Introduction

All of us face risks and uncertainties in our lives. No less is it so for those responsible for planning and managing water resource systems. Their job is to provide the desired quantity and quality of water, at reasonable costs, when and where it is needed. They are asked to reduce the extent, and adverse impacts, of floods and droughts, and provide the water needed for a variety of purposes, some of which are conflicting, and at the same time satisfy environmental and ecological goals. They must do this in ways that will best meet society's needs for water now and into the future without knowing how much water will be available in the future and what the temporal and spatial demands for it will be.

Not only are supplies and demands variable and uncertain, this variability and uncertainty is changing in ways we cannot predict. And to add to all this complexity, we cannot predict with certainty many of the economic, environmental, ecological, and social impacts resulting from various system infrastructure design and operating policy decisions – that are made in an effort to meet these needs and demands of society and which themselves are changing in uncertain ways over time.