Appendix C-2 Water Balance 2008 (PDF)

Golder Associates Inc. 18300 NE Union Hill Road, Suite 200 Redmond, Washington 98052 Telephone: (425) 883 0777 Fax: (425) 882 5498

TECHNICAL MEMORANDUM

TO: Joe Simmler and Larry Grimm., OTAK, Inc. DATE: December 19, 2008 FR: Alyssa Neir and Carl Einberger, L.Hg. OUR REF: 083-93430.001 ESTIMATED AVERAGE ANNUAL WATER BALANCE, GREATER HALL LAKE, RE: HALL CREEK, CHASE LAKE, ECHO LAKE, LAKE BALLINGER AND MCALEER CREEK WATERSHED

This technical memorandum describes the estimated average annual water balance for the greater Hall Lake, Hall Creek, Chase Lake, Echo Lake, Lake Ballinger, and McAleer Creek Watershed (‘entire watershed’) and the portion of the watershed that drains to Lake Ballinger (‘Lake Ballinger watershed’). The entire watershed is comprised of twelve subbasins that drain to Lake Washington and includes the Lake Ballinger watershed and the downstream watersheds. The entire watershed covers approximately 5,249 acres. The Lake Ballinger watershed covers approximately 3,566 acres and includes the drainage to Hall Lake, Hall Creek, Echo Lake and Lake Ballinger. Estimated water balances were developed for forested conditions (pre-development conditions) and existing conditions for both the entire watershed and Lake Ballinger watershed. The estimated water balances were then compared to provide an estimate of the effect of development on components of the water balance for the entire watershed and the portion associated with the Lake Ballinger watershed.

1.0 ANNUAL WATER BALANCE MODEL

A Geographic Information System (GIS) model was developed based on a series of overlay calculations to develop a water balance and to identify evapotranspiration, groundwater recharge, and surface runoff components of the water balance under forested land use conditions (assuming no development in the watershed) and existing land use conditions (development as of 2008). The model incorporates GIS information on surficial geology, precipitation, and land use. These data are linked using the relationship between surficial geology and groundwater recharge rates developed by Thomas et al. (1997) (Figure 1) and the effective impervious area (EIA) factors used in the 2002 Marshland Tributaries and Sunnyside Creek Drainage Needs Report (Snohomish County, 2002) (Table 1).

The land use data used for this model was developed through interpretation of land cover data provided by Otak (on November 25, 2008). The land cover data is the product of a satellite data land cover classification based on spectral signature, with a group of pixels with similar spectral signature defining a specific land cover. When reviewing this data, it appeared that Hall Lake, Echo Lake and Lake Ballinger were not entirely classified as water, based on the pixel interpretation. To correct for this, the water component of the model was redefined with a more conservative approach, with all area within the lakes correctly reclassified as water. These boundaries were delineated using Edmonds East 1:24,000 USGS topographic map.

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The average annual water balance is calculated based on the average annual precipitation rate for each land use unit (one land use unit corresponds to one pixel of the land use dataset):

PA = RR + ET+ SR, where

PA= Average annual precipitation rate from PRISM (inches/year (in/yr)) RR = Groundwater recharge rate (in/yr) ET = Evapotranspiration rate (in/yr)

SR = Surface runoff rate (in/yr)

These rates were then converted into volumes using the area of the pixel ((3.28)2 feet2) and summed for the twelve subbasins.

1.1 Precipitation

A detailed map of precipitation was incorporated based on the PRISM Model (Spatial Climate Analysis Service at Oregon State University and Climate Source, LLC, 2000). PRISM is an analytical model that distributes point measurements of monthly, seasonal, and annual precipitation to a geographic grid of 4 kilometers by 4 kilometers. These grids are produced in a GIS-compatible latitude-longitude grid or a gridded map projection. The PRISM map represents average annual precipitation from 1960 to 1991.

1.2 Groundwater Recharge

Groundwater recharge within the subbasins was calculated using precipitation, surficial geology, and land use data available in a GIS format and a series of calculations developed in the GIS ModelBuilder tool. The model calculates the amount of groundwater recharge associated with each land use unit to obtain an area-based volume that can then be summed for the land uses located within the subbasins. Groundwater recharge will generally discharge to surface water after some amount of retention time in the subsurface. Our approach is focused on estimating changes in natural recharge to groundwater, but it is not intended to address the ultimate timing, location, and amount of any groundwater discharge to surface water. Such an analysis would require more sophisticated numerical modeling that is beyond the scope of this study.

