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Trend Analysis of Hydro-climatic Historical Data and Future Scenarios of Climate Extreme Indices over Mono River Basin in West Africa

H. Djan’na Koubodana*, Julien Adounkpe, Moustapha Tall, Ernest Amoussou, Kossi Atchonouglo, Muhammad Mumtaz

West Africa Science Service Centre on Climate change and Adapted Land Use, WASCAL-Climate Change and Water Resources, University of Abomey Calavi, 03 BP 526 Cotonou, Benin

Abstract: Climate change impacts considerably on water balance components and needs to be evaluated through trend analysis or climate models scenarios extremes. The objective of this paper is to perform non-parametric Mann Kendall (MK) trend analysis on historical hydro-climatic data (1961-2016), to validate an ensemble climate model and to compute temperature and rainfall extremes indices. The climate indices are evaluated using MK test and annual trend analysis for two future scenarios (2020- 2045) over Mono River Basin (MRB) in Togo. Results show positive and negative trends of hydro-climatic data over MRB from 1961 to 2016. The average temperature increases significantly in most of the stations while a negative non-significant trend of rainfall is noticed. Meanwhile, the discharge presents a significant seasonal and annual trend Corrokope, Nangbéto and Athiémé gauge stations. Validation of the ensemble climate models reveals that the model under-estimates observations at Sokode, Atkakpamé and Tabligbo stations, however linear regression and spatial correlation coefficients are higher than 0.6. Moreover, the percentage of bias between climate model and observations are less than 15% at most of the stations. Finally, the computation of extreme climate indices under RCP4.5 and RCP8.5 scenarios shows a significant annual trend of some extreme climate indices of rainfall and temperature at selected stations between 2020 and 2045 in the MRB. Therefore, relevant governmental politics are needed to elaborate strategies and measures to cope with projected climate changes impacts in the country.

Keywords: trend analysis, extremes indices, climate change, ETCCDI

Received May 01, 2020; Revised June 02, 2020; Accepted June 09, 2020

Citation: H. Djan’na Koubodana, Julien Adounkpe, Moustapha Tall, Ernest Amoussou, Kossi Atchonouglo, Muhammad Mumtaz, Trend Analysis of Hydro-climatic Historical Data and Future Scenarios of Climate Extreme Indices over Mono River Basin in West Africa. American Journal of Rural Development, 8 (1). 37-52. January 2020. doi:10.12691/ajrd-8-1-5

1. Introduction

According to the Intergovernmental Panel on Climate Change [1], climate change is happening due to natural and anthropogenic factors. The emissions of greenhouse gas (GHG) from anthropogenic activities are causing to increase temperature and affect precipitation frequencies [2,3]. Therefore, it is important to evaluate the long-term changes of hydro-climatic variables to track their impacts on natural water resources and biodiversity.

A set of historical and present state of hydro-climatology can be analyzed through climate model and Representative Concentration Pathways (RCPs) scenarios impact studies. Many previous studies already demonstrated that West Africa has suffered from extreme events [4,5,6,7,8]. For example, West Africa has experimented drought of 1970s and 1980s and recently floods events [9]. [10] highlighted a dry trend over West Africa in a long period from 1951 to 2012.

Trend analysis of climate variables is commonly assessed by hydrologists and climatologists using Global Climate Models (GCMs) and or Regional Climate Models (RCMs) from global to local scales. GCMs are more global, coupled with atmospheric and ocean model reflects the earth climate system However, local impact studies rarely used GCMs outputs without downscaling GCMs. This is due to errors from the limited spatial resolution, simplified physics, thermodynamic processes, and numerical schemes. Thus, downscaling and bias correction of GCMs from high to small grids are more representative local agriculture, biodiversity or hydrological modelling analysis. Many bias correction methods already exists such as bias correction with variability; bias correction no variability, change factor no variability, quantiles mapping and raw data [11,12,13]. A number of studies have concluded better performances of quantiles mapping bias correction for precipitation data and bias correction with variability for temperature [14,15,16,17].

