Biogeosciences, 3, 229–241, 2006www.biogeosciences.net/3/229/2006/© Author(s) 2006. This work is licensedunder a Creative Commons License.BiogeosciencesLand-surface modelling in hydrological perspective – a reviewJ. Overgaard1,2, D. Rosbjerg1, and M. B. Butts21Institute of Environment & Resources, Technical University of Denmark, Building 115, DK-2800 Kongens Lyngby,Denmark2DHI Water & Environment, Agern All´e 5, DK-2970 Hørsholm, DenmarkReceived: 18 October 2005 – Published in Biogeosciences Discuss.: 13 December 2005Revised: 15 March 2006 – Accepted: 20 March 2006 – Published: 17 May 2006Abstract. The purpose of this paper is to provide a reviewof the different types of energy-based land-surface mod-els (LSMs) and discuss some of the new possibilities thatwill arise when energy-based LSMs are combined with dis-tributed hydrological modelling. We choose to focus onenergy-based approaches, because in comparison to the tradi-tional potential evapotranspiration models, these approachesallow for a stronger link to remote sensing and atmosphericmodelling. New opportunities for evaluation of distributedland-surface models through application of remote sensingare discussed in detail, and the difficulties inherent in var-ious evaluation procedures are presented. Finally, the dy-namic coupling of hydrological and atmospheric models isexplored, and the perspectives of such efforts are discussed.1 IntroductionWith the growing population of the Earth and predictedchanges in the global climate, the pressure on the alreadyscarce water resource is likely to increase in the comingyears. This has created a need for integrated models that canassess the available water resource as well as predict the im-pact of future changes in management and climate. Makingsuch accurate predictions is an immense task that can only beachieved through joint co-operation between scientists acrossmultiple disciplines. Located at the borderline between theatmosphere and hydrology, the land-surface provides the linkbetween several scientific disciplines, and land-surface mod-elling has been subject to intense research in the hydrologi-cal, atmospheric, and remote sensing communities in the lastdecades. Combining these efforts is of vital importance forthe successful predictions of future changes.Correspondence to: J. Overgaard([email protected])Especially LSMs that are based on a solution of the energybalance equation at the land surface have been subject to in-tense research. Since the late 1980s, a large number of ad-vanced energy-based LSMs containing sophisticated param-eterisations of vegetation and root zone have been developed,and currently, the energy-based models are probably the mostfrequently applied LSMs in the scientific community. Threefactors are mainly driving the interest in energy-based LSMs.First of all there is the desire to gain better physical un-derstanding of the land surface-vegetation system throughdevelopment of more and more sophisticated and advancedmodels. In this regard, the physical basis of the energy-based LSMs makes them an attractive alternative to the moreconceptual types of evapotranspiration models that have tra-ditionally been applied in hydrological modelling. Mostenergy-based LSMs are one-dimensional column models thatdescribe the root zone and vegetation in great detail. Ap-plied at the plot scale and evaluated against measured land-surface fluxes, these models have helped to gain importantinformation on the fluxes of heat and water between theland surface and atmosphere for many different vegetationtypes and under many different climatic conditions. In somecases energy-based LSMs have been applied in spatially dis-tributed frameworks, but these rarely include the lateral sur-face and subsurface flows between the columns.Secondly, the atmospheric scientific community has con-tributed to the development of advanced energy-based LSMs.Recognizing the close connection between the atmosphereand land surface, great effort has, since the early 1990s, beenput into developing advanced LSMs for atmospheric mod-els working at all scales ranging from storm scale to globalscale. Providing information on all fluxes and state variablesrequired at the land-surface boundary in atmospheric mod-els, the energy-based LSMs have a great advantage over theconceptual evapotranspiration models that tend to provideonly actual evapotranspiration. This has made energy-basedLSMs the preferred choice of atmospheric modellers. InPublished by Copernicus GmbH on behalf of the European Geosciences Union.230 J. Overgaard et al.: Land-surface modelling in hydrological perspectiveatmospheric models, these LSMs are inherently distributed,but also here, the lateral surface and subsurface flows be-tween cells are rarely considered.Finally, the remote sensing community has an importantrole in the rapid development of energy-based LSMs. Thephysical basis of the energy-based LSMs makes them wellsuited for utilizing the growing amount of land-surface dataavailable from remote sensing, and energy-based modelshave therefore become the preferred LSMs in the remotesensing community.The LSMs have, beyond any doubt, benefited from thecombined efforts across multiple scientific disciplines, and inmany studies state-of-the-art models have proven to performwell at the plot scale, and in some cases, also when appliedin spatially distributed frameworks.However, when moving from the one-dimensional columnmodels to fully distributed models, it becomes increasinglyimportant to describe the spatial variations in soil moistureto ensure an accurate simulation of the land-surface fluxes.Variations in soil moisture may be induced by factors such asprecipitation, soil texture, drainage, irrigation, flooding andshallow groundwater. While most of the energy-based LSMsinclude a detailed description of the vegetation and root zone,the interactions between groundwater, root zone and surfacewater, as well as the lateral surface and subsurface flows, arenormally neglected, and consequently these models will failto produce accurate results in areas where such interactionsare important.Contrary to this, hydrologists have a long tradition fordeveloping and applying distributed models that take theseinteractions into consideration, but so far the energy-basedLSMs are rarely applied in integrated hydrological mod-elling, and ironically, most of the integrated hydrologicalmodels that claim to be physically based do, in fact, containrather