The groundwater recharge rate for each land use is a function of the average annual precipitation rate, surficial geology or soil type, and extent of impervious surface. The following equation is used to determine the groundwater recharge rate:

RR = RN*(1-EIA), where

RR = Groundwater recharge rate for land use unit (in/yr)

RN= Natural recharge (in/yr) EIA = Effective impervious area factor

The natural recharge rate (RN) is dependent upon the average annual precipitation rate and surficial geology. The USGS developed a relationship for natural recharge based on surficial geology in western Snohomish County (Thomas et al, 1997). Linear equations were developed based on the information in Figure 1. These relationships are provided below:

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Till: RN = 0.5*PA - 5

Glacial Outwash: RN = 0.875*PA – 11.25, where

RN= Natural recharge rate (in/yr)

PA= Average annual precipitation rate from PRISM (in/yr)

For any land use unit, the natural recharge can be calculated by linking the land use unit with the surficial geology and precipitation maps, and calculating volume based on the pixel size, surficial geology (till- or outwash-equivalent), and precipitation. The surficial geology, grouped into outwash- or till-equivalents, is presented in Figure 2. Grouping of surficial geology units into either higher recharge outwash-equivalent material or lower recharge till-equivalent material was based on interpretation of surficial geology mapped by Booth et al. (2004).

The EIA factor represents the fraction of natural recharge that does not reach the groundwater (i.e., becomes surface runoff or evapotranspires). Snohomish County has developed EIA tables for categories of existing development (Snohomish County, 2002). The EIA is dependent to some extent on the age of development. Table 1 summarizes EIA factors used by Snohomish County for more recent development. Older residential areas frequently have a smaller EIA because, for example, roof downspouts may be discharged to splashpads instead of being tied into the storm drain system. Roads in older areas may have a smaller area of pavement but the same overall width of right-of-way (smaller gross impervious area). Roads in older areas may also have relatively ineffective open ditch drainage (low effective percentage) as compared to curb and gutter for new developments. The EIA factors for more recent developments are used in this model, because much of the watershed consists of roads with curbs and gutters and rooftops are more likely to be tied into the storm drain system. Figure 3 presents the land cover categories associated with each EIA factor for the existing conditions scenario.

The EIA factors in Table 1 are assumed to represent the fraction of natural recharge that would not become groundwater recharge. For vegetation land cover (e.g., forest, pasture, grass), the EIA is equal to zero (the land use unit is not impervious) and recharge represents the natural recharge rate. This is the assumption used in the forested conditions scenario where all the land not covered by surface water is assumed to be forested.

1.3 Evapotranspiration

The fraction of natural recharge that does not reach the groundwater was assumed to either evapotranspire or runoff. Evapotranspiration rates for the forested and existing conditions in the watershed were used to identify the fraction of precipitation that evapotranspires. Table 2 identifies the evapotranspiration rates for forested and existing conditions.

1.4 Surface Runoff

Surface runoff is calculated as the remaining precipitation that does not recharge the groundwater or evapotranspire.

SR = PA - RR - ET, where

PA= Average annual precipitation rate from PRISM (in/yr) RR = Groundwater recharge rate (in/yr) ET = Evapotranspiration rate (in/yr)

SR = Surface runoff rate (in/yr)

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2.0 MODEL RESULTS

2.1 Forested Conditions Results

The forested conditions scenario assumes that there is no development in the watershed and that the land in the watershed not covered by surface water is completely forested. Table 3 presents the estimated water balance results for the Lake Ballinger watershed, the entire watershed and each subbasin. Based on the model results, the majority of the average annual precipitation that does not evapotranspire (84% and 87% of the remaining amount in the Lake Ballinger watershed and entire watershed, respectively) is estimated to recharge groundwater. This water then ultimately either discharges after retention in the groundwater system to surface water bodies in the watershed or, to a more limited extent, recharges deeper aquifers.

