Forecasting Exempt Well Demand in Spokane County

Presented by:

Mike Hermanson Water Resources Specialist

Spokane County Utilities

Exempt Well Conference

May 17, 2011

Forecasting Exempt Well Demand in Spokane County


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

PublicSupply

Self Supply Residential

Self Supply

IndustryAgricultural

CommercialIndustrial

UrbanIrrigation

Public-SupplyAgriculture

Single Family

Multi-Family

Non RevenueWater

Residence& Yard

SmallAgriculture

ThermoelectricPower

Golf Courses

Large Industry

Livestock

IrrigatedAcres


• 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


• Separated into forecast units – rural areas based on watershed boundaries

• Data from: – Spokane Regional

Transportation Council forecast &

– Spokane County Assessor

– US Census


Quantity of Demand Units



• 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.

Water Use Per Unit


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



• 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


• 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



• Public Supplied Residential Econometric Model

– Indoor model (r2 = 0.55): • Water use is a function of:

– household income

– Outdoor model (r2= 0.74) : • Water use is a function of:

– home value,

– temperature,

– precipitation, and

– lot size.




• Adjustment for Exempt Wells Households – Well Yield, Land Cover & Livestock


agricultural/pasture

forested



• 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.


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 .


• For more information

– www.spokanecounty.org – Spokane County>Water Resources>Projects>Water Demand Forecast Model

http://www.spokanecounty.org/

Documents

Forecasting Exempt Well Demand in Spokane County