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The water footprint and land footprint of water storage systems : an analysis of the evaporation losses from differently sized water storages within a semi-arid catchment area

Bos, A.S. (2018) The water footprint and land footprint of water storage systems : an analysis of the evaporation losses from differently sized water storages within a semi-arid catchment area.

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Abstract:Water scarcity is a global challenge, affecting billions of people around the world. At the global level, enough freshwater is available to meet a rising demand, however, spatial and temporal variations are large, resulting in a lack of water availability. By capturing water in times of excess and releasing water in times of deficit, implementing water storages is a promising part of the solution. On the contrary, stored water is exposed to evaporation, leading to losses of the available water resources. Therefore, more knowledge is required about the quantities of the water losses occurring with different systems of storing water. The aim of this study is to estimate the differences in blue water footprints and land footprints between water storage systems consisting of multiple decentralized small-sized water storages and centralized large-sized reservoirs used for water supply. The blue water footprint is expressed as the ratio between the evaporative water losses and the total available withdrawable water in m3/m3 over a period of time. The land footprint is expressed as the area required for the available withdrawn water to be stored in m2/m3. The evaporation calculation is based on the method of Finch (2001). The blue water footprint and land footprint calculations were performed using a storage water level fluctuation model with multiple in- and outflows from the storages within four systems. Moreover, the calculations were performed for six scenarios. Three scenarios consisting of irrigational purposed storages and three scenarios consisting of multi-purposed storages were analysed, differing in amount of precipitation during the year from a dry to a wet year. From the irrigational purposed storage systems water is only abstracted during the 100-day cropping season. For the multi-purposed storage systems water abstraction occur year-round. The total yearly water abstractions are kept equal for all scenarios. The systems are based on the Challawa reservoir (Global Water System Project, 2017), located in Nigeria and multiple small-scale water harvesting storages (Hagos, 2005; Rämi, 2003). The climatological data of Challawa reservoir from 1997 to 2016 were retrieved from the ERA-Interim database (ECMWF, 2017). System 1 has the largest inflow volume and maximum surface area, water depth and water volume per storage unit, followed by system 2, system 3 and, respectively, system 4. The total maximum volume and inflow volumes are however equal for all four systems. System 1 is assumed to have one large storage. The second system is designed to have 64 medium-large storages, the third system consists of 3,950 medium-small storages and the fourth system consists of 252,000 small-sized storages. The storage water level fluctuation was similar for all four storage systems. Even though the systems have different dimensions, resulting in lower volumes and depths, the systems followed almost the same pattern throughout the year. More decentralized systems consisting of smaller storages were more often empty within the year than centralized systems consisting of larger storages. The differently purposed scenarios showed different storage water level fluctuations throughout the year, however both scenarios were empty during part of the cropping season under normal precipitation conditions. The multi-purposed storage systems also showed empty storages just before the raining season. The dry, normal and wet year showed yearly precipitations of 268 mm, 386 mm respectively 464 mm. As a result, the storages were more often empty during the dry year than during the wet year. Under normal precipitation conditions, for irrigational purposed water storage systems 15% to 30% of the total seasonal water abstractions is lost through evaporation. For multi-purposed water storage systems 12% to 24% of the total annual water abstractions is lost through evaporation. For both the irrigational purposed and multi-purposed water storage systems, under normal precipitation conditions, 0.12 to 0.39 square meter is required to abstract one cubic meter of water. It can be concluded that the, seasonal and yearly, blue water footprint and land footprint are positively correlated with the amount of storages within a storage system. This correlation happens for three reasons. Firstly, with aggregated water capacities being equal for all four systems, the probability of water supply, and thus the amount of abstracted water, is lower for systems consisting of many smaller storages than for systems consisting of fewer large reservoirs. Additionally, this correlation becomes stronger due to the occurrence of water partly not being captured from the land by the storages. This occurs more often in systems consisting of many smaller storages. Thirdly, the systems differ in flatness of the storages. The flatter (depth / surface area) the storages are, the more evaporation relatively occurs, resulting in higher blue water footprints and land footprints for water storage systems consisting of smaller, decentralized storages than systems consisting of larger, centralized storages. The blue water footprint and land footprint of a storage system consisting of one large-sized reservoir is about twice as low as the blue water footprint and land footprint of a storage system consisting of many small-sized storages. Furthermore, it can be concluded that the be the blue water footprints are higher for irrigational purposed storages than for multi-purposed storages. Moreover, the blue water footprint and land footprint are positively correlated with yearly precipitation. These correlations occur due to differences in probability of water supply. The probability of water supply is strongly correlated with yearly precipitation.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/74921
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