2.2 Existing Conditions Results

The existing land use conditions scenario estimates the current average annual water balance based on existing development within the watershed. Table 4 presents the total acreages of each land use type within the Lake Ballinger watershed, the entire watershed and each subbasin. The proportion of the different types of land cover in the Lake Ballinger watershed and entire watershed are very similar (within 1 to 2%). Over half of the area of both watersheds is covered by impervious surfaces.

Table 5 presents the estimated water balance results for the Lake Ballinger watershed, the entire watershed, and each subbasin. Under existing land use conditions, a significant decrease in the average annual precipitation that does not evapotranspire (31% and 34% of the remaining amount in the Lake Ballinger watershed and entire watershed, respectively) is estimated to recharge groundwater. In this case, the majority of the average annual precipitation that does not evapotranspire becomes surface or impervious runoff in the watershed.

2.3 Discussion

Table 6 compares the forested and existing water balance components. The changes applying to the runoff components of the Lake Ballinger watershed include an increase of 4,854 acre-feet (AF) of impervious surface runoff, a reduction of 2,078 AF of groundwater recharge and the reduction of 248 AF of surface runoff. This suggests that under existing conditions, there is estimated to be over six times the amount of total runoff (surface and impervious runoff combined) and 46 percent less groundwater recharge in the Lake Ballinger watershed than there was prior to development of the watershed. The changes that apply to the water balance of the entire watershed include an increase of 6,806AF of impervious surface runoff, a reduction of 202 AF of surface runoff and a reduction of 2,956AF of groundwater recharge. This suggests that under existing conditions, there almost eight times the amount of total runoff and 43 percent less groundwater recharge in the entire watershed than there was prior to development of the watershed.

Figures 4 and 5 illustrate another way to look at the change in the estimated average annual water balance for the Lake Ballinger watershed and entire watershed, respectively. The gray columns represent the existing water balance components (impervious runoff, surface runoff, groundwater recharge, and evapotranspiration). The green columns represent the forested water balance components. The significant estimated increase in surface runoff in both watersheds is evident from the chart.

Figure 6 illustrates the estimated Lake Ballinger watershed average annual water balance as a percentage of average annual rainfall (approximately 38 inches) for the forested and existing land use conditions. The development of the watershed is estimated to have decreased evapotranspiration by

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22% of the total rainfall. This suggests that 2,528 AF of water that used to be returned to the atmosphere as evaporation or transpiration is now becoming runoff and flowing into Lake Ballinger.

Figure 7 illustrates the estimated entire watershed average annual water balance as a percentage of average annual rainfall (approximately 38 inches) for the forested and existing land use conditions. The development of the entire watershed is estimated to have decreased evapotranspiration by 22% of the total rainfall. This suggests that 3,648AF of water that used to be returned to the atmosphere as evaporation or transpiration is now becoming runoff and contributing to surface water flows into Lake Washington.

The analysis presented in this technical memorandum suggests based on the model estimates that development of the watersheds has resulted in an additional 2,528 AF of water flowing into Lake Ballinger and 3,648 AF flowing into Lake Washington every year. This change is a direct consequence of a decrease in evapotranspiration and groundwater recharge, and an increase in impervious runoff. Because this analysis is based on annual average flows, the short term timing of the increased surface water flows is not directly addressed; however, it is well established that decreases in groundwater recharge and associated retention, and increases in impervious runoff in watersheds lead to shorter duration, but higher, peak streamflows. These changes also typically lead to reduced recharge of deeper aquifer systems, as well as decreased baseflows.

3.0 REFERENCES

Booth, D.B., B.F. Cox, K.G. Troost, and S.A. Shimel, 2004, Draft Composite Geologic Map of the Sno-King Area, 1:24,000, http://geomapnw.ess.washington.edu/index.php?toc=maintoc&body=services/publications/m ap/SnoKingDisc.htm.

Clear Creek Solutions. 2008. McAleer Strategic Plan Tech Memo #1 Surface Water Flow Analysis. December 16, 2008.

Snohomish County. 2002. Marshland Tributaries and Sunnyside Creek Drainage Needs Report. Appendix A2. Hydrologic Model Development. Snohomish County Public Works Department, Surface Water Management Division. December 2002.

