2.1 The concept of water footprint (WF)
The water footprint (WF) has been introduced by Hoekstra (2002). It is tool that assesses the total volume of fresh water by considering both direct and indirect uses to produce the goods and services, measured over the full supply chain. It shows water consumption and pollution for a specified geographical region over a particular time horizon. The WF is expressed in water volume per unit of mass (m3/ton or litre/kg), unit of time (m3/month, m3/year), or unit of energy (m3/MJ, m3/GJ) [6].
The WF consists of three components: blue, green and grey water footprint. The blue water footprint refers to surface and groundwater in a catchment area that evaporates during crop growth. The green water footprint refers to the rainwater that evaporated during crop growth. And the grey water footprint is the volume of freshwater for assimilating waste water base on ambient water quality standards [6]. The aim of this paper is to calculate WF of sugarcane and cassava for bio-ethanol production in the cultivated area of northern Thailand.