2015 Washington Metropolitan Area Water Supply Study Demand and Resource Availability Forecast for the Year 2040 Prepared by S.N. Ahmed, K.R. Bencala, and C.L. Schultz August 2015 ICPRB Report No. 15-4 The Section for Cooperative Water Supply Operations on the Potomac Interstate Commission on the Potomac River Basin 30 West Gude Drive, Suite 450 · Rockville, Maryland 20850 2015 Washington Metropolitan Area Water Supply Study: Demand and Resource Availability Forecast for the Year 2040 Prepared by S.N. Ahmed, K.R. Bencala, and C.L. Schultz August 2015 ICPRB Report No. 15-4 Copies of this report are available at the ICPRB website, at www.PotomacRiver.org, under “Publications.” To receive printed copies of this report, please write to ICPRB at 30 West Gude Drive, Suite 450, Rockville, MD 20850; or call 301-984-1908. 2015 Washington Metropolitan Area Water Supply Study Table of Contents Acknowledgements .................................................................................................................................... viii Disclaimer .................................................................................................................................................. viii List of Abbreviations ................................................................................................................................... ix Executive Summary ..................................................................................................................................... xi Recent & Forecasted Water Use .............................................................................................................. xi Upstream Consumptive Demand ........................................................................................................... xiii Ability of Current System to Meet Forecasted Demands ...................................................................... xiv System Performance under Repeat of Historical Drought Conditions ............................................... xv System Performance under Climate Change....................................................................................... xv Recommendations .................................................................................................................................. xvi 1 Study Objective & Background ......................................................................................................... 1-1 Objective .................................................................................................................................... 1-1 Introduction ................................................................................................................................ 1-2 Water Suppliers .......................................................................................................................... 1-4 History of Cooperation............................................................................................................... 1-4 2 Overview of the Washington Metropolitan Area Water Supply System ........................................... 2-1 System Demands ........................................................................................................................ 2-1 Water Service Areas ........................................................................................................... 2-1 Historical Water Production Trends................................................................................... 2-2 System Resources ...................................................................................................................... 2-4 Potomac River .................................................................................................................... 2-4 Shared Reservoirs .............................................................................................................. 2-4 Additional Resources ......................................................................................................... 2-5 3 Annual Demand Forecast ................................................................................................................... 3-1 Introduction ................................................................................................................................ 3-1 Method for Determining Past Unit Use Rates............................................................................ 3-1 Utility Billing Data............................................................................................................. 3-2 Current & Past Demographic Information ......................................................................... 3-2 Past Unit Use Rates .................................................................................................................... 3-4 Unit Use Trends ................................................................................................................. 3-6 i 2015 Washington Metropolitan Area Water Supply Study Method for Forecasting Unit Use Rates ..................................................................................... 3-9 Selecting Unit Use Rate for Beginning of Demand Forecast Period ............................... 3-10 Potential Changes in Customer Demand.......................................................................... 3-10 Unit Use Forecast ..................................................................................................................... 3-11 Method for Developing the Annual Demand Forecast ............................................................ 3-13 Demographic Forecast ..................................................................................................... 3-13 Estimate of Unmetered Water Use ................................................................................... 3-18 Annual Demand Forecast ......................................................................................................... 3-19 Comparison of Annual Demand Forecast with Previous Estimates ........................................ 3-21 4 Modeling Daily Variations in Water Demand ................................................................................... 4-1 Data ............................................................................................................................................ 4-2 Removing the Long-term Time Trends ..................................................................................... 4-3 Monthly Mean Production ......................................................................................................... 4-6 Regression Models ..................................................................................................................... 4-9 ARIMA Model ......................................................................................................................... 4-13 Model Demonstration .............................................................................................................. 4-15 Effects of Water Use Restrictions ............................................................................................ 4-17 5 Modeling System Resources & Operations in PRRISM .................................................................... 5-1 Potomac River Flow .................................................................................................................. 5-1 Potomac Flow Recommendations ...................................................................................... 5-3 Reservoir Operations ................................................................................................................. 5-4 North Branch Reservoirs .................................................................................................... 5-4 Use of the Little Seneca, Occoquan, & Patuxent Reservoirs ............................................. 5-7 Effects of Sedimentation on Reservoir Storage ....................................................................... 5-11 Occoquan Reservoir ......................................................................................................... 5-12 Patuxent Reservoirs.......................................................................................................... 5-12 Little Seneca Reservoir .................................................................................................... 5-13 Jennings Randolph Reservoir ........................................................................................... 5-13 Treated Wastewater Return Flows ........................................................................................... 5-15 Production Losses .................................................................................................................... 5-17 Loudoun Water Quarry & Water Treatment Plant ................................................................... 5-19 6 Upstream Consumptive Demand ....................................................................................................... 6-1 Current Upstream Consumptive Use by Use Type .................................................................... 6-1 ii 2015 Washington Metropolitan Area Water Supply Study Consumptive Demand Estimates for Potomac River Basin Public Water Suppliers ................. 6-3 Forecasts of Upstream Consumptive Use .................................................................................. 6-8 7 Climate