Impact of Rainfall Variability

Potential implications of rainfall variability along with Land Use and Land Cover Change (LULC) in the Black Volta basin on the Bui hydropower plant have been assessed.

The aridity index pro le of the Black Volta catchment was rst developed showing three climatic conditions in the basin: Semi-arid zone, dry sub-humid and humid zone.

The spatio-temporal variability of rainfall over the Black Volta was assessed using the Mann-Kendall monotonic trend test and the Sen’s slope for the period 1976-2011. The statistics of the trend test showed that 61.4% of the rain gauges (8 stations out of a total of 13) presented an increased precipitation trend whereas the rest of the stations showed a decreased trend. However, the test performed at the 95% con dence interval level showed that the detected trends in the rainfall data were not statistically signi cant. Only rainfall data at Boura in dry sub-humid zone revealed a statistically signi cant increase (pvalue less than 0.05). LULC is an important factor controlling the hydrology of a basin. Three Landsat satellite images were collected including Landsat 5 Thematic Mapper for the year 1987, Landsat 7 SLC-on for the year 2000 and Landsat 8 OLI for the year 2013. Land use trends between the year 2000 and 2013 show that within thirteen years, land use classes like bare land, urban areas, water bodies, agricultural lands, forest deciduous and forest evergreen have increased respectively by 67.06%, 33.22%, 7.62%, 29.66%, 60.18%, and 38.38%. Only grass land has decreased by 44.54% within this period. To evaluate the combined e ects of rainfall variability and land use change on the discharge at Bui, the hydrological model, SWAT has been selected.

A rst calibration was performed for the period 2000-2005 and validation for the period 2006-2010. During calibration, the model performance was quali ed as very good with NS = 0.9 and R2 = 0.91. The strong correlation between measured and simulated
ows showed that the physical processes implicated in the generation of the stream
ow in the basin are well captured by the model. For the validation period, the model performance was good with NS = 0.7 and R2 = 0.8. However, the graphical representation of the observed and the simulated
ows during validation revealed that there was a delay in peak
ows starting from 2008. It was assumed this e ect may be due to the Bui reservoir construction. To con rm the hypothesis, the model performance was tested for a period prior to the Bui dam construction.This second calibration was performed for the period 1990-1995 and validated for the period 1996-2000. The calibration was quali ed as very good (NS = 0.9 and R2 = 0.82) and validation as good (NS = 0.7 and R2 = 0.85). The graphical representation of the observed and the simulated
ows during calibration and validation do not show important delay in the peak
ows proving that the construction of the Bui dam may e ectively a ect the dynamic of the river system. Changes in seasonal stream
ow due to LULC was assessed by de ning dry season (February, March and April) and wet season (August, September and October).

The results showed that from year 2000 to year 2013, the dry season discharge has increased by 6% whereas the discharge of wet season has increased by 1%. The changes in stream
ows components such us surface run-o (SURF Q), lateral
ow (LAT Q) and ground water contribution to stream
ow (GW Q) and also on evapotranspiration (ET) changes due to LULC was evaluated. The results showed that between the year 2000 and 2013, SURF Q and LAT Q have respectively increased by 27% and 19% while GW Q has decreased by 6%. At the same time, ET has increased by 4.59%.

The resultant e ects is that the water yield to stream
ow has increased by 4%. We believe that the overall impacts of rainfall variability and LULC may bene t the Bui hydropower plant.