’ South America Central America
2. Southern Africa River Flow Database
Anthony Andrews, Ipyana Mwakalinga and Andrew Bullock,
in association with National Hydrological Services of the Southern African FRIEND region
2.1 Introduction
The principle objective of the River Flow Database Project, co-ordinated by the Institute of Hydrology and the University of Dar-es-Salaam, is to underpin the research components of the Southern African FRIEND programme by establishing an international database of river flow on a common hydrological database platform. This objective placed emphasis on the collation of historic daily flows (as demanded by the regional water resources and drought assessment and daily rainfall-runoff model research programmes) and instantaneous peak discharges (as demanded by the regional flood frequency research programme). There is no intention of establishing an additional operational networked flow database for the region, or to regularly update the flow series with recent data, although future occasional updates are not precluded.
The principal accomplishment has been the transfer of a regional database of suitable hydrological time series from National Hydrological Services to the FRIEND research programmes. Specifically, daily river flow data were contributed by the National Hydrological Services of each of the eleven Southern African FRIEND countries and transferred to the Coordination Centre at the University of Dar-es-Salaam where it was loaded onto the FRIEND HYDATA database. Annual instantaneous maxima were also contributed and archived at the Coordination Centre. The HYDATA database of daily flows was transferred to the Regional Water Resources and Drought Assessment Research Programme at Wallingford and to the Rainfall- Runoff Modelling Research Programme at Grahamstown.
Flood data remained at the University of Dar-es-Salaam for analysis. Rainfall data required by the Rainfall-Runoff Research Programme were made available by the National Hydrological and Meteorological Services in a variety of formats and durations, but principally in CLICOM format.
2.2 Identification and selection of flow series
Selection of suitable catchments for analysis within the Southern African FRIEND had two components: first the construction of a regional Master Register of gauging stations (Section 2.2.1) and second, the selection of suitable flow records according to pre-defined guidelines (Section
2.2.2). There was also a further categorization of selected flow records according to their suitability for different research programmes (Section 2.4.2).
2.2.1 Master register of gauging stations
In association with the National Hydrological Services, a regional Master Register of all Southern African gauging stations was compiled comprising me&data for over 10,500 stations, although not all stations have yielded processed flow data. The Master Register provided a foundation for the selection of appropriate flow series with data characteristics that meet the selection criteria specific to the demands of the research projects. Gauging station registers produced by each National Hydrological Service in either paper or computer format, supplemented by information from national reports under the Sub-Saharan Africa Hydrological Assessment Programme, were compiled into the Master Register. Me&data items include local station code, river name and location, latitude and longitude, area, start and end dates of the period of record.
2.2.2 Selection of appropriate flow series
The target in selecting river flow series were hydrological records which were long, continuous, of good quality, representative of relatively natural flow conditions and available in computer format. The selection process was implemented by a combination of pre-defined guidelines and advice ofNational Hydrological Service, encompassing aspects of data quality, reliability, availability and degree of artificial influence. The application of deterministic criteria and local knowledge varied from station to station, and country to country.
This phase of the selection procedure identified 732 gauging stations for inclusion in the river flow database. A sub-set of the Master Register, termed the FdEND Register, contains .meta-data of those stations on the river flow database. The FRIEND Register contains supplementary information to the Master Register, including the actual period of flow record of data provided, the FRIEND station code (Section 2.3. l), research data flag, period of record for low flow, mean annual runoff and flood andfor runoff analysis (Section 2.4.2), and catchment area calculated using the GIS database (Chapter 3). Figure 2.1 illustrates the location of the selected FRIEND gauging stations.
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Andrew. Mwakalinga & Bullock
500 0 500 Kilometers
Figure 2. I Location of FRIEND gauging stations
2.3 Database construction
Construction of the river flow database involved four steps:
first, the development of a new regional unifying gauging station numbering scheme (Section 2.3.1); second, a common flow archiving software system (Section 2.3.2);
third, establishment of data transfer agreements (Section 2.3.3), and fourth, assembly of the database (Section 2.3.4).
2.3.1 A new regional unifying gauging station numbering scheme
The National Hydrological Services of Southern Africa have historically adopted different numbering schemes for gauging stations, with resulting differences between neighbouring countries, with no attempt previously to unify schemes within international rivers. National numbering schemes involve simple numeric schemes in Angola, Mozambique and Swaziland, a structured numeric scheme in Botswana and Zambia, and alpha numeric schemes (each with different structures) in Lesotho, Malawi, Namibia,
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South Africa, Tanzania and Zimbabwe. The FRIEND numbering scheme enhances commonality of data handling within the FRIEND project, and is not intended as a replacement for national schemes. The new scheme has eight digits, which conforms to the maximum number of digits allowed in HYDATA (the common hydrological archiving software, described in Section 2.3.2), and is made up of six components:
a country code (NATION) identifier (first two digits) which is represented by the last two digits of the country’s International Direct Dialing telephone code (IW;
a pre-defined Level 1 - primary river basin identifier (third digit) of which there is a maximum of nine, e.g.
