Building an agro-hydrologic model of Europe: model calibration issues

A combination of driving forces are increasing pressure on local, national, and regional water supplies needed for irrigation, energy production, industrial uses, domestic purposes, and the environment. In many parts of Europe groundwater quantity, and in particular quality, have come under sever degradation and water levels have decreased resulting in negative environmental impacts. Rapid improvements in the economy of the eastern European block of countries and uncertainties with regard to freshwater availability create challenges for water managers. At the same time, climate change adds a new level of uncertainty with regard to freshwater supplies. To address the above issues, a calibrated and reliable model of the region is of paramount importance. The Soil and Water Assessment Tool (SWAT) was used to build an agro-hydrological model of Europe. We simulated water resources as well as wheat, barley, and maize yields, and nitrate concentration of the surface and percolating water at the Hydrological Response Unit (HRU) level. The use of large-scale, high-resolution water resources models enables consistent and comprehensive examination of integrated system behavior through physically-based, data-driven simulation. However, calibration of large-scale models suffers from a number of conceptual and technical issues, which we believe require a more careful consideration by the scientific community. The issues with large-scale model calibration include: 1) parameterization, 2) non-uniqueness, 3) objective function definition, 4) use of different optimization algorithms, 5) model conditionality, 6) inadequate definition of the base model, and 7) time constraints. A software for calibration of SWAT, SWAT-CUP was develop to enable the analysts to deal or to analyze some of these issues. In SWAT-CUP, i) detailed parameterization scheme is included to enable regionalization of parameters at the smallest SWAT unit (HRU); ii) Nonuniqueness is captured by including programs that address uncertainty issues; iii) 11 different objective functions are included to highlight the effect of using different objective functions; iv) 5 different optimization methods are included to highlight the effect of using different algorithms; and v) a Windows-based parallel processing scheme is includes to speed up the calibration procedure. In light of the above issues, “conditionality” of the calibrated model becomes an important issue, which can partially be addressed by using different variables in the objective function. Finally, we emphasize the importance of performing pre-calibration processing of the hydrologic model and the use of an initially adequate model to be sued for calibration. The SWAT agro-hydrological model developed for Europe was calibrated based on a large number of river discharge outlets and measured nitrate, mainly in the Danube Basin. Results of major river discharges and river nitrate concentration were quite well simulated all over Europe and in the Danube Basin, respectively. The results produced here could provide information support to the European Water Framework Directive and lay the basis for further assessment of the impact of climate change on water availability and quality. Many applications of this model could be foreseen such as: conducting policy and impact studies, calculating crossboundary water transfers, calculating quantities of nitrogen loads being transferred from upstream to downstream of a river, and calculating nitrogen loads entering the seas and ocean. The approach and methods developed are general and can be applied to any large region around the world.