TRENDS AND CHANGE IN CLIMATE OVER THE VOLTA RIVER BASIN

TRENDS AND CHANGE IN CLIMATE OVER THE VOLTA RIVER BASIN VOLTRES PROJECT WORK PACKAGE 1a: CLIMATE KEY RESULTS E. Obuobie, H.E. Andersen, C. Asante-Sasu, M. Osei-owusu 11/9/217

OBJECTIVES Analyse long term historical climate trends over the Volta Basin; and Generate and analyse downscaled future climate projections over the basin for use in impact analysis

STUDY AREA Volta Basin (Lake region verses upstream catchment) VBA - Volta Basin Portal, 215

STUDY AREA 85 climate stations 49 observed stations 36 virtual stations

DATA Data type Specific data Data period Resolution Data sources Observed climate time series Modeled climate time series Rainfall, Tmax, Tmin Rainfall, Tmax, Tmin 1961-25 Daily GMET & BFMET 1961-21 Daily CORDEX 1 -AFRICA (Giorgi et al. 29) Semi-observed rainfall time series (CHIRPS 2 ) Rainfall 1981-25 Daily (.5 o x.5 o ) Climate Hazard Group (Funk, et al., 214) Re-analysis data Rainfall, Tmax, Tmin 1979-25 Daily (.5 o x.5 o ) WFDEI 3 (Weedon et al., 2114) 1. CORDEX - Coordinated Regional Downscaling Experiment 2. CHIRPS - Climate Hazards Group InfraRed Precipitation with Stations 3. WFDEI - WATCH-Forcing-Data-ERA-Interim

DATA List of RCMs and driving GCMs that provided climate projection for the study Model name Model type Modelling Centre Reference SMHI-RCA4 CLMcom- CCLM RCM RCM Swedish Meteorological and Hydrological Institute Panitz et al. (213) Rockel et al. (28) MPI-ESM-LR GCM Max Planck Institute for Meteorology Popke et al. (213) EC-EARTH GCM EC-EARTH consortium Hazeleger et al. (21) CNRM-CM5 GCM Centre National de Recherches Météorologiques/Centre Européen de Recherche et Formation Avancée en Calcul Scientifique Voldoire et al. (212) Previous studies (e.g. Panitz et al., 214; Kim et al., 213, Dosio et al., 215; Klutse et al., 214; Abiodun et al., 215) have shown that CORDEX RCMs simulate well the spatial and temporal distributions of rainfall over West Africa, though with some seasonal and sub-regional biases

DATA IPCC Scenarios 2 most recent IPCC emission scenarios: Representative Concentration Pathways (RCPs): RCP4.5, RCP8.5 RCP4.5: represents medium-low emission scenario RCP8.5: represents the high end emission scenario Name Radiative Forcing Concentration (ppm) Emissions Pathway RCP8.5 >8.5 W/m 2 in 21 >137 eq-co 2 in 21 Rising RCP4.5 ~4.5W/m 2 at the stabilization ~66 eq-co 2 at the stabilization Stabilization without level after 21 level after 21 overshoot Baseline/reference period: 1961-25 Future time horizon: 251-28

METHODS Downscaling/bias correction of modeled data Method: Quantile-Quantile transformation (Maraun et al., 21; Themeßl et al., 211; Amadou et al., 215; Sarr et al., 215) Trend analysis Magnitude and direction of trends computed with Sens slope estimator (Sen, 1968; Theil, 195) Statistical significance of trends estimated with Mann-Kendal test (Mann, 1945;Kendall, 1975) at 95% confidence level

METHODS - 2 Trend analysis in mean parameters & extreme events 2 mean climate parameters (annual rainfall, mean temperature) and 1 extreme events (5 temperature & 5 rainfall related) Index id Index name Definitions Units TN1p Cool nights Percentage of days when TN<1th percentile Days TX1p Cool days Percentage of days when TX<1th percentile Days TN9p Warm nights Percentage of days when TN>9th percentile Days TX9p Warm days Percentage of days when TX>9th percentile Days WSDI Warm spell duration indicator Annual count of days with at least 6 consecutive days when TX>9th percentile Days R2 Number of days above 2 mm Annual count of days when PRCP>=2 mm Days CWD CDD Consecutive wet days Consecutive dry days Maximum number of consecutive days with RR>=2 mm Maximum number of consecutive days with RR<1mm R99p Extremely wet days Annual total PRCP when RR>99 th percentile mm SDII Simple daily intensity index Annual total precipitation divided by the number of wet days (defined as PRCP>=1.mm) in the year Days Days mm

Rainfall (mm) Rainfall (mm) Rainfall (mm) KEY RESULTS - 2 Trends in annual rainfall over the Volta Basin Volta Basin : Rainfall Anomaly 6 4 2-2 -4 1961 1971 1981 1991 21 -------- Trend : -5.2 mm per year 4 2-2 -4-6 Volta Basin: Rainfall 1981 1991 21 4 Upper Volta : Rainfall Anomaly 2 Upper Volta: Ranifall trend 2-2 -2-4 1961 1971 1981 1991 21 -------- Trend : -3.8 mm per year Lower Volta : Rainfall Anomaly 8 6 4 2-2 -4-6 1961 1971 1981 1991 21 -------- Trend : -6.6 mm per year -4 4 2-2 -4 1981 1991 21 Lower Volta: rainfall trend 1981 1991 21

