Advances in ecohydrological modelling with SWAT—a review

Ecohydrology is an integrative science studying the relationships between hydrological, biogeochemical and ecological processes in soils, rivers and lakes, and at the catchment scale. It proposes a “dual regulation” of a system by simultaneously studying ecological and hydrological processes to enhance the overall integrity of aquatic ecosystems in the face of human-driven alterations and Global Change (see definition in the UNESCO Ecohydrology Programme at http://typo38.unesco.org/en/ecohydrology.html). Ecohydrology deals with hydrological factors which determine the dynamics of natural and human-driven terrestrial ecosystems, and which, together with ecological factors, influence water dynamics and water quality. River basins have a hierarchical structure and natural boundaries, and can be considered as inherent integrators of the effects of many climatic and non-climatic factors. That is why river basins represent a suitable scale for integrated ecohydrological studies and modelling. An ecohydrological river basin model includes a hydrological submodel as a basic component. Other components describing biogeochemical cycles (carbon, nitrogen and phosphorus) and vegetation are coupled with the hydrological component, in order to include important interactions and feedbacks between the processes, such as water and nutrient drivers for plant growth, water transpiration by plants, and nutrient transport with water. Generally, vertical and lateral fluxes of water and nutrients in catchments are modelled separately, whereas climate and land-use related parameters are treated as external drivers. The spatial and temporal resolution of a model depends on data availability and the aim of the study. The scale of application, spatial resolution and objective of the study are connected: a fine spatial resolution may be needed for a small catchment in order to study water flow components and their pathways using tracers; a lumped model may be sufficient for the case where “precipitation–runoff” relationships are investigated in a homogeneous medium-scale catchment; whereas a coarser resolution could be applied for a mesoscale or large river basin for climate impact assessment. There are many different classifications of river basin models. The differentiating principle could be the modelling approach or the scale of model application. For example, one can distinguish physically-based, conceptual, or black-box models; lumped and distributed models; and deterministic and stochastic models. A physically-based hydrological or ecohydrological model describes the natural system using mainly basic mathematical representations of physical laws governing the transfer of mass, momentum and energy. As a rule, such a model has to be fully distributed by accounting for spatial variations in all variables and parameters. However, the inclusion of physical laws in a model does not by itself guarantee its high quality. Even if physical laws included in the model represent a rigorous mathematical description for a soil column under laboratory conditions where soil has been well mixed, this may not automatically be the case at the scale of the grid elements used in distributed hydrological models: hundreds of metres or even kilometres (Beven, 1996). Besides, the so-called physically-based models often include empirical and statistical equations, especially to represent non-hydrological processes. In contrast, the description of important physical processes is lacking in the simplified conceptual hydrological models (e.g. water movement through soil layers) and, therefore, it is