5. DERIVATION OF NET BASIN SUPPLY TIME SERIES FOR THE EQUATORIAL
5.3 Results from application of the Equatorial Lake Model and segmentation
5.3.2 Lake Kyoga
The observed variation of annual net basin supply in Figure 5.4 is similar to the findings in a study for development of a new lake Victoria water release policy (Centre for Ecology and Hydrology, 2008), where the values were derived from 108 years of record (Sept 1899 – August 2008) and the difference is in their definition of a hydrological year i.e. September to August as opposed to the calendar year definition of January to December that is utilised in this study.
|The Institute of Hydrology (1993) evaluated the accuracy of net basin supply time series derived using the indirect approach (Figure 5.4) against those derived using a conventional water balance approach i.e. with lake rainfall and evaporation inputs for the period 1925 to 1990. It was concluded that there is reasonable agreement for both inflow models but the conventional water balance approach particularly provides a better representation of the increase in net basin supply during the 1961-64 period. The errors in net basin supply vary randomly in many periods, and are typically in the range 10,000-20,000 MCM/year. These error magnitudes were established to translate into about 0.15 - 0.30 m per year in terms of depth over the lake surface.
An assessment of the sensitivity of lake levels to variations in lake rainfall or land use change was also undertaken by the study conducted by Institute of Hydrology (1993). The findings demonstrated that Lake Victoria water balance is about 5 times more sensitive to long term changes in rainfall than to changes in basin runoff coefficient.
Lake Kyoga NBS = Kyoga Nile Outflow + Change in Storage – Victoria Nile outflow (5.4) Simulated and observed water levels for Lake Kyoga are shown in Figure 5.5.
Figure 5.5 Model simulation of Lake Kyoga levels.
The results are similar to those obtained by Mot Macdonald (1998). It appears that observed water levels lag simulated water levels by approximately one month, but the magnitude of the simulated levels matches the observed closely from 1899 to 1979. After the year 1980 and particularly from 1978 to 1984 and in the early 1990’s, simulated levels clearly exceed the observed. The discrepancies arise due to the fact that during extension of the data, some water levels were estimated using a rating relationship between mean monthly levels at Masindi Port and monthly Lake Kyoga outflow data. This relationship is expressed as Equation 4.2 in Section 4.4.3. Where the levels for in-filled periods are mean monthly rather than end of month, but are treated by the model as end of month, a shift will be evident.
During the duration of the blockage of Lake Kyoga, the model is unable to match the observed and simulated levels due to distortion of the natural level-outflow relationship after the
9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0
1899 1908 1917 1926 1935 1944 1953 1962 1971 1980 1989 1998 2007
Masindi gauge (m)
Year Observed Simulated
blockage and during the remedial dredging works that took place to remove the blockage.
Figure 5.6 illustrates model simulation of Lake Kyoga discharges.
Figure 5.6 Model simulation of Kyoga discharges.
In Figure 5.6, the simulated discharges also appear to be slightly ahead of the observed discharges, with a reasonably good match until the onset of the blockage in May 1998. After the removal of the blockage in 2005, observed discharges increase but are not reproduced by the model due to unsteady flow conditions at the time of the dredging works.
Another contributing factor to the failure of the model to match observed and calculated discharges is the considerable scatter inherent in plots of long term lake level – out flow rating relationship at Masindi Port (WMO, 1977). All periods of the historic discharge record do not match the rating curve used by the model in computing lake discharge. Figure 5.7 shows the Kyoga outflow rating based on the observed data input to the model, and that used by the model to compute outflows. Clearly this is partly the source of differences between simulated and observed discharges.
0 500 1000 1500 2000
1899 1908 1917 1926 1935 1944 1953 1962 1971 1980 1989 1998 2007 Discharge (m3.s-1)
Year Observed
Calculated
Figure 5.7 Lake Kyoga outflow relationships.
This indicates the need to regularly review and update the lake level-outflow and elevation area curves for Lake Kyoga since they are prone to alteration arising from both flood events and floating papyrus movements. The resulting Lake Kyoga annual net basin supply time series obtained through application of the Equatorial Lake Model were subjected to the segmentation algorithm developed by Hubert (1997, 2000) and the results are illustrated in Figure 5.8.
