Water For Senegal
Understanding The Water Issues Faced By A Developing Country
GIS in Water Resources - Fall Semester 2002 - Dr. Maidment
PROJECT OBJECTIVES:
- Provide a general history of the country, including its cultural, religious, economic, geographic and environmental components and use GIS to represent the important features
- Create a basemap and appropriate cartographic representations of the country to display and analyze the water components of the environment, particularly temperature and precipitation variations and trends
- Attempt to construct a water balance for the country with available data
- Begin to develop educational materials and tools that may be of use to the volunteers and trainers in the Peace Corps - Senegal program
Please note - all of the maps, pictures, and charts displayed on this page are thumbnails. Click on the item to display the full-sized representation.
INTRODUCTION/COUNTRY PROFILE:
Senegal is the westernmost country in continental Africa (14.00° N, 14.00° W). While a relatively small country with an area of 196,190 square kilometers (approximately the same size as South Dakota), the capital, Dakar, serves as one of the major seaports for West Africa. Senegal was a French colony that earned its independence in 1960. The country is governed as a republic with multi-party democratic rule. The current president (elected in April 2000) is Abdoulaye Wade. The country is divided into 10 political regions, each with its own regional capital (with the same name as the region itself). The regions are: Dakar, Diourbel, Fatick, Kaolack, Kolda, Louga, Saint-Louis, Tambacounda, Thies, Ziguinchor.
Senegal is bordered on the
west by the Atlantic Ocean, on the north by Mauritania, on the east by
Mali, and on the south by Guinea (also called French Guinea or Guinea
Conakry) and Guinea Bissau. The Gambia, a former British colony,
lies entirely within Senegal (except on the west side where they have
their own coastline), and surrounds, on the north and the south, a large
portion of the Gambia River. (In 1982, Senegal and the Gambia joined
to form a union called the Senegambia, but the integration was never
carried out and the union dissolved in 1989.) The border between
Senegal and Mauritania is defined by the Senegal River; the Casamance
River runs through the southernmost portion of the country; the Gambia
River through the southeast region; and the Saloume River runs through the
central-western area of Senegal.
Senegal has a population of 10.6 million which is made up of numerous ethnic groups, the most prevalent being Wolof (43.3%), Pulaar (23.8%), Sereer (14.7%), Diola (3.7%), and Mandinka (3%). The country's national language is French and each ethnic group within the country speaks their own ethnic language. There are also dialects of ethnic languages that vary by region. (For example, in the region of Kolda, the Pulaar people speak a dialect of Pulaar called Fuulakunda; whereas the norther Pulaars speak a dialect called Pulafuta.) In general, the Pulaars are concentrated in the north (along the border with Mauritania), and in the southern and western regions of Tambacounda and Kolda. The Diolas are most commonly found in the region of Ziguinchor, the Mandinkas in Ziguinchor and Kolda, and the Sereers in the Kaolack region and along the cost from the Gambia to Dakar. The Wolofs, being the most prominent ethnic group in Senegal are found across the entire country, but their highest density is in the Dakar, Thies, Diourbel, Louga, and Saint-Louis regions. The country is 94% Muslim, 4% Catholic, and 2% "other" including Animist. In must be noted, however, that many Animist influences and beliefs are still practiced along with the Muslim religion and culture, especially in the South.
Other than a few densely populated urban-industrialized areas, Senegal still functions primarily as an agrarian society. The major crops include peanuts, rice, corn, beans, sorghum, cotton, and millet. The southern areas of the country also produce large volumes of mangos, many of which are shipped to northern countries to be sold. Technically considered to be part of "sub-Saharan" Africa, the Sahara desert has been growing and creeping further and further south into Senegal. Indeed, the northern areas of Senegal are extremely desert-like, whereas the southern areas are densely forested. Because of the changing desert conditions and the country's high dependence upon agriculture, the aqua-environment of the country is extremely important. In general, Senegal has a rainy season that extends from May until September. However, there is much variation in both the actual length and the intensity of the rainy season across the country. In the north, the rainy season is considered to be good if it rains twice a week from the middle of June until the middle of September. On the other hand, in the southern regions, a rainy season is considered to be "good" if it rains at least every other day from the end of May until the middle of October. During the dry season, many of the fields are used as grazing area for cattle (being herders by origin, the Pulaars have the majority of the cattle, and the cattle are therefore concentrated in the far northern areas and in the sounthern areas of Kolda and Velingara). (Click on various areas of the country to see pictures from those areas of what the terrain, landscape, and environment look like.)