Parametric or non-parametric trend analysis methods used as a statistical approach are frequently used for trend detection by fixing a certain level of confidence. Several studies have been discussed regarding trend analysis of hydro-climatic variables at over watersheds. For example, Yan et al. [3] in China found a decrease and an increase of flood precipitation and non-flood precipitation between 1969 to 2011. Gocic et al. [18] found negative and positive trends of streamflow for the last 50 years and have compared the factors due to human activities and climate change. In west and south Africa [19] analyzed the trend in daily climate extremes and have observed an evident warming over most of the region. Oguntunde et al. [5] determined a significant increase of runoff trend by analyzing the long-term trend hydro-climatologic data of Volta river basin in West Africa from 1901 to 2002. Diallo et al. [20] have analyzed rainfall inter-annual variability using several climate models over Sahel region and concluded that the models can reproduce rainfall variability with correlation exceeding 0.6 compare to observations. Amoussou et al. [21] have reviewed hydro-climatic variability and flood risk in two small forests located in the Mono River Basin (MRB). Others studies in this basin have assessed extreme climate trends of temperature and rainfall using a single model of Coordinated Regional Downscaling Experiment (CORDEX) and later with Regional MOdel (REMO) and by considering a few numbers of stations [22,23]. These studies pointed out an increase of temperature and a high variability of rainfall for historical and future baseline. According to the literature, in the MRB, none study in MRB have incorporated water balance components and as well utilization of ensemble of climate models for future scenarios climate extreme indices assessment.

The present study objectives is to analyze a set of historical hydro-climatic data over MRB in order to investigate trend in streamflow, rainfall, potential evapotranspiration and mean temperature over MRB, to validate an ensemble climate model and to compute and evaluate future scenarios climate extreme indices in the MRB. As added value, MK is applied on hydro-climtic variables and validated ensemble climate model outputs are used for future extreme climate indices evaluation in MRB.

The method adopted is Man-Kendall (MK) test trend on historical and extreme indexes computation using RclimDex package [24]. MK method was widely used and has demonstrated to be reliable for climate change detection on data time series [25,26,27,28].

Section 2 provides (i) a description of the study area, (ii) hydro-climatic data and climate model description, (iii) MK methodology description, ensemble climate model validation and future scenarios climatic extremes indices computation. Section 3 presents the results of MK trend analysis applied on hydro-climatic variables, ensemble climate model spatio-temporal validation as well of future scenarios of extreme climate indices and section 4 provides perspectives and future directives.

2. Materials and Methods

2.1. Study Region

The MRB is the second largest river in Togo bordering with the Benin Republic. The basin is located between 6.16° and 9.2 °N and 0.42° and 1.40° E (Figure 1). The whole river basin represents 38% and 2.14% area of Togo and Benin Republics respectively. At the outlet at Athiémé, the basin covers an area of 22,014 km2 with 88% of its area in Togo and 12% in Benin [29]. The MRB is 309 km long and has its source in the Alédjo mountains in the north of Benin and drains into the Atlantic Ocean vias “Boca Del Rio”. The elevation of the basin ranges from 12 to 948 meters [30]. The biggest dam on the way of this river is at Nangbéto and produces 20% of the total electricity used by Togo and Benin.

The watershed area encompasses two climate zones. In the south, from 6° to 8°N two rainy seasons and two dry seasons exist with mean rainfall between 1200 and 1500 mm/year in the mountainous area of the south-west and 800 to 1000 mm/year in the coastal zone [31].

The natural vegetation is mainly savanna and is composed of the bush and tree savanna, gallery forests and grassland [32]. The relief is generally flat except for the mountainous regions of the West and the Northwest. During the last general population census in 2011, the MRB was populated by about 5.1 Million inhabitants. The main socio-economic activities are agriculture, trade, fisheries and livestock husbandry [29,33,34]. According to Food and Agriculture Organization, the population in Togo has triplicate since 1975 and it will be constantly increasing in future (http://worldpopulationreview.com/countries/togo-population).

2.2. Data

2.2.1. Discharge

Daily discharge data are analyzed for trend analysis in this study and are collected at the General Direction of Water and Sanitation of Togo (DGEA-Togo), Central Electric of Benin (CEB) and the General Direction of Water of Benin (DGEau-Benin), abbreviations in French. The maximum of discharge usually occurs between August and September corresponding to one month after the maximum rainfall in the basin. Details regarding hydrologic data are provided in Table 1.

