JETSE D. KALMA CSIRO Division of Water Resources, Canberra Laboratories, GPO Box 1666, Canberra City, ACT 2601, Australia

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1 Exchange Processes at the Land Surface for a Ranee of Space and Time Scales (Proceedings of the Yokohama Symposium, July 1993). IAHS Publ. no. 212, Goals and implementation of the UNESCO/IHP Project H-l-1 "Review of the scientific aspects of the interface processes of water transport through the atmospherevegetation-soil system at elementary plot, catchment and large grid-size scales" JETSE D. KALMA CSIRO Division of Water Resources, Canberra Laboratories, GPO Box 1666, Canberra City, ACT 2601, Australia REINDER A. FEDDES Department of Water Resources, Agricultural University, Nieuwe Kanaal 11, 6709 PA Wageningen, The Netherlands THE fflp/h-1-1 PERSPECTIVE International Hydrological Programme, Project H-l-1 The International Hydrological Programme (IHP) is an UNESCO international scientific cooperative programme in water resources with as general objective the improvement of the scientific and technological basis for the development of methods and the human resource base for the rational development and management of water resources, including the protection of the environment (UNESCO, 1990). IHP addresses itself to the long-term goal of advancing our understanding of the processes occurring in the water cycle and integrating this knowledge into water resources management. The fourth phase of IHP ( ) - IHP IV - has been divided into three subprogrammes, including (H) Hydrological research in a changing environment. The objectives of sub-programme H are: (i) To improve knowledge of the processes involved in the hydrological cycle and to determine the manner in which these processes might be most appropriately described to meet the demands for planning, design, construction, maintenance and operation of water management schemes within changing environmental situations including the possibility of climate change; and (ii) To provide in cooperation with the international scientific community a general framework for the national, regional and international development of hydrology and related water sciences (UNESCO, 1990). Sub-programme H has several themes including: Interface processes between atmosphere, land and water systems (Theme H-l). Theme H-l includes Project H-l-1: Review of the scientific aspects of the interface processes of water transport through the atmosphere-vegetation-soil system at an elementary plot, catchment and grid size scale (UNESCO, 1990). In the implementation of this Programme close cooperation and coordination is maintained with several international non-governmental organizations such as the International Association of Hydrological Sciences (IAHS). IAHS has been engaged as lead organization for the implementation of Project H-l-1.

2 14 Jetse D. Kalma & Reinder A. Feddes IAHS and ICASVR The International Association of Hydrological Sciences (IAHS) General Assembly at its plenary session in Vienna (August 1991) established an International Committee on Atmosphere-Soil-Vegetation Relationships (ICASVR). This Committee was preceded by an IAHS Working Group on Atmosphere- Soil-Vegetation Relationships since the IAHS Symposium in Baltimore in May The terms of reference of ICASVR are: (a) to promote the study of the physical and biological processes of water and energy fluxes within the atmosphere-soil-vegetation system; (b) to promote the study of the transport of chemicals within the atmosphere-soilvegetation system; (c) to promote the modelling of these processes on different time and space scales. ICASVR is committed to contributing to the science of hydrology and to the application of hydrological process understanding in the management of water resources. IAHS has agreed to the execution of IHP project H-l-1 by ICASVR. The current workplan of ICASVR is the workplan agreed between IAHS and UNESCO/IHP for the implementation of Project H-l-1. This workplan called for (i) the evaluation of several recent international meetings and (ii) the convening of two international symposia. ad (i) ICASVR evaluated the IAHS/WMO/UNESCO Ljubljana April 1990 Symposium on "Regionalization in Hydrology" (see Beran et al., 1991) and the IAHS/UNESCO/IGBP Vadstena June 1990 Workshop on "Plantwater interactions in large-scale hydrological modelling" (see IGBP, 1990). ad (ii) In 1991 ICASVR convened on behalf of IAHS, UNESCO and WMO a symposium during IUGG XX in Vienna on "Hydrological Interactions between Atmosphere, Soil and Vegetation". The current (July 1993) joint IAHS/IAMAP symposium in Yokohama on "Exchange processes at the land surface for a range of space and time scales" is the second symposium co-convened by ICASVR Vienna Symposium This IAHS/ICASVR symposium on "Hydrological Interactions between Atmosphere, Soil and Vegetation" was organized jointly with UNESCO and WMO. It had four subtopics (see Kienitz et al, 1991; Hydrographischen Dienstes in Oesterreich, 1991): (i) Moisture, radiation and temperature fluxes at the atmosphere-soil-vegetation interfaces; (ii) Modelling of the movement of water and chemicals in the soil; (iii) Physiological behaviour of plants in relation to water and chemicals; (iv) Case studies and field investigations at plot and catchment scale. A large number of papers reported on studies of local-scale energy exchange and/or mass transfer in the soil-vegetation-atmosphere transition zone. However few new developments and insights emerged from these papers.

