A Dutch study has assessed the water requirements of 13 bioenergy crops across the world. The findings could help select the best crops and locations to produce bioenergy.

The EU climate action and renewable energy package has set a target of increasing the share of renewable energy to 20 per cent of energy used by 20201. This includes a minimum 10 per cent share for transport, which could include biofuels. This study used the concept of a water footprint in order to compare the water needs of various crops.

A water footprint is the total annual volume of fresh water used to produce goods and services at the place of production and in this case is measured in m3 of water per Gigajoule of energy produced (m3/GJ). In this study it consists of two components: rainwater used during crop growth and surface and groundwater for irrigation.

The study investigated 12 crops that contribute most to global crop cultivation, currently making up 80 per cent of total agricultural production. It also investigated the plant jatropha, used to make biodiesel in India, Indonesia, Nicaragua, Brazil and Guatemala. Water footprints were calculated for the production of various types of bioenergy, including heat and electricity, biodiesel and bioethanol. Since climate and production vary according to location, the footprints were calculated by country.

Water footprints varied greatly across countries, but generally the footprint of electricity produced by biomass is smaller than that of biofuels. Biomass contains large amounts of cellulose in stalks and leaves, which can be burnt for heat and electricity. It also contains starch, sugar and oil, which can be converted into biofuels. It is more efficient to use all of the biomass's cellulosic content to make electricity or heat than it is to convert a fraction of a crop, such as the sugar, starch or oil, to make biofuel. Next-generation biofuels currently under development seek to convert the cellulosic content into biofuels, which would increase biofuel yield per unit of crop considerably.

The most favourable crops for electricity production are sugar beet, maize and sugarcane, which have an average footprint of about 50 m3/GJ. Rapeseed and jatropha, which are typical bioenergy crops, have an average footprint of about 400 m3/GJ.

The footprint for bioethanol appears to be smaller than that of biodiesel. For ethanol, sugar beet and potato (global averages of 60 and 100 m3/GJ respectively) are the most water efficient, whilst sorghum (400 m3/GJ) is the most inefficient. For biodiesel, soybean and rapeseed have the most favourable footprints (global average of 400 m3/GJ) and jatropha has the least favourable (600 m3/GJ).

As well as varying with climate, location and crop the water footprint also varied with agricultural practice. Low yields caused high water footprints, for example, in Kazakhstan yields of barley, potato and wheat are low and this produces high water footprints.

The study makes a number of assumptions. For example, that crop water use is equal to crop water requirements. This means that when water availability is low, crop water use could be overestimated. In addition, the data do not reflect annual variations and do not include energy requirements in the agricultural system such as energy used to produce fertilisers or energy use during the industrial production of biofuels. Including the water footprint of those related energy requirements is likely to further increase the total water footprint per GJ energy. Nevertheless, with further refinement, this type of research could be used to select crops and countries that could produce bioenergy in the most water-efficient way.