Presentation by Geoff Zimmel, Deltares, at the iMOD - User Day, during Delft Software Days - Edition 2019. Tuesday, 12 November 2019, Delft.

1. i M O D U s e r D a y 2 0 1 9 – D S D - I N T 2 0 1 9
Regional groundwater and geological voxel
models for the Cauca Valley, Colombia
Geoff Zimmel

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Cauca River Valley
• 3rd Iteration of a groundwater modelling
program within the Cauca valley
• Joint project between
• CVC, IHE Delft, Deltares
• Regional Groundwater Models
• Better understand the
hydrogeological system
• Many stakeholders in the region with
concerns about climate change
altering groundwater availability
• Voxel model
• Phreatic aquifer protection
• Groundwater Management Plan

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Previous Work
• ESCACES 1 & 2
• Hydrogeological model
• Develop the understanding of river –
groundwater interaction
• Monitoring system
• DAGMA
• ESCACES 3
• All focused on water use and the effects
of changing climatic conditions
• Strategy development coupling technical
and social workflows.
ESCACES 1
ESCACES 2
ESCACES 3
PMAA
Cali

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QuestionsClimate Change ScenariosVoxel ModelGroundwater ModelOverview

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Geology
• Wellbore lithology data availability
• Kh (dark) and Kv (light) shown on the
right
• Depth averaging within the
hydrogeological unit to derive Keq
values → transmissivity and vertical
resistance values
• Values consistent with previous model
versions in the cental and south
models
A1 A2 B C
202 141 115 71

7. Groundwater Model
• Head levels
• Generally following the surface elevation.
• Higher heads along the east and decreasing
values entering the valley
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8. Groundwater Model
• Groundwater recharge areas
• Flow direction between the A1 and A2 units
• Recharge along in the highlands to the east and
discharge onto the valley floor.
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9. Groundwater Model
• Flow lines
• Forward flow lines
• Starting points 1km x 1km spacing
• Release point from the top of the phreatic aquifer
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10. Groundwater Model
• Flow lines
• Reverse flow paths
• Starting points ever 500m along the Cauca
River
• Release point from DEM elevation
• Flow from the river to the recharge location
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11. Groundwater Model
• River Recharge / Discharge
• Cauca River (Predominantly Gaining)
• Most rivers have a combination of gaining and loosing sections
• Only considers the portion in the boundary area
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12. Groundwater Model
• Problems
• Poor observation well spacing for validation
• Infrequent recordings only twice per year
• Lack of drains under urban areas
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13. Groundwater Model
• Transient Model
• Built to better understand the relationship
between the wet and dry seasons
• Difference between wet and dry seasons
A1 (left) and A2 (right)
• Negative – dry season levels higher then
wet season
• Positive – wet season levels higher then dry
season levels
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14. Groundwater Model
• Transient
• Positive values greater around pumping
wells
• Indication that there is pumping and
drawdown during the dry season
• Wet season and dry season are noticeable
in time series taken close to the Cauca
River
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QuestionsClimate Change ScenariosVoxel ModelGroundwater ModelOverview

16. Voxel Model
• Purpose
• Visualize the lithology distribution
• Used and an input for a aquifer risk
assessment
• 500m x 500m grid size
• Completed using iMOD XYZtoIDF batch
function
• 3 models were generated
1. (unsaturated) Between the surface and gwl
(-5m)
2. (aquifer 1) Between gwl and a constant
depth of 30m below DEM elevation
3. (aquifer 2) Between gwl and top of Layer B
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17. Voxel Model
• 3 models were generated
1. (unsaturated) Between the surface and gwl (-5m)
• Voxel depth set at 0.5 meters
• Cutoff originally set at gwl but 5 meters had to be
added to deal with data loss issues.
2. (aquifer 1) Between gwl and a constant depth of
30m below DEM elevation
• Voxel depth set at 10 meters
3. (aquifer 2) Between gwl and top of Layer B
• Voxel depth set at 10 meters
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18. Voxel Model
• 3 models were generated
1. (unsaturated) Between the surface and gwl (-5m)
• Voxel depth set at 0.5 meters
• Cutoff originally set at gwl but 5 meters had to be
added to deal with data loss issues.
2. (aquifer 1) Between gwl and a constant depth of 30m
below DEM elevation
• Voxel depth set at 10 meters
3. (aquifer 2) Between gwl and top of Layer B
• Voxel depth set at 10 meters
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QuestionsClimate Change ScenariosVoxel ModelGroundwater ModelOverview

20. Climate Change Scenarios
• 2 scenarios were identified
• Baseline (2000 -2017 data)
• RCP4.5 (Estimate recharge based on 2040 climate change
predictions)
• RCP4.5 data calculated from data gathered through downscaling
of a global climate change model
• 99 different recharge values were generated for each scenario,
198 new recharge scenarios in total
• Each recharge value was then used as an input to groundwater
models to estimate the effects on groundwater level of changing
climatic (recharge) conditions
• Head levels measured at five comparison points.
• Differences between baseline and RCP4.5 models were
generated at 10th, 50th and 90th percentile values.
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21. Climate Change Scenarios
• Slight changes can be seen throughout the valley
but rarely exceeds +/- 0.5m on the valley floor
• All 4 units see similar results and spatial
variability and relationships
• Average results from all 99 model runs are
represented by p50 values
• Low recharge represented by p10 and higher
recharge by p90
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p10 p50 p90

22. Climate Change Scenarios
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• Box plots including all 99 iterations
for Baseline (Left) and RCP4.5(right)
scenarios were calculated at each
location
• Mean values were compared to
understand the potential effects of
climate change as represented by the
models
• Overall the model results showed a
low effect of climate change on
groundwater levels for this area of
interest given the model parameters

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Conclusions
• Drawdown from pumping during the dry
season has a much greater effect on local
groundwater levels then climate change
at a single point in time. But…
• The cumulative effects of multiple dry
years (due to climate change) could
greatly reduce water availability in the
valley
• A strong water management plan is a
crucial next step to proper resource
management

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QuestionsClimate Change ScenariosVoxel ModelGroundwater ModelOverview