3 Goals and implementation of the UNESCO/IHP Project H-l-1 15 Spatial and temporal heterogeneity at various scales was discussed throughout the symposium. On a small scale, the role of non-homogeneity in soils affecting water and chemical movement was discussed in section (ii). On a local and regional scale, advection problems at forest edges and in irrigation areas were discussed in several papers in section (i). A small number of papers considered the problems associated with scaling up from local scale to catchment scale and larger scales. Few papers considered remote sensing techniques and very little reference was made to its potential role in spatial integration. Catchment models were described in several papers addressing the impact of land use change (especially forestry operations) and land management on the local water balance and soil water quality in different climatic zones. Spatial variability was only considered implicitly. Several general points received special attention during the symposium. They are briefly discussed below. (Individual Vienna symposium contributions have been given in italics). (i) Atmospheric physics, biophysics and soil physics all have crucial roles to play in understanding energy and mass exchanges at the land-atmosphere interface. Although theory of individual parts must precede theory of whole complex systems, it was noted that most current approaches are too compartmentalised (ii) [see Milly; Shuttleworth; Caspar?]. Study of the component processes is hampered by insufficient understanding of the relevant processes, inadequate measuring techniques and uncertainties in modelling. Many of these problems are caused by spatial and temporal nonhomogeneity at a variety of scales in the soil [see Milly; Van Genuchten; Villholth et al; Shouse et al; Taniguchi], at the land surface (soil type, cover and terrain) and in the atmospheric boundary layer [see Henderson-Sellers; Pitman; Veen et al; Hutjes et al; Kruyt et al; Gay and Bernhofer]. (iii) Modelling of water movement and chemicals in the unsaturated zone is of crucial importance in understanding hydrological interactions at the land surface. Continued effort will be required to address the problems associated with the use of classical approaches such as the Richards equation and the Fickian convectiondispersion equation for field applications These problems include: preferential flow through micropores, spatial and temporal variability in soil hydraulic properties, the assumption of isothermal soil conditions, and non-equilibrium processes affecting chemical transport (see Van Genuchten). (iv) Priority areas for research in plant water relationships include study of the role of roots as sensors of water stress, root water relationships in conjunction with soil water deficit and hydraulic and chemical interactions between roots and shoot. Evaporation research needs to address regional and large-scale processes with due consideration to spatial heterogeneity in climate, vegetation and soil conditions (see Jensen et al). (v) (vi) Vegetation pays a crucial role in energy and mass exchanges at the land surfaceatmosphere interface and in coupling interactive biosphere models to general climate models. A great deal more emphasis must be placed on the role of vegetation in hydrological models and on study of the impact of land surface modifications [see Henderson-Sellers; Pitman; Wood et al; Martin]. There is an urgent need of coupling land-surface hydrology models and meso-