A rainy season that produces less than average amounts of rainfall in the north will greatly impact the crop outputs. Whereas excessive amounts of rainfall in the south during the early stages of the rainy season may cause many early crops to be washed out entirely. The amount and availability of water in Senegal's environment also has an effect upon people's daily lives. For example, in rural areas, all water for cooking, bathing, laundry, etc., is drawn from wells. It is not uncommon in the northern areas of the country to find wells that are 100 meters deep and wells in the southern regions that may be only 15m deep. At certain points during the year, particularly at the end of the long dry season, wells run dry and great efforts must be put forth to obtain water for daily living. There is one major railroad system in Senegal which runs from the capital, across the country and into Mali. The major road system connects the cities indicated on the above map.
DATA SOURCES:
Unlike within the United States, and within Texas in particular, finding data for international locations isn't simple.. The data is not abundant and isn't as straightforward to track down. Especially for developing countries in Africa, it seems that little to almost no data is available (quality continental data is available, but not very much for specific countries). For this project, numerous sources were researched and investigated, and the ones that actually had data in some useful form are the following:
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The Geography Network - The Geography Network is a global network of geographic information users and providers. It provides the infrastructure needed to support the sharing of geographic information among data providers, service providers, and users around the world. Through the Geography Network, one can access many types of geographic content including dynamic maps, downloadable data, and more advanced Web services.
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AQUASTAT, United Nations - Food & Agriculture Organization - Land & Water Development Division - AQUASTAT is the United Nation's Food & Agriculture Organization's global information system of water and agriculture developed by the Land and Water Development Division of FAO. The objective of AQUASTAT is to provide users with comprehensive information on the state of agricultural water management across the world, with emphasis on developing countries and countries in transition.
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Digital Chart of the World Server, Penn State University Libraries - The Digital Chart of the World (DCW) is an Environmental Systems Research Institute, Inc. (ESRI) product originally developed for the US Defense Mapping Agency (DMA) using DMA data. The DCW 1993 version at 1:1,000,000 scale was used. The DMA data sources are aeronautical charts, which emphasize landmarks important from flying altitudes. This explains why there is a separate aeronautical theme with all conceivable airports, yet why on some themes small islands and lakes are simply unnamed points. ESRI, in compiling the DCW, also eliminated some detail and made some assumptions for handling tiny polygons and edgematching.
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Digital Atlas of the World Water Balance, Version 2.0, May 1998, Center for Research in Water Resources, University of Texas at Austin - "The Digital Atlas of the World Water Balance is our second attempt at creating an integrated set of data files and models in GIS format that can be used to characterize the water balance of the earth. The concept arises out of a project called the FAO/UNESCO Water Balance of Africa, sponsored by the Land and Water Development Division of the UN Food and Agriculture Organization (Rome, Italy) and the Division of Water Sciences, UNESCO (Paris, France)." (taken from CRWR's website: http://www.crwr.utexas.edu/)
DATA COLLECTED:
From the above sources, the following data components were collected:
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A preliminary water balance for Senegal as well as basic usage and withdrawal data from 1995 from AQUASTAT
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Various line, point, and polygon files representing the political boundary, road networks, railroad lines, hypsography of the country, drainage network, and populated places were collected.from the Penn State Digital Library
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Temperature, Precipitation, Soil Water Holding Capacity, and Net Radiation data were collected from CRWR for the entire African continent (as well as the political boundary)
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Various maps and information were gathered from the Geography Network about Agricultural Threats in West Africa and Senegal
METHODS:
Before compiling the data and creating a basemap for Senegal in ArcMAP, the individual country data for Senegal had to be extracted from the continental dataset of temperature, precipitation, soil water holding capacity, and net radiation. To do this, all of the cells that were either fully or partially within the political boundary of Senegal were highlighted as depicted below, and that data was exported as its own layer. Then, the intersection tool of the Spatial Analyst was used to trim off all of the data within the perimeter cells that was outside of the political boundary.
