The water quantity and quality pressures have been aggravated by our inability to provide a coordinated management and governance framework within which to manage our ground and surface waters conjunctively. Water shortage, deteriorating water quality and persistent flood risks are among the most urgent problems that need attention. However, surface water and groundwater have been, by tradition, managed separately - often in completely different branches of government. The separation in the governance function for ground and surface waters is reflected in the separation of the sciences, professional disciplines and modeling tools that define our governance approach. It is now recognized that water resources problems cannot be treated in isolation to the hydrological cycle. The problems are seldom isolated and their solution requires a holistic approach to water management that must address different, often conflicting, demands for water. Problems like wetland protection or the conjunctive use of surface water and groundwater resources require the integrated management of surface water and groundwater together with the water chemistry and ecology.

In the hydrological cycle, water evaporates from the oceans, lakes and rivers, from the soil and is transpired by plants. This water vapor is transported in the atmosphere and falls back to the earth as rain and snow. It infiltrates to the groundwater and discharges to streams and rivers as baseflow. It also runs off directly to streams and rivers that flow back to the ocean. The hydrologic cycle is a closed loop and our interventions do not remove water; rather they affect the movement, transfer and storage of water within the hydrologic cycle. However, the spatial scales of the processes involved range from the size of soil pores to regional groundwater aquifers of many 1000's of square kilometers. Furthermore, the inherent heterogeneity of natural systems makes it difficult to represent these processes accurately and the impacts of agriculture and urban development are by no means fully understood.

To overcome the problems associated with complex hydrological sciences and limited data availability the science of the hydrological cycle was conceptualized into storages which lumped whole catchment processes into single computational nodes with simplified mathematical descriptions. With the development of the first personal computers in the 1980ís these lumped conceptual models began to develop as simple modeling systems. These lumpedconceptual models have dominated the surface water hydrological sciences within Australia because of their ease of use and limited data requirements. The physicsbased, distributed model approach was developed as an alternative to the traditional lumped-conceptual rainfallrunoff model type models.

Changing to a physics based watershedbased water management system challenges not only our management structures, but it also requires alternative and more sophisticated tools than are currently employed within the water community. The modeling tools that are employed within the water sector are important as they will have some influence (if not all the influence) in almost all decisions are made regarding the management of our water resources. It is therefore critical that we look at alternative modeling tools if we are going to redefine the way we make decisions. These alternative tools are not necessarily ìnewî tools and many of were developed in an era when the limitations of computing power and remote sensing techniques, limited their utility. However, times have changed! Traditional groundwater and surface water models employed within the Australian water community today were not designed to answer questions related to conjunctive use of groundwater and surface water. They have proven wanting when assessing water quality impacts of surface water on groundwater, impact of land-use changes and urban development on water resources, and floodplain and wetland management. Today and into the future we must move towards physics based, fully integrated hydrologic models of the watershed behavior. We can no longer make the arguments of the past in relation to perceived limitations of integrated physics based hydrological modeling. These fully integrated physics based hydrological models must not only describe the water flow processes in an integrated fashion, but they must also be able to describe the movement of sediment, chemicals, nutrients, and waterborne organisms and their role in watershed habitats and ecology.

The SHE Story
From 1977 onwards, The Institute of Hydrology in the United Kingdom, SOGREAH in France, and the Danish Hydraulic Institute in Denmark (now called DHI) developed, and extensively applied a fully integrated hydrological cycle model, the SystËme Hydrologique EuropÈen (SHE). The project was funded by the European Commission in an attempt to realize the vision of Freeze and Harlan from the 1960ís. In the mid- 1980ís, the integrated hydrological modeling system, MIKE SHE, emerged from this work, which has been further developed and extended by DHI. Early applications of MIKE SHE included, the Wye Catchment in the UK in the mid 1980ís. This small fully integrated model was developed to predict flood levels at the catchment outlet. The model required several hours of computational time on a Cray super computer of the day. Today, such a model could be simulated in seconds on a laptop. Applications in Australia in the 1990ís were also challenged by data and computational requirements. In the 90ís the application of SHE on pilot areas within New South Wales demonstrated that the current separation of ground and surface water management was likely producing a double accounting of water availability in some systems. However, the availability of sufficient data and computer facilities limited the opportunities to resolve these accounting issues. Over the last decade, continuing advances in data collection and availability as well as computer resources have now made distributed, physics-based watershed modeling feasible in a wide range of applications.