4 16 Jetse D. Kalma & Reinder A. Feddes scale atmospheric models. The hydrological cycle and the climate system must be linked by coupling interactive models of the biosphere and general climate models. This requires significant advances in modelling and monitoring [see Kavvas et al; Dyck and Baumert; Wood et al; Martin; Shuttleworth; Krenke et al; Caspary]. (vii) Modelling energy and mass exchanges at the scale of catchments, regions and continents requires land surface parameterization. This involves overcoming complex problems of aggregation due to the heterogeneity of natural surfaces. The inverse problem arises when obtaining small-scale information by disaggregation of larger-scale surface conditions. (See also US National Research Council, 1991.) (viii) Scale-issues arise in a wide range of important soil physical, biophysical and atmospheric physics problems. These arise in modelling the movement of water and chemicals in soils, dealing with non-homogenous vegetation covers, in simulating advection and evaporation, in understanding the physical controls of hydrological variability and in climate modelling. Lettenmeyer (personal communication) poses the question: Is the problem of hydrologie scale simply one of parameter estimation or must the underlying parameterizations change? Strategies must be developed for aggregation (scaling up) and disaggregation (scaling down)(see Dyck and Baumert; Shuttleworth; see also Wood, 1991). (ix) Finally, hydrological observations, monitoring and modelling (including land surface parameterization) at local, regional and continental scale are essential in order to understand the close linkages between the global hydrological cycle and climate system. Shuttleworth identified four major areas of work: (a) validation of models of climate and biogeochemical cycles; (b) parameterization and calibration of models of the land surface-atmosphere interface (see also Avissar & Verstraete (1990), Avissar (1991) and Famiglietti & Wood (1991)); (c) prediction and validation of climate models (see also Dickenson & Kennedy (1991), Sellers et al. (1990) and Sellers (1991)), as well as prediction of global climate change; and (d) interpretation of global change impact [See Dyck and Baumert] Yokohama Symposium The present IAHS/IAMAP symposium on "Exchange processes at the land surface for a range of space and time scales" will address the contributions of the hydrological and atmospheric sciences to Project H-l-1 of the International Hydrological Programme and to International Geosphere-Biosphere Program (IGBP) Core Project BAHC (Biospheric Aspects of the Hydrological Cycle). Papers have been invited on the following topics: Techniques for measuring and estimating energy exchange and mass transfer with reference to on-ground measurements, atmospheric boundary layer sounding approaches and remote sensing techniques. Strategies for observing and modelling the aggregation/disaggregation of energy exchange and mass transfer. Subtopics include (i) the relative roles of soil, vegetation

5 Goals and implementation of the UNESCO/IHP Project H-l-1 17 and atmosphere in controlling spatial and temporal variability in hydro-ecological processes and their interconnection; (ii) hierarchical structures in process studies and modelling to facilitate movement between scales; and (iii) parameterization of soil physical, hydrological, meteorological and related biochemical processes. Models for energy exchange and mass transfer at patch, regional and continental scales, incorporating the relevant vertical and horizontal transport processes. Implications of the interactions between land surface properties and climate: (i) Implications of climate change for plant growth and water use; (ii) Implications of climate change for regional and continental hydrology. REFERENCES Avissar, R. (1991) A statistical-dynamical approach to parameterize subgrid-scale landsurface heterogeneity in climate models. Special Issue of Surveys in Geophysics 12(1-3), Avissar, R. & Verstraete, M. M. (1990) The representation of continental surface processes in atmospheric models. Reviews of Geophysics 28(1), Dickenson, R. E. & Kennedy, P. J. (1991) Land surface hydrology in a general circulation model - global and regional fields for validation. Special Issue of Surveys in Geophysics 12(1-3), Famiglietti, J. S. & Wood, E. F. (1991) Evapotranspiration and runoff from large land areas : land surface hydrology for atmospheric general circulation models. Special Issue of Surveys in Geophysics 12(1-3), Hydrographischen Dienstes in Oesterreich (1991) Extended abstracts of poster papers presented at IAHS symposia held during the XXth General Assembly of IUGG in Vienna (11-24 August 1991). Mitteilungsblatt Nr 65/66, Kienitz, G., Milly, P. C. D., Van Genuchten, M. Th., Rosbjerg, D. & Shuttleworth, W. J. (eds) (1991) Hydrological Interactions Between Atmosphere, Soil and Vegetation (Proc. IAHS/UNESCO/WMO Symp., Vienna, August 1991). IAHS Publ. no. 204, IAHS Press, Wallingford, UK. Sellers, P. J., Rasool.S. I. &Bolle, H. J. (1990) A review of satellite data algorithms for studies of the land surface. Bull. American Meteorol. Soc. 71(10), Sellers, P. J. (1991) Modelling and observing exchanges between the terrestrial biosphere and the atmosphere. In: Proc. Int. Hydrology and Water Resources Symposium (Perth, Australia, October 1991), United States National Research Council (1991) Opportunities in the hydrological sciences. National Academy Press, Washington, DC. Wood, E. F. (ed.) (1991) Land surface- atmosphere interactions for climate modelling. Special Issue of Surveys in Geophysics 12(1-3),

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