Forecasting Exempt Well Demand in Spokane County Presented by: Mike Hermanson Water Resources Specialist Spokane County Utilities Exempt Well Conference May 17, 2011
Background Spokane County Water Demand Forecast Model Project Completed in January 2011 Model included all water use sectors including self supplied residential, or exempt wells County Forecast Public Supply Self Supply Residential Self Supply Industry Agricultural Commercial Industrial Single Family Residence & Yard Thermoelectric Power Livestock Urban Irrigation Multi-Family Small Agriculture Golf Courses Irrigated Acres Public-Supply Agriculture Non Revenue Water Large Industry Forecasting Exempt Well Demand in Spokane County
Inventory & Forecasting Approach 4 objectives for Exempt Well Water Use Model Inventory and forecast for sub-areas Seasonal aspects of use Identify and incorporate factors that influence water use Model differences between geographic areas of the county Two primary aspect to the model The number of demand units Residences Water use per residence
Inventory & Forecasting Approach Quantity of Demand Units Separated into forecast units rural areas based on watershed boundaries Data from: Spokane Regional Transportation Council forecast & Spokane County Assessor US Census Forecasting Exempt Well Demand in Spokane County
Inventory & Forecasting Approach Water Use Per Unit Limited available data Approach: Survey self supplied residential water users & suburban public supply water users Determine if survey results can be used to develop a relationship between the two groups Use relationship to estimate exempt well water use.
Inventory & Forecast Approach Residential Water Use Survey 1,249 responses 53% Public 47% Self Supplied Survey: Household information Water supply information Indoor water use information Quantity of irrigated landscape Irrigation quantity/timing Livestock quantity Location of Survey Responses
Inventory & Forecast Approach Survey Results & Statistical Analysis: Indoor water use was not significantly different between self supplied and public supplied residences. There is not a significant difference in the amount of irrigated landscape between self supplied and public supplied residences 8,000 sq ft. The average watering time per week was not significantly different. Self supplied residences with a well yield 5 gpm and below have on average 50% less irrigated landscape. Self supplied residences located in forested areas have on average 25% less irrigated landscape. 20% of self supplied residences have at least one livestock
Inventory & Forecast Approach Determined that using the public supplied residential model with correction factors for limited water availability areas and forested areas is appropriate
Demand Forecast Model Single Family Residential Model: Analyzed 20 years of monthly water use data from 15 water systems Used Cross Sectional Time Series Regression to develop an econometric model Household Characteristics Weather
Demand Forecast Model Public Supplied Residential Econometric Model Indoor model (r 2 = 0.55): Water use is a function of: household income Outdoor model (r 2 = 0.74) : Water use is a function of: home value, temperature, precipitation, and lot size. Forecasting Exempt Well Demand in Spokane County
Demand Forecast Model Forecasting Exempt Well Demand in Spokane County
Demand Forecast Model Adjustment for Exempt Wells Households Well Yield, Land Cover & Livestock agricultural/pasture forested
Demand Forecast Model Data needs for each forecast unit: # of current & forecasted residences Average household income Average lot size Average assessed value Temperature & Precipitation Land cover Well yield
Forecast & Analysis Subbasin Analysis Evaluated increases in withdrawals from exempt wells. Forecasting Exempt Well Demand in Spokane County
Forecast & Analysis Summer Low Flow in Marshall Creek = 1.60 cfs Increase in July withdrawals from Marshall Creek Subbasin = 1.47 cfs Summer Low Flow in Deep Creek = 1.5 0.6 cfs Increase in July withdrawals from Deep Creek Subbasin = 1.78 cfs Local Hydrogeology must be understood to determine actual impacts of withdrawals on stream flow.
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