Review of River Discharge Records and Gauging Stations in the Rwenzori Mountains of Uganda
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Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda Richard Taylor Department of Geography University College London, UK Kyewe Aggrey Water Resources Management Department Directorate of Water Development Entebbe, Uganda November 2004 Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 1 Summary This report provides an overview of the inspections of river gauging stations and a quality control analysis of discharge records carried out in July 2004 for rivers draining the Rwenzori Mountains in Uganda. The impetus for the review is an on-going investigation of the impact and implications of rapid glacial recession on alpine riverflow. Based on the review, recommendations pertaining to monitoring of riverflow in the Rwenzori Mountains and analysis of discharge records include: re: RIVER-DISCHARGE MONITORING 1. Re-establishment of a river gauge on the River Mubuku upstream of diversions for HEP generation (Kilembe, KCCL) is strongly recommended. An automated gauge that is able to record discharge (or stream stage) on an hourly basis to account for expected diurnal changes in river discharge is suggested. 2. Repair of the gauging station on the River Nyamagasani that on inspection on July 18 2004 is missing a stage board (plate). re: RIVER-DISCHARGE RECORDS 1. Review stage data and applied rating equations for Rivers Mubuku, Rukoki and Rwimi. It is necessary to resolve whether identified but unexplained anomalies drive from stage observations or application of rating equations to stage data. 2. Review the duration, frequency and availability of measurements of river stage from automated gauge recordings on the River Nyamagasani. If available, these may be used to assess expected diurnal changes in alpine riverflow from the Rwenzori Mountains. Support from University College London and Rwenzori Beverage Company Limited for this survey, and support for the Rwenzori Expedition 2003 from Royal Geographical Society, The Royal Society, University of London and Rwenzori Beverage Company Limited are gratefully acknowledged. Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 2 Table of contents Executive summary 2 1.0 Purpose of the survey and review 4 2.0 Available records of river discharge in the Rwenzori Mountains 6 3.0 Field surveys 6 4.0 Results of field surveys and discussions 7 4.1 River Mubuku 7 4.2 River Rukoki 10 4.3 River Rwimi 12 4.4 River Nyamagasani 13 5.0 Discussion of observations 15 6.0 Recommendations 16 Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 3 1.0 PURPOSE OF THE REVIEW Surveys of watercourses and associated river gauges, both active and historical, at the base of the Rwenzori Mountains (Figure 1) were conducted in 2003 and 2004 in order to investigate anomalies and uncertainties in records of river discharge that are outlined in a discussion paper, “Assessing the impact of glacial recession on alpine riverflow in the Rwenzori Mountains: resolving uncertainties in the observational record” by Richard Taylor. Anomalies were discussed in the Water Resources Management Department (Entebbe) on July 9th 2004 and subsequently with the Acting District Water Officer for Kasese District on July 18th, 2004. Collaborative research under the “Rwenzori Expedition 2003” project involving University College London (UCL), Water Resources Management Department (WRMD), Makerere University, and University of Innsbruck (Austria) observed that deglaciation, which has occurred more or less steadily in the Rwenzori Mountains from 1906 to 1991 (Kaser and Osmaston, 2002), continues unabated to the present (Figure 2). Subsequent investigation of the impact of glacial recession on riverflow (seasonality, annual volume) and the climatic implications of rapid deglaciation is, however, currently constrained by uncertainties detected in available records of river discharge. No long-term, meteorological records in alpine areas of the Rwenzori Mountains exist to assess directly links between glacial retreat and climate. It hoped, however, that possible variations or trends in precipitation may be inferred from records of riverflow. Figure 1. Topographic map of the study area showing alpine river channels and the location of river-gauging and local meteorological stations. Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 4 Figure 2. Changes in the glacial extent of glaciers on the Rwenzori Mountains from 1906 to 2003 showing the predicted elimination of glacier cover in 2023 assuming a continued, linear rate of glacial recession. 