Application of WEAP model for assessment of current and future water resources availability and demands of Mukungwa catchment, Rwanda

Abstract

Water managers and policy makers require tools in order to achieve a balance in water supply and demand, to ensure equitable allocation of water resources, protect the environment, promote efficient use of water and develop priorities in shared water resources. The Water Evaluation and Planning System (WEAP) Model is one of such tools used for many river basins when dealing with the stochastic nature of stream flow and water use variables. The overall objective of this thesis is to contribute to sustainable water resources management in Rwanda through assessment of current and future water resources availability & demands and developing water allocation plan for Mukungwa catchment using WEAP model as a Decision Support System (DSS). The WEAP hydrologic modelling of Soil Moisture Method (SMM) was used to allow the most comprehensive analysis of characterization for land use and soil type. For allowing simulation of water balance and water allocation, the elements that comprise the water demand-supply system and their spatial relationships were characterized for the Mukungwa catchment. The data used in this study were obtained through site visits and from different sources, including Government Agencies, private companies, NGO’s and different websites for published papers and books. In order to address a broad range of "what if" questions, three projection scenarios (2030 and 2050) were developed and their possible impacts on the catchment water balance evaluated.  Scenario 1 considers an increase in population growth rate (increased population generates increased water demand);  Scenario 2 considers an increase in economic development (sectoral water demands such as increase in irrigated areas, in industry and manufacturing, in mining, etc.);  Scenario 3 deals with the implementation of an environmental flow requirement (ecosystems and downstream water demand). The economy of the Mukungwa catchment relies strongly on rain-fed agriculture, both for rural livelihoods and exports. Horticulture is common in the catchment as the fertile soils in the area lead to high production levels of e.g. vegetables and Irish potato. Mining, both commercial and artisanal, is common and ‘sand mining’ in riverbeds is also an important economic activity. Volcanoes National Park is the main source of foreign currency from tourism activities, commercial activities, and fisheries development in Burera and Ruhondo Lakes. There is a large number of Hydropower Plants, supplying electricity to the whole country. A very small percentage, urban dwellers only, have house water connections; the remainder only have access to an unprotected spring or a river as their only (unimproved) source of water supply. The results show that the total water availability in the catchment is estimated at 883.4 MCM annually, unevenly distributed in months. This includes surface runoff from the catchment area, base flow, and return flows. The minimum river flow occurs in the month of August and peak flows occur in the month of April. The results of water demand during baseline year of 2020 show that the total annual water demand is about 48 Mm3, where domestic is the largest water use sector, which comprises of v 66% of total water demands equivalent to 31.7 Mm3, followed by irrigation with 22% of total water demands equivalent to 10.4 Mm3. The projected water demands are mainly influenced by population growth and socio-economic development in the catchment including irrigation sector. In 2030 projection for all scenarios including EFR, the annual water demands is expected to be 346.3Mm3, while in 2050, water demands are expected to be 366.8Mm3. The trend of consumptive water demand from 2020 up to 2050 shows that it is expected to increase at almost double in 2050. The WEAP Modelling results on water availability confirm the statement of the National Water Resources Master Plan, saying, “Mukungwa catchment is water tower of Rwanda”. However, it is recommended that water allocation should be applied based on water resources availability, by carefully considering the location point of abstractions vis a vis the location of existing hydropower plant, to avoid potential conflict. The development of artificial water storages is recommended for balancing water availability and demand across seasons