Figure 1. Location of the study

Table 1. Discharge data for available stations in MRB

2.2.2. Climatic Data

Historical daily rainfall data of 21 rain gauge stations are collected at General Direction of Meteorology of Togo (DGMN-Togo) and National Meteorology Agency of Benin (METEO-Benin). The minimum and maximum temperatures (1961-2016) are collected at three meteorological stations from Togo (Table 2). The period of 1961 to 2016 is chosen because of the availability of the data with fewer gaps. There are no gaps in temperature data while the daily missing values in the selected others variables are less than 10%. Rainfall missing values are filled by using Pearson correlation with a neighboring station [35]. The daily values of potential evapotranspiration (PET) collected at DGMN- Togo are available only for three stations between 1981 and 2016 (Table 2). PET is computed using Hargreaves method [36] which requires only minimum and maximum temperature [37,38]. Monthly values are computed by average of daily temperature, discharge and PET and rainfall. Finally, a year is divided in four sub periods: spring as MAM (March, April and May), summer as JJA (June, July and August), autumn as SON (September, October and November) and winter as DJF December, January and February according to the climatology in the region [39].

Table 2. Detail of climatic stations

Table 3. Global climate models used for ensemble climate model generation

2.2.3. Climate model Used

In this study, available downscaled and bias corrected historical daily temperature and rainfall (1980-2005) are used. The RCP4.5 &RCP8.5 future scenarios (2020-2045) provided by Climate Change Agriculture and Food Security (CCAFS) portal. More information about downscaling and bias correction method are described on (http://ccafs-climate.org/data_bias_correction/). The model data are from Global Climate Models (GCMs) derived from Princeton Reanalysis datasets. Princeton Reanalysis is a meteorological forcing dataset which is used to drive GCMs for land surface hydrology studies (Table 3). The dataset is constructed by combining a suite of global observation-based datasets with the National Center for Environmental Predictions/ NCEP/NCAR reanalysis.

2.3. Methods

2.3.1. Computation of Mean Hydro-climate data over the whole basin

A primary analysis is conducted using a simple average of three stations of PET and temperature in the MRB. The mean rainfall over MRB is obtained by computing Thiessens polygons [44] using 21 stations of MRB. In order to achieve accurate estimation of rainfall over the whole basin, it is necessary to use Thiessen polygons methods. Thiessen polygons method assigns weight at each gauge station in proportion to the catchment area that is closest to that gauge.

2.3.2. Test of Auto-correlation of Hydro-climatic time Series

Initially, auto-correlation test is applied on hydro-climatic time series for determining the randomness of the data. This criterion involves using parametric and non-parametric methods for trend analysis. Therefore, depending for the correlation coefficient values ranges (0 ≤ r1 ≤ 1), parametric or non-parametric MK test is used [45]. For this analysis two methods MK and Sen’s estimator were used to evaluate the variables trend for a long period.

2.3.3. Mann Kendall (MK) Trend Analysis

The method consists to compute a climatic index and apply non-parametric MK test for trend detection analysis with MAKESENS version 1.0 application developed by Finnish Meteorological Institute [46] for MK test and slope estimation. The method was applied successfully in many studies around the world [5,47,48,49]. In this study, analysis is applied on mean temperature, rainfall and PET series over the MRB.

The MK test analysis S [50,51] is computed as:

[picture] | (1)

With n the number of data point, Xi and Xj are the data values in the time series i and j (j>i), and sgn(Xj-Xi) is the sign of the following system:

[picture] | (2)

This statistic computes the number of positive differences minus the negative differences for all the differences considered. The variance is computed as:

[picture] | (3)

Where n is the number of data point available, m is the number of tried groups in the series and ti denotes the number of ties of extent i. A tried group is defined as the set of sample data that have the same value. If no ties between observations are present and no trends in the time series, the test statistic is asymptotically normal distributed with:

[picture] | (4)

For the case where the size of the sample data is bigger than 10 (n>10), the standard normal test statistic Zs is computed using the flowing equation:

[picture] | (6)

A positive value of Zs shows the increase of the trend while a negative value indicates the decrease of trends. To test the trend, α significant level can be used. When | Zs | >Z1-α/2, the null hypothesis is rejected and a significant trend exists in the time series. Z1-α/2 is computed by from the standard normal distribution table. In this study we used α =99% and α = 95%. So at 5% significance level the null hypothesis of no trend is rejected if |Zs|>1.96 and rejected if | Zs |>2.576 at 1% significance level.

2.3.4. Sen’s Slope Estimator

Sen and Niedzielski [52] has developed the non-parametric procedure in order to estimate the slop of trend in the sample of N pair of data.

[picture] | (6)

Where Xj and Xk are the data value at tile j and k (j>k), respectively.

