Department of Civil Engineering
The University of Texas at Austin
Kwabena Asante
2.0 Process Description
2.1 Reservoir Database
2.2 Niger Basin Coverages
3.0 Discussion
3.1 Recommendations
3.2 Conclusions
References
Since the first major river basin simulation by the Harvard Water Program in 1960, tremendous strides have been made in the analysis of water systems. These development have stemmed out of the realization that water is a scarce commodity that must be conserved. Despite the fact that water makes up over 70 percent of the earth's surface, only about two and a half percent of this water is fresh and even less useable for human consumption and agriculture. With the present rate of population growth, it has been estimated that the water will exceed the available supply within the next 30 years, unless substantial advancements are made in the storage and reuse of water. The prospect of this shortage is even more frightening when one takes into account the fact that the developing countries which have the least resources for exploring new water sources or extracting potable water from non-potable sources such as sea water, are recording the highest population growth rates. Development agencies such as the World Bank are already involved in a number of large scale project that are aimed at improving the planning of resources in this region of West Africa. Within the last two years alone the World bank has embarked on number of such projects in Guinea, Guinea Bissau, Niger, Burkina Faso and Nigeria. However, a lot of this effort has focused on national scale planning. Since most river basins in the region tend to cross national boundaries there has been a need to continually renegotiate water rights between countries, leading to a lot of conflict. The need for the planning of water resources by river basins is thus apparent.
The logical starting point in the adoption of strategies for the prevention of water shortages is an understanding of the behavior of the natural system of water flow. This was the first objective of the Niger Basin water balance project. In meeting this objective, data was collected from various sources to define the processes that take place in the basin. Some of the systems defined include the precipitation, evapotranspiration, surface water, ground water and the runoff in the region. The next step in the process was to represent these processes in such a format as to enable the flow of water and constituents in the system to be simulated. To this end, a Geographical Information System model with subroutines of the various processes and how they interact with each other have been developed and calibrated to ensure consistency with the observed processes in the field. The final step in the process is to determine what the likely demands on the system are and to determine the ability of the system to meet those demands and the possible effect of these on the other users of the system. This study is meant to be the starting point in the examination of the effects of the non-natural processes, those over which we have control, as a basis for planning of future activities.
While GIS technology has only been around since the 1960's, only recently has it been recognized as a versatile tools in the management of natural resources. It has already found extensive use in fields such as forestry management, fish and wildlife management as well as in the management of resources during crises such as fires and oil spills. The adoption of GIS as the water resource inventory and planning tool is therefore inline with the goal of integrated resource management, espoused in the UN Agenda 21 [5].
In developing the point coverage, the data containing the properties of the reservoirs had to be entered into a data base. To do this a table was created in ArcView with 28 fields representing the various attributes of the reservoirs and dams. It was decided that even though the geographic coordinates could only be obtained for 7 of these reservoirs, the reservoirs would be referenced by joining the river name from the reservoir data base to the equivalent name in the stream delineation network.
To test the model, the PC version of the model was loaded and run with minimal instruction from the developer to enable its user-friendliness to be accessed. Sample flow data was obtained from past river gauging records to ensure that they were representative of actual field conditions. Difficulties encountered were noted as a basis for making recommendations for future improvement.
arcview &
From the resulting screen,
select new project under the File pulldown menu.
A project window appears from which a table can be created by
double clicking on the table icon.
This opens up a blank table. With the table window still active, select start editing from the under table in the menu bar. Next
select Add Field from under the Edit menu.
An input screen appears from which the name of the field, the type of variable and the number of decimal places (for numerical variables) can be entered. This process was repeated until all the twenty eight fields had been defined. The tables developed with the procedure outlined above are shown in section 2.1 of this report.
It was initially anticipated that the reservoirs would be located by their geographic coordinates. When this was not possible due to the absence of geographic coordinates, it was decided that the reservoirs would be linked by the names of the rivers along which they are located. The datasets from the FAO, however, did not have the river names attached. Consequently, only the seven dams of known coordinates were used to generate the coverage. Three of those reservoirs were located on the Niger River itself while the other four were located on other rivers in the basin. (Two reservoirs actually fell outside of the defined basin boundaries but this was not considered a major problem as the reservoir boundaries were defined only as a guide to limiting the study area). The three reservoirs on the Niger River, Kainji, Jebba and Goronyo, were then used to test the model.
