Hydrological Modelling to Assess the Impact of Climate Change at Gilgel Abay River, Lake Tana Basin - Ethiopia

Sammanfattning: Climate changes have had marked impacts on the natural systems. However its impact will be significant with the hydrological cycle. It is agreed that climate change have adverse impacts on socio-economic development of all nations. But, it is expected that its impact will hit developing countries the worst. The objective of this thesis is to assess the impact of climate change on Gilgel Abay River. Gilgel Abay River is the largest tributary to Lake Tana basin. Lake Tana basin is located in the north-western Ethiopia. It lies between latitude 10.95o and 12.780N, and longitude 36.89o and 38.25oE. Lake Tana is the key socio-economic focal point in the area. However, due to climate variability and change, the water level in the lake fluctuates. GCM derived scenarios of climate change were used for predicting the plausible future climate of the study area. The HaDCM3 A2a and B2a scenario experiments were used for the climate projection. As the GCM data are too coarse for impact assessment at regional level, SDSM was used to downscale the GCM data into finer scale. A physically based hydrological model, SWAT, was developed, calibrated and validated. SDSM downscaled climate outputs were used as an input to the SWAT model and used to assess the impact of climate change on the Gilgel Abay River and Lake Tana basin. The climate projection analysis was done dividing the coming 90 years into three time periods. The 1990-2001 was taken as baseline period against which comparison was made. The mean annual precipitation may decrease in the 2020s and increase in the 2050s and 2080s. However, the mean monthly precipitation may both increase and decrease. The decrease in mean monthly precipitation may be up to 30% in 2020s and the increase may reach up to 34% in 2080s. The maximum and minimum temperature indicated an increasing trend. The change in monthly mean maximum temperature ranges between -2.5 0c in the 2020s and +5 0c in the 2080s. The change in monthly mean minimum temperature ranges between -1.4 0c in the 2020s and +4.2 0c in the 2080s. The impact of climate change may cause a decrease in monthly flow volume up to 46% in the 2020s and increase up to 135% in the 2080s. It is observed that climate change has negligible effect on the low flow condition of the river. Seasonal flow volume may show increase up to 136% and 36% for Belg and Kiremit respectively. The increase in Belg season flow will have a paramount importance for small scale irrigation activities practiced by local farmers. It is observed that there may be a net annual increase in flow volume in Gilgel Abay River due to climate change. As Gilgel Abay is the largest tributary river feeding into Lake Tana, any effect on this river is reflected in the Lake water level. The increase in flow will help to harness a significant amount of water for the ongoing dam projects in the Gilgel Abay river basin. However, it may also aggravate the recurrent flooding problems in the Lake Tana surrounding area.