U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2018–5084

Prepared in cooperation with the City of Centralia

Water Budget of the Upper Chehalis River Basin, 

Southwestern Washington
Cover: Photograph showing Chehalis River at Grand Mound, Washington. 

Photograph by Kevin Linn, U.S. Geological Survey, October 9, 2014.
Water Budget of the Upper Chehalis River Basin, 

Southwestern Washington

By Andrew S. Gendaszek and Wendy B. Welch

Prepared in cooperation with the City of Centrailia

Scientific Investigtions Report 2018–5084

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Gendaszek, A.S., and Welch, W.B., 2018, Water budget of the upper Chehalis River Basin, southwestern Washington: 

U.S. Geological Survey Scientific Investigations Report 2018-5084, 17 p., https://doi.org/10.3133/sir20185084.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope .......................................................................................................................................2

Description of Study Area ............................................................................................................................2

Water Budget .................................................................................................................................................4

Methods...........................................................................................................................................................5

Precipitation, Recharge, Evapotranspiration, Interception, and Outflow—Soil Water

Balance.....................................................................................................................................5

Surface Runoff and Base Flow—Hydrograph Separation ............................................................6

Estimation of Groundwater Pumping.................................................................................................8

Water Budget Results ..................................................................................................................................9

Discussion.....................................................................................................................................................12

Summary........................................................................................................................................................16

References Cited .........................................................................................................................................16

Figures

1. Map showing location of upper Chehalis River Basin in southwestern Washington.......2

2. Map showing land-cover classifications for the upper Chehalis River Basin,

southwestern Washington..........................................................................................................3

3. Map showing mean annual precipitation, estimated from DAYMET precipitation

data for the upper Chehalis River Basin, southwestern Washington, October

2001–September 2015 ..................................................................................................................4

4. Map showing hydrologic soil group for the upper Chehalis River Basin,

southwestern Washington..........................................................................................................6

5. Map showing available water capacity for the upper Chehalis River Basin,

southwestern Washington..........................................................................................................7

6. Graph showing estimates of water-budget components calculated by

Soil-Water-Balance model of the upper Chehalis River Basin, southwestern

Washington, water years 2001–15 ............................................................................................9

7. Map showing distribution of groundwater and surface-water withdrawals by

Group A public-water supply systems, upper Chehalis River Basin, southwestern

Washington, water years 2001–15...........................................................................................10

8. Map showing distribution of groundwater withdrawals by Group B public-water

supply systems and self-supply domestic wells, upper Chehalis River Basin,

southwestern Washington, water years 2001–15 .................................................................11

9. Graph showing comparison of mean base flow estimated by streamflow

hydrograph-separation and analysis computer program and recharge estimated 

by Soil-Water-Balance model, upper Chehalis River Basin, southwestern 

Washington, water years 2001–15...........................................................................................12

10. Map showing mean annual recharge estimated by the Soil-Water-Balance model,

upper Chehalis River Basin, southwestern Washington, water years 2001–15...............13
iv

Figures—Continued

11. Map showing difference in precipitation in the upper Chehalis River Basin,

southwestern Washington, between water year 2015 and the average of 

water years 2001–15...................................................................................................................14

12. Map showing mean annual recharge estimated by the Soil-Water-Balance model

for the upper Chehalis River Basin, southwestern Washington, water year 2015 ..........15

Tables

1. Soil-Water-Balance model lookup table for runoff-curve numbers and root zone...........8

2. Estimates of water-budget components describing the fate of precipitation for 

the upper Chehalis River Basin, southwestern Washington, averaged for water 

years 2001–15 ...............................................................................................................................9

3. Estimates of mean annual water-budget components describing the fate of

recharge for the upper Chehalis River Basin, southwestern Washington .......................11

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

acre 0.4047 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

inch per day (in/d) 25.4 millimeter per day (mm/d)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

million gallons per year (Mgal/yr) 0.04381 cubic meter per second (m3/s)

International System of Units to U.S. customary units

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 
v

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations

APWL accumulated potential water loss

NLCD National Land Cover Database

SSURGO Natural Resources Conservation Service Soil Survey Geographic

SWB Soil-Water-Balance

USGS U.S. Geological Survey

WY water year

Water Budget of the Upper Chehalis River Basin, 

Southwestern Washington

By Andrew S. Gendaszek and Wendy B. Welch

Abstract

Groundwater and surface water collectively supply 

the domestic, agricultural, and industrial needs of the 

895-square mile upper Chehalis River Basin upstream of 

Grand Mound, Washington, while providing streamflow 

for fish and other aquatic species in the Chehalis River and 

its tributaries. To support sustainable water management 

decision-making, a water budget (including precipitation, 

interception, groundwater recharge, surface runoff, and 

groundwater pumping) was developed for the upper Chehalis 

River Basin during October 2001–September 2015. Waterbudget components were estimated from the U.S. Geological 