If there is only one datum in each, time period, the

[picture] | (7)

where n is the number of periods. If there are multiple observations, the N values of Q are ranked from smallest to largest and the median of slop or Sen’s estimator is computed as

[picture] | (8)

The Qmed sign reflects data trend reflection, while its value indicates the steepness of the trend. To determine whether the median slope is statistically deferent than zero, one should obtain the confidence interval of Qmed at specific probability.

The confidence interval about the tile slope [53] can be defined by:

[picture] | (9)

Where Var(S) is defined in equation (3) and Z1-α/2 is obtained from the standard normal distribution table. In this study, the confidence interval is computed at two significance level (α =99% and α =95%).

Then, and are computed. The lower and upper limits of the confidence interval, Qmin and Amax are the M₁th largest and the (M₂ +1)th largest of the N ordered slope estimates.

The slope Qmed is statistically different than zero if the two limits (Qmin and Amax) have similar sign. Sen’s slope estimator has been widely in hydro-climatic time series [5,27,37].

2.3.5. Ensemble Climate Model Validation

Validation gives the fitness between model outputs and observation data and indicates the confidence of future scenarios results. A very good spatial and temporal representation of the ensemble means climate model and observation data involves that data from the model can be used for future scenarios impacts studies in this region [54,55]. In the study downscaled and bias corrected ensembles mean GCM is validated with observation data get from the meteorological services of Togo and Benin Republics. Monthly and annual time series of observation are plotted against ensemble climate model value and by determining the coefficient of determination. Additionally percent of bias computed as the ratio of difference between ensemble model and observations over observation was performed during the same period to determine if the change is sensitive or not as well monthly and annual time steps [17]. A value of bias values lower than ±10% indicated a very good model, between ±10% and ±15% indicates a good model, between ±15% and ±25% reveals satisfactory model whereas value greater than ± 25% displays an unsatisfactory model prediction [56].

The period of validation (1980-2005) is chosen where model outputs and observation data are both available, without gaps and less large events [11]. The linear regression coefficient of observations against model outputs was computed at each station of mean temperature and rainfall data.

The spatial distribution of rainfall ensemble climate model data against observation data were mapped for two periods JJA and DJF. Kriging interpolation method developed in ArcGIS 10.5 is used for rainfall spatial distribution [57,58,59]. Kriging is a linear interpolation method which allows estimating areal values as a weighted mean of observations [60]. In opposite to precipitation, temperature observed stations number cannot be used for spatial validation.

2.3.6. Data Quality Control and Future Scenarios Climate Extreme Indices Computation

The important key for indices concept is the calculation of the temperature and rainfall indices with a fixed threshold. The indices calculation aim is to monitor climate change and detections. Initially 27 cores indices are recommended by CCI/CLIVAR Expert Team for Climate Change Detection Monitoring and Indices (ETCCDI). RClimDex allows a rapid analysis of data quality before indices calculation. Data quality corrects all the unreasonable values, missing value, negatives precipitation and temperature and identifying outliers of daily values outside of user region. More informations about the indices and its computation developed by Expert Team on Climate Risk and Sector-specific Climate Indices can be found on http://www.wmo.int/pages/prog/wcp/ccl/opace/opace4/expertteam.php.

In this study, we computed extreme indices using the packages in R environment (version 1.1.463) developed by [24]. A total number of sixteen (16) indices of temperature and rainfall computed were evaluated by non parametric MK test and annual trend analysis for two climate scenarios RCP4.5 & RCP8.5. Only three representatives’climate stations of MRB (Tabligbo, Atakpamé and Sokodé) were selected for extreme indices analysis. Table 4 shows the selected indices and definition. RClimDex is available through http://cccma.seos.uvic.ca/ETCCDI/. The trend of rainfall or temperature is considered significant if the estimated p ≤ 95%.

Table 4. List of ETCCDI indices used to analyze climate extremes

Table 5. Results of statistical tests for seasonal and annual

3. Results

3.1. Annual and Seasonal Trend of Hydro-climatic Time Series over MRB

Results of MK statistical test on rainfall, mean temperature, PET, and discharge over the whole MRB are shown in Table 5. On annual time scale, Athiémé, Corrokopé and Nangbéto discharge stations indicate none significant negative and positive trend respectively. The result of average values of rainfall and temperature over the basin indicates none significant negative trend of rainfall. However, none of positive trend of inflow and outflow at Nangbéto is detected whereas mean temperature reveals a significant trend at 95% and 99% significance levels.

