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Table 4

Hypotheses for temporal variation in sub-trends of the White Nile flows

HypothesesResultDecision/Conclusion
Changes in precipitation The H0 (no correlation between precipitation and river flow sub-trends) was rejected (p < 0.05). On a 15-year time scale, up to about 75% of the variance in the sub-trends in White Nile flow was explained by the variability in precipitation. The upward step jump in White Nile flow mean was potentially caused by the upward step jump in precipitation over the Equatorial region. The precipitation variability in the Equatorial region could be linked to changes in the Indian Ocean Dipole and the El Nino Southern Oscillation. 
Changes in PET The H0 (no correlation between PET and river flow sub-trends) was rejected (p < 0.05). Variability in PET explained up to about 30% of the changes in the White Nile flow variability. Although the River Nile loses about 50% of its water by evaporation in the floodplains of the Sudd region in South Sudan, the PET changes over the areas upstream of the Sudd region has less influence on White Nile flow than precipitation variability. 
Joint influence of PET, precipitation, and soil moisture on rainfall-runoff generation On average, modeled runoff explained more variance (about 90%) in White Nile flow than when individual predictors including precipitation (75%) and PET (30%) were used. Temporal variation in the sub-trends in White Nile flow is potentially influenced jointly by precipitation, PET and other factors. 
Changes in the Lake Victoria water level as a reservoir supplying the White Nile Variance in the White Nile flow explained by the changes in Lake Victoria water level was found to be more than 94%. Temporal variability in White Nile flow sub-trends significantly (p < 0.01) depends on the variation in the Lake Victoria water level. In turn, about 80% of the Lake Victoria water directly comes from precipitation. 
Changes of recording methodology and equipment Records of Nile flows from several hydrological stations along the upper White Nile were found to temporally resonate closely with each other as well as the Lake Victoria water level. Inconsistencies in data records were very unlikely important to explain the upward step jump in the White Nile flow in 1961. 
Land-use and land cover (LULC) changes and/or human factors such as, urbanization, deforestation, wetland reclamation, and overgrazing Although not investigated (for brevity), the White Nile or upper Nile sub-basin has high population growth. LULC changes follow transition in policy-institutional factors such as, shifts in land laws among the countries in the region. LULC changes and/or human factors were unlikely important to explain the variation in the Lake Victoria water level and the White Nile flow. 
Other human intervention such as, water abstractions, diversions, and installation or construction of hydraulic structures Owen Falls Dam (now Nalubale Dam) through which the Lake Victoria water discharges into the Victoria Nile (where White Nile starts) operates based on a curve or level–discharge relationship agreed by Egypt and Britain. The ‘agreed curve’ constructed using data observed between 1939 and 1950 was to originally vary from 10.3 to 12.0 meters of water level on the Jinja gauge. The upward step jump in mean of the Lake Victoria water level in 1961 prompted water to rise above the stipulated ‘12-meter’ mark on the Jinja gauge. This forced extension of the ‘agreed curve’ to operate between 10.3 and 15 meters with the main aim of ensuring that the pre-dam or original natural relationship between the Lake Victoria level and the outflow in to the Victoria Nile was retained. Other human interventions, such as abstraction were unlikely important to explain the changes in White Nile sub-trends. Control of the Victoria Nile flow via the ‘agreed curve’ was unlikely to explain the upward step jump in mean of the White Nile flow. 
HypothesesResultDecision/Conclusion
Changes in precipitation The H0 (no correlation between precipitation and river flow sub-trends) was rejected (p < 0.05). On a 15-year time scale, up to about 75% of the variance in the sub-trends in White Nile flow was explained by the variability in precipitation. The upward step jump in White Nile flow mean was potentially caused by the upward step jump in precipitation over the Equatorial region. The precipitation variability in the Equatorial region could be linked to changes in the Indian Ocean Dipole and the El Nino Southern Oscillation. 
Changes in PET The H0 (no correlation between PET and river flow sub-trends) was rejected (p < 0.05). Variability in PET explained up to about 30% of the changes in the White Nile flow variability. Although the River Nile loses about 50% of its water by evaporation in the floodplains of the Sudd region in South Sudan, the PET changes over the areas upstream of the Sudd region has less influence on White Nile flow than precipitation variability. 
Joint influence of PET, precipitation, and soil moisture on rainfall-runoff generation On average, modeled runoff explained more variance (about 90%) in White Nile flow than when individual predictors including precipitation (75%) and PET (30%) were used. Temporal variation in the sub-trends in White Nile flow is potentially influenced jointly by precipitation, PET and other factors. 
Changes in the Lake Victoria water level as a reservoir supplying the White Nile Variance in the White Nile flow explained by the changes in Lake Victoria water level was found to be more than 94%. Temporal variability in White Nile flow sub-trends significantly (p < 0.01) depends on the variation in the Lake Victoria water level. In turn, about 80% of the Lake Victoria water directly comes from precipitation. 
Changes of recording methodology and equipment Records of Nile flows from several hydrological stations along the upper White Nile were found to temporally resonate closely with each other as well as the Lake Victoria water level. Inconsistencies in data records were very unlikely important to explain the upward step jump in the White Nile flow in 1961. 
Land-use and land cover (LULC) changes and/or human factors such as, urbanization, deforestation, wetland reclamation, and overgrazing Although not investigated (for brevity), the White Nile or upper Nile sub-basin has high population growth. LULC changes follow transition in policy-institutional factors such as, shifts in land laws among the countries in the region. LULC changes and/or human factors were unlikely important to explain the variation in the Lake Victoria water level and the White Nile flow. 
Other human intervention such as, water abstractions, diversions, and installation or construction of hydraulic structures Owen Falls Dam (now Nalubale Dam) through which the Lake Victoria water discharges into the Victoria Nile (where White Nile starts) operates based on a curve or level–discharge relationship agreed by Egypt and Britain. The ‘agreed curve’ constructed using data observed between 1939 and 1950 was to originally vary from 10.3 to 12.0 meters of water level on the Jinja gauge. The upward step jump in mean of the Lake Victoria water level in 1961 prompted water to rise above the stipulated ‘12-meter’ mark on the Jinja gauge. This forced extension of the ‘agreed curve’ to operate between 10.3 and 15 meters with the main aim of ensuring that the pre-dam or original natural relationship between the Lake Victoria level and the outflow in to the Victoria Nile was retained. Other human interventions, such as abstraction were unlikely important to explain the changes in White Nile sub-trends. Control of the Victoria Nile flow via the ‘agreed curve’ was unlikely to explain the upward step jump in mean of the White Nile flow. 
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