most sensitive galleries Increase abstraction of the less sensitive galleries Maintain the total 1,660 m³/d during the 3-year drought Current Management Results: yr projection Salinities remains below during the drought

11 yr projection Dry Dry Climate and Abstraction Scenarios Worst Case Scenario 3 yr Drought 3 yr Drought Management 20 years of historical rules: rainfall Re-arrangement ( ) of abstraction distribution / Projected total 1,660 period: m³/d abstraction maintained ,500 μs/cm yr projection Results: 3-yr Droughts and Reduced salinities at the Abstraction trunk main -for 1,660 the first m 3 /d drought, but still above Sustainable yield is lower than 1,660 m³/d during the droughts Need of additional management rules

12 Optimised Management Strategy General rules for Action IF salinity at the trunk main above 1,000µS/cm (average for 7 days) THEN reduce abstraction of galleries with salinity above 1,000 µs/cm Reduction rate varies depending on sensitivity of each gallery Revision of reduction rates every 3 months (allows for additional monitoring of impacts, community awareness, introduction of water conservation measures)

13 Optimised Management Strategy Rainfall analysis provides guidance on warning: 12 months rainfall index Watch (below 40 percentile): inform community of potential drought event Warning (below 25 percentile): alert community of high risk of drought AND increase monitoring of galleries and boreholes

14 yr projection Dry 3 yr Drought Dry 3 yr Drought Example of the implementation of the Optimised Management Strategy yr projection 2,500 μs/cm

15 01/08/2018 Warning 1660 m 3 /d Galleries Reduced

16 01/02/2019 Warning Action 1310 m 3 /d Galleries Reduced months 22

17 01/05/2019 Warning Action 1150 m 3 /d Galleries Reduced months 22

18 01/08/2019 Warning Action 1130 m 3 /d Galleries Reduced months 22

19 01/11/2019 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

20 01/02/2020 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

21 01/05/2020 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

22 01/08/2020 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

23 01/11/2020 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

24 01/02/2021 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

25 01/05/2021 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

26 01/08/2021 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

27 01/11/2021 Warning Action 1050 m 3 /d Galleries Reduced months 21 22

28 01/02/2022 Warning Action 1280 m 3 /d Galleries Reduced months 22

29 01/05/2022 Warning Action 1500 m 3 /d Galleries Reduced months

30 01/10/2022 Normal 1660 m 3 /d Galleries Reduced 45 months

31 With 1,660m³/d from 2015 Management Strategy (1,660m³/d m³/d ) Results: Salinities below at the trunk main for entire simulation Abstraction varies between 1,660 m³/d and 800 m³/d Additional water sources are required to supply the reduced abstraction from Bonriki Actions are not only limited to the 3-years drought. The management is required also during low-rainfall periods. Operational management strategy is a proactive approach

32 Conclusions Pre-conditions are important drivers on the response of the lens to droughts Management of the lens is focus on the top fresher part Optimization management strategy: Maintain water quality at the trunk main during droughts by reduced abstraction of galleries based on its sensitivity Prescriptive approach that gives specific task to different government agencies (KMS, PUB, MWPU, Drought Committee) The rules provide operational management (in 3-months steps) A range of values of the sustainable abstraction is determined for worst case scenarios, which allows for planning and preparation of future droughts

33 Thank You Sandra C. Galvis-Rodriguez Groundwater Researcher Geoscience Division SPC