At seasonal time scale (MAM, JJA, SON and DJF), there are positive of negative trends in most hydro-climatic variables and particular there increase at 95% and 99% significant level of is temperature. A positive significant at α = 99% trend is detected in outflow at Nangbéto are for DJF season whereas for the four seasons there is negative trend of rainfall.

During MAM period, negative and none significant trends on PET is observed. In opposite, Athiéme and Corrokope discharge stations, present a positive and none significant trends. A similar situation is observed during JJA and SON seasons. Indeed, discharge at Athiéme decreases at 95% significant levels during JJA and SON and increases during DJF. Afterward, over JJA period, it appears a positive trend of PET and negative trend of discharge at Athiémé and Corrokopé. Further, none significant negative trend of PET and discharge (Athiémé and Corrokopé) is computed.

Appendix shows the annual times series of precipitation, temperature, discharge and PET over the whole MRB. There is a positive annual trend of PET and mean temperature from 1961 to 2015 while negative trend of cumulative discharge at Athiémé (1961-2011) and of annual rainfall (1961-2015) with a breakpoint at the year of 1987 (see figure in Appendix).

3.2. Annual and Seasonal Trend at Individual Stations of Climatic Variables

3.2.1. Mean Temperature (TMP) and Potential Evapotranspiration (PET)

MK statistic trend applied for the mean temperature at individual station in the south, center and north of the basin (Tabligbo, Atakpamé and Sokodé) over the period of 1961-2016 is summarized in Table 6. Consequently, at Tabligbo, Atakpamé and Sokodé stations, positive significant trend at 95% and 99% significant levelsof mean temperature at seasonal and annual time scaleperiod is observed.

Table 6. Results of statistical tests for seasonal and annual

PET analysis results at individual station have almost the same trend with the result of mean PET over the whole basin. However, during MAM a negative (Kara) and positive (Sokode and Tabligbo) trends are discovered but not significant. During JJA season at Kara station, the trend is negative and positive Tabligbo and Sokodé stations. During DJF the trends are positives in the three stations at 95% or/and 99% significant level. Finally at annual scale, the trend is positive with no significance except at Tabligbo where the significant trend is at 95% level.

3.2.2. Rainfall Stations over Mono river Basin Trend Analysis Result

The results of rainfall stations trend analysis over MRB are displayed in Table 7.

The seasonal and annual MK test analysis was performed at each rainfall gauge stations used to compute the mean rainfall over the entire basin and results are shown in Table 7. The analysis reveals that, 6 out of 21 stations present positive or negative significant trend. The rest of station show negative trend in MAM, 12 out of 21 stations in JJA positive and 9 negative trends, at SON there are 10 stations with positive trend and 11 with negative trend while during the period of DJF only 7 stations have positive trend, 13 negative trend and one displays no trend over MRB. According to Table 7 results, positive or negative and not significant trends are displayed for annual rainfall in each station. On seasonal scale, there are positive trend at Sotouboua and negative trend at Blitta at 95% significance level in MAM. Positive and negative trend are also detected respectively at Akaba and Yegue stations in JJA at 95% significant level. During SON, Notsè is the only station where there is positive trend at 95% significant level while in the others stations the trend is not significant. In DJF, a decreasing trend at 95% significant level is shown at Bassila, Blitta and Yegue stations.

3.2.3. Spatial Distribution at Seasonal and Annual Time Scale of Rainfall

The spatial distribution of rainfall trends from 21 stations over the MRB in Figure 2 shows positive, no and negative trend during the seasonal and annual period between 1961 -2016. At annual period, nine of twenty-one station show an increase trend of rainfall and the rest ones a decrease. All the stations with increase rainfall are in the West from north to south. These areas are in the high slope of mountain except Tchetti station in the east. The similar situations are observed during JJA and SON with some exceptions. During MAM and DJF seasons 6 stations are with positive trends of rainfall. In Figure 2, during MAM all the hydrological stations show an increase of discharge while in JJA and SON there are two over three stations of decrease trend. Only at Nangbéto the discharge is increasing because it is the rainy season. During the period of DJF and at annual scale the upstream stations of Corrokopé indicates a decrease of discharge while at Nangbéto there is, increase during DJF at the outlet and decrease at annual time scale.

The spatial trend distribution of rainfall, discharge and temperature are showed in Figure 2.

