All vegetation will transpire water during growth and evaporation is relentless where water is freely available. Evapotranspiration (Figures 2 & 3) accounts for ≈89% of rainfall in Australia with the remaining water either running off the soil surface (9%) or percolating into the soil below the root zone of plants as groundwater recharge (2%) (Prosser, 2011). Agriculture, including rangelands, uses the majority of available water.
Water is one of the most significant limiting parameters in most Australian agricultural enterprises. This limitation will be a challenge for the utilisation of biomass for energy purposes as feedstock supply will be variable (especially due to the climate) and potentially compete for land and water resources for food production.
Competition for water will increase if woody species, with deep roots, are planted as they utilise more than annual crops (White et al., 2002). A general conclusion that ‘trees use more water than pasture and annual crops’ is widely accepted (e.g., Zhang et al., (2001)). The increased capacity to intercept water below the annual crop root zone is important to promote tree survival and increase biomass productivity and potentially enhance environmental benefits (Robinson et al., 2006). But this increased water use increases competition with other agricultural crops (Sudmeyer et al., 2012).
The impact of vegetation on local and catchment scale hydrology though fundamentally understood is difficult to accurately estimate and predict. In Australia there is a need for better understanding of soil water movement at the farm level and the implications for larger-scale catchment implications (Barrett-Lennard, 2002; Herron et al., 2003; Vertessy et al., 2003). Changes in cropping practice leading to increased or decreased water movement on a small scale (e.g., in a single paddock) can have consequences that only become evident years, decades or even centuries later; often at a considerable distance of kilometres or even hundreds of kilometres from the area of land use change.
Though hydrological impacts can be significant at local and larger scale catchments (Herron et al., 2003; Vertessy et al., 2003), recent studies in Western Australia indicate that a large proportion of catchments would need to be planted to mallees to significantly reduce salinity. Bennett et al. (2011) found at sites located in 450 mm rainfall zones that “a belt canopy area of 3–10 per cent of the landscape accounted for up to a 30 per cent net decrease in recharge to groundwater systems”. However, this had “no discernable effect on catchment-scale groundwater levels”. They concluded that two-row mallee systems would have a strong competitive effect with crops whilst not significantly ameliorating secondary salinity. That is, unless significant portions of the catchment are planted, the introduction of woody crops would not have a significant effect on the catchment hydrology.
Comprehensively developing and measuring spatially and temporally sensitive metrics capable of reporting the quantity of water utilised for production whilst accounting for environmental needs is a significant task.
In growing biomass and converting to a useable energy (carrier) we can differentiate between consumptive and non-consumptive use of water and the partitioning between ‘grey’, ‘blue’ and ‘green’ components as outlined by the Water Footprint Network (WFN). We can then estimate the quantities used for various production and environmental requirements. This is based on the concept developed earlier by Hoekstra and Hung (2002) and developed by others.
Whilst an onerous task, quantification of the amount of water, availability and use is but an initial step; the impact of water use for bioenergy production is required to allow value judgements about the appropriate use of this scarce resource. Several approaches try to quantify the impact of water use including: water footprints (Hoekstra and Chapagain, 2007); water stress indicators (and indices) (Rijsberman, 2006); and water scarcity (Pfister et al., 2011). A challenge for these indicators is to integrate temporal and spatial information that is timely and at a scale where analysis will match the capacity to adapt to or mitigate water- related issues.
We can readily agree with the logic described by Young & McColl (2009) where they suggest “that if entitlement and allocation regime are set up in ways that have hydrological integrity, the result should be a regime that can autonomously adjust to climatic shifts, changes in prices and changes in technology without compromising environmental objectives”. Critical to their logic is the interpretation of the ‘hydrological integrity’; this is often either poorly or partially considered, or overlooked7.
7 There is significant information in the existing literature regarding hydrology and hydrological implications of
vegetation across the landscape. This topic, whilst fundamental to this discussion, is not detailed in this paper. Instead we focus on the hydrological implications of bioenergy systems and interpretation of impacts.
Case Study - the Murray-Darling Basin
The Murray-Darling Basin (MDB) in the south eastern Australian mainland is an area (1 060 000 km2) covering some 14% of Australia receiving on average 530 600 GL of water as rainfall with 94% of this transpiring or evaporating, 2% entering the groundwater and 4% becoming run-off (Pink, 2008). The MDB encompases five jurisdictions including the States of Queensland, New South Wales, Victoria and South Australia; and the federal jurisdiction of the Australian Capital Territory.
Agriculture (66.7%) and native forestry (31.9%) are the dominant land-uses in the basin. Plantation forestry, covering an area of approximately 3 600 km2, represents approximately 0.3% of the basin (Pink, 2008). ‘Production’ forestry represents approximately 3.2% of land use. Clearing of native vegetation is limited under respective state legislation and new forest plantations are established on cleared agricultural land.
Water consumption in the basin, like the rest of the continent, is variable and dependent on the climate (rainfall and temperature). In 2008-09 ≈6 000 GL was consumed in the basin. In 2009-10 consumption decreased to ≈5 700 GL; equating to 42% of total water consumption in Australia (Pink, 2012). The agriculture industry consumed nearly 4 000 GL representing ≈70% of water use and more than 50% of water consumed for agriculture across the nation. In contrast household consumption during 2009-10 was 185 GL. The process of distributing the water is inefficient with losses of an estimated 1 280 GL (28%) across the basin.
Production of biomass for bioenergy is not directly controlled by legislation (with the exception of an inability to utilise native forest residues). However, instruments such as the National Water Initiative, especially where planted forests may have to meet specific water- based regulations, will influence production amounts. Whilst this is occurring other initiatives aim to increase water use efficiency of the agricultural systems potentially leading to reduced water availability downstream (e.g., increased utilisation of perennial pastures).
Competition for land and water resources in the MDB is significant and likely to increase in response to demand (e.g., population and market opportunities) and expectations (e.g., changing living standards) whilst resource limits are tested (e.g., climate change impacts, increasing fossil fuel and fertiliser costs). Management of resources, including water, in the MDB are going to remain complex and controversial.