2.0 AVAILABLE RECORDS OF RIVER DISCHARGE IN THE RWENZORI MOUNTAINS Records of river discharge are available from four catchments with their headwaters in the Rwenzori Mountains (Table 1, Figure 1) though only 2 are currently monitored. The possibility that catchments that drain toward the Democratic Republic of Congo (e.g., River Butawu, River Luusilibi) have been or are gauged (Figure 1), remains to be investigated. Analysis of discharge records for the River Semliki (Figure 1) is in progress. Table 1. River gauging stations at the base of the Rwenzori Mountains (see Figure 1) River basin station no. UTM coordinates UTM Grid elevation1 catchment (km2) Record Period Mubuku-Bujuku 84222 179433mE, 29197mN 36N 1077 256 1954-1971 Mubuku-Bukuju KCCL 177360mE, 34161mN 36N 1210 no data 1999- Rukoki 84224 177696mE, 21871mN 36N 970 181 1954-1984 Rwimi 84221 no data 266 1952-1982 Nyamagasani 84228 816498mE, 9986636mN 35M 932 507? 1954-80, 1999- 1: metres above mean sea level (mamsl) 3.0 FIELD SURVEYS Facilitated by a travel grant from University College London to Richard Taylor and logistical support from Rwenzori Beverage Company Limited, a field survey was conducted in July 2004 over a three-day period. After travelling from Kampala by road on July 17th, Richard Taylor (UCL) and Kyewe Aggrey (Hydrological Inspector, WRMD) met with the Acting District Water Officer for Kasese District, Mr. Steven Kanyanatonwa, on the morning of July 18th to discuss observed anomalies and uncertainties in records of river discharge listed in Table 1. This informative meeting was followed by a detailed survey of the flow diversions along the course of the River Mubuku-Bujuku for hydro-electric power (HEP) generation. Inspection of the stream gauge for the River Nyamugasani was then conducted before the team returned to Kampala by road. An earlier survey of gauging stations was performed by Richard Taylor in July 2003 as part of the Rwenzori Expedition 2003 field programme. Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 5 4.0 RESULTS OF FIELD SURVEYS AND DISCUSSIONS Review of river discharge records and gauging stations for each river basin is given below: 4.1 River Mubuku-Bujuku From 1954 to 1971, daily discharge of the River Mubuku was monitored by government where the river intersects the Kasese – Fort Portal Road (Figure 3, Table 1). The River Mubuku includes the River Bukuju, a tributary that drains the highest glaciated Rwenzori peaks, and has the greatest mean discharge of the rivers that flow from the Rwenzori Mountains in Uganda. Maximum and minimum measurements of riverflow each month (based on daily observations) are plotted in Figure 4. The gauging station is, however, no longer operable (Figure 5). Although there are a few gaps in the record (1955, 1959-1960), the cause of a dramatic reduction in peak riverflow (and, hence, total river discharge) beginning in the first half of 1966 (Figure 4) remains unclear. Figure 3. Photograph of the River Mubuku floodplain and bridge for the Kasese – Fort Portal Road (UTM Grid – 36N: 179433mE, 29197mN, 1077mamsl) where river discharge was monitored from 1954 to 1971. max. Q min. Q 100 90 80 70 60 50 40 Q (cumecs) 30 20 10 0 7 8 3 4 7 54 5 5 -60 6 6 6 -69 -70 n- an- an- Ja Jan-55 Jan-56 Jan- Jan- Jan-59 Jan Jan-61 Jan-62 J J Jan-65 Jan-66 Jan- Jan-68 Jan Jan Jan-71 Figure 4. Maximum and minimum measurements of riverflow each month (based on daily observations) for the River Mubuku (1954 to 1971 inclusive). Mean river discharge from 1954 to 1971 is 12.8 m3·s-1. Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 6 Figure 5. Photo of (presumably) former river gauging station (now inoperable) for the River Mubuku under the Kasese – Fort Portal road bridge. Flow of the River Mubuku was (and continues to be) diverted for HEP generation (Kilembe Mines) at an intake (Figure 6) in Nyakalengija (UTM Grid – 36N: 170469mE, 38349mN, 1546mamsl). Flow is then directed via a conveyor (Figure 7) to the Ibanda HEP generating facility (UTM Grid – 36N: 173733mE, 36498mN, 1370mamsl) and returned to the River Mubuku via one of its tributaries, River Isha (Figure 8). Sluices and drains at the Nyakalengija diversion permit peak flows to continue along the river channel. There is, furthermore, no apparent consumptive use of peak flow associated with the diversion for HEP generation. A time lag in the response of the river gauge to precipitation might be expected as a result of the longer and more circuitous route taken by diverted flow but the virtual elimination in 1966 of a difference between maximum and minimum flows is puzzling. There is no log for this station. No further insight was gained from the Acting District Water Officer or the manager of the Ibanda HEP Station, Mr. Balyebulya Jonas. Communication with the former (and potentially new) operators of Kilembe Copper Mines is suggested in an attempt to resolve this query. Other possibilities are that the change in 1966 reflects either a field measurement error or an error in the application of the rating curve (i.e., translation of stage records to an estimation of river discharge) (Figure 9).