Survey Soil-Water-Balance (SWB) model except for 

groundwater pumping, which was estimated from public water 

purveyor records, annual system data from the Washington 

State Department of Health, census population data, and 

water-use estimates. Groundwater recharge estimated 

from the SWB model was compared to base flow, a proxy 

for groundwater recharge, independently estimated from 

separation of the hydrograph recorded by the U.S. Geological 

Survey streamgage at the outlet of the basin. Mean annual 

precipitation for the basin was estimated at 72.6 inches, of 

which 35 percent was lost to evapotranspiration, 30 percent 

was recharged to groundwater, 30 percent was surface runoff, 

and 5 percent was lost to interception. SWB model estimates 

of groundwater recharge were 17 percent less than estimates of 

base flow from hydrograph separation. Groundwater pumpage 

in the basin was estimated at 1 percent of groundwater 

recharge estimated by SWB and 0.8 percent of base flow 

estimated by hydrograph separation. These estimates form a 

baseline for understanding future changes to components of 

water use and may be used to inform numerical groundwater 

models to support sustainable management of water resources 

in the upper Chehalis River Basin.

Introduction

The Chehalis River, upstream of Grand Mound, 

Washington, drains an 895-mi2 area of Lewis, Thurston, and 

Pacific Counties in southwestern Washington State, including 

part of the Willapa Hills and the foothills of the Cascade 

Range (fig. 1). The water supply for people, fish, and wildlife 

is met in this geographical area (hereinafter referred to as 

the upper Chehalis River Basin) through a combination of 

groundwater and surface-water features including rivers, 

wetlands, and springs. Groundwater generally discharges to 

the Chehalis River and its tributaries from underlying aquifers 

(Ely and others, 2008), but also is recharged through losing 

stream reaches depending on the hydraulic relation between 

rivers and the underlying aquifers (Gendaszek, 2011). To 

sustainably manage future water resources for residential, 

industrial, and agricultural users while maintaining adequate 

water supply for fish and other wildlife, water managers in 

the upper Chehalis River Basin need to account for individual 

components of the hydrologic cycle through a water budget. 

In addition to basin-averaged water-budget components, the 

spatial distribution of groundwater recharge is an important 

boundary condition for the future development of a numerical 

groundwater-flow model to inform management of water 

resources of the upper Chehalis River Basin.
2 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

tac18-1224_fig01

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

Centralia

Napavine

Chehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum RiverNewaukum River

ChehalisC

River

hehalis River

SouthFork

Chehalis River SouthForkChehalis River

Skookumchuck RiverSkookumchuck River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

WASHINGTON

Cascade Range

Willapa Study Area

Hills

Purpose and Scope

The purpose of this report is to describe an accounting of 

water-budget components for the upper Chehalis River Basin. 

Mean annual water-budget components were calculated for the 

15 years starting in water year1 (WY) 2001 through the end of 

WY 2015, a period selected because of the availability of input 

data and to account for inter-annual variability. Mean annual 

water-budget components were then compared to waterbudget components calculated during a drought year (WY 

2015) to examine the effect of drought conditions on recharge. 

The water budget presented in this report informs ongoing 

efforts by the U.S. Geological Survey (USGS) to characterize 

the groundwater-flow system (Gendaszek, 2011) and 

groundwater/surface-water interactions (Ely and others, 2008) 

in the Chehalis River Basin in preparation for development of 

a numerical groundwater-flow model.

Figure 1. Location of upper Chehalis River Basin in southwestern Washington.

Description of Study Area

Most residential, industrial, and agricultural development 

and demand for water in the upper Chehalis River Basin are 

restricted to the valleys of the Chehalis River and its primary 

tributaries, the Newaukum and Skookumchuck Rivers 

(Gendaszek, 2011; fig. 2). These valleys dissect forested 

uplands, which have some rural residential development. The 

cities of Centralia, Chehalis, and Napavine meet their watersupply requirements with municipal public-water systems, 

whereas rural water users typically are self-supplied or 

supplied by smaller public-water systems.

Groundwater typically is pumped from unconsolidated 

alluvial and glacio-fluvial aquifers in the valleys of the 

Chehalis River and its tributaries (Gendaszek, 2011). 