Deg. Celcius Deg. Celcius Deg. Celcius KEY RESULTS Trends in annual temperature over the Volta Basin 1. Volta Basin : Temperature Anomaly.5. -.5-1. 1961 1971 1981 1991 21 ------------ Trend : +.35 Deg. C per decade Upper Volta : Temperature Anomaly 1.5 1..5. -.5-1. 1961 1971 1981 1991 21 ------------ Trend : +.3 Deg. C per decade 1.5 1..5. -.5-1. -1.5 Lower Volta Basin: Temperature anomaly 1961 1971 1981 1991 21 ------------ Trend : +.39 Deg. C per decade

KEY RESULTS Trends in extreme temperature indices TEMPERATURE INDICES HISTORIC (1961-25) RCP4.5 (251-28) RCP8.5 (251-28) Index Slope STD of slope P-Value Slope STD of slope P-Value Slope STD of slope P-Value TX1P -.637-23.3 days/decade -.38 13.8 days/decade.7 -.876-31.9 days/decade TN1P -1.34-37.7 days/decade -.484-17.7 days/decade.17-1.157-42.2 days/decade TX9P.736 +26.9 days/decade.433 +15.8 days/decade.9 1.47 +38.2 days/decade TN9P 1.157 +42.2 days/decade.592 +21.6 days/decade.4 1.23 +43.9 days/decade WSDI.56 +2.4 days/decade 1.7 +36.7 days/decade.7 1.88 +39.7 days/decade.1 Index Positive (significant) (%) Positive (non-significant) (%) Negative (significant) (%) Negative (non-significant) (%) TX1P.. 64.17 35.83 TN1P 4.17 4.17 65. 26.66 TX9P 5.83 2.. 29.17 TN9P 7.3 25.8. 4.17 WSDI 41.11 29.72. 29.17

KEY RESULTS Trends in extreme rainfall indices RAINFALL INDICES HISTORIC (1961-25) RCP4.5 (251-28) RCP8.5 (251-28) Index Slope mean P-Value Slope STD of slope P-Value Slope STD of slope P-Value R2.52 +2 days/decade.589.11 +3.6 days/decade.516.45 +1.6 days/decade.776 CWD -.273-13.7 days/decade -.32.421-15.3 days/decade.112 -.318-11.6 days/decade.511 R99p.375 +9.9%/decade.117.398 +14.5%/decade.355.175 +6.4 %/decade.77 SDII.1 +1.3 mm/ decade.954.3 +3.9 mm/decade.53.1 +1.3 mm/ decade.798 CDD.192 +7. days/decade.376.232 +8.5 days/decade.569.287 +1.5 days/decade.636 Index Positive (significant) (%) Positive (non-significant) (%) Negative (significant) (%) Negative (non-significant) (%) R2 CWD R99p SDII CDD. 86.11 8.69 5.2. 2.17 65. 79.66 4.83 72.. 23.17. 88.83. 11.17 4.11 66.72. 29.17

Temperature ( o C) Temperature ( o C) Temperature ( o C) KEY RESULTS Projected maximum temperature over the Volta Basin from 1961-21 4 38 36 34 32 3 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 4 38 36 34 32 Upper Volta Basin (upstream of Lake Volta): Maximum Temperature ( o C) RCP4.5: +2.6 o C RCP8.5: +4.7 o C 3 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 4 38 36 34 32 RCP4.5: +2.6 o C RCP8.5: +4.7 o C Volta Basin : Maximum Temperature ( o C) RCP4.5 RCP8.5 ENS-MEAN4.5 ENS-MEAN8.5 MODEL-HISTORICAL RCP4.5 RCP8.5 ENS-MEAN4.5 ENS-MEAN8.5 MODEL-HISTORICAL Lower Volta Basin (Region of Lake Volta): Maximum Temperature ( o C) RCP4.5: +2.7 o C RCP8.5: +4.7 o C 3 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 CCLM-CNRM4.5 CCLM-CNRM8.5 ENS-MEAN4.5 ENS-MEAN8.5 MODEL-HISTORICAL ENS-M-MEAN OBSERVED 4 38 36 34 32 3 4 38 36 34 32 3 4 38 36 34 32 3

