HYDROLOGICAL PROCESSESINVITED COMMENTARY

Hydrol. Process. 18, 567–570 (2004)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5500

Rainfall-runoff modelling for assessing impacts of climateand land-use change

Axel Bronstert*University of Potsdam, Departmentfor Geoecology, Potsdam, Germany

*Correspondence to:Axel Bronstert, Department forGeoecology, University ofPotsdam, PO Box 60 15 53, 14415Potsdam, Germany. E-mail:axelbron@rz.uni-potsdam.de

Since the start of industrialization of our Earth (i.e. for aboutthe last 150 years), the use of fossil energy has created a new(additional) source of carbon dioxide emissions from the Earthinto the atmosphere. Though these emissions are small comparedwith the natural emissions (mainly from the biosphere), they havedisturbed the carbon balance of the atmosphere, i.e. the carbondioxide concentration of the lower atmosphere has increased froma rather stable pre-industrial level (280 ppm) to a present levelof 360 ppm. Besides carbon dioxide, the concentration of other‘greenhouse gases’ has increased through mankind’s activities. Themost important of these increases are for methane, which rose froma pre-industrial value of 0·7 ppm to a present value of 1·74 ppm, andnitrous-oxide, which rose from a pre-industrial value of 275 ppb toa present value of 313 ppb.

Carbon dioxide and the other ‘greenhouse gases’ affect the atmo-spheric absorption properties of longwave radiation, so changing theradiation balance. The most obvious impact of this altered radiationbalance is the warming of the lower troposphere, which has beenobserved through an increase of global temperature of about 0·6 °Cover approximately the last 50 years. Depending on the differentemission scenarios the IPCC (Houghton et al. 2001) project a fur-ther increase of global temperature in the range of 1 to 5 °C, pointingout that higher latitudes and land surfaces will have to cope with amuch higher temperature increase than lower latitudes and sea sur-faces. For the hydrological cycle, the projections on the developmentof precipitation are even more relevant than those for tempera-ture; however, they are even more uncertain. From the viewpointof global atmospheric energy, one can conclude that an increase ofglobal atmospheric temperature of 3 °C, for example, would yieldan intensification of the hydrological cycle by about 10%, i.e. 10%higher global evaporation and precipitation rates. The projectionsof the development of regional precipitation are even more uncer-tain, and even today (if derived directly from general circulationmodel) results are of rather limited value for climate-change impactassessment.

A second major issue of human-induced changes of our globalenvironment originates from the intense human utilization of landresources, which almost unavoidably results in significant changes oflanduse and land cover. This effect has been proven throughout theentire history of mankind. However, since the era of industrialization

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and rapid growth of population, land-use changephenomena have accelerated in many regions, suchas deforestation of tropical forest. (LUCC, 2002) orurbanization of formerly agricultural or forestedland (e.g. Krausmann et al., 2003). It is ratherclear that the change in ‘landuse’ or ‘land cover’are of major relevance for rainfall-runoff pro-cesses, in particular if runoff-generation processesare influenced by the land surface conditions of thecatchment area under investigation.

The impacts of these changed boundary con-ditions on the hydrological system have been ofconcern ever since the relationships between exter-nal forces (boundary conditions) and the internaldynamics (sub-processes and their interactions) ofthe hydrological cycle have been known. In thiscontext, the term ‘environmental change’ is used tosummarize changes in climatological and land-useconditions. There are other changes to the bound-ary conditions that could be included in the term‘environmental change’, e.g. river training mea-sures, water retention in reservoirs, or groundwa-ter use, but those are not discussed here.

So, in principle, we need a (model) descrip-tion of the boundary conditions, including theirchanges and a model description of the internalsystem dynamics. It is argued that rainfall runoffmodels can serve as a tool for such a purpose,because they transform the meteorological forc-ing (in particular rainfall) into the hydrologicalresponse of a catchment (in particular runoff).In that context, climate change impact assess-ment mostly concentrates on the changed meteo-rological forcing, and the land-use-change impactassessment focuses more on the internal dynam-ics of the hydrological system. In recent years, awide range of rainfall-runoff model applicationshave been used to assess impacts of climate andland-use change on the hydrological cycle, e.g. seeBronstert et al. (2002) or Niehoff et al. (2003) foran overview. Actually applying a rainfall-runoffmodel is the only economically possible way (land-use-change impact assessment) or even the onlyfeasible way (climate-change impact assessment) toobtain some quantitative figures about the impactof such environmental changes on the hydrolog-ical cycle. So what are the (new) problems andwhat are the (new) challenges for rainfall-runoffmodelling in that context?

