HYDROLOGICAL PROCESSESHydrol. Process. 21, 3647–3650 (2007)Published online 22 October 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.6901

Biofuel and water cycle dynamics: what are the relatedchallenges for hydrological processes research?

Stefan Uhlenbrook1,2*1 UNESCO-IHE, Westvest 7, 2601 DADelft, The Netherlands2 Delft University of Technology,Faculty of Civil Engineering andApplied Geosciences, Water ResourcesSection, PO Box 5048, 2600 GA Delft,The Netherlands

*Correspondence to:Stefan Uhlenbrook, UNESCO-IHE,Westvest 7, 2601 DA Delft,The Netherlands.E-mail: s.uhlenbrook@unesco-ihe.org

Received 7 August 2007Accepted 17 August 2007

The accurate analysis of environmental impacts of large-scale biofuel(Definition: Fuel with a minimum of 80% content by volume ofmaterials derived from living organisms harvested within 10 years ofits manufacture.) production is important for many reasons, but inparticular it is essential to protect water resources and to assureecological integrity as far as possible. Biofuels, or more generallybioenergy (including biodiesel, etc.), are promising renewable energysources intended to satisfy the escalating global energy demand (e.g.Somerville, 2006) and to limit greenhouse gas emissions. The advantagesof biofuels are manifold: (i) security of supply (renewable energy;biofuels can be produced locally in comparatively sustainable systems)(ii) lower net greenhouse gas emissions (biofuels recycle carbon dioxidethat was extracted from the atmosphere in producing biomass) (iii) lesspollution in respect to other emissions (less sulfur, carbon monoxide andparticulates; biodegradable byproducts) (iv) well suited for transportuses (high energy density and handling convenience) and (v) support foragriculture. Somerville (2006) discusses further substantial economic andstrategic advantages, as for instance a larger independence of westerncountries with regard to fossil fuel originating from politically unstablecountries. The expectations in this energy source are tremendous (e.g.Ragauskas et al., 2006); the combination of intensive agriculture, modernbreeding and transgenic techniques should result in achievementsgreater than those of the Green Revolution in food crops, and in farless time (e.g. Koonin, 2006).

Already about 15% of the total global energy consumption is nowderived from biomass and the envisaged future uptake of biofuel istremendous, particularly in the North where the energy demand ishighest. The US Department of Energy proposes that 30% of groundtransportation fuel should be replaced by biofuels by 2030. The currentEU target is a 10% biofuel replacement by 2020, but many expertsstate that this is not enough to reach its environmental objectives byfar. Some European countries like Sweden and Germany want to gofurther and would like to become much more independent from fossilfuel for transportation purposes. Brazil currently derives 25% of itstransportation fuel from ethanol produced from sugarcane (Somerville,2006), and it uses only 0·5% of its land area for this.

Biofuels can be produced from different crops: (i) corn and soybeans(primarily in USA) (ii) flaxseed and rapeseed (Europe) (iii) sugarcane(Brazil) and (iv) palm oil (South-East Asia). New research is focussing onmicro-algae for biodiesel and ethanol production. Recently the demandfor biofuel has increased significantly and resulted in an increase in theprices of food crops (Laney, 2006). This is likely to affect food secu-rity, particularly in poor countries. This situation becomes aggravatedparticularly as energy efficient production of biofuels is best possiblein sub-humid and humid tropical regions, due to suitable climate andsoils. Consequently, on the one hand, larger biofuel production offersgreat economic chances for developing countries located in the tropics.The farmers could become ‘energy farmers’ in the South and sell highly

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demanded products to the North for good prices. Onthe other hand, the impacts in the developing world onfood security and environmental integrity (includingwater resources) need to be evaluated carefully.

Ongoing ResearchCurrently, research efforts in this field concentratemainly on the technical and engineering aspects ofbiofuel production. Significant efforts are directedtowards, for instance, the development of efficientmethods for converting plant lignocelluloses to fuels(i.e. without using unacceptably large amounts ofenzymes to produce sugar), methods for efficientlyfermenting sugars other than glucose, different andnew sources of biofuels (e.g. micro-algae), and othertechnical (e.g. Hill et al., 2006; Somerville, 2006) andagricultural issues (e.g. Pimentel and Patzek, 2005).Other projects focus on the related socio-economicchallenges also (UN, 2007). However, Farrell et al.(2006) concluded that many of the important environ-mental effects of biofuel production are poorly under-stood. Further research into environmental metricsis needed to consider all environmental and socio-economic impacts in a holistic way.

