Hindasting and Calibration of Mongalla Gauge Flows
Mongalla, a town situated at the upper reach of the White Nile, here called the Bahr el Jebel, is the key gauging station for inflows into the Sudd swamps of southern Sudan, one of the world largest wetlands. Due to political instability, flow measurements have been suspended in 1983, leaving the inflow into the swamps ungauged. The flows at Mongalla are a combination of Lake Victoria discharge, influenced by evaporation, damping and storage effects of the Equatorial Lakes (Albert, Edward, Kyoga) and seasonal torrent runoff during the rainy season.
The project comprised the estimation of the important missing flow data at Mongalla from the two components of Lake Albert flow and torrent runoff. As the torrent flows are not gauged, they are deduced from spatial rainfall fields using the overlapping time periods where both Lake Albert outflows and Mongalla flows are available for calibration of the torrent flows. Lake Albert flows have been generated from lake levels measured at Butiaba utilizing newly generated rating curves.
Approach to the problem and flow data sources
Data for locations within the area of interest are available for different periods of time, providing sufficient overlapping information to estimate missing periods of data based on correlation factors. Two overlapping periods were used for establishing these correlations. Panyango flows were used to calibrate calculated Lake Albert discharges, and combined Mongalla flow and rainfall runoff datasets for the period from 1961 to 1983 were used to calibrate and validate correlation coefficients for Mongalla from which new flows were estimated for the period from 1984 to 1996. Further estimating would be possible with additional rainfall datasets using the same principles.
Torrent flows were historically not directly measured but can be estimated by the difference between Lake Albert outflows and flows measured at Mongalla. As there is no permanent gauging station at the outlet of Lake Albert, outflows have to be deduced from a rating curve based on lake water levels measured at Butiaba and flows measured at Mongalla during the dry season. Losses through evapotranspiration and gains through dry season catchment runoff have to be taken into account with an estimated value of 5% of the Mongalla flows. The calculated Lake Albert outflow datasets were validated with flow records measured at Panyango.
The rating curve for the Lake Albert discharges is altered by vegetation dynamics which in return are influenced by high or low flow conditions. The curve was deduced by plotting monthly average Butiaba gauge readings against discharges at Mongalla for the dry season, split up into different periods of years which were selected according to significant flow conditions. The flows were validated with data measured at Panyango near the outlet of Lake Albert. For the White Nile at Mongalla, they provide dry season flows and base flows during the rainy season.
The rainfall data used in the analysis are based on CHARM data, a grid-based rainfall dataset called the Collaborative Historical African Rainfall Model which spans the time period 1961 to 1996 in daily resolution. The two key sources of data for CHARM are the National Centre for Environmental Prediction (NCEP) reanalysis time-series and station data. The daily estimated precipitation fields from the reanalysis are smoothed with a spatial filter. This generates a set of 'synoptic' rainfall fields at a cell size resolution of 0.1 degree, equal to 11.2 km in the area of interest.
Rainfall data have been extracted from the CHARM grids for the catchment area between Lake Albert and Mongalla for the period from 1961 to 1996. For the extraction of time series data from the daily spatial CHARM layers a GIS tool was developed. The tool was built for a cell by cell extraction of daily rainfall values within a period of 36 consecutive years. The area of interest consists of 1224 single cells, which results in 421,575 values for one year and 15,186,700 values for the whole dataset (with some values missing). The tool extracts the values of all overlaying layers at a specific location for a specified cell. The data were analyzed calculating average, minimum and maximum values as well as the spatial distribution and extent of rainfall events resulting in an averaged rainfall time series for the area of interest. The values were then converted to runoff flow quantities considering a runoff function to capture initial infiltration and losses which was derived through calibration with overlapping flow datasets. For the calibration, the torrent flow was assumed to be the difference between flow at Mongalla and flow at Lake Albert outlet, i.e. Panyango less some estimated losses.
Adding Lake Albert discharges and the torrent runoff, the discharges at Mongalla were calculated. For calibration of the flows, overlapping pre 1983 datasets were used and a correlation factor applied to cater for damping effects. Assuming that the Lake Albert flows were sufficiently accurate, the flow data was calibrated in order to match the Mongalla flows when combined with the Lake Albert discharge for the overlapping period from 1961 to 1983. The level of confidence in the estimated Mongalla flow data was established taking into account points of potential uncertainty including uncertainties in Lake Albert outflow, the CHARM data and the assumptions made in calculating the runoff using sensitivity calculations. Sensitivity trials have been conducted to gauge the effect of the different parameters in the overlapping period. The method was then applied to the post 1983 data to generate the Mongalla flow to 1996. For periods of missing Lake Albert records, outflow was estimated using linear interpolation, crosschecking qualitatively with outflow data from upstream Lake Victoria.
Results for Lake Albert outflow
Lake Albert outflows are based on a rating curve converting lake water levels at Butiaba into flows at Mogalla. These are assumed to be similar to those at Panyango. As the flows leaving the lake are sensitive to changing vegetation conditions which depend on high or low flows, different rating curves have been established for the changing flow regimes in different years.
While no calibration data are available, it can be assumed that the trend of decreasing discharges continued after 1983 due to decreasing Lake Albert levels and related vegetation dynamics leading to a decrease of channel capacity. Based on some rating curves, regression functions were established. The resulting outflow data was compared with Panyango flow data for validation and shows good correlation.
Results for torrent runoff and Mongalla flows
Comparing the CHARM derived torrent flows with the flows derived from historical sources, good qualitative results were found during normal flow conditions while peak events were not captured sufficiently. The same result was found when comparing the calculated flows at Mongalla with measured values. While qualitatively the time series are comparable, peaks during extreme events are not represented well by the modelling approach.
The result of the study have been published in: Estimation of ungauged Bahr el Jebel flows based on upstream water levels and large scale spatial rainfall data, G. Petersen, H. Bast, N. Fohrer, Adv. Geosci., Volume 18, 2008 (Link)
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