Table 7. Annual and seasonal trend statistic of rainfall selected stations

Figure 2. The spatial distribution of rainfall (top), PET (middle) and discharge (bottom)

At the downstream, the PET is increasing during at seasonal and annual periods. In the upstream of the basin the station outside of the river shows a decrease trend during MAM, JJA and SON whereas an increase during DJF and annual time scale. The stations being outside PET is link with the river hydrology because of land -atmosphere exchange. Finally, the second stations at the upstream show a positive trend for MAM, DJF and annual while no trend is detected during JJA and negative trend in SON.

3.3. Ensemble Climate Model Temporal Validation of Mean Temperature (1980-2005)

Table 8 shows the coefficient of determination and bias between ensemble climate model and observations time series at three major stations of mean temperature. There is an excellent coefficient of determination (R²> 0.95) between model and observation at each station. There are excellent biases less than ± 10% in the three stations at monthly and annually scale even equal to zero in November at Tabligbo station .The ensemble climate model slightly underestimates the observations with a small positive deviation as given in Figure 3 and confirmed the low bias values. There is good Pearson coefficient at Tabligbo (R²=0.96), Atakpamé (R²= 0.98) and Sokodé (R2= 0.97) coefficient of regression between the model outputs and observations. At seasonal variability between 1980 and 2005, the seasonality is well represented in the stations. As conclusion; the climate model outputs matches with observations data in the MRB. The bias or error between model and observations are too low and range between -0.15% and 6% at Sokodé, Atakpamé and Tabligbo stations at annual scale between 1980 and 2005.

Table 8. Pearson coefficient, bias values for mean temperature (1980-2005)

3.4. Multi Ensemble Model Temporal and Spatial Validation of Rainfall (1980-2005)

3.4.1. Time series Comparison

The validation of rainfall over MRB was performed using 21 gauge stations. Figure 4 and Table 8 show the bias between the ensemble climate model outputs and observations. The coefficient of determination between model and observation is displayed in Table 9 and reveals a very good Pearson correlation between the two datasets. The bias computed as the ratio over observations of the difference of model value to observation. There is high bias at Tchamba and Adjarala stations in July, August and September. The bias in the others stations are quite acceptable. In detail Table 9 show the monthly bias per station. The model underestimates the observation globally in these stations (Figure 3 & Figure 4). From the Table 9 there are some stations with high bias (bias value ≥ ± 25%). Particulary, highest bias is observed at Sotouboua, Yegue, Tchetti, Anié, Agouna, Lonkly and Afanyangan.

Figure 3. Observed and model mean temperature time series comparison

Figure 4. Observed and ensemble model precipitation time series comparison

Table 9. Bias [%] between climate model and observations rainfall

The very highest bias is observed at Yegue in November (71.8%). For the other station value never exceed ±25%. However the months of March, June and July bias are all lower than ± 25%. Globally at annually the bias are acceptable for the 21 stations. The highest bias obtained can be due to downscaling methods.

Figure 5. Spatial distribution of rainfall (mm/month) of ensemble climate model and observations

3.4.2. Spatial Representation of Climate Model and Obseravtionsrainfall

In order to determine the difference between ensemble climate model and observation, spatial interpolation representation of rainfall in DJF and JJA are computed in Figure 5. DJF months represents the dry seasons and the rainy season in JJA when the rainfall is at the maximum. In DJF, the model and observation show high values of rainfall are in the southwest of the basin (16.8 mm/month and 16.6 mm/month respectively). The low values of rainfall are in the north such as in the model (7 mm/month) and in observations (10.3 mm/month). However model shows in the south east a low value of rainfall. As the model, observations mean values of rainfall are seen in the center of the MRB.

The maximum values of rainfall in the model (196.7mm/month) and observations (215.98mm/month) during JJA period are located in the Northwest of MRB. The low values of rainfall during this period are situated in the south of the basin for the two datasets.

The spatial coefficient of correlation between observations and ensemble climate model are 0.62 and 0.82 respectively in DJF and JJA which are acceptable. However the extremes value from the model and observation are closer in DJF and JJA. The rainfall variability (seasonality) such as in the model and observations is accurate. It is exactly predicted by the model in the south (Tabligbo) two zones of maximum rainfall in June and October and in the center and north (Atakpamé and Sokodé) one peak of maximum rainfall in August reflecting the two different climate zone cover by MRB as displayed the time series in section 3.4.1.