Zambezi;
a pre-defined Level 2 - secondary sub-basin within the primary basin identifier (fourth digit) of which there are a maximum of nine per primary basin, e.g. Lower Zambezi;
a pre-defined level 3 - tertiary sub-basin within the secondary river basin identifier (fifth digit) of which there are a maximum of nine per secondary basin, e.g.
Shire;
a pre-defined Level 4 sub-basin within the tertiary river basin (sixth digit) of which there are a maximum of nine per tertiary basin, and which is based on individual or groups of national hydrometric zones (occasionally across national borders);
the unique station number based on the local code (seventh and eighth digits) of which there is a maximum 99 in any one hydrometric region.
The numbering scheme is founded on the assignment of numerical codes to a set of nine newly categorized principal river basins, and the secondary, tertiary and quatemary sub- divisions within them. The delineation was implemented on the principle that the lowest level of basin sub-division adopted by National Hydrological Services should be accommodated within the numbering scheme. The sub- division into the nine primary basins is presented in Table 2.1.
Example: the Malawi gauging station 07.G. 14 on the South Rukuru at Phewzi.
The FRIEND code for this station is 65312414 which is constructed as follows:
65 Malawi country code.
3 Zambezi primary river basin.
1 Lower Zambezi (below Cabora Bassa) secondary basin.
2 Shire tertiary river basin.
4 North and South Rukuru hydrometric basin.
14 Gauging station identifier which can be the same as the original local code.
Southern Africa FRIEND : IHP IV Technical documents in &irolog~ NO IS
Table 2.1 Prima91 basins in the Southern Africa region used in the FRIEND numbering scheme
Number Primary river basin
Application of the new scheme enables rapid identification of all FRIEND stations in a country or river basin, or any combination. The WMO basin numbering scheme is applied to promote commonality. The FRIEND code also provides a relational link between the catchment/hydrometric basin spatial databases described in Chapter 3.
In a limited number of cases, two gauging stations from different countries in the same hydrometric region possess the same national gauging station identifier (the last two digits). In this case the two stations are distinguished by the use of the country code. For example, gauging station WSH008 (Bonnie Brook in South Africa) has a FRIEND code 27632208 and gauging station 08 (Ngwavuma Nsoko in Swaziland) has a FRIEND code 68632208. Both stations are located in the Usutu hydrometric region of the Lower hydrological data, and encourage compatibility by providing standard facilities for data validation, analysis, presentation.
Most critically in the context of establishing the international river flow archive, the adoption of a common hydrological database would facilitate the transfer of hydrological data between countries.
Within the past decade, several countries in the southern Africa region have moved towards establishing national hydrological archives on personal computers. One system that had been widely adopted in the region prior to the FRIEND programme is the HYDATA database system.
Chapter 2: Southern Africa River Flow Datahmr
HYDATA is used in 22 African countries and, as a result of the FRIEND initiative, is the adopted national hydrological database archive system in eight of the eleven countries in SADC (Table 2.2). Furthermore, initiatives in training and the dissemination of the HYDATA system has supported and encouraged its continued use throughout southern and eastern Africa.
HYDATA was developed over a decade ago and evolved from the UK National Surface Water Database on a mainframe computer. The current HYDATA version (~3.2 l), supplied to or purchased by participating countries in the FRIEND project, has various analysis functions (for example, flow duration analyses) in addition to the storage of water level data, discharge, rainfall, reservoir and lake storage, and general data series such as evaporation. River flow and rainfall data can be stored at intervals of 1.5 minutes and data is converted automatically from instantaneous readings to daily, monthly and annual values. Other HYDATA modules include tools for developing rating equations and for generating output in a ‘yearbook’ style.
HYDATA is operated using on-screen menus designed so that staff with limited knowledge of computers can use the system to its full application potential. A further advantage of HYDATA is that the DOS-based versions will be maintained for at least the next ten years and that HYDATA version 4.0 for MS-WINDOWS is currently under development.
