Water Year Definition

To define each non-Normal water year type (Very Dry, Dry, Wet, Very Wet), specify how much more or less water flows into the system in that year relative to a Normal water year. For example, if a Wet year has 25% more inflow than a Normal year, enter 1.25 for the Wet year. Typically, you would derive these fractions from a statistical analysis of historical flows. First you would group the years into five bins (quintiles), then compute how they vary from the norm, perhaps by month. Note: the Current Accounts year is not necessarily a Normal water year.

You may specify a single variation fraction for an entire water year type, or you may specify a different fraction for each month. Your data might show, for example, that the winter months of a Dry year average 50% of a Normal winter, while the summer months are closer to 75% of Normal summer inflows.

A simple way to explore sensitivity to climate change would be to define two scenarios. The first would use the Water Year Method to reproduce the observed variation in hydrology from the historical record. The second scenario would use the first as a starting point, but alter each water year type according to predicted effects of climate change (e.g., wetter winters and dryer summers).

Entered on: Data View, Branch: Hydrology \ Water Year Method, Tab: Definitions (applies to scenarios only, not to the Current Accounts)