Spatial Climate Analysis Service at Oregon State University and Climate Source, LLC. 2000. United States Average Monthly or Annual Precipitation, 1961-90. Spatial Climate Analysis Service at Oregon State University (SCAS/OSU): Corvallis, Oregon, USA.

Thomas, B.E., J.M. Wilkinson, and S.S. Embrey. 1997. The Ground-Water System and Ground- Water Quality in Western Snohomish County, Washington. US Geological Survey Water- Resources Investigations Report 96-4312. Prepared in cooperation with Snohomish County, Public Utility District 1 of Snohomish County, and Washington State Department of Ecology.

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List of Tables

Table 1 Effective Impervious Area Factors used to Estimate Groundwater Recharge Table 2 Evapotranspiration Rates Table 3 Forested Conditions Estimated Average Annual Water Balance Table 4 Summary of Acreages of Existing Land Use Conditions for the Lake Ballinger Watershed and each Subbasin Table 5 Existing Conditions Estimated Average Annual Water Balance Table 6 Comparison of Existing and Forested Average Annual Water Balance Components

List of Figures

Figure 1 Precipitation-Recharge Relationships for Western Snohomish County, Washington Figure 2 Surficial Geology Identified as Outwash- or Till- Equivalent Figure 3 Existing Conditions Land Cover Figure 4 Lake Ballinger Watershed Estimated Average Annual Water Balance Figure 5 Entire Watershed Estimated Average Annual Water Balance Figure 6 Lake Ballinger Watershed Estimated Average Annual Water Balance as Percentage of Rainfall Figure 7 Entire Watershed Estimated Average Annual Water Balance as Percentage of Rainfall

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TABLES

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TABLE 1 Effective Impervious Area Factors used to Estimate Groundwater Recharge

1 General Land Use Effective Impervious Area (EIA) Factor 2 Category Specific Land Cover Categories Existing Development Open Water water light, water dark 0 greenfield, canopy mixed, canopy evergreen, Forest, pasture, grass grassland, dirt, shadow4 0 road, sidewalk, parking lot, roof (light, grey, 3 Impervious Surfaces dark, brown) 0.855

Notes 1. The effective impervious area factor represents the fraction of natural recharge that does not reach the groundwater. The fraction of natural recharge that does reach the grgroundwateroundwater is equivalent to one minus the EIA factor times 100. For example, the EIA for undeveloped land is 0. The percent of recharge that reaches the groundwater equals 100 percent ((1- 0)*100). 2. Effective Impervious area factors based on Existing EIA#2 which is applicable to more recent development where roads have curb and gutter, and rooftops are more likely tied into the storm drain system. 3. Impervious surfaces assumed to have an EIA factor consistent with roads. 4. Shadows are assumed to be grassland surfaces because they would generally be from buildings that would be expected to be surrounded by grassland surfaces.

Source: Snohomish County (2002)

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TABLE 2 Evapotranspiration Rates

Evapotranspiration (ET) Rate Soil Specific Land Use Categories (inches/year) Source Outwash canopy mixed, canopy evergreen 20 Clear Creek Solutions (2008) Outwash greenfield, grassland 14.8 Clear Creek Solutions (2008) Till canopy mixed, canopy evergreen 20.2 Clear Creek Solutions (2008) Till greenfield, grassland 15.8 Clear Creek Solutions (2008) road, sidewalk, parking lot, dirt, roof Impervious 7.1 Clear Creek Solutions (2008) (light, grey, dark, brown)

Thomas, et al (1997) reports ET rates of 20 to 24 in/year based on a WSU study in 1966 that used a Palmer-Haven application of Thornwaite's method with None water light, water dark 19.2 an average soil waterholding capacity of 6 in. Assumed ET rate is 0.80 percent of the maximum ET rate reported in Thomas, et al (1997).

