Water Model and Impacts
The groundwater model estimates the impact of changes in water demand
and land cover on groundwater storage in the region and forecasts
the wells at risk of saltwater intrusion for each of the scenarios.
MODFLOW, (McDonald and Harbaugh 1988; 1996) the most widely used and
respected groundwater modeling program, was used to develop a
preliminary groundwater model for the San Juan Londó aquifer.
Currently, the San Juan aquifer is the only major source of potable
water supplying the citizens of the City of Loreto and surrounding
areas. In constructing a groundwater model, estimates of the water
entering the aquifer (recharge), water leaving the system (well
pumping), and hydraulic properties (an estimate of how fast the
water moves through the system) are entered into the model. MODFLOW
then produces a map of groundwater elevations using a set of partial
differential equations.
To prevent saline water from entering the San Juan aquifer, and thus
contaminating the aquifer, water must flow out of the basin into the
Sea of Cortez. If this stops and the elevation of the Sea becomes
greater than the elevation of the groundwater, saline water will
flow into the system. If pumping then continues, the saline water
will reach the wells resulting in the loss of potable water.
In 1986, the Institute of Geophysics at the Universidad Nacional
Autónoma de México (UNAM, 1986) used a basic water balance equation
and rudimentary flow net analysis to estimate recharge. Given the
sparse data available and high level of uncertainty, they estimated
a recharge rate of 10 Mm3 per year, plus or minus an order of
magnitude, which translates into a rate of recharge that falls
within a range of 1 Mm3 and 100 Mm3 per year.
In this study, we update this estimate using a different
methodology. To calculate the amount of recharge entering the
aquifer, sub-basins were delineated using digital elevation models
and the total volume of water that fell within each sub-basin was
determined from annual rainfall rates and storm data. In semi-arid
areas such as Loreto, only a small portion of this volume becomes
recharge; most is lost to run-off and evaporation. In average or dry
years - years with average annual rainfall of 11.5 cm or less -
rainfall is not sufficient to produce aquifer recharge. Potential
recharge in wet years was estimated by including data from larger
2-year, 5-year, 25-year, and 50-year storm events. The 2-year storm
event yielded the highest amount of annual recharge: 2 Mm3/yr. Using
improved data analysis tools that were not available at the time of
the 1986 study, our estimates of average annual recharge are close
to the lower of the earlier estimates. The updated analysis carried
out for this study indicates that given the level of rainfall and
the aridity of the area, a recharge of 10 Mm3 per year is not
possible.
To ascertain the effects of pumping on the aquifer, the model was
applied with the projected water consumption rates for each of the
growth scenarios (see Table 4). Both the high and low recharge
estimates (10 and 2 Mm3 per year) were tested in different runs of
the models.
In all cases, the pumping resulted in saline intrusion into the
aquifer, although the estimated date of saline intrusion varies with
different recharge and pumping rates (Figure 11).
More importantly, at current population levels and a recharge rate of
2 Mm3 per year, the municipal wells will see saline intrusion by the
year 2025 (Figure 12). The onset of saline intrusion could be
significantly delayed with infrastructure improvements. Even with an
optimistic recharge estimate of 10 Mm3 per year, the maximum amount
of pumping above current rates that can be sustained is 0.9 Mm3 per
year. This is approximately equal to the amount of water needed for
an additional 4000 residents in the region. In summary, the model
results conclude that any future development must find an
alternative water source for that development and the associated
growth in supporting population.
More importantly, at current population
levels and a recharge rate of 2 Mm3 per year, the municipal wells
will see saline intrusion by the year 2025 (Figure 13). The onset of
saline intrusion could be significantly delayed with infrastructure
improvements. Even with an optimistic recharge estimate of 10 Mm3
per year, the maximum amount of pumping above current rates that can
be sustained is 0.9 Mm3 per year. This is approximately equal to the
amount of water needed for an additional 4000 residents in the
region. In summary, the model results conclude that any future
development must find an alternative water source for that
development and the associated growth in supporting population.
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