Chapter 19 Fitting Hydrological Models on Multiple Responses Using the Multiobjective Evolutionary Annealing-Simplex Approach A. Efstratiadis and D. Koutsoyiannis Abstract Most complex hydrological modeling schemes, when calibrated on a sin- gle observed response (e.g., river flow at a point), provide poor predictive capability, due to the fact that the rest of the variables of basin response remain practically un- controlled. Current advances in modeling point out that it is essential to take into account multiple fitting criteria, which correspond to different observed responses or to different aspects of the same response. This can be achieved through multiob- jective calibration tools, thus providing a set of solutions rather than a single global optimum. In addition, actual multiobjective optimization methods are rather ineffi- cient, when real-world problems with many criteria and many control variables are involved. In hydrological applications there are some additional issues, due to un- certainties related to the representation of complex processes and observation errors. The multiobjective evolutionary annealing-simplex (MEAS) method implements an innovative scheme, particularly developed for the optimization of such problems. Its features and capabilities are illustrated by solving a challenging parameter estima- tion problem, dealing with hydrological modeling and water resource management in a karstic basin in Greece. Keywords Parameter estimation · conjunctive hydrological models · evolutionary multiobjective optimization · irregular pareto front · model uncertainty 19.1 Introduction The parameter estimation procedure of hydrological models aims toward a faith- ful reproduction of observed outputs, in addition to establishing “behavioral” (i.e., A. Efstratiadis Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Heroon Polytechneiou 5, 157 80 Zographou, Greece, e-mail: andreas@itia.ntua.gr D. Koutsoyiannis Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Heroon Polytechneiou 5, 157 80 Zographou, Greece, e-mail: andreas@itia.ntua.gr R.J. Abrahart et al. (eds.), Practical Hydroinformatics. Water Science 259 and Technology Library 68, c  Springer-Verlag Berlin Heidelberg 2008