Watershed-at-risk analysis as a guide for ground water management

Presentation (pdf document)

Loiselle, Marc C. and Marvinney, R.G., Maine Geological Survey

Over the past year, the Maine Geological Survey has been meeting with representatives from state agencies, agricultural irrigators, commercial water bottlers, water utilities, and interested parties to review the regulations that govern ground water withdrawals. To help focus this effort, the Maine Geological Survey undertook a preliminary analysis of watersheds at risk using available datasets. For this analysis, a “watershed-at-risk” is defined as a watershed where the sum of the required instream flows plus the consumptive water withdrawals exceeds some percentage of the total runoff generated within the basin or watershed. For this GIS exercise, the 12-digit Hydrologic Unit Code (HUC) watersheds were used for comparing net runoff to minimum instream flow requirements plus consumptive water use. The long-term average annual runoff generated in a watershed was estimated from a regional regression equation developed by the USGS (Dudley, 2004). We used the provisional instream flow rules adopted by the BEP to determine the instream flow requirements. Finally, we assigned to each watershed consumptive water uses from a number of sources:

• public water supply withdrawals as reported to the Public Utilities Commission
• reported water use by other major water users as required by law.
• estimates of withdrawals by smaller water users regulated by the Maine Drinking Water Program.
• aqricultural water withdrawals as reported to the Department of Agriculture, Food and Rural Resources.
• private domestic water use estimated from 1990 Census data on percent of households on private wells (the latest data of this type available) and 2000 Census population data.

Using a 90-percent threshold, we found 11 watersheds with minimum required instream flow plus estimated consumptive water use equal to or greater than the threshold. This analysis is the basis for recommendations to the Legislature for improved stewardship of Maine’s ground water resources.