To develop a coverage of the reservoirs, a text file containing the locations of the dams was created and saved under the file name dm.dat in the following format. The first number represents the dam number while the second and third are the longitude and latitude respectively.
1 6.667 12.167
2 6.167 12.517
3 5.733 13.450
4 4.117 11.383
5 4.833 9.167
6 8.033 11.700
7 8.400 11.467
end
The dam coverage was then generated from the UNIX command prompt as follows:
arc to open the arc prompt
generate reservoirs
This is the command that starts up the dialogue for generating the coverage and results in the generate prompt.
input dm.dat
specifies the source of the coordinates for building the coverage.
points
to specify the type of coverage. This results in points with the coordinates specified being created. When this is complete, exit generate by typing
quit
From the arc prompt, create the coverage from the created points by typing
build reservoirs points
Add the coordinates as x and y coordinates by typing
addxy reservoirs
Next, the coverage created was projected into the coordinate system used in developing the Niger basin coverages by creating a text file called ngprj, containing the inputs for the projection as follows:
input
projection geographic
units dd
parameters
output
projection flat_polar_quartic
units meters
parameters
00 00 00
end
(Be sure to hit return after the end statement to ensure that the end command is executed).
To project the coverage reservoirs to a new coverage dams using the projection file ngprj, the following command was entered from the arc prompt:
project cover reservoirs dams ngprj
The creation and projection of the coverage is now complete. To leave arc, type
quit
The model was initially obtained in a PC format and sent to the UNIX system by File Transfer Protocol (ftp) in a compressed form and then uncompressed. The steps for transferring the model and running it on the UNIX machine are listed below. First, the compressed file NGFLOW.ZIP was copied to the hard drive of the PC. Then the following command sequence was entered from the MS DOS command prompt:
ftp
open alpha1.ce.utexas.edu
The user is then asked to enter the username and password. Next, change directory to the working directory with the command
cd [directory name]
binary
changes the transfer mode to binary
put NGFLOW.ZIP
results in the file being transferred to the UNIX
bye
exits ftp.
On the UNIX machine, the file was uncompressed with following command:
unzip NGFLOW.ZIP
ArcView was then opened with the command
arcview &
A program contained in the project start.apr had to be run to correct the directories of the files contained in the NGFLOW model. To do, this select open project from the file menu of the project window and double click on the start.apr from the working directory. (An input screen appears after selecting open project).
The coverages required for running the model include the basin polygon coverage (NGBASIN), the river line coverage (NGRIVER) and the dam point coverage (DAMS). These themes were added to a new view as theme by selecting add theme from under the theme menu of the view window and double clicking on the required coverage from the resulting input screen. To run the simulation with the reservoirs in place, the dam locator was activated by selecting the icon with the pennant red flag (the third icon from the left on the second row of the project window). An input screen appears from which the user is required to specify whether a reservoir, a flow check or a flow diversion point is to be inserted. By electing to insert a reservoir, the user can subsequently input reservoirs by clicking on the desired location. At each location, the user is prompted to input the attributes of the reservoir together with the demands from the reservoir. The attributes of the three reservoirs used in the simulation were input using this procedure. A new data file was generated for each reservoir that was input. Next, the number of simulations to be run was specified by selecting add records (AddingRec) from the surface water model (SFwModel) menu in the project window. Enter the number of simulations to be run for each of the dam data files from the resulting input screen. The model was then run by selecting SFlowSim from under the SFwModel menu. Samples of the tables resulting from the model run are included in section 2.2.