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TABLE 3

Forested Conditions Estimated Average Annual Water Balance

Annual Groundwater Precipitation Evapotranspiration Recharge Surface Runoff Geographic Area (acre-feet/year) (acre-feet/year) (acre-feet/year) (acre-feet/year) Lake Ballinger Watershed1 11,353 5,981 4,490 882 Entire Watershed2 16,624 8,791 6,847 986 Subbasin 1: drainage to Hall Lake 1,100 575 418 107 Subbasin 2: drainage to Hall Creek 6,067 3,202 2,422 443 Subbasin 3: drainage to Echo Lake 757 400 279 78 Subbasin 4: drainage of brookside and hillside tributaries west of Towne Center 1,501 813 653 34 Subb asi n 5 : di rect d rai nage t o L ak e B alli nger 3,428 1,805 11370,370 254

Subbasin 6: drainage of McAleer Creek upstream of NE 205th east of I-5 822 432 356 34 Subbasin 7: drainage of McAleer Creek upstream of 15th Ave NE 779 412 351 17

Subbasin 8: drainage of McAleer Creek upstream of NE Perkins Way 698 369 326 3 Subbasin 9: drainage of McAleer Creek upstream of NE 180th St 781 413 350 17 Subbasin 10: drainage of McAleer Creek upstream of NE 178th St 381 203 178 0

Subbasin 11: drainage of McAleer Creek upstream of Bothell Way NE 226 122 104 0 Subbasin 12: drainage of McAleer Creek upstream of mouth at Lake Washington 84 45 39 0

Notes 1. The Lake Ballinger Watershed includes Subbasins 1, 2, 3, and 5.

2. The entire watershed includes Subbasins 1 through 12 and represents drainage to Lake Washington from the Lake Ballinger and McAleer Creek Watersheds.

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TABLE 4 Summary of Acreages of Existing Land Use Conditions for the Lake Ballinger Watershed and each Subbasin

Impervious Total Area surfaces Water Forest Pasture, grass Bare soil Geographic Area (acres) (acres) (acres) (acres) (acres) (acres) Lake Ballinger Watershed1 3,566 2,021 118 680 687 61 Entire Watershed2 5,249 2,868 118 1,128 1,008 128 Subbasin 1: drainage to Hall Lake 342 193 6 81 55 7 Subbasin 2: drainage to Hall Creek 1,907 1,159 0 371 352 24 Subbasin 3: drainage to Echo Lake 238 144 11 37 40 6 Subbasin 4: drainage of brookside and hillside tributaries west of 198 0 170 97 21 Towne Center 487 Subbasin 5: direct drainage to Lake Ballinger 1,079 525 100 191 240 23 Subbasin 6: drainage of McAleer Creek upstream of NE 205th east of I- 139 0 59 53 8 5 258 155 0 45 39 9 Subbasin 7: drainage of McAleer Creek upstream of 15th Ave NE 247 109 0 66 38 8 Subbasin 8: drainage of McAleer Creek upstream of NE Perkins Way 221 Subbasin 9: drainage of McAleer Creek upstream of NE 180th St 248 136 0 52 49 11 53 0 40 25 4 Subbasin 10: drainage of McAleer Creek upstream of NE 178th St 122 38 0 15 16 5 Subbasin 11: drainage of McAleer Creek upstream of Bothell Way NE 73 Subbasin 12: drainage of McAleer Creek upstream of mouth at Lake Washington 27 19 0 1 5 3 Percent of Lake Ballinger Watershed 57% 3% 19% 19% 2% Percent of Entire Watershed 55% 2% 21% 19% 2%

Notes 1. The Lake Ballinger Watershed includes Subbasins 1, 2, 3, and 5. 2. The entire watershed includes Subbasins 1 through 12 and represents drainage to Lake Washington from the Lake Ballinger and McAleer Creek Watersheds.

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TABLE 5 Existing Conditions Estimated Average Annual Water Balance

Annual Groundwater Impervious Precipitation Evapotranspiration Recharge Surface Runoff Runoff Geographic Area (acre-feet/year) (acre-feet/year) (acre-feet/year) (acre-feet/year) (acre-feet/year) Lake Ballinger Watershed1 11,353 3,453 2,411 634 4,854 Entire Watershed2 16,624 5,143 3,892 784 6,806 Subbasin 1: drainage to Hall Lake 1,100 336 221 70 473 Subbasin 2: drainage to Hall Creek 6,067 1,780 1,208 306 2,772 Subbasin 3: drainage to Echo Lake 757 221 135 53 348 Subbasin 4: drainage of brookside and hillside tributaries west of Towne Center 1,501 536 464 51 450 Subbasin 5: direct drainage to Lake Ballinger 3,428 1,116 847 205 1,260 Subbasin 6: drainage of McAleer Creek upstream of NE 205th east of I-5 822 252 210 31 329