The number of operational HYDATA copies in each of the SADC countries is presented in Table 2.2 together with the uptake of HYDATA within each of the National Hydrological Services. HYDATA in Mozambique and Angola is in the Portuguese language. Adoption and establishment ofHYDATA within the FRIEND programme can be viewed within some countries as contributing directly to the upgrading of national capacity for river flow archiving, quality control and data analysis, Other countries, notably South Africa, Namibia and Zimbabwe adopt HYDATA for the purposes of the FRIEND project alone. and continue to operate their bespoke national river flow archiving systems.
The decision to transfer from one database system to another carries with it a number of important considerations which include training of staff, equipment procurement and transferring of existing data from the old system to the new.
A PC-based support and training package provided additional information such as data transfer methods, tutorial notes, data disaster recovery service and help-line.
The number of staff trained in each of the countries and the number trained through involvement with the FRIEND project are presented in Table 2.2. A HYDATA training workshop coinciding with the Regional Postgraduate Diploma course in Operational Hydrology at the Institute
Table 2.2 Smmaty of HYDATA usage and attendance at HYDA TA training within Southern Africa (extractedfrom Sene et al., 1995)
HYDATA USAGE STAFF TRAINED (BY LOCATION)
COUNTRY No ALL NATIONAL A B C D
Angola 2 a,c Yes 2
Botswana 4 a,d Yes 1 5
Lesotho 3 a,b,c,d Yes 7 2 6
Malawi 3 a,d Yes 2 2 13
Mozambique 4 a Yes 1 3
Namibia 1 d 1
South Africa 1 d
Swaziland 2 a,d Yes 2 1 3
Tanzania 4 a,b,c,d Yes 5 3 8
Zambia 3 a,b.c.d Yes 1 2 14
Zimbabwe 3 b,d 1
Total 30 19 15 51
Key to usage:
a - National database b - Major user in parallel with national database c - Project use d - Research
Key to staff training:
A - Institute for Meteorological Training and Research (IMTR) B - Southern Africa FRIEND workshop, Dar-es-Salaam, September 1993 C - Visits by FRIEND Time Series Database Project to participating National Hydrological Services D - Visits by hydrologists from participating countries to the FRIEND Time Series Database Project at Wallingford, UK
for Meteorological Training and Research (IMTR) in Nairobi was attended by 19 hydrologists and technicians from the region. The Southern Africa FRIEND project implemented a training course on HYDATA database development for a minimum of one representative of each National Hydrological Service, and has provided ad-hoc training and supervision on specific aspects of HYDATA to 5 1 staff ofNational Hydrological Services. Over 70 different individual staff of National Hydrological Services, Meteorological Services and research organizations have received HYDATA training during the first phase of the FRIEND project. In addition to the provision of HYDATA within National Hydrological Services of participating
countries, HYDATA was also implemented within the
Coordination Centre at the University of Dar-es-Salaam.
2.3.3 Establishment of data transfer agreements River flow data was provided freely by the National Hydrological Services under a data exchange agreement defining conditions of use during and beyond the FRIEND project. The agreement stipulates that the database is to be managed by the Programme Coordination Centre and that
use of data is restricted (according to different guidelines from different countries) to FRIEND-related research projects.
2.3.4 Assembly of the database
Visits were made by members of the database project team to National Hydrological Services of participating countries to facilitate the process of station selection, establishment of HYDATA software, allocation of FRIEND station numbers and release of river flow time series. Following visits to each of the participating countries, data were transferred to the Coordination Centre where data validation procedures (Section 2.4) were implemented.
2.4 Data validation
2.4.1 Validation of flow data
It is inevitable that errors are introduced into the database when transferring large quantities of data. Simple tests were implemented, such as identifying periods with consecutive days showing the same flow, correct allocation of data strings to water years, and verification of allocated station numbers.
Hydrographs were plotted for each station and the period of record inspected visually. Typical errors included:
l truncation of peak flows;
l scale changes in flow (causing order of magnitude rises and falls in flow);
l confusion between zero flows and missing data codes;
l anomalous spikes and troughs in the hydrograph.
These errors were managed in terms of data fulfilling the requirements of the research projects. Obvious detectable errors such as anomalous spikes and troughs, and misallocation of missing data codes were resolved. However, some errors on the database did not preclude their use on some of the research projects. For example, periods of record with truncated peak flows would not affect the analysis of annual minima within the regional water resources and drought assessment project but clearly could not be analyzed by the flood project. Periods of record that contained errors relevant to all of the research projects were removed from the database and individual days were set to missing unless they could be interpolated accurately. During this exercise, stations fulfilling the data requirements for a research project were assigned flags defining the research project for which the data were to be used. This procedure is outlined in section 2.4.2.