The next task that needed to be completed was the merging of the two sets of data (from the Penn State Library and CRWR) into one database. The PASDA (Penn State) data was downloaded for free from their website and was presented in the GCS_Clarke_1866 coordinate system. The data from CRWR (temperature, precipitation, etc.) were available in GCS_Assumed_Geographic_1. All available and known methods for converting/transforming coordinate systems were tried in ArcGIS, but neither coordinate system would transform properly. Each time, the political boundaries would not line up with each other, and the error in the overlap was too significant to be overlooked. In the end, a stalemate was called and it was decided to leave the two datasets separate. Obviously, it would be more effective if the two sets of data (the temperature, precipitation, etc. and the drainage network, road network, hypsography, etc.) could be merged and one, comprehensive basemap could be created. But, c'est la vie. Two separate datasets and maps will have to be used.
The dataset of precipitation, temperature, soil water holding capacity, and net radiation from CRWR was initially presented as the political boundary and overlying data grid with all of the data tucked away in the attribute table. In order to display each of the data categories on the basemap, multiple copies of the data layer were created, and each one was colored/shaded according to one particular data category (i.e., a layer was created that would display the temperature gradient over the country, another layer was created to show the variation in precipitation amounts, etc.). From this, the various trends across the country could be seen. While at first glance, none of these phenomena are terribly surprising, the gradient maps provide a useful and interesting cartographic display of just how the water-related environmental factors vary over the country.
The next portion of the project involved creating a water balance for Senegal. To do this, values including the following needed to be determined: precipitation, runoff, evaporation, withdrawals, and renewable resources. All of these values are known based on the existing data except for evaporation. The potential evaporation can be calculated using the following formula: Ep = 1.3(d/(d+g))*Er, where Ep is the evaporation rate; d is the gradient of saturated vapor pressure curve at the prevailing temperature; g is the psychrometric constant; and Er is the water evaporation equivalent of the net radiation. The values of d and g as a function of temperature were determined according to Table 4.2.1 of The Handbook of Hydrology by D. R. Maidment (McGraw Hill, 1993). The value of Er is given by Er = 86400*Rn/L, where Rn is the net radiation and L is the latent heat of vaporization of water (and 86400 is a conversion factor). After performing these calculations for every data cell in Senegal, the total potential evaporation for the country was calculated to be 324.7km^3/yr.
Difficulty was encountered when attempting to find the runoff measurement for the country. A shapefile was imported from CRWRs site that included runoff measurements made by the Global Runoff Center in Koblenz, Germany of 160 sites worldwide. However, as can be seen from the map below, only two of those measuring stations were actually in Senegal (one along the Senegal river and the other along the Gambia river). Based on those two measuring points, the runoff was calculated to be 26.50km^3/yr. The actual runoff value for the country, however, is probably much higher than that.
According to the AQUASTAT water balance, the average annual precipitation is 146km^3/yr, the total renewable water resources are 39.4km^3/yr, and the total water withdrawals (domestic, agricultural, and industrial) are 1.54km^3/yr.
Based on these figures, an initial water balance was constructed:
Obviously, something is wrong. The outputs far exceed the inputs, and the evaporation level is extremely high compared to the precipitation level. Further investigation and research will be needed in order to correct this water balance.
DISPLAYS AND ANALYSIS OF TRENDS:
(in the next four maps, the same major cities shown above are represented by the black dots)
Mean Annual Precipitation:
As can be seen from the maps below, the mean annual precipitation is much higher in southern Senegal than in northern Senegal. The values range from 270 mm/yr in the northern region (lightest shades of blue) to 1793 mm/yr in the far southern regions (darkest shades of blue).