Process-based hydrologic modeling
MIKE SHE, in its original formulation, could be characterized as a deterministic, physics-based, distributed model code. It was developed as a fully integrated alternative to the traditional lumped-conceptual rainfall-runoff model type models. A physics-based model is one that solves the partial differential equations describing mass flow and momentum transfer. The parameters in these equations can be obtained from measurements and used in the model. For example, the St. Venant equations (open channel flow) and the Darcy equation (saturated flow in porous media) are physics-based equations. There are, however, challenges to the applicability of such physics-based models.

For example,

• it is widely recognized that such models require a significant amount of data and the cost of data acquisition may be high;
• the relative complexity of the physicsbased solution requires substantial execution time; 
• the relative complexity may lead to overparameterized descriptions for simple applications; and 
• a physics-based model attempts to represent flow processes at the grid scale with mathematical descriptions that are often only valid for small-scale experimental conditions.

Therefore, it is often practical to use simplified process descriptions as substitution in some cases. In most watershed problems one or two hydrologic processes dominate the watershed behavior. For example, flood forecasting is dominated by river flows and surface runoff, while wetland restoration depends mostly on saturated groundwater flow and overland flow. Thus, a complete, physics-based flow description for all processes in one model is not necessary. A sensible way forward is to use physics-based flow descriptions for only the processes that are important, and simpler, faster, less data demanding methods for the less important processes. The downside is that the parameters in the simpler methods are usually no longer physically meaningful, but must be calibrated-based on experience.

The process-based, modular approach used in MIKE SHE has made it possible to add multiple numerical methods for each of the hydrologic processes. In the simplest case, MIKE SHE can use fully distributed conceptual approaches to model the watershed processes. For advanced applications, MIKE SHE can simulate all the processes using physics-based methods. Alternatively, MIKE SHE can combine conceptual and physics-based methods-based on data availability and project needs. The flexibility in MIKE SHE's process-based framework allows each process to be solved at its own relevant time step and spatial scale. For example, evapotranspiration varies over the day and surface flows respond quickly to rainfall events, whereas groundwater reacts much slower. In contrast, most non-commercial, researchoriented integrated hydrologic models are tied to a single numerical method, which is often either too simple or too numerically challenging. For example, in many modeling systems, all the hydrologic processes are solved implicitly at a uniform time step, which can lead to intensive computational effort for watershed scale models.

Agriculture
Pasture and crops take up nearly 40% of the Earth's land area and worldwide about 70% of all fresh water usage is directed at irrigation. However, in most settings water for irrigation is neither metered, nor easily forecast because irrigation is applied only when it is required. Irrigation is further complicated by water rights and complex management rules. MIKE SHE can simulate a wide range of irrigation practices with multiple sources. Irrigation management can be simulated using distributed temporal crop water demand. It includes the conjunctive use of surface and groundwater with the option of setting irrigation priorities and control strategies based on soil moisture levels.

Water Quality
Advection/dispersion methods are used to address problems where the exchange of contaminants between the hydrologic processes is important - that is transport in and exchange between overland flow, channel flow, unsaturated flow, and saturated groundwater flow. MIKE SHE can simulate transport of water and solutes in macropores and fracture systems, with exchange to the surrounding bulk matrix. It can also simulate equilibrium and kinetic sorption (including hysteresis), first-order decay that depends on soil temperature and soil moisture content, sequential biodegradation, and plant uptake with transpiration. In addition to the advection/dispersion method, a random walk particle tracking method is also available for the saturated groundwater zone.

Ecological modeling is a relatively immature discipline that involves many different processes and networks of interacting subsystems. To this end, a general ecological modeling tool (ECOLab) has been developed that enables engineers and ecosystem experts to develop their own ecosystem models appropriate to site-specific ecological conditions. ECOLab can address problems such as eutrophication in surface water, and the retention and removal of nutrients and pollutants in wetlands.

Water Budgeting
The hydrologic cycle is all about water exchange and the analysis of this exchange is the water budget. Questions regarding sustainability and environmental impacts are directly related to the water budget. Since MIKE SHE includes all of the processes in the hydrologic cycle, MIKE SHE includes a sophisticated water budgeting tool for summarizing, mapping and plotting the exchange of water between all of the hydrologic processes.