2.4.2 Data categorization for research purposes Acknowledged variations in data quality across the flow range at an individual gauging station necessitated a categorization of data for suitability for research analysis in terms of mean flow, low flows and flood flows. For this
purpose, long-term hydrographs for each gauging station were inspected visually for data errors and inconsistencies in the flow record, against the background of known limitations with individual gauging stations. As a result of this exercise flow records were deleted from the FRIEND register if part of the record showed inconsistencies, and 56 stations were rejected from the database because of toti many missing data, unreliable flow record or too short a period of record. Each station was designated a flag denoting whether the station is suitable for low flow, flood and rainfall runoff modelling or some combination of these. The data flags are stored on the FRIEND register of gauging stations. together with the reliable period of record for analysis for each regime component.
Based on this categorization of the 676 FRIEND stations it is recommended that 668 (99%) are suitable for low flow analysis. 633 (94%) for flood and 647 (96%) for runoff analysis.
2.5 Data distribution
The HYDATA database of daily flows was transferred to the Regional Surface Water Resources and Drought Assessment Research Programme at Wallingford, where it was loaded on an ORACLE Relational Database System and to the Rainfall-Runoff Modelling Research Programme at Grahamstown, where it was loaded into HYMAS software. The flood peak data series remain at the Coordination Centre at Dar-es-Salaam.
2.6 Content of the river flow archive
The Southern African FRIEND river flow archive contains data from 11 countries in southern Africa, within the nine defined major basins. The 11 participating countries have between them contributed 676 time series ofdaily flow data.
This is approximately 15% of the number of gauging stations contained on the Master Register.
2.6.1 Record lengths and catchment areas
The period of record ranges from 1940 to 1992 and equates to 15.190 station years of data with an average of 23 years per station. Figures 2.2 and 2.3 illustrate the distribution of period of record and record length for all FRIEND gauging stations, respectively. The number of station years varies from 1 to 51 and the mode of the frequency distribution shown in Figure 2.3 is 27 years.
Table 2.3 presents the distribution of gauging stations by catchment area. Following initial intentions to restrict selection to flow records from basins with a maximum catchment area of 1000 km’ it became evident that, if adhered
Chapter 2: Southern Afn’ca River Flow Datahasr
-1940--1945--1950--1955--1960--1965--1970-1975-1960-1965-1990-
1944 1949 1954 1959 1964 1969 1974 1979 1964 1969 1994
5 year period
Figure 2.2 Frequency distribution ofperiod of record amongst Southern African FRIEND gauging stations
- I lto5' 610 'llto'16to‘2lto~26to 31to 3610 4110 z-46
IO 16 20 25 30 35 40 45
5 year period
Figure 2.3 Frequency distribution of record length amongst Southern African FRIEND gauging stations
to strictly, a substantial number of good quality, long time series data of interest to National Hydrological Services would be disregarded. Consequently catchment area became a secondary consideration in the selection procedure, although over 60% of catchments are less than this original threshold.
Table 2.3 Distribution of Southern African FRIEND gauging stations bJ> catchment area
Catchment Number of Percentage Cumulative
area (krr?) gauging stations of total percentage
Undefined 22
0t0100 142 22 22
101to 500 166 25 47
501t01000 96 15 62
1001t05000 140 21 a4
5001t010000 41 6 90
10001t0 100000 55 a 98
> 100000 14 2 100
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It was not possible. even using GIS technology, to determine the upstream catchment area of 22 gauging stations. The principal reason for this was low topographic variation in swampy, pan and desert regions, making catchment boundary deduction extremely difficult at l : 250K scale.
2.6.2 Geographic extent of the archive
The number of gauging stations selected for inclusion on the FRIEND database by each country is presented in Table 2.4 together with the number of station years of data. These contributions reflect the extent of the hydrometric network and the status of existing databases in each country. South Africa possesses the largest network of gauging stations in the SADC region, reflected in the capability of the Department of Water Affairs and Forestry to contribute a large number of stations suitable for the FRIEND project.
The unavailability of processed daily river flow data in computer format accounted for the exclusion of many gauging stations that appear on the MASTER register. For example, during the data collection phase of the project, the National Hydrological Service of Mozambique was in the process of a complete review of the rating of their gauging
The unavailability of processed daily river flow data in computer format accounted for the exclusion of many gauging stations that appear on the MASTER register. For example, during the data collection phase of the project, the National Hydrological Service of Mozambique was in the process of a complete review of the rating of their gauging