These aquifers are confined locally by low-permeability 

hydrogeologic units consisting of unconsolidated fine-grained 

1 The 12-month period from October 1, for any given year, through September 30 of the following year. The water year is designated by the calendar year in 

which it ends.
Description of Study Area 3

tac18-1224_fig02

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

Centralia

Napavine

Chehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum RiverNewaukum River

ChehalisC

River

hehalis River

SouthFork

Chehalis River SouthForkChehalis River

Skookumchuck RiverSkookumchuck River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

Emergent herbaceous wetlands (95)

Woody wetlands (90)

Cultivated crops (82)

Pasture/hay (81)

Grassland/herbaceous (71)

Shrub/scrub (52)

Mixed forest (43)

Evergreen forest (42)

Deciduous forest (41)

Barren land (31)

Developed, high intensity (24)

Developed, medium intensity (23)

Developed, low intensity (22)

Developed, open space (21)

Open water (11)

Land-cover classifications

EXPLANATION

Figure 2. Land-cover classifications for the upper Chehalis River Basin, southwestern Washington. Source: 2001 

National Land Cover Database (Homer and others, 2007).

sediments of alluvial and lacustrine origin. Some groundwater 

in rural upland areas is pumped from bedrock, but yields are 

minimal and pumping is limited to self-supply for domestic 

water use.

The climate of the upper Chehalis River Basin is 

characterized by cool, wet winters and warm, dry summers. 

Altitude of the basin ranges from a low of 124 ft at the outlet 

of the basin at Grand Mound to a high of 3,825 ft in the 

foothills of the Cascade Range. Precipitation falls primarily 

as rain during autumn and winter. Mean annual precipitation 

averaged over the upper Chehalis River Basin was 72.6 in. 

and ranged from an annual mean of less than 50 in. in the 

northern part of the basin around the city of Centralia to more 

than 140 in. at the higher altitudes of the basin (DAYMET 

precipitation data averaged during WYs 2001–15; Thornton 

and others, 2017; fig. 3). Mean annual precipitation for the 

upper Chehalis River Basin during October 2001–September 

2015 was estimated from DAYMET precipitation data 

(Thornton and others, 2017).
4 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

tac18-1224_fig03

Skookumchuck River

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

CentraliaCentralia

NapavineNapavine

ChehalisChehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

45 to 59

60 to 79

80 to 99

100 to 119

120 to 139

> 140

Mean annual precipitation, 

in inches

EXPLANATION

Figure 3. Mean annual precipitation, estimated from DAYMET precipitation data (Thornton and others, 2017), for the upper 

Chehalis River Basin, southwestern Washington, October 2001–September 2015.

Water Budget

A water budget for a hydrologic system equates inflows 

of water to the system (Qin) to outflows of water (Qout) and 

changes in storage (ΔS):

Qin = Qout + ΔS. (1)

Changes in storage may occur over short inter-annual to 

monthly timescales, but over the long term, steady-state 

conditions generally are present where inflows to the 

hydrologic system are balanced by outflows. Inflows to a 

hydrologic system can include precipitation, surface-water 

inflows, and groundwater inflows from adjacent hydrogeologic 

units. Outflows from a hydrologic system can include 

evapotranspiration, groundwater pumpage, and outflows of 

surface water and groundwater from the system. Water also 

moves within a hydrogeologic system between surface and 

groundwater storage through hydrologic processes, including 

groundwater recharge and groundwater/surface-water 

exchange between aquifers and rivers and lakes.
Methods 5

Methods

Precipitation, Recharge, Evapotranspiration, 

Interception, and Outflow—Soil Water Balance

A Soil-Water-Balance (SWB) model (SWB version 

1.0.1; U.S. Geological Survey, 2016) was developed for the 

study area to estimate the spatial distribution of recharge, 

evapotranspiration, interception, and outflow as surface 

runoff during WYs 2001–15 at a daily time step (Dripps and 

Bradbury, 2007; Westenbroek and others, 2010). The daily 

spatial distribution of precipitation was supplied as an input to 

the SWB model. A 22-month model initialization period from 

January 1999 to September 2000 was completed prior to the 

WY 2001015 study period to estimate antecedent conditions 

including soil moisture and snow cover. The study area was 

divided into 500-ft grid cells consisting of 378 rows and 546 

columns. Cells outside the Chehalis River watershed upstream 

of the USGS streamgage at Grand Mound were inactive, 

which resulted in a total of 100,142 active model cells. The 

500-ft grid cell resolution was selected to be comparable to 

current (2018) regional-scale numerical groundwater-flow 

models of western Washington under development by the 

USGS in western Washington.