ž First, the uncertainty involved in this type ofclimate-change impact assessment limits the valueof the results. Although any model predictionsin hydrology and other environmental sciencesare uncertain, and this uncertainty (data, pro-cess, parameter uncertainty) should be takeninto consideration, one has to acknowledge thatclimate-change impact assessment introduces anew important source of uncertainty: the uncer-tainty associated with the climate scenario. Cli-mate scenarios (global or regional) are hardlyever given in a probabilistic quantity, i.e. theyinclude no description of occurrence probabil-ity. The latest development of ‘ensemble’ sce-narios (simulated climate with different initialclimate conditions) are a step towards probabilis-tic scenarios, but it is not enough to considerthe effects of different initial conditions. Fora ‘comprehensive probabilistic climate scenario’one should also include other sources of uncer-tainty in the derivation of the ensemble scenar-ios, such as different emission estimates, differ-ent parameterization approaches for regional cli-mate physics (e.g. cloud and rainfall formation,land-surface interaction), etc. These ‘comprehen-sive probabilistic climate scenarios’ may yieldone important basis for an uncertainty-basedrainfall-runoff modelling of climate-changeimpacts.

ž The standard calibration methods of rainfall-runoff models need to be adjusted or extended forsuch kinds of impact analysis. The main chal-lenge here lies in the adequate representation ofthe altered internal dynamics of the hydrologicalsystem if the boundary conditions of the land-cover change. That is, the hydrological processesand their interdependencies have to be repre-sented by the rainfall-runoff model in such a waythat possible system changes can be covered bythe model. There are, in principle, two options tocome closer to this target: multi-process calibra-tion and multi-site calibration. The first optiontries to validate the model dynamics of differ-ent hydrological processes by looking at varioushydrological variables, e.g. runoff (maybe con-sidering different components), snow cover, soilmoisture. The second option aims at validatingthe rainfall-runoff model by applying it to vari-ous catchments with different typical governing

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runoff-generation processes (e.g. groundwater-dominated runoff, snowmelt-dominated runoff,Hortonian-infiltration excess-dominated runoff)and proving its applicability for these differinghydrological dynamics.

ž The boundary conditions, i.e. climate change orland-use change scenarios and the rainfall-runoffmodel, need to be adjusted or improved for extremehydrological conditions. Floods and droughts areof particular interest for many impact assess-ments. However, most climate and land-use sce-narios are not tailored for such conditions, e.g.precipitation values are given as large spatialaverages and/or the occurrence probability ofextreme rainfall intensities or drought lengths isnot represented realistically by climate modelsfor ‘today’s climate conditions’. The IPCC hasgiven a primary focus on anomalies for its forth-coming assessment report, so let us hope thatthe hydrological community will profit from animproved representation of climate anomalies inthe climate models.

Furthermore, many rainfall-runoff modelshave not been thoroughly tested for extremerunoff conditions, in particular the runoff-gene-ration processes under extreme rainfall and/orwet catchment conditions. So the challenge is alsoto test such models for extreme hydrological con-ditions. This is not an easy task, because data forextremes are rare (by definition!) and often lackreliability and comprehensiveness. One impor-tant step towards this aim is to make use of con-tinuous runoff records for calibration of rainfall-runoff models for assessment of flood condi-tions. Lamb (1999) has shown that a continu-ous rainfall-runoff simulation and a calibrationwith a special emphasis on the peak dischargescan yield flood frequency distributions that aremuch closer to the observed ones than event-based rainfall-runoff modelling, which does notsimulate the antecedent catchment conditions.

ž Climate and land-cover conditions may interactso strongly with one another that it might beappropriate to link the climate and land-use sce-nario. However, in rainfall-runoff model appli-cation for environmental-change impact assess-ment, the ‘standard’ (mostly applied) procedureis in a ‘one-way’ or ‘uncoupled’ mode. That iswe use a set of climate or land-use scenarios

as varying boundary conditions for a rainfall-runoff model without bothering about feedbackeffects, neither direct climate–land-surface feed-backs nor the effects of changed forcing on therelevance of different hydrological processes. Forexample, a change of characteristics in rainfallintensity (in a changed climate) may result indifferent relevance of Hortonian runoff. Or achange of soil surface conductivity (e.g. causedby different land use) alters infiltration condi-tions. Most rainfall-runoff models applicationsdo not account for these changes by applyingthe same set of equations or even the same setof model parameters as for the original set ofboundary conditions.

Furthermore, possible long-term interdepen-dencies between different types of boundary con-dition (such as climate or land-use conditions)are not considered. For example, a warmer anddrier climate will be accompanied by a changein vegetation dynamics and composition, possiblyby altered soil surface conditions and definitelyby changed water management. Or, as an exam-ple of an impact of land-use change on climate,a large-scale change of land-surface conditions(for instance, a conversion of forest to arableland) results in a changed energy balance of theland surface, thus altering the climate system.The consideration of these long-term interde-pendencies and linked changes of the internaldynamics require the definition of ‘integratedscenarios’ (i.e. scenarios of adjusted future cli-mate and land-use/land-cover conditions) andoffer a future area of application for rainfall-runoff models.

