非洲之角的水文循環與可利用水資源已經明顯受到氣候變遷與變動的影響。本研究採用六個大氣環流模式(General Circulation Models, GCMs) 搭配IPCC AR4高排放(A2)與低排放(B1)兩種情境的氣候推估來評估2020-2039期間Gilgel Abbay與Gumara集水區的水文受氣候變遷的衝擊。並進一步採用七個大氣環流模式搭配IPCC AR5高(RCP 8.5)與中-低(RCP 4.5)代表濃度路徑 (Representative Concentration Pathways, RCP)的氣候推估來評估在2021-2040與2081-2020期間，Gilgel Abbay 集水區受到氣候變遷的影響。研究資料蒐集包含集水區的觀測日均溫及日雨量，氣候變遷情境推估資料採用對應於基期資料之溫度差值與降雨變化率為基礎，透過氣象資料繁衍用以驅動GWLF 水文模式進行集水區水文模擬，再分析降雨、溫度、蒸發散與逕流所受到的影響。此外也針對AR4與AR5的情境評估結果比較Gilgel Abbay集水區在2021-2040間衝擊評估結果的差異。最後挑選三個大氣環流模式搭配AR5的氣候推估，評估在集水區尺度下Lake Tana湖水在2021-2040與2081-2100期間相對於1993-2012的水體平衡。
儘管逕流量變化的推估在不同大氣環流模式與選用情境間有極大差異，以AR4的氣候推估評估結果為例，在兩個研究集水區的乾季與濕季逕流量皆預測為增加，主要原因為大氣環流模式的雨量推估皆增加。相對於逕流量的增幅，蒸發散量的變化較不顯著，以AR4的評估結果而言，乾季逕流量增加將有助於農業灌溉。但若以AR5的氣候推估評估結果，在兩個研究集水區的乾季逕流量則為減少、濕季逕流量為增加，且RCP 8.5的變化大於RCP 4.5、2081-2010期間的變化大於2021-2040期間。有鑑於AR4與AR5在乾季逕流變化有不同評估結果，本研究也進一步比較Gilgel Abbay集水區在2021-2040間採用AR4與AR5的衝擊評估結果在乾季與溼季水文量的異同與可能成因。本研究根據AR5氣候推估情境評估塔納湖(Lake Tana)在2021-2040與2081-2100期間的水文平衡變化，結果顯示未來塔納湖的水位有增加趨勢，主要由於溼季逕流增加所帶來較多入流，尤其是在九至十二月的湖水面有明顯上升的可能。
未來逕流量的增加有助於提高塔納湖(Lake Tana)的入流量，將有益於水資源的利用與水力發電，但濕季的降雨增加所導致的較高逕流則提高淹水危害的機率。而乾季的降雨減低則影響了原本用於灌溉的儲水。所以些微的水文狀態改變皆會對這個區域的社會、經濟與農業生產造成衝擊。某些不利的改變也會危害到周遭國家的經濟命脈，因此需要規劃與推動調適措施來減低可能的災害。因此，未來的研究工作應該著重於調查不同集水區的變遷，以及氣候變遷影響區域性及國家經濟議題。
;Climate change and variability have significant influences on hydrological cycles and the availability of water in the Horn of Africa. Both IPCC AR4 and AR5 scenarios were applied to assess the impact of climate change on watershed hydrology of Tana Basins the upper Blue Nile, Ethiopia. Projections for six General Circulation Models (GCMs) in association with high (A2) and low (B1) emission scenarios were adopted from the Special Report on Emission Scenarios (SRES) for the period 2020-2039 to assess the impacts of climate changes on the Gilgel Abbay and Gumara watershed hydrology. Concurrently projections of seven global circulation models (GCMs) associated with high and medium–low Representative Concentration Pathways (RCP 8.5 and RCP 4.5) for the period 2021–2040 and 2081–2100 were adopted to assess changes on runoffs in the Gilgel Abbay watershed in this dissertation. The GCMs selected were screened in accordance with the study areas baseline climate statistics. A weather generator was employed to generate daily temperature and precipitation to drive the GWLF hydrological model for simulating runoffs. Projected changes in temperature differences and precipitation ratios relative to the baseline were analyzed to explain the variations in evapotranspiration and the influences on runoff. Assessment results on Gilgel Abbay watershed by the AR4 and AR5 scenarios were compared for the period 2021-2040. Finally, three GCMs from RCPs scenarios were selected and employed to assess the impact at basin scale and to estimate the lake water balance at present 1993-2012 and future time windows 2021-2040 and 2081-2100.
Despite the fact that the projected magnitude varies among GCMs and scenarios, increasing runoff in both wet and dry seasons was observed for both watersheds, attributable mainly to the increase in precipitation projected by most GCMs. In contrast to the great increases in runoff, variations in evapotranspiration are less significant. The projected runoff in both watersheds implies increased potential for promoting agricultural irrigation in the dry season in case of AR4 SRES. Alongside, despite the projected magnitude of changes varied among different GCMs, increasing runoffs in wet-season and decreasing in dry-season are observed in both periods, mainly attributed to the change in projected precipitation. Such changes are profound in cases of RCP 8.5 with respect to those of RCP 4.5 and in cases of 2081–2100 with respect to those of 2021–2040 in case of RCPs. Finally, three GCMs from the RCPs scenarios at basin scale indicated Climate change has a potential to perturb the water balance of the lake due to inflows as it do affect runoffs and rate of changes in water storage through water evaporation. The water balance prediction by the three GCMs for 2021-2040 and 2081-2100 time windows shown a general increase in the predicted lake level in the month of September to December relative to baseline this may attributed to seasonal shift occurred in both precipitation and inflow.
Although the increasing runoffs would provide greater inflow to Lake Tana, the increase of precipitation in wet-season would imply a higher possibility of flash floods. On the other hand, decrease runoffs in dry-season further intensify existing shortage of irrigation water demand. So, any changes in the hydrological or ecological behavior of the Lake will have far reaching consequences on the economy of the region. These changes will have deleterious consequences on the economic wellbeing of the country and require successful implementation of adaption measures to reduce vulnerability. Therefore, future studies shall investigate further the interlinkages between watershed and basin level physical impacts of climate change on the regional and national economy.
