1. Kingsley Dunham Centre Keyworth Nottingham NG12 5GG Tel 0115 936 3100 © NERC All rights reserved Investigative modelling for the ZIGARF project… Chris Jackson, Paul Hulme, Majdi Mansour and Andrew Hughes Groundwater Modeller’s Forum Meeting Birmingham, 28 th March 2007 …or the perils of modelling stream depletion.

2. © NERC All rights reserved Outline Background to (Z)IGARF Description of the Candover test Hydrogeology Test itself Investigative modelling Conclusions

3. © NERC All rights reserved Outline Background to (Z)IGARF Description of the Candover test Hydrogeology Test itself Investigative modelling Conclusions

4. © NERC All rights reserved Background IGARF ZOOM for IGARF ZIGARF Candover

5. © NERC All rights reserved Outline Background to (Z)IGARF Description of the Candover test Hydrogeology Test itself Investigative modelling Conclusions

6. © NERC All rights reserved Candover test Chalk in Southern England River augmentation boreholes Three sites, six boreholes Identified and developed in early 1970s Commissioning test in 1976 Undertaken during a drought!!!

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8. Hydrology Rainfall/recharge Rainfall 875 mm/a Recharge 400 mm/a River flows – mean flow to end of 1977 Candover at Borough Bridge – 53.8 Ml/day Alre at Drove Lane – 138 Ml/d Cheriton at Sewards Bridge – 54.4 Ml/day Itchen at Easton – 396 Ml/day

9. © NERC All rights reserved Geology and Hydrogeology Hydrographs Smaller range (1 – 5 m) than rest of Hampshire Chalk ? High T Transmissivity Analysis of tests show high T 4300 m 2 /d and storage coefficient of between 0.4 and 2.6 %. Abstraction Majority for SWA (as was then) Abstraction for cress beds

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11. Outline Background to (Z)IGARF Description of the Candover test Hydrogeology Test itself Investigative modelling Conclusions

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13. Abstraction during test

14. © NERC All rights reserved Depletion calculation Regression analysis of the flows in the Cheriton with the Candover, Alre and Dever. Abstraction from scheme boreholes assumed to have negligible impact on the Cheriton. Abstracted water added to the Candover, therefore need to calculate Net Gain. Difficulties as don’t have access to original calculation and resulting data.

15. © NERC All rights reserved Depletion calculation - 1 Measured flow

16. © NERC All rights reserved Depletion calculation - 2 Convert cumulative totals to monthly time series.

17. © NERC All rights reserved Outline Background to (Z)IGARF Description of the Candover test Hydrogeology Test itself Investigative modelling Conclusions

18. © NERC All rights reserved Runs undertaken 1.IGARF s/sheet 2.ZOOM straight-line river 3.Modified straight-line river 4.Three boreholes used for abstraction 5.Grid refinement to represent River Candover more accurately 6.Addition of Rivers Itchen, Alre and Cheriton 7.Adding in rest of rivers 8.River conductances from Entec Itchen model 9.T distribution from Entec Itchen model 10.Higher Sy in valleys 11.T varies with saturated thickness 12.Correct river elevations

19. © NERC All rights reserved Run 1 – IGARF spreadsheet

20. © NERC All rights reserved Run 7 – Most complex homogeneous model - Adding in rest of rivers

21. © NERC All rights reserved Run 12 – Heterogeneous properties incl. Correct river elevations and recharge

22. © NERC All rights reserved Depletion results

23. © NERC All rights reserved Run 13 – High T line representing fracturing

24. © NERC All rights reserved Outline Background to (Z)IGARF Description of the Candover test Hydrogeology Test itself Investigative modelling Conclusions

25. © NERC All rights reserved Why doesn't it work? 1.Errors in the data: Is the depletion calculation wrong? It’s a model (regression analysis), after all! Good relationship between Cheriton and Candover, but not with Cheriton and Alre. 2.Errors in the conceptual understanding: Where does water come from? What is the role of the layering in the system, e.g. Cress beds on the Alre? Vertical variations in properties of the Chalk and how do they change over space and time? Does the drought conditions affect response? 3.Errors in the approach: Is the parameterisation of the model correct? Should we be using a regional-scale model to simulate this test at all?

26. © NERC All rights reserved What do you want to do with the models? Typical use Abstraction – Depletion Water balance – CAMS Final thoughts: Scale – space and time Complexity - refinement of river mechanisms, is one coefficient enough? What about the Chalk? Simplicity of models, simple but the right simplicity… What do we need to include?