Subbasin 7: drainage of McAleer Creek upstream of 15th Ave NE 779 220 180 21 359

Subbasin 8: drainage of McAleer Creek upstream of NE Perkins Way 698 227 208 12 252

Subbasin 9: drainage of McAleer Creek upstream of NE 180th St 781 235 209 22 315

Subbasin 10: drainage of McAleer Creek upstream of NE 178th St 381 131 124 5 121 Subbasin 11: drainage of McAleer Creek upstream of Bothell Way NE 226 69 67 5 85 Subbasin 12: drainage of McAleer Creek upstream of mouth at Lake Washington 84 20 19 2 42

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TABLE 6 Comparison of Existing and Forested Estimated Average Annual Water Balance Components

Forested Existing Change Change Water Balance Components (acre-feet/year) (acre-feet/year) (acre-feet/year) (%) Lake Ballinger Watershed1 Evapotranspiration 5,981 3,453 -2,528 -42% Impervious Runoff 0 4,854 4,854 100% Surface Runoff 882 634 -248 -28% Groundwater Recharge 4,490 2,411 -2,078 -46% Entire Watershed2 Evapotranspiration 8,791 5,143 -3,648 -41% Impervious Runoff 0 6,806 6,806 100% Surface Runoff 986 784 -202 -21% Groundwater Recharge 6,847 3,892 -2,956 -43%

Notes 1. The Lake Ballinger Watershed includes Subbasins 1, 2, 3, and 5. 2. The entire watershed includes Subbasins 1 througgph 12 and represents drainag e to Lake Washing ton from the Lake Ballinger and McAleer Creek Watersheds.

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FIGURES

Golder Associates FIGURE 1 PRECIPITATION-RECHARGE RELATIONSHIPS FOR WESTERN SNOHOMISH COUNTY, WASHINGTON LAKE BALLINGER WATERSHED STUDY/WA 08393430001fig01.ai | Mod: 12/03/08 | AMP h

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Title Forested Conditions Lake Ballinger Watershed Estimated Average Annual Water Balance Drawn JV Checked AMN Existing Conditions Project No. Project Name Lake Ballinger Watershed Study 083‐93430.001 Reviewed CME

Client Name Date FIGURE Otak, Inc. December 19, 2008 4

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0 0 Impervious Runoff Surface Runoff Groundwater Recharge Evapotranspiration Water Balance Component Forested Conditions Title Entire Watershed Estimated Average Annual Water Balance Drawn JV Checked AMN Existing Conditions Project No. Project Name Lake Ballinger Watershed Study 083‐93430.001 Reviewed CME

Client Name Date FIGURE Otak, Inc. December 19, 2008 5

V:\PROJECTS\_2008 PROJECTS\083-93430 Lake Balllinger Watershed Study\Water Balance Model\Figures 4 to 7_121908_JV.xlsx 100%

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60% 7.8% Rainfall 42.8%

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Title Groundwater Recharge Lake Ballinger Watershed Estimated Average Annual Water Balance as Percentage of Rainfall Drawn JV Checked AMN Surface Runoff Project No. Project Name Lake Ballinger Watershed Study 083‐93430.001 Reviewed CME Impervious Runoff Client Name Date FIGURE Evapotranspiration Otak, Inc. December 19, 2008 6

V:\PROJECTS\_2008 PROJECTS\083-93430 Lake Balllinger Watershed Study\Water Balance Model\Figures 4 to 7_121908_JV.xlsx 100%

90% 23.4%

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Title Groundwater Recharge Entire Watershed Estimated Average Annual Water Balance as Percentage of Rainfall Drawn JV Checked AMN Surface Runoff Project No. Project Name Lake Ballinger Watershed Study 083‐93430.001 Reviewed CME Impervious Runoff Client Name Date FIGURE Evapotranspiration Otak, Inc. December 19, 2008 7

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