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0 500 1000 1500 2000 2500
Gauge level (m)
Discharge (m3.s-1)
Lake Kyoga water level - outflow relationships
Observed Simulated
Figure 5.8 Historical variation in net basin supply to Lake Kyoga.
Application of the segmentation algorithm to the derived annual net basin supply time series of Lake Kyoga produced seven contiguous segments with different means at a 0.05 significance level of the Scheffe test. Several shifts occur in the averages of the dominant phases, and it appears that the time series exhibit alternating periods of net gains and net losses, where the duration of contiguous mean of net gain or net loss appear to be a random variable. The long term average for the period 1899 – 2008 is 157 MCM.
The alternating episodes of net gain and net loss in the various segments presents a markedly different pattern when compared against the results for Lake Victoria, where the change points in 1961 and 1964 do not correspond to the change points identified in the Kyoga data, particularly in 1962 and 1996. It appears that the sudden increase in lake rainfall in 1961 that occurred in Lake Victoria basin does not coincide in time over Lake Kyoga basin. It appears that the increase occurred a year later in 1962, and from then until 1995, there were net gains in inflow. It is not expected that the hydrological response time of the Lake Kyoga and Lake Victoria basin is similar owing to large differences in basin size and the significant influence attributed to the existence of extensive wetlands around Lake Kyoga. The Lake Victoria net basin supply has been shown to be more sensitive to net rainfall over the lake while Lake Kyoga
-12000 -7000 -2000 3000 8000 13000
1899 1908 1917 1926 1935 1944 1953 1962 1971 1980 1989 1998 2007
Annual NBS (MCM)
Year Lake Kyoga net basin supply 1899 - 1911
1912 - 1916 1917 - 1918 1919 - 1961 1962 - 1995 1996 - 2000 2001 - 2008
is more susceptible to the effects of attenuation caused by the large surrounding wetlands (Sene, 2000).
The unique pattern of variation of the annual net basin supply time series over Lake Kyoga clearly illustrates the net effect of Lake Kyoga on the Nile flows, which is to cause net losses in dry years and net gains in wetter years (Hurst and Phillips, 1938). Extreme net gains in net basin supply are recorded in 1918, 1962, 1965 and in 2006 while the largest net losses occurred in 1916, 1926 and 1998. The segment for the period 1996 – 2001, coincides with the period when the blockage at the mouth of Lake Kyoga raised lake levels and increased the total surface area of the lake. It is associated with net loss in net basin supply. The reason as to why the net basin supply during the entire period is negative can be attributed to excessive evapotranspiration. The general pattern of net basin supply variability up to the year 1997 is similar to the results obtained by Mott MacDonald (1998) where data was analyzed for the period 1899 - 1997.
Brown and Sutcliffe (2013) have assessed the accuracy of net basin supply calculated from the difference between outflow and inflow, allowing for changes in lake storage for the period 1940 – 1977 when precise measurements are available. The assessment was conducted by comparing net basin supply with basin rainfall (Figure 5.9).
Figure 5.9 Lake Kyoga net basin supply vs basin rainfall (Brown and Sutcliffe, 2013).
The moderate precision of the derived relation between the two can be explained by errors inherent in the method of deriving the two datasets. There is a standard error introduced by utilizing measurements at Kamdini and Masindi Port to determine Lake Kyoga outflows. It can be deduced by computing the standard deviation of the differences over the periods 1940–1950 and 1971–1977 and was determined to be 1.30 km3, or 5% of the average flow of 26.0 km3 (Brown and Sutcliffe, 2013).
Lake Kyoga annual basin rainfall time series were estimated based on the average of annual totals at all available stations. The availability of stations varied throughout the assessment period (1902 – 1985) from 5 to 22. The standard error of basin rainfall estimate was quantified as the standard deviation of the station rainfall annual totals divided by the square root of the number of observations. The magnitude of the stand error was found to be high in the early years, reaching a fairly steady value of about 50 mm, and then increasing because of the decreasing station coverage after 1977 (Brown and Sutcliffe, 2013).