Jess Rodrguez Martnez and Jos M' Rodrguez

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Effects of Changes in Irrigation Practices and
Aquifer Development on Ground-Water Discharge to
the Jobos Bay National Estuarine Research
Reserve, South Central, Puerto Rico.

• Jesús Rodríguez Martínez and José M. Rodríguez
  

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Purpose and Objectives

• Develop a digital gw model of the aquifer
• Calibrate the model by simulating the effects
  (heads) of gw withdrawals between 1986 and 2004
• gw discharge rate to the Jobos
  estuary (mangrove
• area)
• use the calibrated model to anticipate the
  effect of various scenariosgw withdrawals and
  implementation of artificial recharge on gw flow
  to the Jobos Bay and the potentiometric surface.

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Location of ground-water flow area and extent of
fan-delta plain in Southern Coastal Plain of
Puerto Rico
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Aerial extent of sugar-cane cultivation in the
study area as inferred from the distribution of
flood-irrigation areas (Torres-González and
Gómez-Gómez, 1987 Quiñones-Aponte and
Gómez-Gómez, 1987)
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Distribution of active agricultural areas mostly
under irrigation with micro drip methods and
center-pivot overhead sprinklers
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Irrigation water deliveries from Canal de
Patillas and rainfall from, 1985 to 2004.
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Surficial geology of the study area.
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Sand and gravel percentage in the fan-delta
aquifer in the vicinity of Salinas and Bahía de
Jobos.
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Generalized distribution of hydraulic
conductivity in the fan-delta aquifer between the
Río Jueyes and Río Guamaní
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Regional direction of ground-water flow in the
study area in April 1986, with average rainfall
and when flood irrigation was still prevalent.
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Regional direction of ground-water flow in the
study area during July 2002, with average
rainfall and when irrigation with micro drip
methods and center-pivot overhead sprinklers was
prevalent
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Regional direction of ground-water flow in the
study area during May 2004, with above average
rainfall and when irrigation with micro-drip and
center-pivot overhead sprinklers was prevalent.
13
Estimated spatial distribution of the base of the
fan-delta aquifer in the study area.
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Approximated altitude of the top of the permeable
aquifer deposits (equivalent to the base of
semi-confining clay and silt deposits).
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Location of continuous resistivity surveys and
corresponding resistivity sections.
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Fence diagrams showing the spatial variation in
thickness of undifferentiated gravel and sand
units of the fan-delta deposits in the study area.
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Finite-difference grid and boundary conditions
for layers 1 through 5.
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Horizontal hydraulic conductivity values assigned
to each of the 5 model layers
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Recharge rates assigned to model cells in layer 1
or 2 for the steady-state simulation (1986)
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Storage values assigned to the 5-layer model
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Sensitivity analysis for the steady-state
simulation (condition for 1986)
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Simulated potentiometric surface for model layers
1, 2 and 3 with posted residuals during 1986 for
the steady-state simulation.
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Observed and simulated water levels during
1986-2004 at selected wells in the study area
(transient simulation)
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Classical sensitivity analysis based on transient
simulation using the residual standard deviation
from water level hydrographs
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Calibrated transient model simulated water budget
for annual stress periods 1986 through 2004
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Flux to mangroves as obtained by the transient
calibration simulation 1986-2004
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Various management alternatives

• 1. GW flux after 10 years of average recharge and
  2004 pumpage with injection wells to the north of
  JBNERR
• 2. GW flux after 10 years of average recharge and
  2004 pumpage with artificial recharge applied
  over an area north of JBNERR
• 3. GW flux after 10 years of average recharge and
  cease of gw withdrawal from all wells north of
  JBNERR
• 4. GW flux after 10 years of average recharge
  and a 50 reduction in 2004 pumpage from
  agricultural wells
• 5. GW flux after 10 years of average recharge
  with artificial recharge applied over an area of
  about 590 acres north of JBNERR and a 50
  reduction in 2004 pumpage from agricultural wells

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Various management strategies and resulting
fluxes to mangrove area of Jobos Bay
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Three of the simplifications involved in this
modeling effort are

• 1. simplified homogenous hydraulic conductivity
  zones
• 2. use of the fresh/saltwater interface as a
  no-flow boundary for a constant density model,
  Rather than using a variable density model code
• 3. use of non-varying general head boundaries
  along the coast
• The greatest source of errors in the model
  calibration are a result of the lack of metered
  ground-water withdrawals, especially from wells,
  and the lack of continuous streamflow gaging
  stations on streams that lose flow to the Salinas
  fan-delta aquifer or the canals

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Questions