Human water withdrawals and return flows alter the magnitude and temporal variability of natural streamflow. Water resource assessments use streamflow indicators to measure and map the degree of alteration and potential ecosystem stress in a watershed. The spatial scale at which a water resource assessment is performed can affect the accuracy and usefulness of the results. Indicators applied at large spatial scales emphasize the conditions of large streams and potentially mask stress conditions in headwater areas. Recent developments in spatial geographic and hydrologic information databases enable regional and local water resource assessment to be performed at a much finer spatial scale than was previously possible, leading to very different and more accurate descriptions of water resource stress. A case study is presented which demonstrates a new method for regional water assessment and documents that in Massachusetts, spatial scales larger than USGS 12-digit hydrologic units lead to a generally unacceptable level of accuracy in the interpretation of resulting indicator maps.
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