Hydrosystem management is a demanding and challenging task due to the complexity of the system dynamics, the number of decision variables and the hydrological uncertainty. In order to overcome those issues the parsimonious parameterization-simulation-optimization (PSO) framework is employed coupled with stochastically generated hydrological time-series. However, this coupling imposes a computational barrier to the framework. The purpose of this paper is threefold: a) Investigate the potential of Efficient Global Optimization (EGO) algorithm which is capable of reaching global optima within a few simulation model evaluations b) Extend the capabilities of WEAP21 water resources management model by using it within PSO framework (WEAP21-PSO) and c) Validate and compare the results of WEAP21-PSO using the well-known hydrosystem management model Hydronomeas DSS. Results confirm that EGO has the potential to handle effectively computationally demanding optimization problems performing few simulation model evaluations and that the WEAP21-PSO framework performs well at the task at hand.