3.5. Future Scenarios Temperature and Precipitations Climate Extreme Indices

The trend analysis results for RCP4.5 and RCP8.5 in three major climatic stations of MRB are summarized in Table 10. The values in bold indicate the significant trend at 95% confidence level. The results show at annual scale that the significant negative, positive or none trend will be observed at some station from 2020 to 2045 and depending of the scenarios and the selected climate extremes indices. The monthly maximum value of daily temperature indices (TNn), and rainfall indices such as simple daily index (SDII) number of heavy precipitation days (R10), number of days above 25mm of precipitation (R25) and consecutive dry days (CDD) don’t present significant trend in the three stations. In opposites these indices present positive or negative slope reflecting increase or decrease of annual rainfall in RCP4.5 or RCP8.5.Rainfall intensity indices like annual precipitation amount (PRCPTOT) reveals no significant increase (decrease) trend at Sokodé of 6.014mm/year (5.269mm/year) of RCP4.5 (RCP8.5) respectively, positive trend of 5.635mm/year of RCP4.5 and 6.159mm/year of RCP8.5 at Atakpamé while 5.469 mm/year of RCP4.5 and 5.975mm/year of RCP8.5 at Tabligbo. These results show an increase of rainfall at two selected stations of MRB and where p_value < 0.05.

Table 10. Annual extremes indices of temperature and precipitation for scenarios (2020 to 2045)

The indices (R99p) increase significantly at Tabligbo for RCP8.5 whereas not significant at Atakpamé and Sokodé stations. The consecutive wet days (CWD) for RCP8.5 are significantly increasing at Sokodé and Atakpamé whereas there is none significant trend at Tabligbo. The maximum 5-day precipitation amount (RX5day) is only significant for RCP4.5 at Sokodé with a slope of 3.18mm.

The monthly maximum of daily maximum temperature (TXx), the monthly minimum of maximum temperature (TXn) and monthly maximum value of daily minimum temperature (TNx) for RCP8.5 show a significant trend in Sokodé, Atakpamé and Tabligbo representative stations respectively of -0.03 °C, 0.012°C and 0.015°C at Sokodé while 0.061°C, 0.016°C, 0.06°C at Atakpamé and 0.039°C; 0.009°C and 0.011°C at Tabligbo.

The diurnal temperature (DTR) indices are only significant at Sokodé for RCP4.5 with positive slope showing the increase of monthly mean difference TX and TN. Temperature indices warm days (TX90p) show positive and negative trend at 95% level of confidence for the Scenarios RCP8.5 in the north of the basin at Sokodé station, positive significant trend of RCP4.5 and RCP8.5 at Atakpamé station while at Tabligbo station in the south the significant trend is negative. This reveals that cool days and warm day annually trend depend from station and of RCP scenarios in MRB from 2020 to 2045. Nevertheless,warm nights (TN90p) are all showing significant trend at Tabligbo, Atakpamé and Sokodé at 95% significant levels and for the two considered RCP4.5 and RCP8.5. A particularly TN10p slope is negative while positive for TN90p. The warm spell duration indicator (WSDI) shows only a significant RCP8.5 negative trend at Tabligbo.

4. Discussion

The historical mean temperature data trend analysis shows a significant upward of mean temperature over the whole basin and at individual temperature station which is in concordance with most of the previous studies investigated in this region [1]. Similar finding have reported a low and high increase of temperature over MRB from 1961-2010 [22,23]. A highest increase of temperature of 3°C around 14°N in May -June and low increase of 0.5°C below 8°N were also observed over West Africa [61,62,63]. Over the basin, negative significant trend of precipitation at annual and seasonal time scale whereas positive and negative significant trend are observed in PET at annual and seasonal may be due to temperature increasing. During the last forty years, West African countries have been experimented very driest periods with drought and extremes [4,5,37,64].

The ensemble Climate model underestimates observations over the period of validation (1980- 2005) with very excellent coefficient of determination higher than 0.9 in most of selected climate stations over the basin. Therefore, uncertainties associated with model are minimized with rainfall percent bias less that ± 25%. The spatial coefficient of correlation between observations and model outputs is over 0.6 during the extremes values of rainfall (1980-2005). The ensemble climate model underestimation can be explained by weakness of primitives equation used during climate model generations. Indeed, climate models are not able to reproduce all the process of the earth and computation uncertainties increase errors in models [65,66].

