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	IPAT Decomposition	Water Footprint Analysis
Ämnesområde	Environmental Sociology	Environmental Sociology
Familj	Process / pipeline	Process / pipeline
Ursprungsår	1971	2011
Upphovsperson	Paul R. Ehrlich & John P. Holdren (IPAT); Yoichi Kaya (Kaya identity)	Arjen Y. Hoekstra (with Chapagain, Aldaya & Mekonnen)
Typ	Multiplicative accounting identity and decomposition of environmental impact	Volumetric freshwater-appropriation accounting pipeline
Ursprungskälla	Ehrlich, P. R., & Holdren, J. P. (1971). Impact of Population Growth. Science, 171(3977), 1212-1217.	Hoekstra, A. Y., Chapagain, A. K., Aldaya, M. M., & Mekonnen, M. M. (2011). The Water Footprint Assessment Manual: Setting the Global Standard. Earthscan. ISBN: 9781849712798
Alias	IPAT Identity, Ehrlich-Holdren Identity, Kaya Identity Decomposition, Impact Equation	Water Footprint Assessment, Blue-Green-Grey Water Accounting, Virtual Water Analysis, Hoekstra Water Footprint
Närliggande	4	3
Sammanfattning	IPAT decomposition expresses environmental impact as the product of three factors, population, affluence, and technology, providing a simple accounting framework for attributing degradation to its proximate human drivers. The identity was crystallized in the debate between Paul Ehrlich, John Holdren, and Barry Commoner around 1971, with Ehrlich and Holdren's Science article on the impact of population growth a foundational statement. In the equation, affluence is output per person and technology is impact per unit of output, so the three factors multiply back exactly to total impact, making IPAT a definitional identity rather than an empirical claim. Its best-known specialization, the Kaya identity, decomposes carbon emissions into population, GDP per capita, energy intensity of output, and carbon intensity of energy, and underpins much emissions-scenario work. By taking growth rates, IPAT also yields a clean additive decomposition that apportions the change in impact among its drivers. Because the identity assumes each factor contributes proportionally, it was the stimulus for the stochastic STIRPAT model, in which Dietz and Rosa relaxed that assumption to test the drivers statistically.	Water footprint analysis is a volumetric accounting method that measures the appropriation of freshwater used to produce the goods and services consumed by an individual, community, business, or nation. Formalized in Arjen Hoekstra's Water Footprint Assessment Manual of 2011, it decomposes water use into three components: the green water footprint (rainwater consumed, mainly through crop evapotranspiration), the blue water footprint (surface and groundwater consumed), and the grey water footprint (the volume of freshwater needed to dilute pollution to meet ambient quality standards). By tracing water through supply chains and aggregating these components, the method reveals how much and what kind of water lies behind products and consumption — including virtual water embedded in trade — and then assesses whether that appropriation is sustainable relative to local water availability and pollution-assimilation capacity.