GIS based Estimate of quantity of rainwater available for artificial recharge on New Providence, Bahamas

1. Water Resources Assessment and GIS-Based Stormwater Runoff Estimates for Artificial Recharge of
Freshwater Aquifers in New Providence, Bahamas
Genevieve Diamond (co-author Assefa Melesse)
Department of Earth and Environment, Florida International University 11200 SW 8 St., Miami, FL 33199
AWRA 2010 SPRING SPECIALTY CONFERENCE GIS and Water Resources VI March 20 – 31, 2010
ABSTRACT
The Bahamas is a small island nation that is dealing with the
problem of freshwater shortage. All of the country's freshwater is
contained in shallow lens aquifers that are recharged solely by
rainfall. The country has been struggling to meet the water
demands by employing a combination of over-pumping of aquifers,
transport of water by barge between islands, and desalinization of
sea water. The problem is greatest on the island of New Providence
where the capital city of Nassau is located. As rainfall is the single
natural source of freshwater, a careful study of ways to efficiently
utilize this resource is vital to any water management plan. In
recent decades there has been a great deal of new development on
New Providence which has created a large area of impervious
surfaces and thereby a substantial amount of runoff. Government's
way of dealing with the increased flooding from this runoff has
been, and is, to drill more and deeper injection wells to dispose of
it. However, this runoff represents a supply of freshwater that could
be utilized by the island. It is the purpose of this project to quantify
this runoff and to identify areas where artificial recharge of the
shallow lens aquifers might be accomplished. Remote sensing
images were used to update the existing land cover maps. Since
there are no natural streams on the island, the roads network was
used to create streams and basins to calculate runoff and identify
areas suitable for artificial recharge. By using this runoff in an
artificial recharge plan, it will be possible not only to make
additional freshwater available but also to mitigate the frequent
flooding events experienced in some parts of the island.
OBJECTIVES
DATA
To complete an analysis
of surface runoff and potential
recharge sites;
To determine the quantity of
rainwater that can be captured
for recharge
To select potential areas
based on presence of impervious
surfaces capable of producing
runoff
STUDY AREA RESULTS
DISCUSSION
CONCLUSIONS & RECOMMENDATIONSMETHODOLOGY
December, 2000
Study Area
Total Area: Approximately 80 mi2 Population density: 2,665 per mi2
Dominant Soil Type: Calcaric regosols developed from sand, silt and loam
Parent Material: Limestone
Rainfall Annual Average 1971-2000 – 55 inches
Climate marine tropical with marked wet/dry pattern, most rainfall May-Oct
Aquifer type: Ghyben-Hertzberg with total storage capacity: 120,448,000m3
New Providence International Airport Rainfall Totals
1971 - 2000
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
1971
1974
1977
1980
1983
1986
1989
1992
1995
1998
Total in inches
2 per. Mov. Avg.
(Total in inches)
2004ArcInfo dataset with aerial, two foot
contours, and paved roadway coverages
Spreadsheets from Ministry of Works and
Transportation with drainage wells GPS points
The Nature Conservancy landcover, aquifers,
political boundary layers
Update existing maps as needed e.g. impervious areas on land cover layer
Edit tables – assign curve numbers, add rainfall
Calculate total runoff for impervious and semi-impervious areas
Estimate quantity of rainwater potentially available for artificial recharge
Create drainage basins and points
Determine best locations for recharge based on slope and land cover
New Providence Monthly Average Rainfall (ins)
1
2
3
4
5
6
7
8
9
10
Jan
Feb
M
ar
Apr
M
ay
Jun
Jul
Aug
Sep
O
ct
Nov
Dec
Month
Rainfall(Inches)
Rainfall (ins)
Office of Meteorology rainfall data for Nassau
International Airport 1971 - 2000
Calculations
Curve Number Method
Q= (P-Ia)2/P-Ia+S
Where Q = Total runoff
P = rainfall (inches)
Ia = Initial losses (interception, infiltration, depression
storage, etc.), for this calculation taken as 0.2S
S = recharge capacity of watershed (inches)
S is determined using the equation CN=1000/10 + S, CN = Curve number
from table (weighted)
New Providence, like all the islands of the Bahamas, has no streams or rivers because, in the natural state, the rate of
infiltration is high and little runoff occurs. Therefore, the roads network was substituted for a stream network in this
analysis as it is these impervious surfaces that provide channels for runoff. Anecdotal evidence suggests that rainfall
amount varies over the island, but not enough data exists to verify this so the annual average rainfall of 55 inches at
Nassau International Airport for the years 1971 to 2000 was used for the entire island.
Red arrows indicate potential recharge locations that
coincide with dense clusters of drainage wells in flood-
prone areas on NP
References
Cant, Richard V., and P. S. Weech. Water Resources Evaluation of the Bahamas. Tech. Nassau, Bahamas: Ministry of Works and Utilities, 1980. Print.
England, Gordon. Nassau Stormwater Masterplan. Stormwater Solutions Inc. Cocoa Beach, FL 2009
Acknowledgments
The grant for this project was provided by USDA Cooperative State Research, Education, and Extension Service (Grant Number USDA- CSREES 2006-51160-
03409) through the Agroecology Program at Florida International University.. Thanks to Bahamas Department of Meteorology for providing information on rainfall
data, to Bahamas Ministry of Works and Transportation for GIS and drainage data and to The Nature Conservancy for additional GIS data.
Thanks to Daniel Gann at Florida International University GIS Center for his willing assistance and invaluable advice.
The volume of runoff available for artificial recharge to aquifers on New Providence Island is estimated
by this analysis to total 5.336x107 m3 per year. This represents a significant resource that can help the
island meet its water demands in future. The capture and storage of this runoff will also help alleviate
the problem of flooding in those areas prone to it as the map below illustrates.
While understanding the extent of this resource is a
hopeful beginning to the solution of the problems
being faced by the island of New Providence, it is
only a beginning. With any artificial recharge
project that may be envisioned, there will be other
problems to be solved, such as acquiring the
necessary land to construct collection basins and
how to deal with the pollution carried in the runoff
water. However, the benefits that will accrue to the
population will be well worth the effort.
Data Sources