SWB calculates the water budget for each cell at a daily 

time step through a modified Thornthwaite-Mather soil-waterbalance approach (Thornthwaite and Mather, 1955, 1957):

Recharge = Rainfall + Snowmelt + Inflow 

– Interception – Outflow – Evapotranspiration

– Soil Moisture. ∆ (2)

Spatially distributed inputs to the SWB model included daily 

precipitation, daily minimum temperature, daily maximum 

temperature, land cover data, overland flow direction, and two 

related soil properties—hydrologic soil group and available 

soil-water capacity. For each model cell, input data for each 

of these properties were resampled to a 500-ft grid cell size 

and aligned and projected to a common grid cell for the 

SWB model. Daily precipitation, minimum temperature, and 

maximum temperature for WYs 1999–15 were assigned to 

each model cell from the DAYMET (Thornton and others, 

2017) database. Land-cover classification was obtained from 

the Multi-Resolution Land Characteristics Consortium’s 2001 

National Land Cover Database (NLCD; Homer and others, 

2007). The NLCD classifies the conterminous United States 

into 16 land-cover classes at a 30-m resolution, which was 

clipped to the model extent and resampled to a 500-ft cell 

size (fig. 2). Fifteen NLCD land-cover classes occur in the 

model extent. Hydrologic soil groups and available water 

capacity data were obtained from the Natural Resources 

Conservation Service Soil Survey Geographic (SSURGO) 

database (Natural Resources Conservation Service, 2014), 

clipped to the extent of the model, and resampled to the model 

500-ft grid size. Soils in the SSURGO database are classified 

into four hydrologic soil groups (A, B, C, and D), which range 

from group A soils that have a high-infiltration and low-runoff 

capacity to group D soils that have a low-infiltration and highrunoff capacity (fig. 4). SSURGO also specifies the available 

water capacity, defined as the amount of water that the soil can 

hold for each soil series at several soil depths. For the model, 

the SSURGO available water capacity was averaged over the 

top 39 in. of soil and assigned to each 500-ft grid cell (fig. 5).

Calculation of water-budget components by SWB for 

each model cell is summarized here and described in detail 

by Westenbroek and others (2010). Daily meteorological data 

(including precipitation, minimum temperature, and maximum 

temperatures) were specified for each model cell; depending 

on the temperature, precipitation fell as either rain or snow. If 

precipitation occurred when the temperature was lower than 

the freezing point, the model calculated precipitation as snow 

and it accumulated at each model cell until the temperature 

exceeded the freezing point. Above the freezing point, the 

snow melted and then that water equivalent became available 

for evapotranspiration, recharge, and runoff. The SWB 

flow routing of surface-runoff option was not used because 

of the resolution of this model was too large to adequately 

represent routing of surface water; therefore, we assumed 

that surface runoff that left a cell immediately left the model 

area and that the inflow term for each cell was zero. As a 

result, recharge estimated may have been underestimated by 

the model because surface runoff did not have an opportunity 

to recharge within downstream cells. Rainfall that did not 

reach the ground because of vegetation was modeled as 

interception loss. The interception rate was specified for 

each land-use type for the growing and dormant seasons. 

Outflow from each cell was calculated using the NRCS curve 

number rainfall-runoff relation (Cronshey and others, 1986) 

and immediately exited the model domain as surface runoff. 

Potential evapotranspiration for each cell was calculated using 

the Hargreaves-Samani method (Hargreaves and Samani, 

1985), which uses daily minimum and maximum temperature 

data provided by DAYMET for this model. Changes in soil 

moisture were calculated by first subtracting precipitation from 

potential evapotranspiration. If potential evapotranspiration 

exceeded precipitation, then there was a potential deficiency of 

water, which was tracked by the accumulated potential water 

loss (APWL) term. Soil moisture was estimated from the 

APWL using the non-linear relation described by Thornwaite 

and Mather (1957). Conversely, if precipitation exceeded 

potential evapotranspiration, soil moisture increased and, if 

moisture exceeded the maximum water-holding capacity, the 

excess water became recharge. 
6 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

tac18-1224_fig04

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

Centralia

Napavine

Chehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum RiverNewaukum River

ChehalisC

River

hehalis River

SouthFork

Chehalis River SouthForkChehalis River

Skookumchuck RiverSkookumchuck River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

Hydrologic soil group

A

B

C

D

EXPLANATION

Figure 4. Hydrologic soil group for the upper Chehalis River Basin, southwestern Washington. Soil classification ranges 

from Group A soils with high infiltration capacity and low overland flow potential to Group D soils with low infiltration 

capacity and high overland flow potential. Data from Natural Resources Conservation Service (2014).