Temperature ( o C) Temperature (oc) Temperature (oc) KEY RESULTS Projected minimum temperature over the Volta Basin from 1961-21 28 27 26 25 24 23 22 21 2 RCP4.5: +2.5 o C RCP8.5: +3.5 o C Volta Basin :Minimum Temperature ( o C) 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 28 27 26 25 24 23 22 21 2 28 27 26 25 24 23 22 21 2 RCP4.5 RCP8.5 ENS-MEAN4.5 ENS-MEAN8.6 MODEL-HISTORICAL ENS-M-MEAN Upper Volta Basin (upstream of Lake Volta): Minimum Temperature ( o C) RCP4.5: +2.6 o C RCP8.5: +3.8 o C 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 RCP4.5 RCP8.5 ENS-MEAN4.5 ENS-MEAN8.6 MODEL-HISTORICAL ENS-M-MEAN 28 27 26 25 24 23 22 21 2 28 27 26 25 24 23 22 21 2 Lower Volta Basin (Region of Lake Volta): Minimum Temperature ( o C) RCP4.5: +2.4 o C RCP8.5: +3.1 o C 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 28 27 26 25 24 23 22 21 2 RCP4.5 RCP8.5 ENS-MEAN4.5 ENS-MEAN8.5 MODEL-HISTORICAL ENS-M-MEAN OBSERVED

KEY RESULTS Projected annual changes in mean temperature for 251-28, relative to 1961-25 Volta Basin Upper Volta Lower Volta Tmeam Scenario MIN MAX ENS-MEAN RCP4.5 1.4 2.5 2.1 RCP8.5 2.2 3.2 2.8 RCP4.5 1.5 2.5 2.2 RCP8.5 2.3 3.2 2.9 RCP4.5 1.4 2.4 2. RCP8.5 2.1 3.1 2.7

Deg. Celcius Deg. Celcius Deg. Celcius KEY RESULTS Projected monthly temperature over the Volta Basin 4 Volta Basin: Tmean 35 3 25 2 15 1 5 RCP4.5: 251-28 RCP4.5: 251-28 Historical: 1961-25 Observational: 1961-25 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 4 Upper Volta Basin: Tmean 4 Lower Volta Basin: Tmean 35 35 3 3 25 2 15 1 RCP4.5:251-28 RCP8.5:251-28 Observational:1961-25 Historical:1961-25 25 2 15 1 RCP4.5:251-28 RCP8.5:251-28 Observational:1961-25 Historical:1961-25 5 5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Rainfall mm) Rainfall mm) Rainfall (mm) KEY RESULTS 2 15 1 5 Projected annual rainfall over the Volta Basin for 1961-21 Volta Basin : Annual Rainfall (mm) 2 15 1 5 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 RCP8.5 RCP4.5 ENS-MEAN8.5 ENS-MEAN4.5 MODEL-HISTORICAL OBS-MEAN ENS-M-MEAN 2 15 1 5 Upper Volta Basin (upstream of Lake Volta): Annual Rainfall (mm) 2 15 1 5 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 RCP8.5 RCP4.5 ENS-MEAN8.5 ENS-MEAN4.5 MODEL-HISTORICAL ENS-M-MEAN OBS-MEAN Lower Volta Basin (Region of Lake Volta) 25 2 15 1 5 1961 1971 1981 1991 21 211 221 231 241 251 261 271 281 291 RCP8.5 RCP4.5 ENS-MEAN8.5 ENS-MEAN4.5 MODEL-HISTORICAL 25 2 15 1 5

Rainfall Amounr (mm) Rainfall Amounr (mm) KEY RESULTS Projected monthly rainfall over the Volta Basin 25 2 Upper Volta Basin (upstream of Lake Volta): Monthly Rainfall RCP4.5:251-28 RCP8.5:251-28 Observation:1961-25 Historical:1961-25 25 2 Lower Volta Basin (Region of Lake Volta): Monthly Total Rainfall RCP4.5:251-28 RCP8.5:251-28 Model-Historical:1961-25 Observation:1961-25 15 15 1 1 5 5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

KEY RESULTS Projected annual changes in annual rainfall for 251-28, relative to 1961-25 Volta Basin Upper Volta Lower Volta Rainfall (%) Scenario RANGE ENS-MEAN RCP4.5-17.7 to +11.6 +2.1 RCP8.5-23.3 to +19.5 +1.6 RCP4.5-18.2 to +8.2-1.7 RCP8.5-23.3 to +15.4 -.4 RCP4.5-17.2 to+15.4 +5.1 RCP8.5-23.4 to +22.8 +3.2 Similar findings have been obtained by Sylla et al. 216; IPCC 213; Ly et al. 213; Druyan 211; Giorgi et al. 21; Sylla et al. 215;

CONCLUSIONS The Volta River Basin has experienced a statistically significant increase of about 1.6 o C (about.35 o C/decade) in the mean temperature from 1961 to 25. The rate of warming was faster in the region of Lake Volta, compared to the upstream catchment areas. The warming is projected to exacerbate by the end of the 21 st century, with projected increases of 1.4 o C to 2.5 o C and 2.2 o C to 3.2 o C for 251-28, relative to 1961-25, for RCP4.5 and RCP8.5, respectively. The basin has experienced significant reduction (average of 5 mm per year) in annual rainfall in the 1961-25 period, the Lake region experienced a higher reduction compared to the upstream areas. In recent times, from 1981 to date, the rainfall has been on recovery even though not yet at the levels of the 196s. There is high variability and uncertainty regarding the trend of future rainfall over the basin. Model projections indicate a change of -23% to +2%, with a mean of about +2% for the two RCPs.

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