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

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1.0 Purpose of the survey and review

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2.0 Available records of river discharge in the Rwenzori Mountains

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3.0 Field surveys

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4.0 Results of field surveys and discussions 4.1 River Mubuku 4.2 River Rukoki 4.3 River Rwimi 4.4 River Nyamagasani

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5.0 Discussion of observations

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6.0 Recommendations

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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.

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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.

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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). Resolution of the rating equations that have been applied to stage data is required to assess the latter possibility.

Figure 6. Photo of the intake for the diversion of flow from the River Mubuku in Nyakalengija (UTM Grid – 36N: 170469mE, 38349mN, 1546mamsl) to the Ibanda HEP generating facility.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 7

Figure 7. Photo of the conveyor directing diverted flow from the River Mubuku to the Ibanda HEP generating facility.

Figure 8. Photo of the return of the outflow from Ibanda HEP generating facility (UTM Grid – 36N: 173733mE, 36498mN,

1370mamsl) to the River Isha, a tributary of the River Mubuku.

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Figure 9. Measurements of river discharge versus river stage (i.e., rating curve) for the River Mubuku. Measurements are plotted according to rating equation (e.g., A, B) were taken from 1954 to 1973 with 4 measurements taken from 1997 and 1998.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 8

A second diversion of the River Mubuku for HEP generation by the Kasese Cobalt Company Limited (KCCL) occurs at Bugoye (UTM Grid – 36N: 177360mE, 34161mN, 1210mamsl). The intake occurs just after the river gauge (Figure 10). Records of river discharge begin in early 2000 (Figure 11) and were kindly provided by Mr. Evaristo Byekwaso, KCCL’s Environment Officer on July 20, 2003. Gaps in the monitoring record exist though it is presently unclear whether this reflects inconsistent monitoring or problems of data retrieval. At Bugoye, flow is directed via a conveyor (Figure 12) to the KCCL HEP generating facility east of the Kasese – Fort Portal Road (UTM Grid – 36N: 182684mE, 29069mN, 988mamsl). Diverted riverflow is returned to the outlet of the River Mubuku (Figure 13) in the topographical depression that includes Lake George.

Figure 10. Photo of the stream gauge (foreground: Mr. Kyewe Aggrey, WRMD) and intake for the diversion of flow from the

River Mubuku at Bugoye (UTM Grid – 36N: 177360mE, 34161mN, 1210mamsl) to the KCCL HEP generating facility.

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Figure 11. Daily records of discharge for the River "Mubuku III" collected by KCCL from January 2000 to July 2003. Mean discharge over this period is 11.8 m3·s-1.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 9

Figure 12. Photo of the conveyor at Bugoye directing diverted flow from the River Mubuku

to the KCCL HEP generating facility.

Figure 13. Photo of the return of the outflow from the KCCL HEP generating facility (UTM Grid – 36N: 182684mE, 29069mN,

988mamsl) to the outlet of the River Mubuku in the topographical depression including Lake George. 4.2 River Rukoki From 1954 to 1984, daily discharge of the River Rukoki was monitored by government where the river intersects the Kasese – Fort Portal Road (Figure 14, Table 1). A dramatic, sustained increase in the discharge begins in 1975 (Figure 15). Mean river discharge from 1955 to 1975 is 4.5 m3·s-1 whereas from 1976 to 1984 the mean discharge is 14.3 m3·s-1. As the last direct measurement of river discharge (i.e., relating river stage to discharge) was taken on December 12th 1969 and there are very few measurements above 10 m3·s-1 (Figures 15 and 16), significant imprecision in discharge estimates above this value (e.g., 1976 to 1984) is expected from application of the rating curve. An increase in precipitation in the late 1970s is not evident from lowland records at Kilembe Mines and Kasese Airport (Figure 17). Resolution of the rating equations that have been applied to stage data is required to assess the magnitude of these errors. Errors in discharge estimates may also derive from the non-uniform nature of the gauging station itself (Figure 14).

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 10

Figure 14. Photo of the stream gauge (far left) for the River Rukoki at the

Kasese – Fort Portal Road (UTM Grid – 36N: 177696mE, 21871mN, 970mamsl).

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Figure 15. Daily records of discharge for the River Rukoki from 1954 to 1984. Mean river discharge from 1955 to 1975

is 4.5 m3·s-1 whereas from 1976 to 1984 mean discharge is 14.3 m3·s-1.

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Figure 16. Measurements of river discharge versus river stage (i.e., rating curve) for the River Rukoki.

Measurements are plotted according to rating equation (e.g., A, B) were taken from May 1954 to December 1969.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 11

Figure 17. (a) Monthly precipitation (13-month moving average - circles) at Kasese Airport (0°12'N, 30°06'E, 960mamsl); (b)

monthly precipitation (13-month moving average - circles) at Kilembe Mines (0°13‘N, 30°00’E, 1370mamsl). Note: interruptions in climate records reflect intervals if interrupted observation. Mean precipitation represent periods of uninterrupted observation.

4.3 River Rwimi From 1952 to 1982, daily discharge of the River Rwimi was monitored by government where the river intersects the Kasese – Fort Portal Road (Figure 18, Table 1). Apart from a 13-month gap in records starting in October 1960, river discharge reduces to <0.05 m3·s-1 in June 1982 and extends until June 1983 when the period of observation ceases. (Figure 19) Crude estimation of the river’s discharge on July 20 2003 indicated a flow of approximately 5 m3·s-1. Local records of precipitation do not indicate drought conditions that would be expected to correspond with a significant reduction in river discharge from June 1982 to June 1983 (Figure 17). Annual precipitation recorded at Kasese Airport in the calendar years of 1982 and 1983, 876mm and 839mm respectively, is only marginally less than annual mean precipitation of 883mm from 1987 to 2002. The decrease in river discharge may alternatively reflect a change in the course of the river though this remains highly speculative.