For each unique land-cover/hydrologic soil group 

combination, runoff-curve numbers, vegetation routing 

depths, interception rates, and maximum daily recharge 

values were defined in a lookup table used by SWB. The 

lookup table used for the upper Chehalis River Basin SWB 

model was populated with values used by Tillman (2015). 

Runoff-curve numbers obtained from the NRCS National 

Hydrology Handbook (Natural Resources Conservation 

Service, 2004) and Westenbroek and others (2010), and root 

zone depths obtained from Canadell and others (1996) and 

Westenbroek and others (2010), are presented in table 1. 

Interception rates and maximum daily infiltration rates were 

obtained from Westenbroek and others (2010). An interception 

rate of 0.0835 in. was applied to all land-cover types during 

the growing season except for open water, barren land, and 

emergent herbaceous wetlands (for which the interception rate 

was 0 in.); during the dormant season, the interception rate 

was 0 in. (Tillman, 2015). For all land-cover types, maximum 

recharge rates were set to 2 in/d for hydrologic soil group A, 

0.6 in/d for hydrologic soil group B, 0.24 in/d for hydrologic 

soil group C, and 0.12 in/d for hydrologic soil group D 

(Tillman, 2015).

Surface Runoff and Base Flow—Hydrograph 

Separation

Hydrograph separation was used to partition streamflow 

into its two components—base flow from groundwater 

discharge and surface runoff from overland flow. Groundwater 

discharge to streams, or base flow, has often been used to 

approximate groundwater recharge (Risser and others, 2005). 
Methods 7

tac18-1224_fig05

Skookumchuck River

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

CentraliaCentralia

NapavineNapavine

ChehalisChehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

EXPLANATION

Available water capacity, 

in inches

5.4

0.6

Figure 5. Available water capacity for the upper Chehalis River Basin, southwestern Washington. Data from Natural 

Resources Conservation Service (2014).

Hydrograph separation assumes losses of groundwater 

due to other processes including evapotranspiration from 

riparian vegetation, exchange with underlying aquifers, 

and groundwater exports from the basin due to pumping or 

underflow are minimal. Many methods have been developed 

to differentiate streamflow into these two components 

based on graphical, chemical, and isotopic methods (Healy, 

2010). Although chemical and isotopic base flow separation 

methods such as specific conductance or isotopic data offer 

improvements over graphical separation methods (Burns, 

2002), only streamgage records were available in the upper 

Chehalis River Basin. Therefore, graphical methods were 

used to estimate base flow in the upper Chehalis River Basin. 

The accuracy of graphical methods, however, is limited. For 

example, Stewart and others (2007) reported an accuracy of 

base flow estimated by graphical separation of as much as a 

factor of two for basins in the Southeastern United States.

The daily mean discharge hydrograph recorded by the 

streamgage at the outlet of the upper Chehalis River Basin 

(USGS streamgage 12027500) during WYs 2001–15 was 

separated into base flow and surface runoff by using the 

streamflow hydrograph-separation and analysis computer 

program (HYSEP; Sloto and Crouse, 1996). HYSEP includes 

three methods for separating streamflow into base-flow and 

surface-runoff components by using geometric algorithms 

including the fixed interval, sliding interval, and localminimum algorithms. Following Smith and Westenbroek 

(2015), an estimate averaged from the three streamflow 

hydrograph separation methods calculated by HYSEP was 

used in subsequent analyses in this report to avoid potential 

biases associated with choosing a single method.
8 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

Table 1. Soil-Water-Balance model lookup table for runoff-curve numbers and root zone.

[From Tillman (2015) and references within. Abbreviations: HSG, hydrologic soil group; NLCD, National Land Cover Database; ft, foot]

NLCD land cover Curve No. Depth of root zone (ft)

