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-Rodriguez1,2, Amandine Bosserelle1, Peter Sinclair1, Vincent

Post2, Adrian Werner2

1Secretariat of the Pacific Community - Geoscience Division, Suva, Fiji

2Flinders University, Adelaide, Australia

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

Case Study I: Bonriki Island

• Bonriki water reserve supplies 67% of the households in South Tarawa

South Tarawa: 56,000 inhabitants

• Surface area: 0.768 km²

• Extensively used aquifer in the Pacific

Credit: Marie Bourrel

Bonriki Water Reserve

Buota Island

Bonriki Village

Infiltration galleries and monitoring network

Quantity

• Average annual rainfall since 2004: 1,740 mm/yr

• Estimated mean annual net recharge: 947 mm/yr (1,992 m3/d)

• Average abstraction rate since 2004: 1,480 m3/d

• Design Yield: 1,660 m3/d

Salinity

• Maximum EC recorded at the Trunk Main: 1,200 μS/cmwith up to 3,800 μS/cm at individual galleries

• EC water quality threshold at the trunk main 1,500 μS/cm

3D Numerical Groundwater Model

Model Domain Cross Section

3D Modelling Simulation –Bonriki Lens

5 % Seawater20 % Seawater50% Seawater

Long term simulation / Calibration

Observed

Modelled

• Period: 01/1997 to 06/2014

• Modelled VS. Observed EC at each gallery and at the trunk main

PUB water treatment plant

1,500 μS/cm

Application of the model

Management of the Lens

• Different climate/rainfall scenarios

• Verify necessity to reduce design abstraction (1,660 m3/d) during drought

– EC must stay below 1,500 μS/cm

• Define which galleries shall be reduced (sensitivity analysis)

2015

3-yr

Drought

3-yr

Drought

Climate and

Abstraction Scenarios

Base Case Scenario

• 30 years of historical rainfall (1986-2014)

• Projected period: 2015 -2044

• 3-yr Drought (similar to 1998-2000)

• Abstraction - 1,660 m3/d

1997-2014 2015-2044 30 yr projection

Wet

2015-2044 30 yr projection1997-2014

1,500 μS/cm

2,500 μS/cm

Sensitivity Analysis

One simulation per gallery (total of 21 simulations):

• Increment of abstraction at the gallery - 18 m3/d (1,678 m3/d)

• Resulting increment of salinity at the trunk main

Sensitivity = ΔEC/ΔAbstraction

Which galleries produced higher increments of salinity at the trunk main ?

Management Option

1997-20142015-2044 30 yr projection

Management of the lens

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

1997-20142015-2044 30 yr projection

Current Management

3 year drought Similar to1998-2000 event

Results:

• Salinities remains below 1,500 μS/cm during the drought 1,500 μS/cm

1,500 μS/cm

3 yrDrought

3 yrDrought

Dry

Climate and Abstraction

Scenarios

Worst Case Scenario

1997-20142015-2044 30 yr projection

1997-20142015-2044 30 yr projection

1,500 μS/cm

2,500 μS/cm

• 20 years of historical rainfall (1996-2014)

• Projected period: 2015-2044

• 3-yr Droughts 2018-2020 and 2037-2039

• Abstraction - 1,660 m3/d

Management rules:

• Re-arrangement of abstraction distribution / total 1,660 m³/d abstraction maintained

Results:

• Reduced salinities at the trunk main for the first drought, but still above 1,500 μS/cm

• Sustainable yield is lower than 1,660 m³/d during the droughts

� Need of additional management rules

Dry

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)

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

3 yrDrought

3 yrDrought

Dry

Example of the

implementation of the

Optimised Management

Strategy

1997-20142015-2044 30 yr projection

1997-20142015-2044 30 yr projection

1,500 μS/cm

2,500 μS/cm

Dry

Strategy Implementation

1660 m3/d

Galleries Reduced

Warning01/08/2018

1,500 μS/cm

Galleries Reduced

456

89

11

16

19

22

1310 m3/d

Warning01/02/2019

Strategy Implementation

+5 months

Action

1,500 μS/cm

Galleries Reduced

456789101112

1516

19

22

1150 m3/d

Warning01/05/2019

Strategy Implementation

+3 months

Action

1,500 μS/cm

Galleries Reduced

456789101112

15161719

22

1130 m3/d

Warning01/08/2019

Strategy Implementation

6 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/11/2019

Strategy Implementation

9 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/02/2020

Strategy Implementation

12 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/05/2020

Strategy Implementation

15 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/08/2020

Strategy Implementation

18 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/11/2020

Strategy Implementation

21 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/02/2021

Strategy Implementation

24 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/05/2021

Strategy Implementation

27 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/08/2021

Strategy Implementation

30 months

Action

1,500 μS/cm

Galleries Reduced

3456789101112

1415161719

2122

1050 m3/d

Warning01/11/2021

Strategy Implementation

33 months

Action

1,500 μS/cm

Galleries Reduced

567891011

19

22

1280 m3/d

Warning01/02/2022

Strategy Implementation

36 months

Action

1,500 μS/cm

Galleries Reduced

56

9

19

1500 m3/d

Warning01/05/2022

Strategy Implementation

39 months

Action

1,500 μS/cm

Galleries Reduced

1660 m3/d

01/10/2022

Strategy Implementation

45 months

Normal

1,500 μS/cm

With 1,660m³/d from 2015

Strategy Implementation

Management Strategy (1,660m³/d - 800 m³/d )

Results:

• Salinities below 1,500 μS/cm 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

1,500 μS/cm

1,500 μS/cm

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

Thank You

Sandra C. Galvis-Rodriguez

Groundwater Researcher

Geoscience Division

SPC

sandrar@spc.int