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Moving Beyond Mean Annual River Runoff (marr): Redefining Water Resources Assessments Through Stores

Congress: 2015
Author(s): Simon Damkjaer (London, UK)

Keyword(s): Sub-theme 8: Revisiting water paradigms,
AbstractWater resources, fundamentally vital for human and environmental existence, are subject to pressures such as increasing demands, population rise and the uncertain effects of climatic changes. The impacts of these stresses on global water resources are popularly assessed through Global Water Balance Models (GWBMs), such as WaterGap and PWR-GLOBWB. Notwithstanding these model's challenges related to temporal-spatial scales and issues of integrating routing, their underlying hydrological parameters are hydrollogically lacking. The reliance on Mean Annual River Runoff (MARR) to derive water availability focuses solely on hydrological fluxes, assuming global hydrological stationarity and thus neglect important stores such as groundwater, dams, soil moisture and water quality status in addition to environmental flow requirements and society's adaptive ability to import virtual water. Assessments that rely on MARR further undertake such studies at the inter-annual scale, neglecting the seasonal intra-annual variability of precipitation in (semi)-arid and humid areas. A comprehensive review presented here starts off by revealing that both GWBMs and the hydrological components of even the most ambitiously holistic metrics that address water scarcity and sustainability have for more than three decades consistently applied one or both of two measures used to estimate degrees of stresses on available water resources: the Falkenmark Indicator and the water Withdrawal to Availability (wta) ratio. Whereas the former assumes fixed per capita water requirements to availability, the latter factors in societies' differing demands. However, both have in common that they derive available water resources from MARR and have universally and interchangeable defined intervals and thresholds for depicting when an area faces water stressed or scarce conditions. Although some encouraging attempts have been made to address the problems associated with the reliance on MARR in water resources assessments, it is acknowledged that more local scale work is required. The recurring application of measures which do not consider water availability across the full hydrological cycle for over thirty years begs to question the role of indicators. A critical systematic review of the literature on water indicators identify that in addition to being tools of communication and politics, indicators are also social constructs, which within the hydrological domain have managed to "naturalise" a state of global water scarcity, through the use of flawed hydrological metrics that automatically show a worsening situation. The consequences of this risks giving uncontested legitimisation to economic approaches and interventions to water resources management given that the current metrics applied automatically show an increasing and impending global water crisis which some argue can only be avoided if financial approaches to water are taken. This paper aims to encourage the water resources assessment discourse to move away from defining water scarcity as an imbalance between demand and flux-derived annual supply, and explore the idea that water scarce conditions occur only when stores (surface and ground) are unable to meet the proportion of demand that is in excess of those flow-derived supplies, factoring in issues such as environmental flow requirements and change the time-scale of analysis to an intra-annual scale. In all other natural resource management approaches, the contribution of stocks to supply have been very well considered. Given that the water-land-food-energy nexus idea has stemmed out of the water community, it remains surprising to see this continuous lack of regard of water store contributions in the field of hydrology. As groundwater resources are becoming increasingly frequent relied-upon sources of water their contributions must be addressed. This becomes even more important in light of recent anticipated discoveries of more than hundred times groundwater stores than that derived from surface water in Sub-Saharan Africa. As such, this on-going work rigorously interrogates the assumptions that global water resources assessments (GWBMs and indicators) make, through a comparison of global and local scale data using the Great Ruaha Basin in Tanzania as a case-study. Investigations into the perceptions of water scarcity at the local scale compared to that portrayed at the global scale are explored as are the contributions of stores to water use during the dry-season in terms of water uses. 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