Value Description HSG A HSG B HSG C HSG D HSG A HSG B HSG C HSG D

11 Open water 100 100 100 100 0 0 0 0

21 Developed, open space 49 69 79 84 8.53 8.53 8.53 8.53

22 Developed, low intensity 77 86 91 94 8.53 8.53 8.53 8.53

23 Developed, medium intensity 89 92 94 95 8.53 8.53 8.53 8.53

24 Developed, high intensity 98 98 98 98 8.53 8.53 8.53 8.53

31 Barren land 77 86 91 94 1 1 1 1

41 Deciduous forest 32 48 57 63 9.5 9.5 9.5 9.5

42 Evergreen forest 39 58 73 80 12.8 12.8 12.8 12.8

43 Mixed forest 46 60 68 74 11.15 11.15 11.15 11.15

52 Shrub/scrub 49 68 79 84 3.5 3.5 3.5 3.5

71 Grassland/herbaceous 64 71 81 89 8.53 8.53 8.53 8.53

81 Pasture/hay 49 69 79 84 8.53 8.53 8.53 8.53

82 Cultivated crops 71 80 87 90 2 2 2 2

90 Woody wetlands 88 89 90 91 4.5 4.5 4.5 4.5

95 Emergent herbaceous wetlands 89 90 91 92 4.5 4.5 4.5 4.5

Estimation of Groundwater Pumping

Groundwater pumping by public-supply, self-supply, 

and irrigation wells in the upper Chehalis River Basin was 

estimated by using a combination of reported pumping rates 

by water purveyors, annual system data from the Washington 

State Department of Health, census population data and 

domestic water-use estimates, and irrigated acreage and 

irrigation application rates. Twenty-four Group A publicwater systems, which serve 15 or more connections or more 

than 25 people, served an estimated total of 30,963 people 

in the upper Chehalis River Basin as of 2010. The largest 

of the public-water systems, the City of Centralia, supplied 

monthly groundwater pumpage records from 2000 to 2017. 

Annual groundwater pumpage for the remaining Group A 

public-water supply systems was estimated by the annual 

totals reported to the Washington State Department of Health 

(2018). In addition to withdrawals from groundwater, several 

public-supply systems in the basin obtain water from surface 

water from the Chehalis River and its tributaries. The spatial 

distribution of groundwater pumpage by Group A publicsupply systems was determined from overlaying 2010 census 

blocks on the public-water supply service areas (Washington 

State Office of Financial Management, 2018); the residual 

census blocks that were not assigned to Group A public-supply 

systems were assumed to be either self-supplied or part of 

a Group B system, which serves fewer than 15 connections 

and 25 people. Census blocks, which range in size depending 

on factors including population density, are the smallest 

geographic unit for which population is tabulated by the 

U.S. Census.

As of 2010, self-supply domestic and Group B wells 

serve an estimated population of 23,896 people. Unlike Group 

A systems, Group B and self-supply systems were not required 

to report pumpage rates to the Washington State Department 

of Health; therefore, water-use estimates were based on 2005 

and 2010 per-capita water-use rates for Lewis and Thurston 

counties reported by Lane (2009) and Lane and Welch (2015). 

Total irrigation withdrawals were estimated by determining the 

area of irrigated agricultural land from the Washington State 

Department of Agriculture agricultural land use geodatabase 

(Washington State Department of Agriculture, 2018) and 

multiplying by the irrigation application rate for Lewis County 

(17.04 in/yr) reported by Lane and Welch (2015). Irrigation 

was separated into its groundwater component by multiplying 

the fraction of Lewis County irrigation from groundwater 

pumpage (0.55) reported by Lane and Welch (2015).
Water Budget Results 9

Water Budget Results 

Components describing the fate of precipitation 

including surface runoff, evapotranspiration, groundwater 

recharge, interception, and change in soil moisture are 

reported from SWB model output; the sum of these 

components equals total precipitation supplied as an input 

to the SWB model. The average estimate of water-budget 

components from WYs 2001–15 is reported in table 2, and 

water-budget components for individual water years are 

presented in figure 6. On average, SWB estimated that during 

WYs 2001– 15, nearly equal parts of the gross precipitation 

were recharged to groundwater (22.0 in.; 30.3 percent), left as 

surface runoff (21.6 in.; 29.8 percent), or were evapotranspired 

(25.6 in.; 35.3 percent). Vegetation intercepted less water 

(3.6 in.; 5.0 percent), with higher rates of interception 

occurring in the summer when deciduous vegetation has 

leaves. Changes in soil moisture were almost negligible, 

with an overall change in soil moisture of -0.2 in. during 

WYs 2001–15 (-0.3 percent), indicating that soil moisture was 

nearly in steady state during this 15-year period. 

The fate of the groundwater recharge component of 

the water budget was calculated in two ways: (1) by using 

groundwater recharge estimated by the SWB model, and 

(2) by calculating groundwater recharge from base flow 

estimated by hydrograph separation. In the first case, 

Table 2. Estimates of water-budget components describing 

the fate of precipitation for the upper Chehalis River Basin, 

southwestern Washington, averaged for water years 2001–15.