Figure 18. Photograph of the River Rwimi and bridge for the Kasese – Fort Portal Road

where river discharge was monitored from 1952 to 1982.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 12

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Figure 19. Daily measurements of discharge for the River Rwimi from April 25 1952 to May 31 1983. Mean river discharge from 1952 to 1983 is 5.7 m3·s-1.

4.4 River Nyamagasani The River Nyamagasani is the only river draining the Rwenzori Mountains (in Uganda) that is currently monitored by government (Figure 20). Available records of discharge from 1954 to 2002 are presented in Figure 21. Long gaps in observations begin in 1968 and 1979. An automated recording gauge operated at this site for an indeterminate period of time before being destroyed by flooding in 2001. The station was rehabilitated on September 4, 2001 and is shown in Figure 20 (automated gauge has blue cover in the foreground). On July 18 2004, the third of four gauge board (plate) (indicated by the Hydrological Inspector, Kyewe Aggrey in Figure 20) was missing and required replacement.

Figure 20. Photo of the stream gauge (midground: Mr. Kyewe Aggrey, WRMD) for the River Nyamagasani near Katwe at the

conjunction with the Kaba – Beni (DRC) road (UTM Grid – 35M: 816498mE, 9986636mN, 932mamsl).

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 13

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Figure 21. Daily measurements of discharge for the River Nyamagasani from January 1, 1954 to December 31 2002.

Mean discharge over this period is 8.3 m3·s-1.

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5.0 DISCUSSION OF OBSERVATIONS 5.1 River Mubuku re: RIVER-DISCHARGE MONITORING Monitoring of the discharge of the River Mubuku is currently conducted by KCCL at Bugoye, a station that is downstream of the Nyakalengija diversion for the Ibanda HEP generating station. Records starting from January 2000, accessed from KCCL in July 2003, highlight gaps that reflect either inconsistent monitoring or inadequate data storage. The importance of this river merits consideration of its inclusion within Uganda’s hydrological monitoring network. The River Mubuku via its tributary, River Bujuku, drains the only remaining glaciated highlands in Uganda and has the greatest (annual) mean discharge (~12 m3·s-1) of the major rivers flowing from the Rwenzori Mountains in Uganda. Two HEP generating facilities currently rely upon its flow. Re-establishment of a stream gauge on the River Mubuku is recommended. To monitor the river’s discharge effectively, the gauge should be placed upstream of current flow diversions at Nyakalengija and Bugoye. A gauge could, for instance, be rather easily installed just in front of the intake for the Ibanda HEP generating facility where flow is channelled (see cover photo). A potential complication is that this site lies adjacent to a reservoir supplied by the River Ruboni, a tributary of the River Mubuku, where there are managed releases (weekly) bypassing the gauge. If these managed releases are recorded, their volume releases could be included alongside stream gauge data. An automated gauge with a measurement frequency of every hour (or more frequent) is desirable. Single daily readings of a stream gauge (e.g., taken at 9AM) may miss a significant volume of riverflow that derives from afternoon rainfalls that commonly occur in the Rwenzori Highlands. re: HISTORICAL DISCHARGE RECORDS The significant reduction in peak riverflow observed in historical records of the River Mubuku’s discharge, starting in 1966, remains unclear. Further investigation should begin by resolving whether the reduction is reflected in stage observations and not potentially application of rating equations to stage data. 5.2 River Rukoki (historical records) The more than tripling in mean discharge of the River Rukoki from the period of 1955 to 1975 to the period 1976 to 1984 is unclear. Several measurement problems may account for this increase in river discharge that is unsupported by precipitation records. The paucity of discharge measurements above 10 m3·s-1 is expected to give rise to significant imprecision in discharge estimates above this value that commonly occur from 1976 to 1984. Review of stage data and applied rating equations should be conducted in order to resolve whether the dramatic rise in river discharge is reflected in stage observations and not potentially application of rating equations to stage data. The non-uniform nature of the gauging station itself may, nevertheless, preclude effective use of discharge datasets. 5.3 River Rwimi (historical records) The reduction in the discharge of River Rwimi to <0.05 m3·s-1 over the final year of observation from June 1982 to June 1983 is puzzling and not corroborated by local records of precipitation. Review of stage data and applied rating equations should be conducted in order to confirm that the dramatic reduction in river discharge is reflected in stage observations and not potentially misapplication of rating equations to stage data. 5.4. River Nyamagasani re: RIVER-DISCHARGE MONITORING A stage board was missing from the gauging station on July 18 2004 and requires replacement. The duration, frequency and availability of measurements of river stage from automated gauge recordings are unclear but highly valuable to studies of alpine riverflow. Review of the current and historical datasets from automated gauges on the River Nyamagasani is recommended.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 15

6.0 SUMMARY OF RECOMMENDATIONS 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: DISCHARGE RECORDS

3. Review of stage data and applied rating equations for Rivers Mubuku, Rukoki and Rwimi is recommended in order to resolve whether identified but unexplained anomalies drive from stage observations or application of rating equations to stage data.

4. Review of the duration, frequency and availability of measurements of river stage from automated gauge

recordings on the River Nyamagasani is recommended as these may be used to assess expected diurnal changes in alpine riverflow from the Rwenzori Mountains.

Review of river discharge records and gauging stations in the Rwenzori Mountains of Uganda 16