Different modelling purposes result in differentrequirements for the rainfall-runoff model to beapplied. For instance, before starting the assess-ment, one has to define clearly the appropriatetemporal and spatial scale, and whether the focusis on the average water balance or on extremehydrological conditions (floods and droughts). Aswith any model, the chosen rainfall-runoff modelcannot give a full picture of reality; however, it isimportant that the model represents the main partof the system dynamics. This implies, of course,that we know (or at least can make a well-educatedguess about) the main dynamics of the catchment

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under consideration. The more we know about thecatchment to be modelled and the better the modelis able to ‘represent’ this knowledge, the more reli-able the model results will be.

As mentioned before, the scientific basis of theboundary conditions is of similar importance asthe performance of the rainfall-runoff model itself.This means that the whole modelling proceduremust contain an adequate, scientifically sound han-dling of the boundary conditions. A changed cli-mate or an altered land-use condition should notbe chosen arbitrarily. Instead, it should be basedon reasonable assumptions about the trend of driv-ing forces influencing climate (e.g. greenhouse gasconcentrations) or landuse (e.g. economic develop-ment), and the expression of the changed climateor land-use conditions for the spatial and tempo-ral scale that is relevant for the study. Today, anattribution of occurrence probability to the var-ious scenarios of land-use or climate change isstill absent. If this can be provided in the future,then a very important step towards a comprehen-sive, probability-based estimation of environmental-change impact assessment will be accomplished.

The ideas presented above are condensed into afew statements, yielding a (rather ambitious andoften rather hard to follow) scheme for applyingrainfall-runoff models for environmental impactassessment:

1. Define the modelling purpose clearly, includingthe definition of the important hydrological pro-cesses, the relevant temporal and spatial scales,and the role of hydrological anomalies.

2. Check if the chosen rainfall-runoff model con-tains/represents the relevant processes of the sys-tem to be modelled, under the given purposes.

3. Check model performance, e.g. by comparingthe model results obtained based on currentboundary conditions with observed data (of dif-ferent variables, if available), and by evaluatingmodel performance in several catchments withdiffering land use or different climatic condi-tions.

4. Define the overall trend of the land-use and/orclimate change and develop the land-use and/orclimate scenario at the temporal and spatialscales relevant for the specific study.

5. Check if feedback effects of climate and land useare important for the specific study, and, if so,develop integrated scenarios.

6. Assess the uncertainty of the whole system mod-elled, including both the uncertainty within therainfall-runoff modelling procedure (data, pro-cess, parameter uncertainty) and the uncer-tainty due to the definition of the scenarios(boundary condition).

By following the essence of this scheme, I amquite convinced that rainfall-runoff models canserve as most adequate tools to assess the impactsof climate and land-use changes on the hydro-logical cycle. There are no better tools available,and the hydrological community should be confi-dent that they can give the answers to the ques-tions raised by the public. But scientists have tobe aware that these answers can be different fordifferent catchments, i.e. specific regional investi-gations are required. Furthermore, the results arealways associated with uncertainty, and it is ofmajor importance to include this uncertainty issuein the answers given!

References

Bronstert A, Niehoff D, Burger G. 2002. Effects of climate andland-use change on storm runoff generation: present knowl-edge and modelling capabilities. Hydrological Processes 16(2):509–529.

Krausmann F, Haberl H, Schulz NB, Erb K-H, Darge E,Gaube V. 2003. Land-use change and socio-economic metabolismin Austria—part I: driving forces of land-use change:1950–1995. Land-use Policy 20(1): 1–20.

Houghton JT, Ding Y, Griggs DJ, Noguer M, van de Linden,Dai X, Maskell K, Johnson CA. (eds). 2001. Climate Change2001. The Scientific Basis. Contribution of Working Group I tothe Third Assessment of the Intergovernmental Panel on ClimateChange. Cambridge University Press; Cambridge.

Lamb R. 1999. Calibration of a conceptual rainfall-runoff modelfor flood frequency estimation by continuous simulation. WaterResources Research 35(10): 3103–3114.

LUCC. 2002. New estimates of tropical deforestation and terres-trial carbon fluxes: result of two complementary studies. LUCCNewsletter (December).

Niehoff D, Fritsch U, Bronstert A. 2002. Land-use impacts onstorm-runoff generation: scenarios of land-use change and sim-ulation of hydrological response in a meso-scale catchment inSW-Germany. Journal of Hydrology 267(1–2): 80–93.

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