Flooding
MIKE SHE is being used to meet the growing need for flood modeling, flood forecasting and flood hazard assessment. MIKE SHE, is being used to calculate the impacts of urban flooding. This is especially true when questions related to evaporation, infiltration and ecology are important.

MikeSHE Today
Today, MIKE SHE is an advanced, flexible framework for hydrologic modeling. It includes a full suite of pre- and postprocessing tools, plus a flexible mix of advanced and simple solution techniques for each of the hydrologic processes. MIKE SHE covers the major processes in the hydrologic cycle and includes process models for evapotranspiration, overland flow, unsaturated flow, groundwater flow, and channel flow and their interactions. Each of these processes can be represented at different levels of spatial distribution and complexity, according to the goals of the modeling study, the availability of field data and the modeler's choices. The MIKE SHE user interface allows the user to intuitively develop a numerical model based on the user's conceptual model of the watershed. The model data is specified in a variety of formats independent of the model domain and grid, including native GIS formats. At run time, the spatial data is mapped onto the numerical grid, which makes it easy to change the spatial discretisation.

MIKE SHE uses MIKE 11(a fully dynamic 1D River Model) to simulate channel flow. MIKE 11 includes comprehensive facilities for modeling complex channel networks, lakes and reservoirs, and river structures, such as gates, sluices, and weirs. In many highly managed river systems, accurate representation of the river structures and their operation rules is essential. In a similar manner, MIKE SHE is also linked to the MIKE URBAN (a fully dynamic 1D sewer and pipe model), which can be used to simulate the interaction between urban storm water and sanitary sewer networks and groundwater. MIKE SHE is applicable at spatial scales ranging from a single soil profile, for evaluating crop water requirements, to large regions including several river catchments, such as the 350,000 km2 Senegal Basin.

MIKE SHE has proven valuable in hundreds of research and consultancy projects around the world covering a wide range of climatological and hydrological regimes. MIKE SHE is used across Australia in projects ranging from watershed management to wetland-plantation interaction to salinity risk assessment.

Summary
Few modelling systems applied today in industry and science have been designed and developed to fully integrate surface water and groundwater processes. Even less hydrological cycle models have been applied outside of the academic community. MIKE SHE is one of the few publically available and commercially supported models that have been widely used for integrated hydrologic modeling. MIKE SHE's process based framework allows each hydrologic process to be represented according to the problem needs at different spatial and time step scales. This flexibility has allowed MIKE SHE to be applied at spatial scales ranging from single soil profiles, to the field scale, and up to the watershed scale. Furthermore, each process can be represented at different levels of complexity. MIKE SHE has a modern, Windows-based user interface that includes advanced tools for water quality, parameter estimation and water budget analysis.

MIKE SHE is used in Australia and around the world for integrated catchment management, land-use and climate change evaluation, wetland rehabilitation and protection, and integrating water quantity and quality on a watershed basis. Thus, hydrologic modeling has become an essential tool in catchment management, with two fundamental roles. The first role is to improve our understanding of the physical, chemical and biological processes within a watershed and the way they interact. The second, more practical role is to apply this understanding to manage and protect our water resources and the water environment. Many challenges remain on both fronts, but integrated modelling tools, such as MIKE SHE, are making important contributions to these challenges.

References

Abbott, M. B., Bathurst, J. C., Cunge, J. A., O'Connell, P.E. and Rasmussen, J. (1986) An introduction to the European Hydrological SystemóSysteme Hydrologique EuropÈen, SHE. 1 History and philosophy of a physically-based distributed modelling system. J. Hydrol. 87, 45ñ59.

Abbott, M. B., Bathurst, J. C., Cunge, J. A., O'Connell, P. E. and Rasmussen, J. (1986) An introduction to the European Hydrological SystemóSysteme Hydrologique EuropÈen, SHE. 2 Structure of a physically-based distributed modelling system. J. Hydrol. 87, 61ñ77.

Freeze, R.A., and Harlan, R.L. (1969) Blueprint of a physically-based, digitially simulated hydrologic response model, J. Hydrol. 9, 237-258