Assessment of climate and land use change impacts with SWAT

The Soil and Water Assessment Tool (SWAT) is a deterministic continuous-time semi-distributed hydrological model for application at the river basin scale. The model was developed by the USDA Agricultural Research Service in the early 1990s based on Simulator for Water Resources in Rural Basins (SWRRB) and including routing procedures from Routing Outputs To Outlet (ROTO) tool (Arnold et al. 1995, 1998). SWAT is initialized by spatially distributed data on topography, land use/land cover, soils, land management and climate. The spatial disaggregation scheme includes subbasins delineated based on topography, which are further subdivided into nonspatial hydrological response units using the principle of similarity. Several versions of GIS interface were developed for SWAT enabling the model parametrization and initialization from maps and related data, starting from a GRASS GIS version (Srinivasan and Arnold 1994) to the current ArcGIS 10.1/10.2.2 interface version. From the early 1990s, the SWAT model has undergone continuous testing, review, modifications and enhancement of many modules (Arnold et al. 1998; Neitsch et al. 2004, 2005; Gassman et al. 2007). The model is process-based and simulates water flows in soil and groundwater, crop/vegetation growth, nutrient cycling, erosion, pesticides, bacteria, lateral flows of water, sediments and nutrients, as well as environmental impact of land management practices on the main model outputs. A number of agricultural practices can be simulated, such as fertilizer and manure application, tillage, irrigation, subsurface drainage, buffer strips, as well as wetlands and ponds. The river routing component in SWAT routes the simulated water flows, sediments, nutrients, pesticides and bacteria along the river network in a basin. The reservoir module accounts for transformation of water fluxes, nutrients and sediments when passing the reservoir. The main model outputs include water, sediments, nutrients, pesticides and bacteria yields that can be provided for every subbasin outlet at a daily time step. The SWAT tool belongs to a family of models, with several versions that are currently being distributed (see more information in Douglas-Mankin et al. 2010 and on the SWAT website: http://swat.tamu.edu/). Besides, several currently applied models were developed based on SWAT and still have many similar or identical components. The main reasons for this development were to adapt the model to other data formats outside USA, to modify/enhance some specific components, and to add new components. Among them are the following: the SWIM model (Soil and Water Integrated Model) (Krysanova et al. 1998, 2000), aiming mainly at climate and land use change impact assessment at a large regional scale; the ESWAT model (van Griensven and Bauwens 2005) with an adjustment for the hourly time step; the ALMANAC model (Kiniry et al. 2008) focused on simulation of crops, perennial grasses and forest ecosystems including successional changes; the coupled surface and groundwater model SWAT–MODFLOW (Kim et al. 2008); and the APEX model (Gassman et al. 2010) developed for assessment of various agricultural management strategies and accounting for economical costs. V. Krysanova (&) Potsdam Institute for Climate Impact Research (PIK), PO Box 601203, Telegrafenberg, 14412 Potsdam, Germany e-mail: [email protected]