SURFACE WATER DEMAND AND SUPPLY OF GABORONE CITY AND SURROUNDING AREAS: RESPONSE TO CLIMATE CHANGE AND POPULATION INCREASE Bosa Mosekiemang1, G. Mengistu Tsidu1 1Department of Earth and Environmental Sciences, Botswana International University of Science and Technology, Palapye, Botswana ABSTRACT The adequate supply of the ever-increasing demand of fresh water continues to be a challenge in parts of the globe. This challenge has been aggravated due to increasing population and climate change. The anticipation for better lifestyles and improved water supply has resulted in an increase in migration from rural settlements leading to an increase in the populations of many cities globally. This study therefore investigates the variability and trends in the surface water demand and supply of the city of Gaborone and surrounding areas in response to population growth and climate change using the Water Evaluation and Planning (WEAP) model for future scenarios. The study includes analysis of population trends, water production and consumption rates, hydrological of the study area as well as projected climate data at a high spatial resolution of 1 km2. The current General Circulation (GCM) or Regional Climate (RCM) models are not able provide such data. Therefore, the climate data for existing GCMs is statistically downscaled using the high resolution Worldclim data to spatial resolution of 1 km2 and bias corrected against Global Climatology Precipitation Center (GPCC) precipitation. The GCM data for the mid-range Concentration Representative Pathways (RCP4.5) and high emission RCP 8.5 future scenarios of Coupled Model Inter-comparison Project Phase 5 (CMIP5) are employed in the study. Under both RCP4.5 and RCP8.5 scenarios, the reservoir inflow indicates that the level of reservoirs at Foresthill, Diremogolo, Gabane hill, Oodi hill and Mabutswe will be reduced during 2081-2097 period. The unmet water demand of the whole study area will be 52.5 million m3 in 2050 as compared to 1490 million m3 in 2100 under RCP 8.5 climate and high population growth scenarios. However, the unmet demand under RCP4.5 climate and high population growth scenarios will be 51.14 million m3 in 2050 as compared to 1450 million m3 in 2100. On the other hand, the unmet water demand will be reduced by as much as 50% under both scenarios if low population growth rate of 2.2% is assumed. As an option of water management, increasing water loss reduction by 3% every year could drastically reduce the unmet water demand. Introduction and objectives The prolonged surface water stress experienced in Botswana as a whole has been one of the major problems faced by the government of Botswana. Owing to its flat topography, low rainfall amounts, high seepage generally cause by sandy soils, and high evaporation rates, Botswana has few surface water resources. This inhibits the sufficient supply of water to many of its areas. High water loss rates lead to reductions in the levels of water resources which also contribute to higher water supply deficits. As an urban area Gaborone is also prone to facing such water supply deficits due to its rapidly increasing population which eventually over flows to its neighbouring sub-cities. The general objective of this study is to investigate the surface water demand and supply prospects of the city of Gaborone and its neighbouring villages through the application of the Water Evaluation And Planning (WEAP) model using scenarios of population growth trends and climate change. Materials and Methods Methods WEAP: Water Evaluation And Planning model Experimental design Soil moisture method (5 퐿퐴퐼 푑푧푗 푍푖,푗−2 푧2 Sw = 푃 (푡)−푃퐸푇(푡) (푡)( 1 )−푃 (t)푧 2 − 푓 퐾 푍2 − (1 − 푑푡 푒 푘푐,푓 3 푒 푗 푧,푗 1 2 푓푗)푘푧,푗 푍푖,푗………………………(1) • For every reservoir, water balances are calculated as: 푡 푡−1 푡 푡 푡 푠푗 − 푆푗 = 푄푖푛.푗 − 푄표푢푡,푗 − 퐿푗 …………………………………… (2) • Water demand for each demand node, was calculated as follows: 퐷푒푚푎푛푑 퐴푛푛푢푎푙 = (푇표푡푎푙 푎푐푡푖푣푖푡푦 푙푒푣푒푙 ′ × 푊푎푡푒푟 푢푠푒 푟푎푡푒 " × …) …….......... (3) 퐵푟 퐵푟 , 퐵푟 • For every water demand there is a supply requirement 푆푢푝푝푙푦 푟푒푞푢푖푟푒푚푒푛푡푠 = (퐷푒푚푎푛푑퐷푠푚 × (1 − 푅푒푢푠푒 푟푎푡푒퐷푠푚) ×(1−퐷푆푀 푠푎푣푖푛푔푠퐷푠))/(1−퐿표푠푠 푟푎푡푒퐷푠)………………..(4) Weap then integrates the results of the two balance equations to come up with the amount of water available and supplies it to the demand sites. 퐷푒푚푎푛푑 푆푖푡푒 퐼푛푓푙표푤푠퐷푠 = 푆푟푐 푇푟푎푛푠푚푖푠푠푖표푛퐿푖푛푘 푂푢푡푓푙표푤푆푟푐,퐷푠 ……………………. (5) Water supply • through precipitation • Demand site inflow should be equal to its supply requirement unless; Water demand -there are water shortages due to hydrological, physical, contractual • natural processes (evapotranspiration or other constraints i.e.; ) if 퐷푒푚푎푛푑 푠푖푡푒 퐼푛푓푙표푤푠퐷푠 ≥ 푆푢푝푝푙푦 푟푒푞푢푖푟푒푚푒푛푡푠퐷푠 … … … … … … . (6) Adequate supply and if; 퐷푒푚푎푛푑 푠푖푡푒 퐼푛푓푙표푤푠퐷푠 < 푆푢푝푝푙푦 푟푒푞푢푖푟푒푚푒푛푡푠퐷푠 ……………..