Development of Water Demand Forecasts for the OWASA Comprehensive Water/Sewer Master Plan

PMCL, as subcontractor to CH2M-Hill, developed the water demand forecasts as one component of the Comprehensive Water and Sewer Master Plan for the Orange Water and Sewer Authority (OWASA). OWASA provides water and sewer service to the municipalities of Carrboro and Chapel Hill, North Carolina, as well as the campus and hospital facilities of the University of North Carolina - Chapel Hill. Water use characteristics were evaluated from monthly data for the single-family, multifamily, commercial, irrigation-only, and university sectors. The single-family, multifamily, and university sectors each averaged more than 25 percent of total water consumed. Historical and projected service area demographics (housing, employment, and university facility gross square footage) were prepared by CH2M-Hill. Log-linear regression models were developed with monthly indicators and weather departure variables for each sector and subsector. Water demand forecasts were estimated using the Forecast Manager of the IWR-MAIN Water Demand Management Suite© for all combinations of expected, high, and low growth with normal, cool-wet, and hot-dry weather conditions.

The end-use model of the Conservation Manager of the IWR-MAIN Water Demand Management Suite© was used to estimate the impact of conservation on future water demand. To the extent possible, conservation assumptions were based on previous end-use studies and discussions with OWASA and University of North Carolina staff. A baseline forecast was calibrated to the expected growth/normal weather forecast scenario. Seven potential water demand management programs, plus two emergency restriction programs were stipulated with estimated implementation conditions and assumed program costs. The impact of these programs, as well as the impact of passive conservation, were estimated using the IWR-MAIN Conservation Manager software.

Passive conservation savings were estimated to reduce the estimated 2050 total raw water demand by 18 percent. The baseline characteristics of the OWASA service area indicated that most of the water demand is for indoor use, with about 17 percent of 1995 annual average use for toilet usage. Thus, the impact of passive conservation is significant over the 50-year planning horizon. Active conservation provided an estimated additional 4 percent savings in 2010 and 1 percent savings in 2050. The benefit-cost analysis compared the estimated program implementation costs with the savings from deferral of reservoir treatment plant expansion options in future years. Recommendations were made for improving the understanding of the seasonal water use (especially irrigation use), information about the characteristics of water use (i.e., the distribution of end uses within the residential and university sectors), and refining the conservation program implementation costs. Given the uncertainty of the conservation parameters, future weather, and future growth, the expected growth/normal weather baseline forecast was selected for use in subsequent elements of the overall OWASA Master Plan.