The MIT Water Resource System for the U.S. (WRS-US) component of the Integrated Global System Model (IGSM) framework is modified to be more computationally efficient and more flexible. For this purpose, version 2.0 of the WRS-US model now solves all basins, or Assessment Sub Regions (ASR), simultaneously. Additionally, the model now includes two objective functions and a new calculation procedure is now adopted to estimate initial storage capacity.