The impacts of large-scale land use changes asa consequence of increasing biofuel production onhydrological processes and water cycle dynamics arelikely to be very severe, but they are not fully under-stood. This is a new challenge for hydrological sci-ences, in particular when newly created plant vari-eties, new agricultural techniques, etc., are consid-ered. To the best of the author’s knowledge, there areno existing peer-reviewed articles on the hydrologi-cal impacts! Nevertheless, the general impacts of landuse changes on hydrology have been investigated bymany researchers (e.g. Calder, 1993; McCulloch andRobinson, 1993). Conversion from forest to agricul-tural land generally reduces interception, infiltrationand groundwater recharge, while surface runoff anderosion increase. This has often led to an increase inflood runoff and a decrease in low flows (‘accelerationof the water cycle’). Significant changes of hydrolog-ical processes, including more overland flow and lessrecharge, were observed if the land use was alteredfrom forest or farm land to urban areas (e.g. Niehoffet al., 2002). However, predicting the impacts of landuse changes without detailed experimental and mod-elling studies in data-rich study areas remains difficult(e.g. Sivapalan et al., 2003) and is the subject of manyongoing research initiatives. An additional constraintis that many areas that are likely to be the focus ofenhanced biofuel production (i.e. tropical regions) aregenerally poorly observed in terms of their water bal-ance parameters and hydrological processes. Thus,predicting the impacts of the changed land use willbe very difficult and uncertain, without having gooddata for model verification even under current condi-tions and considering our incomplete understandingof processes in these regions.

Impacts on Hydrological Processes and OpenResearch QuestionsIntensive, large-scale biofuel production would imply:(i) the conversion of existing crop land to farmlandwith biofuel crops (ii) the change of other land uses(e.g. forest, pasture land) (iii) the use of new plantvarieties (resulting from new breeding and trans-genic research) and (iv) intensive agriculture includ-ing monocultures and plantations over large areas,use of fertilizer and agrochemicals, and installationof water management systems for irrigation anddrainage. This will have significant ecological impacts(ecosystem health, biodiversity, etc.), but the impactson hydrological processes (water quantity and waterquality impacts) play a crucial role in this regard.

To estimate the impacts of large-scale biofuel pro-duction on hydrological processes and water cycledynamics, the effects on the water balance should beanalysed over a wide range of spatial and temporalscales. The water balance can be written as:

P = (EI + dSI/dt) + (ES + RS + dSS/dt)

+(EU+ET+RU+dSU/dt) + (RG + dSU/dt)(1)

where P [mm t−1] is precipitation; E [mm t−1] isevaporation from the interception storage (EI), theground surface (ES; including evaporation from openwater), the soil (mainly unsaturated zone storage; EU)as well as transpiration (ET); R [mm t−1] are the dis-charge components for surface runoff (RS; includingstream flow), runoff in the soil zone (unsaturated zone,interflow; RU) and runoff from groundwater (RG);dS [mm] is the change in hydrological storages pertime step dt [t], considering the interception storage(SI), the surface water storage including storage inthe stream network, lakes and reservoirs (SS), thesoil storage (unsaturated zone; SU) and the ground-water storage (SG). Snow and ice storage as well asurban water storage systems are not treated sepa-rately in this simplified equation (summarized in SS).The brackets in Equation (1) sum up the interceptionprocesses (‘white water’ fluxes), surface runoff pro-cesses (‘blue water’ fluxes), interflow as well as plantwater uptake and transpiration processes (the lattercalled ‘green water’ fluxes), and groundwater fluxes(‘deep blue water’ fluxes). The rainbow of the watercycle was introduced by Falkenmark (1995; here mod-ified according to Savenije, 1999) and is nowadaysused frequently in water management.

The interventions into the hydrological cycle con-nected to intensive, large-scale biofuel productionwill change every component of the water balanceEquation (1) and this has, of course, implications forthe coupled energy balance (changes in latent and sen-sible heat fluxes). The evaporation components willshift depending on the interception, transpiration andsoil evaporation processes of the respective vegeta-tion. The soil processes, i.e. infiltration, soil moisture

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storage, recharge and lateral flow, will change depend-ing on the vegetation-dependent net rainfall and soilmoisture storage capacity, and the permeability of theupper soil and its anisotropy that is highly influencedby the plant dependent root system. This will modifythe generation of all runoff components, the storageof water in all hydrological storage systems and, thus,it will change stream flow dynamics. The alteration ofthe physical processes has direct implications on waterchemistry as well as on erosion (impact on overlandflow generation) and sedimentation (i.e. sources, fluxesand fractions of sediments) with numerous impactson the stream’s ecology. Finally, precipitation canbe influenced through the land use changes in itstotal amount, intensity and spatio-temporal variabilitythrough the vegetation-dependent moisture feedbackto the atmosphere (e.g. Savenije, 1995; Rahn, 2007)and the impacts on the energy budget (e.g. Pitman,2003). Consequently, the coupled water and energybudgets will be affected at different spatial and tem-poral scales.