The finding of increase or decrease of PRCPTOT, SDII, CDD and R25 rainfall indices at the selected station or TXn, TXx, DTR and WSDI temperature indices confirmed most of the researches on climate trend analysis and extremes indices experimented in West Africa last year. Past studies show the increase of extremes events like flooding and drought. West African countries are seen warm extremes temperature in the last year (1961-2000) with significant or not decreasing trend of rainfall [67]. For example [68] used RCMs CORDEX to compute CDD, CWD, R10mm, R20mm, PRCPTOT, R95pTOT, RX5day, and SDII extreme rainfall indices under middle and high Representative Concentration Pathway (RCPs) scenarios RCP4.5 and RCP8.5 over West Africa. Their results show a significant decrease of total rainfall, increase of consecutive dry and extreme events for the future period of 2070-2099 over West Africa. In Benin Yabi and Afouda, [6] found an increase of rainfall and drought between 1922 and 2005 with a very dry period noticed between 1970s and 1980s. The study results therefore confirmed the rainfall variability and positive trend of mean temperature over MRB. In previous analysis using REMO model for historical period of 1980-2010 and RCP4.5 and RCP8.5 future emission scenarios (2018-2050), Lawin et al. [22] have proved over the entire watershed increase of rainfall and temperature for historical data and high variability of rainfall during the future scenarios. The same author preformed analysis using different regional climate model during the baseline of 1961-2010 and with high emission scenarios of RCP8.5 from 2051-2100 of temperature, and deduced high extremes trend of temperature between 1961-2010 and an increase of temperature during future change at almost in more station of the river with increase of extremes indices of TX90p, TX10p, TN90p and TN10p. Therefore, for any analysis performed a local scale, the result will be more applicable for decision makers and for local communities’ information. In the case of MRB population growth have an essential impact on land use and land cover changes [32]. The same authors have showed that MRB is characterized by deforestation and savanna decrease and will continue to decrease in the 30 next years if nothing is done and this have effects on climate variables.

The results displayed over the basin a considerable change during the rainy period. Thus in JJA and SON there is an increase of temperature, decrease of rainfall and increase (JJA), decrease (SON) of PET which have impacts on discharge at Corrokopé and Athiémé upstream and downstream respectively gauge stations.

5. Conclusion

This study has contributed to understand the seasonal and annual hydro-climatic variables trend using Mann Kendall trend analysis over the whole MRB and also at individual station for historical period (1961-2016). Afterward, an ensemble Climate model validated with observations was used for 16 ETCCDI climate extreme indices annual trend analysis for RCP4.5 and RCP8.5 in near future (2020-2045). Results can be highly summarized as follows:

• A positive seasonal and annual trend of historical averaged temperature and potential evapotranspiration is observed over the MRB and at selected gauge stations at 99% or 95% significant level

• Positive and negative trends in discharge and rainfall over the entire basin and at selected gauge stations at 99% or 95% significant level.

• During validation period (1980-2005) ensemble climate model underestimates the observations globally but with spatial and temporal coefficient of correlation between observations and ensemble model higher than 0.60

• Mann Kendall test detected significant annual trend at 95% levels of PRCPTOT, SDII, CDD, R25 rainfall indices and TXn, TXx, DTR and WSDI temperature indices of RCP4.5 and RCP8.5 future scenarios at the mains station between 2020 and 2045

By incorporating and utilizing the finding of these studies can bring positive results in implementation of water management policies. Moreover, results of the study can be used for future analysis on modeling the impacts of climate change on streamflow in MRB in order to provide a decision support package for water resource management in the basin in West Africa region.

Funding

Authors would like to thank German Ministry of Education and Research (BMBF) for their financial support of the Graduated Research Programme of Climate Change and Water Resources at the University Of Abomey Calavi (Benin) through the West African Science Service Center on Climate Change and Adapted Land use (WASCAL).

Acknowledgments

Authors acknowledge Togolese and Beninese meteorological and hydrological services for providing historical climatic and discharge data for the analysis. A special thank to Climate Change Agriculture and Food Security (CCAFS) portal (http://ccafs-climate.org/data_bias_correction/) of International Centre for Tropical Agriculture (CIAT) for makingavailable Global Climatic Model downscaled and bias corrected datasets that were used for our analysis. Many thank to Dr Kossi Komi for proofreading this manuscript.

Conﬂicts of Interest

The authors declare no conﬂict of interest.

References

Appendix

Annual time series in the whole MRB

Figure

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