OBSERVED SHIFT AND MERGE OF HYDROLOGICAL REGIMES IN THE SOTA CATCHMENT, BENIN; AN EVIDENCE OF CLIMATE CHANGE

Sub-Saharan Africa have low resilience capacities to the challenges of climate change. This study is aimed at assessing climate trends and regime shifts at the Sota Catchment, Benin. Long term rainfall and river discharge were analyzed from 1950-2010 in order to generate patterns of changes in the basin. Analysis of the hydro-meteorological were based on the two prominent vegetation zones (Sudan and Guinea Savannah) in the catchment. The rainfall and discharge data were subjected to regime shift analysis and Standardized Precipitation Indices (SPI) were computed. Downward trend of precipitation was observed in the Guinea and Sudan vegetation zones of the catchments from 1970. Rainfall and runoff amount at the two assessed vegetation zones was different in the Guinea and Sudan zone up till 1970. Clear merge of rainfall and runoff amount and patterns was witnessed between the two vegetation zones at 2007. This calls for attention of scientists and policy makers in the region to deploy necessary adaptation measures based on such clear evidence of climate change.


Introduction
Climate change refers to a change in the state of the climate that can be identified (e.g. using statistical tests) by changes in the mean and/or the variability of its properties, which persists for an extended period, typically decades or longer (IPCC, 2014a). Renewable freshwater resources in west Africa have been shown by several authors to be highly vulnerable and strongly affected by climate change (Djigbo Félicien Badou Sylla et al., 2015). Rising global temperature is expected to enhance the intensification of the hydrological cycle, resulting in more moisture being released into the atmosphere than the amount received by the soil (Diasso & Abiodun, 2017;IPCC, 2014b). This will lead to soil water deficit, severer dry Http://www.granthaalayah.com ©International Journal of Research -GRANTHAALAYAH [206] seasons and subsequently heightened risks of drought. Changing climate will also have significant impacts on the availability and accessibility of water in west Africa. This will lead to increase in water stress impacts that is already a nagging challenge in the Sota Basin (Oyerinde et al., 2015(Oyerinde et al., , 2017. This study aims at adding additional information to precipitation and runoff patterns in the Sota basin by evaluating shifts and merge in rainfall and runoff patterns in the Sota's vegetation zones.

Objectives
• Assessments of precipitation and runoff trends in the Sota basin • Evaluation of shift and merge of precipitation signals in the vegetations zones of the basin.

Study Area
The River Niger is third longest river in Africa after the Nile and Congo river. The river basin covers 7.5% of the African continent and it's the major water source for most country in the Sahel. The Sota is a catchment of the Niger basin located in Northern Benin (Fig 1). Sota takes source from eastern slopes of sandstone plateau Kalale which is over 400 m above sea level (Djigbo

Data Collection and Analysis
Climate data collected were daily precipitation (mm) and river discharge (m3/s) that is recorded at the Stations of the Benin Meteorological and Hydrological Services located in the Sota basin (Fig.  1). The data were collected from 1950 to 2010 in order to unveil long term fluctuations in climatic trends of the region. The data were subjected to the following analysis procedures:

Hydrological Variability
Inter annual rainfall variability was evaluated in the catchments with the Standardized precipitation index (SPI) ( 1951-1970, 1971-1990 and 1991-2010 in-order to fit available discharge data.

Rainfall Regime Shift
The sequential method (Oyerinde et al., 2015;Rodionov, 2004) was used for detecting significant regime shifts in the mean of precipitation in the catchments using cut-off length of 10 and 1 as Huber's Parameter. A regime shift occurs when a statistically significant difference exists between the mean value of the variable before and after a certain point based on the t-test. Trend lines of significant climate trends and merge of climate regimes were illustrated graphically.

Rainfall Variability
Sudan parts of the Sota basin showed gradual transition from severely wet years in the 1950s to near normal years in the 1960s after which it moved from moderately to extremely dry years from 1970-1973 (Fig 2). This dry trend continued in the northern parts up till 2007 after which we had an increase in rainfall to near normal. SPI in the Guinea region of the basin followed the same observed pattern in the north. The index moved from moderately wet years in the 1950s and 1960s then near normal dry years was observed in the early 1970s and this moved to extremely dry years in the late 1970s to early 1980s. A transient extreme wetness was observed in the beginning and the end of the 1990s; however, the same trend of normal dry years was showcased in other periods of the decade. This transient wetness finally moves into moderately dry years in 1999/2000 and continued till 2010.

Rainfall Regime Shifts
Sudan region of the Sota significantly shifted (p < 0.05) into deficit rainfall in 1959 which make it most vulnerable to drought in the Basin (Fig 3). This dry climatic regime has extended up till 2007 after which a slight upward shift (p < 0.05) was observed. In contrast, a significant dry climate regime shifts (p < 0.05) was only observed in 2007 in the Guinea region of the basin.

Discussions and Conclusions
High inter annual rainfall variability was observed in the sub basins with clear differences in observed signals in different vegetation zones. Sudan zone with higher latitudes were characterized by lower rainfall which made them to be highly susceptible to drought while lower latitudes experience higher rainfall. This follows the pattern of rainfall distribution in West Africa which is as a