The impacts of land use changes due to conver-sion of forest to agricultural land vary significantlyin different study areas, and predicting the impactswithout detailed investigations remains very difficult(e.g. Sivapalan et al., 2003). Therefore, it is not possi-ble with the current incomplete process understandingand available modelling tools to predict with accept-able uncertainty the hydrological impacts of inten-sified biofuel production. To what extent the pro-cesses change will depend on local and regional cir-cumstances. Detailed, multi-disciplinary experimentalinvestigations will have to be carried out to gain abetter understanding of the processes. On the basis ofthese results bio-physically based models will have tobe further developed and regionalized. The direct useof current models faces too many problems relatedto uncertainties of input data, model structure andparameterization to allow an immediate integratedassessment of the impacts.

In this regard, it is important to distinguish betweendifferent spatial scales (e.g. Bloschl and Sivapalan,1995), as different hydrological processes dominate atthe hillslope scale (approx. <1 ha), catchment scale(approx. 100 –103 km2) and river basin scale (approx.>104 km2). It seems more challenging to quantify theimpacts at larger scale, as the interactions betweenland use, climate and the resulting hydrological pro-cesses are more complex at this scale and can becontradictory in their effects. Another challenge isthat investigation will have to be carried out oversufficient time (several seasons), since soil physicalparameters (for instance) often do not change rapidlyafter land use changes (long lasting memory effects),and the natural heterogeneity of processes and hydro-climatological variability, in particular, in tropicalregions do not allow quick reliable assessments. Addi-tionally, many other changes are occurring in thestudy areas over longer time scales with similar or

dissimilar impacts on water cycle dynamics (e.g. cli-mate change, change in water use). It is also crucialto distinguish between different temporal scales (i.e.short-term vs long-term process dynamics) as differ-ent processes are dominant and govern the impacton such processes as floods, interception and surfacerunoff, on the one hand, and groundwater rechargeand low flows on the other.

A number of relevant research questions for hydrol-ogy can be formulated in this regard (see below).However, even more research questions can be for-mulated if the wider aspects of biofuel impact areto be assessed, for instance the impacts on ecology(e.g. biodiversity) or the implications for environmen-tal management and policy (socio-economic research).

ž What are the differences in rain water partition-ing (Including the hydrological processes of inter-ception, evaporation, transpiration, surface runoff,infiltration, water storage in root zone, plant wateruptake and lateral re-distribution of soil water.)of biofuel producing plants in comparison to otheragricultural crops or forest land (hillslope/plot scaleinvestigations)?

ž What is the effect on the different runoff compo-nents generated in catchments where land use ischanged for increasing biofuel production (catch-ment scale investigations)? Here, particular empha-sis should be given to water yields as well as floodsand low flows.

ž What are the impacts of intensified biofuel pro-duction on hydrological processes and water cycledynamics (including change of precipitation throughmoisture feedback) at larger regional scales (riverbasin scale investigations)?

ž What are the impacts of such land use change on thequality of impacted surface water and groundwaterresources considering the nutrients, agrochemicalsand sediments associated with biofuel production?

ž What are the impacts on the interactions betweenwater security (e.g. increase in agricultural waterdemand vs other demands from humans and ecosys-tems), food security (i.e. energy crops vs cropsfor food production) and energy security (satisfy-ing energy demands at local vs regional and globalscales)?

Different experimental techniques (including tracermethods and geophysical techniques) and hydrologicalmodelling approaches (physically based and concep-tual models, statistical data analysis) need to be devel-oped further to answer these questions. The authorvery much believes that it is necessary to assem-ble new, suitable data sets that are also observedusing novel innovative techniques (combined tracerand geophysical data, remote sensing data etc.) andthat a re-analysis of existing data sets alone will notbe sufficient. Such studies will have to be carried outusing a comparative nested-catchment approach (i.e.

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multiple spatial and temporal scales) in different studyareas and environments that are suitable for biofuelproduction, and ideally where previous hydrologicalprocess research has been conducted. The latter war-rants knowledge of the dominating hydrological pro-cesses in the existing land use conditions and the uti-lization of existing research infrastructure for processobservation.

Concluding RemarksBiofuels are promising renewable energy sourcesintended to satisfy the rapidly increasing globalenergy demand. The envisaged future uptake of bio-fuel is enormous and, consequently, large-scale landuse changes particularly in tropical areas can beexpected. The impacts of intensive, large-scale bio-fuel production (including new, fast growing plants,intensive agriculture, etc.) on hydrological processesand water cycle dynamics may be very significant,but they are not well understood and they cannotbe quantified reliably with existing modelling tools.To gain a better fundamental understanding of theimpacts, it is important to distinguish between impactsat local, regional and global scales as well as betweenprocesses dominating at different time scales (short-term vs long-term dynamics). This is an interestingchallenge for hydrological sciences for now and thecoming years.

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