(7) Inadequate supply(unmet demand) RESULTS AND ANALYSIS Impacts of population increase on water demand Model calibration Impacts of climate change on water supply 30 600000 20 500000 1600000 10 400000 1400000 1200000 0 300000 1000000 800000 -10 200000 600000 Number peopleof 400000 -20 Number peopleof 200000 100000 (mm) anomalies Rainfall 0 -30 0 -40 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2014 year 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 Year Gabane Gaborone Gakuto Kumakwane Metsimotlhabe Mmatseta Mmopane Mogoditshane Year Gabane Gaborone Gakuto Kumakwane Metsimotlhabe Mmatseta Mmopane Mogoditshane Tlokweng Fig 4(a):RCP 4.5 anomalies from the mean (1975-2005) Fig 2(b) : Projected population growths of Gaborone and Fig 2(a) : Projected population growths of Gaborone and Fig 1 mean observed and simulated flow of Notwane surrounding areas at 2.2 % low pop growth rate (2051- Inflows to Area surrounding areas at 2.2% low pop growth rate (2014-2050) River. 2100) Scenario: RCP 4.5, All months (12) 100 Fig 1 Shows model calibration results. The WEAP model has 90 9 80 80 been tested on a monthly and yearly time step basis. The model 8 70 70 60 performance was evaluated using statistical parameters such as 7 60 6 50 50 40 the mean error (ME), mean square error (MSE) and the model 5 40 Inflow (MCM) 30 4 coefficient of efficiency (EF). Observed stream flows recorded at 20 3 30 Unmet Unmet demand (MCM) 10 the Notwane gauging station are compared to the simulated flows 20 2 0 Unmet Unmet demand (MCM) produced by the model. Results show that simulated flow at 1 10 0 0 Year Notwane gauge station with the observed flow gives an EF of 2051205320552057205920612063206520672069207120732075207720792081208320852087208920912093209520972099 Year 0.91. Result shows that the simulated flows match the observed Year Diremogolo resevoir Foresthill Resevoir Gabane Hill Resevoir Gabane Mogoditshane Gakuto Kumakwane Metsimotlhabe Mmatseta Mmopane Gaborone Tlokweng Gaborone dam Mabutswe Resevoir Mmamashia Water Works values relatively well. Gabane Mogoditshane Gakuto Kumakwane Metsimotlhabe Mmatseta Mmopane Gaborone Tlokweng Fig 3(a): projected unmet water demands at 2.2% low pop Oodi resevoir growth rate (2014-2050 Fig 3(b): projected unmet water demands at low pop growth rate 2.2% (2051-2100) Fig 4(b): Rcp 4.5 future reservoir inflows Low population growth rate : 2.2% CONCLUSION 30 Pop in 2014,2050 and 2100 = 231k ,506k and 1.5 M (for Gaborone) 20 • Increase in population leads to increased unmet water demand hence water Unmet demand in 2014,2050, and 2100 = 0.45 MCM,7.76 MCM, and 68MCM 10 • The more the population, the more the anticipation for fresh water. management strategies should be drawn and taken into consideration on 0 1600 time. 250 -10 1400 -20 • For the study area, the RCP 8.5 scenario appeared to be drier than the RCP 200 1200 (mm) anomalies Rainfall 1000 -30 4.5 scenario leading to higher unmet water demand in the RCP 8.5 climate 150 800 -40 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 scenario as to RCP 4.5 which means the government should encourage low 100 600 Year 400 50 Unmet demand (MCM) green house gas emissions. Unmet demand (MCM) 200 0 • water re-use and proper infrastructural maintenance are some of the 0 Fig 5(a): Rcp 8.5 anomalies from the mean 2014 2017 2020 2023 2026 2029 2032 2035 2038 2041 2044 2047 2050 2053 2056 2059 2062 2065 2068 2071 2074 2077 2080 2083 2086 2089 2092 2095 2098 2017 2020 2023 2026 2029 2032 2035 2038 2041 2044 2047 2050 2053 2056 2059 2062 2065 2068 2071 2074 2077 2080 2083 2086 2089 2092 2095 2098 Year 2014 (1975-2005) management options that can be used to reduce the future unmet water Year RCP 4.5 RCP 8.5 demands as well as saving it . However cost analysis should be done before RCP 4.5 High population growth rate Rcp 8.5 high population growth rate Inflows to Area implementation. Fig 6(a): unmet demands Rcp 4.5 &8.5 at Scenario: RCP 8.5, All months (12) low pop growth rate Fig 6(b): unmet demands Rcp 4.5 & 8.5 high 100 90 • popRCP4.5 growth climate rate with high population (3.4%) scenario indicates that ) 80 70 REFERENCES • Unmet demand RCP 4.5 @ Low pop growth rate the unmet demand= 51.14 MCM in 2050 as compared to 1450 MCM 60 2050:22.07 MCM 2100:194 MCM 50 in 2100 40 Inflow Inflow (MCM • Unmet demand RCP 8.5 @ Low pop growth rate • RCP8.5 climate with high population scenario unmet demand= 52.5 30 Botswana Statistics Office (2016).