Mean Annual Temperature:
Similarly, the mean annual temperatures are much lower along the coast of Senegal than they are further inland. The temperature values range from 25.1°C along the coastal areas (lightest shades of red) up to 29.3°C along the Mauritanian and Malian borders (darkest shades of red).
Mean Soil Water Holding Capacity:
The soil water holding capacity tends to vary across the country based on both the rivers and ocean presence, and the groundcover. The capacities are highest near the widest part of the Senegal river along the border between Senegal and Mauritania; along the coast from north of Dakar southward to the Gambia; and in the Casamance (Kolda and Ziguinchor), especially along the coast and near the Gambia river. The soil water holding capacities range from 6mm in areas represented by the the lightest shade of purple to 112mm in the areas represented by the darkest shade of purple.
Net Radiation:
The values of net radiation vary from 168 W/m^2 in the areas represented by the darkest orange to 88 W/m^2 in the areas of lightest orange. The net radiation over the country is much higher along the coastline (both north and south) and much lower further inland, especially along the northeast border with Mauritania.
Mean Monthly Precipitation and Temperature:
Since precipitation and temperature are also highly variable by season, graphs were also created to display the mean monthly precipitation and temperature across the country. These values were calculated from the attribute table to be the mean precipitation and temperature for the entire country in a specific month of the year. (If desired, the same technique could also be applied for various regions of Senegal, as opposed to mean values for the entire country, to determine the variations in rainfall and temperature over the year.)
As can be seen from the charts below, Senegal receives either no rainfall or only negligible amounts of rainfall from December through April, and only slight amounts in May and November. Rainfall across the country begins to steadily increase in June, reaching its climax in August, and then decreasing again through September and October. The mean monthly temperatures, on the other hand, display a different trend. The country experiences its coldest temperatures during "winter", December and January. Over the rest of the year, the temperatures tend to steadily rise and fall, reaching their peaks in May and October (the beginning and ending of the rainy season in the southern part of the country).
Mean Temperatures and Precipitation Values for August:
Because the country receives such high levels of rainfall during the month of August, and, not-coincidentally, the month of August is the most important in terms of agricultural production, a map was created to display both the precipitation amounts and temperature levels across the country during that crucial month. The temperatures are represented with the color gradient, the highest temperatures represented in red (mean monthly temperature of 29.9°C) in the northern parts of the country, and the lowest temperatures represented in blue (mean monthly temperature of 25.6°C) in the furthest southeast regions. The precipitation levels are represented by the black dots, and the dot size is weighted according to the precipitation value. During the month of August the highest levels of rainfall are being received in the southern regions, particularly in Ziguinchor and Kolda and along the border with Guinea and Guinea Bissau (the highest levels being 611 mm/month). Not surprisingly, the areas receiving the lowest amounts of rainfall during this month are the northern regions, receiving between 117 and 179 mm/month.
CONCLUSION:
By far, the biggest challenge in this project was the lack of data for developing countries. While a few, good sources were found, it would have been preferable to have an excess of data from which to choose. Also, although water-related data was indeed found for the country, all of that data was either collected in one year, or was compiled to represent one year. To date, no time series data of any sort has been found for Senegal. In order to fully understand the country's water balance and the impacts that drought and excessive temperatures have on the country and their agrarian culture, one really needs to be able to compare the trends over a series of years. In addition, the initial water balance that was constructed for the country appears to be erroneous. Further investigation into this will be needed before a quality water balance can be compiled. With so few sources of data, it is difficult to correct for anomalies and errors that are found.
But, in the end, the data that was found and the subsequent maps, cartographic displays, and charts that were created have provided much insight and clarity to some of the water-related issues in Senegal. The displays of trends and patterns, particularly for precipitation and temperature, are helpful for understanding the aqua-environment of the country and begin to aid in the understanding of just how important education and conservation are in developing countries with such precious and important water resources.
Please note, with only a few exceptions, all of the pictures, maps, and charts displayed on this page are the property of Pam Kearfott. With the exception of the photographs, please feel free to use and reproduce any of them for purposes that you see fit. For the photographs, however, please request permission first before reproducing them elsewhere. Thank you.
Please direct all comments or questions about this project and site to Pam Kearfott.