Fields in the database developed included the following
1. Under Objective,
.......... I = irrigation
.......... P = hydropower production
.......... W = water supply
Repeated letters such as II implies primary objective
Single letters such as I implies secondary objective
2. Irri_area = irrigation area in hectares
3. power = power production in megawatts
4. water = population supplied with water from the reservoirs
5. catchment = catchment area in kilometers squared
6. rainfall = mean annual rainfall in mm
7. inflow = annual inflow in millions of cubic meters
8. area = reservoir surface area in kilometers squared
9. total = total reservoir capacity in millions of cubic meters
10. active = active storage in millions of cubic meters
11. dead = dead storage in millions of cubic meters
12. FWL = flood water level in meters
13. LWL = low water level in meters
14. type = construction material
15. height = dam height in meters
16. length = dam length in meters
17. volume = dam volume in millions of cubic meters
18. outlet = outlet capacity in cubic meters per second
19. spillway = spillway capacity in cubic meters per second
20. cost = cost of construction in millions of Naira (Nigerian currency)
The Niger basin polygon coverage required for locating the reservoirs in the basin. The river coverage was required for ensuring the reservoirs were located on the streams. Since the location of the reservoirs and streams are obtained from independent data sources, this provides an important means of testing the reliability of the stream delineation process (assuming the coordinates of the reservoirs are accurate).
The following coverage shows the location of the 7 dams for which the coordinate were initially available. The 3 reservoirs used in testing the simulation model are labeled in this coverage.
This is the coverage of reservoirs from the entire basin. It was constructed from data that was subsequently obtained from FAO. (These coverages were developed after the completion of the initial simulation).
Sources at FAO were able to compile a more comprehensive list of reservoirs from across the basin with known locations but this information arrived too late to be incorporated in the simulation portion of this study. However, a coverage of these reservoirs has subsequently been developed. Further work is hence required to incorporate these reservoirs in the simulation of the whole basin.
The reservoir routing routine of the model was developed based on the equation below:
S(t) = S(t-1) + I(t) - Q(t) - D(t) - E(t)
where at any time, t
S(t) is the reservoir storage
S(t-1) is the storage for the previous time period
I(t) is the inflow
Q(t) is the release to meet down stream demands
D(t) is the demand diverted for other uses
E(t) is the evaporation and other losses
Two validation checks where included to ensure that the reservoir neither holds more water than its bearing capacity nor releases more water than it contains. This assumes that as long as there is water in a particular reservoir, releases will continue to be made from it. In practice however, reservoirs are not operated independent of each other. Hence, additional rules and restrictions have to be imposed on their operation to make the results more realistic.
The model was on the whole found to be easy to use particularly when one takes the time to read through the operating instructions. The error messages that were obtain were in some cases not very instructive in pointing the user to the source of the error. In addition, it allows the user to run the simulations without inputting the all required information thus making it impossible to tell when a mistake has been made. It also did not display the results of the simulation on the screen after it had finished the computation. It would be helpful if a message appeared on the screen directing the user on where to look for the results of the computations or a summary of the results.
There was also a negative value recorded in the spill during the first simulation. This suggests that there is a bug in the process that needs to be corrected. The spill and evaporation loss fields also had too many decimal places resulting in most numbers being represented as exponential numbers. This needs to be corrected as exponential numbers are difficult to read.
The model in its current state does not allow the incorporation of data sets obtained from other sources as reservoir attributes have to be input individually at each location. This process was adopted because it was initially intended for the model to be used for assessing the impact of introducing a reservoir into an already existing system. This process is rather time consuming and inaccurate especially when dealing with a large number of dams as in the case of the Niger. There is no way of telling whether or not a reservoir has already been placed at a particular location thus introducing the possibility of having multiple reservoirs at some locations and none at others. Provision should be made to incorporate data from already established databases by coding the program to read input from a separate database.
A number of validation checks need to be incorporated in the system to check the practicality of the demands made on the system. Parameters such as the spillway capacity and minimum storage requirements should be incorporated to enable the maximum releases that can be made in any one month to be computed. These could then be used as validation checks for maximum allowable release from each reservoir during a particular month. Also, the number of simulations to be run is a required input for the dams subroutine. However, the user is not prompted to enter this information when beginning a run of the model. Checks should be built in to prevent this from happening.
While the impact assessment process has yet to be completed, I was able to get a feel for the capability of the system and the possibilities that exist in this area. The full set of reservoirs and the demand on these would have to be entered for their full impact on the system to be understood. This will soon be possible as the FAO has recently finished compiling a list of referenced reservoirs from the whole of the Niger Basin. The current model is obviously a little deficient in its ability to assess the impact of developments on the whole system and further development will be required for it to be an effective impact assessment tool.
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