Fate of precipitation Inches per year Percentage of total

Surface runoff 21.6 29.8

Evapotranspiration 25.6 35.3

Groundwater recharge 22.0 30.3

Interception 3.6 5.0

Change in soil moisture -0.2 -0.3

Total precipitation 72.6 100.0

natural discharge was calculated as the difference between 

groundwater recharge estimated by SWB and the sum of 

groundwater pumpage estimated from Groups A and B, 

domestic self-supply, and irrigation wells. In the second case, 

natural discharge was obtained from the base-flow component 

of hydrograph separation and summed with groundwater 

pumpage estimated from Groups A and B, domestic selfsupply, and irrigation wells to obtain total groundwater 

recharge. In each case, it was recognized that natural discharge 

included groundwater discharge to streams as well as springs 

and other surface-water features, which ultimately leaves the 

basin through surface-water flow or evapotranspiration.

2001 2005 2010 2015

0

10

30

50

70

90

-10

Water year

Water, in inches

Evapotranspiration

Groundwater recharge

Surface runoff

Interception

Change in soil moisture

Water-budget components

EXPLANATION

Figure 6. Estimates of water-budget components calculated by Soil-Water-Balance model of the upper Chehalis 

River Basin, southwestern Washington, water years 2001–15.
10 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

Mean annual groundwater and surface-water withdrawals 

were determined for public-supply systems, domestic selfsupply water users, and irrigators. Group A public-water 

systems obtain water from both groundwater pumpage and 

surface-water withdrawals, whereas Group B water systems 

and domestic self-supply wells solely obtain water from 

groundwater. During WYs 2001–15, Group A systems serving 

Chehalis, Pe Ell, and Boistfort Valley obtained a total of 

794 Mgal/yr of water from surface-water sources including 

the Chehalis River and its tributaries, which represented 

45 percent of the total water supplied by Group A systems in 

the upper Chehalis River Basin. The remaining water supplied 

by Group A systems (974 Mgal/yr) was pumped from wells 

(groundwater withdrawal; fig. 7). Group B and self-supply 

domestic wells were estimated to collectively withdraw 

775 Mgal/yr (fig. 8). Averaged across the basin, these 

groundwater withdrawals were 0.06 and 0.05 in/yr for Group 

A wells and Group B/self-supply domestic wells, respectively. 

The total irrigated area of the upper Chehalis River Basin 

was estimated at 5,997 acres and irrigated at an estimated rate 

of 1.42 ft/yr (Lane and Welch, 2015). Averaged across the 

upper Chehalis River Basin, total irrigation was estimated at 

0.18 in/yr with 55 percent of irrigation (0.10 in/yr) sourced 

from groundwater (Lane and Welch, 2015). The combined 

groundwater pumpage from Group A and B public supply, 

domestic self-supply, and irrigation wells was 0.21 in., which 

is 1 percent of groundwater recharge estimated by SWB and 

0.8 percent of recharge estimated from base flow calculated 

from hydrograph separation by HYSEP (table 3—Group A 

well pumpage + Self-supply and Group B well pumpage + 

Irrigation well pumpage).

Mean annual discharge recorded at the USGS streamgage 

at the Chehalis River near Grand Mound (USGS streamgage 

12027500) was 2,760 ft3/s, which equals 41.8 in. averaged 

over the 895-mi2 area of the upper Chehalis River Basin. 

Estimates of the base-flow component of streamflow by the 

three hydrograph separation methods implemented by HYSEP 

were 23.8 in. (fixed interval), 26.5 in. (sliding interval), and 

26.6 in. (local-minimum algorithms). The three-method 

average for the mean annual base-flow estimate was 25.6 in. 

(1,690 ft3/s), and the residual streamflow (16.2 in.; 1,070 ft3/s) 

was inferred to be sourced from surface runoff.

Skookumchuck River

LEWIS PACIFIC

WAHKIAKUM COWLITZ

THURSTON

CentraliaCentralia

NapavineNapavine

ChehalisChehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

EXPLANATION

Surface-water

withdrawal

Groundwater

withdrawal

Millions of gallons

per year

< 5

5 to 9

> 100

50 to 100

10 to 49

Figure 7. Distribution of groundwater and surface-water withdrawals by Group A public-water supply systems, upper Chehalis 

River Basin, southwestern Washington, water years 2001–15.
Water Budget Results 11

tac18-1224_fig08

Skookumchuck River

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

Centralia

Napavine

Chehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

< 1

1

2

3

> 4

Millions of gallons 

per year withdrawn

EXPLANATION

Figure 8. Distribution of groundwater withdrawals by Group B public-water supply systems and self-supply domestic wells, 

upper Chehalis River Basin, southwestern Washington, water years 2001–15.

Table 3. Estimates of mean annual water-budget components describing the fate of recharge for the upper 

Chehalis River Basin, southwestern Washington.

