Different types of bioenergy systems will have different consequences for water and the net effects of establishing a bioenergy project depends on the local context including the previous land use. The use of organic post-consumer waste and residues and by-products from the agricultural and forest industries can mitigate land and water pressures: the water that is used to produce the food and conventional forest products is the same water as that which will also produce the organic waste, residues and by-products potentially available for bioenergy. However, residue extraction needs to consider tradeoffs with soil C management and extraction rates need to reflect what can be sustainably removed without severely impacting soils qualities such as texture and structure, which greatly influence water infiltration, permeability, and water-holding capacity (Blanco-Canqui and Lal, 2009; Ceschia et al., 2010).
Organic post-consumer waste and residues and by-products from the agricultural and forest industries presently contribute a major part of biomass for energy today. These biomass sources can give an important contribution also on the longer term, but they will not suffice to meet the anticipated levels of longer term biomass demand. To illustrate, a recent review by the Intergovernmental Panel on Climate Change (Edenhofer et al., 2011) of 164 long-term energy scenarios showed bioenergy deployment levels in year 2050 ranging from 80 to 150 EJ per year for 440–600 ppm CO2eq concentration targets and from 118 to 190 EJ per year for
less than 440 ppm CO2eq concentration targets (25th and 75th percentiles). In comparison, the
energy content of the present global industrial roundwood production is on the order 20 EJ per year and the energy content in the global harvest of major crops (cereals, oil crops, sugar crops, roots, tubers, and pulses) corresponds to roughly 60 EJ per year. Given that similar magnitudes of organic waste, residues and by-products are generated - and far from all will be available for bioenergy - it is clear that a substantial share of bioenergy feedstock supply would have to come from dedicated production if bioenergy demand grows to these levels in the future.
Beyond the energetic use of forest industry by-flows and residues from silvicultural treaments and final felling, changes in forest management and harvesting regimes may make more biomass available for the energy sector. In forests that are managed with long rotations these changes will not bring with them dramatic changes in water resource use (but might influence water quality - see further below). Water resource flows are to a larger degree influenced when expanding dedicated biomass production for energy is associated with land use change, in this context the change from a previous state (e.g., forest, grassland, agriculture land used for food and fiber production) into a new land use providing bioenergy feedstock, e.g., the
cultivation of annual and perennial plants similar to those used in agriculture today, tree plantations of the type used for pulp and paper production, or the cultivation of specific bioenergy feedstock plants such as various lignocellulosic plants grown in relatively short rotations.
As an illustration of possible magnitude implications in relation to the present land use, Figure 1 illustrates the cropland harvest increase required if a future supply of 1st generation biofuels were to grow to a level corresponding to 20% of the motor fuel consumption in 2005. Countries close to the diagonal line would roughly have to double their crop harvest in order to support such a level of biofuels use, based on domestic feedstocks, while countries far above the line would require less relative increase in harvest. Note that Figure 1 merely indicates the required effort in the agricultural sector and should be complemented with information about resources and competing demand; whether a specific country would be able to achieve the indicated increase depends on the availability of not yet utilized land and water resources, considering also the expected increase in food demand in the coming decades. In addition, technology development might bring about biofuels for transport based on lignocellulosic sources (e.g., forest wood, agricultural harvest residues and lignocellulosic crops) and biomass may also be used for heat and power production, increasing demand further.
Figure 1. An illustration of the crop harvest required for 1st generation biofuels to make a substantial contribution in the world. The y-axis shows the average 2002-2006 domestic production of food and feed crops and the x-axis shows the amount of crops needed as feedstock for the production of 1st generation biofuels corresponding to 20 % of domestic transport fuel consumption in 2005. The red diagonal represents the situation where a country would have to double the domestic crop production in order to reach the 20 % biofuels share. It is assumed that the biomass is converted into biofuels at an average efficiency of 50 % (energy basis). The inset smaller diagram is an enlargement of the lower left part of the larger diagram. Source: (Berndes 2008).
Water scarcity can be partially alleviated through on-site water management and the productivity of agriculture can be improved in large parts of the world through improved soil and water conservation. Investment in agricultural research, development and deployment could produce a further increase in both the water productivity and land use efficiency. In this
context, bioenergy demand may offer new opportunities by opening for new types of crop production that utilizes the water flows more effectively.
As an illustration of possible options and associated consequences for water, Figure 2 shows water flows on the cropland level: if the non-productive evaporation (E) is reduced in favor of plant transpiration (T), total biomass production may increase without necessarily reducing the downstream availability of water. Capture and recirculation of runoff water to the fields can also increase the share of water going to plant transpiration and hence enhance yield levels where water limits crop growth. If, however, total evapotranspiration (ET, which is the sum of E and T) increases this can have consequences for both groundwater recharge and runoff. The ET can increase both as a consequence of measures to enhance the yields of presently cultivated crops, or as a consequence of land use change (LUC) such as when high- yielding biomass plantations are established on lands with sparse vegetation, e.g., degraded pastures. Such LUC may lead to substantial reductions in downstream water availability, which may become an unwelcome effect requiring management of a trade-off between upstream benefits and downstream costs. However, it should be noted that consequences of increased ET need not always be negative. Examples of positive consequences include when biomass plantations are used for salinity management or when plantation establishment on degraded lands reduces runoff intensity and the associated risks of flooding of cultivated areas (Garg et al., 2011).
Figure 2. Overview of rainfall (R) partitioning. Runoff (Roff) and drainage (D) are lost from the field, but is potentially available for downstream use, although part of Roff is lost as evaporation as it flows through the landscape. Field evaporation (E) corresponds to a non-productive water loss, while transpiration (T) by the cultivated plants represents productive water use. The percentages shown correspond to conditions in the semi-arid tropics in Sub-Saharan Africa. Source (Rockstrom et al., 1999)
The water use efficiency varies among crop types; the efficiency of a specific crop varies with climate, growing period and agronomic practice; and there are several options for modification of the water use efficiency. New crops and biomass production systems can also give access to previously little used water flows. Thus, bioenergy demand can be met in many ways that at the same time improves the situation concerning water resource availability and use:
- hardy and drought tolerant plants traits can be cultivated in areas where water scarcity prevents cultivation of conventional food and feed crops (Street et al., 2006 ; Oliver et al. 2009; Hamanishi and Campbell, 2011);
- salt-tolerant plants that can grow in conditions of high salinity are being studied as potential bioenergy crops with the ability to use saline water not suitable for most crops (Ruan et al., 2010; Sotiroudis et al., 2010; Abideen et al., 2011; Li and Qiu, 2012);
- the use of perennial plants and various agroforestry systems for food and bioenergy feedstock production can increase the productivity in rain-fed agriculture by capturing a larger proportion of the annual rainfall in areas where much of the rainfall occurs outside the normal growing season, although productivity of individual species may decrease due to competition for nutrients, water and light (Cesson, 2008, Cardinael et al., 2012, Jose and Bardhan, 2012; Susaeta et al., 2012).
To summarize, the biomass production for energy may grow to a scale similar to the present agriculture and forestry production. The use of organic post-consumer waste and residues and by-products from the agricultural and forest industries can mitigate land and water pressures, but may not suffice to meet the future biomass demand for energy. The requirement for dedicated bioenergy feedstock production may place a new large demand on water resources. However, bioenergy demand also presents new opportunities for using previously little used water resources and improving water use efficiency. One strategy for adaptation to water scarcity can be to use biomass production for energy as a tool for increasing the spatial and temporal accessibility of water resources.