[HYSEP, streamflow hydrograph-separation and analysis computer program]

Fate of recharge

Soil-Water-Balance model HYSEP base-flow separation

Inches per

year

Percentage of 

total

Inches per

year

Percentage of

total

Natural discharge 21.79 99.0 25.6 99.2

Group A well pumpage 0.06 0.3 0.06 0.2

Self-supply and Group B well pumpage 0.05 0.2 0.05 0.2

Irrigation well pumpage 0.10 0.5 0.10 0.4

Total recharge 22.0 100.0 25.81 100.0
12 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

Discussion

Input of water into the upper Chehalis River Basin occurs 

primarily from precipitation in the absence of surface-water 

inputs or groundwater inflow, which is precluded by lowhydraulic conductivity bedrock that bounds major aquifers 

in the basin. Most precipitation falls as rain during autumn 

and winter, with some snow falling at higher elevations of the 

basin. The highest precipitation rates have occurred in areas 

of high topography, including the foothills of the Cascade 

Range and the Willapa Hills where mean annual precipitation 

exceeded 140 in.; precipitation rates were lowest in the lower 

elevations of the northern part of the upper Chehalis River 

Basin, near the city of Centralia, where less than 50 in. of 

precipitation fell (fig. 3). During a drought year (2015), mean 

annual precipitation was 61.4 in. or 15.4 percent less than the 

mean for WYs 2001–15.

Water primarily leaves the basin as evapotranspiration 

or surface-water outflow, assuming minimal groundwater 

discharge at the outlet of the basin where low-hydraulic 

conductivity bedrock largely confines the principal 

alluvial aquifers. A rough basin-scale water budget may be 

constructed by assuming that precipitation (P) is the only 

input, evapotranspiration (ET) and surface-water discharge 

(Qsurface- water) are the only outputs, and that there are 

negligible changes in storage (ΔS) such that equation (1) 

becomes:

P = ET + Qsurface-water. (3)

A first-order estimate of evapotranspiration, therefore, may 

be determined by subtracting surface-water discharge from 

the basin (41.8 in.) from precipitation (72.6 in.), which equals 

30.8 in. To further refine the water budget and determine the 

fate of groundwater recharge, constituents of the water budget 

were estimated using SWB, hydrograph separation, and 

analysis of water-use records and population data. The firstorder estimate of evapotranspiration calculated by equation 2 

(30.8 in.) was 20 percent greater than the mean annual 

evapotranspiration estimated by SWB (25.6 in.). 

Basin-averaged mean annual groundwater recharge 

estimated by the SWB model during WYs 2001–15 was 22 in. 

and broadly agreed with base flow estimated by hydrograph 

separation (fig. 9). During WYs 2001–15, base flow was an 

average of 17 percent greater than recharge. Natural discharge 

accounted for 99 percent of recharge, whereas the combined 

pumping from Groups A and B, domestic self-supply wells, 

and irrigation wells accounted for 1 percent of recharge. 

Recharge ranged from a mean of less than 10 to more than 

50 in/yr across the upper Chehalis River Basin (fig. 10). These 

results are comparable to previously estimated recharge in 

other locations in western Washington including Puget Sound 

(Vaccaro and others, 1998) and Thurston County (Drost and 

others, 1999). Groundwater recharge generally was highest in 

areas of the upper Chehalis River Basin where precipitation 

was highest, such as the Willapa Hills and the foothills of the 

Cascade Range, but soil properties locally limited groundwater 

recharge rates.

40

30

20

10

0

2002 2006 2010 2014

Mean base flow and recharge, in inches

Water year

Mean base flow (streamflow 

hydrograph-separation and 

analysis computer program 

[HYSEP])

Mean recharge (Soil-WaterBalance [SWB] model)

EXPLANATION

Figure 9. Comparison of mean base flow estimated by streamflow hydrograph-separation and analysis computer 

program and recharge estimated by Soil-Water-Balance model, upper Chehalis River Basin, southwestern Washington, 

water years 2001–15.
Discussion 13

tac18-1224_fig10

Skookumchuck River

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

CentraliaCentralia

NapavineNapavine

ChehalisChehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

0 to 4

5 to 9

10 to 14

15 to 19

20 to 24

25 to 29

30 to 34

35 to 39

40 to 44

45 to 50

> 50

Estimated mean annual 

recharge, in inches

EXPLANATION

Figure 10. Mean annual recharge estimated by the Soil-Water-Balance model, upper Chehalis River Basin, southwestern 

Washington, water years 2001–15.

In the Willapa Hills, for example, groundwater recharge 

rates locally exceeded 50 in/yr where Group B soils occurred, 

but recharge rates were less than 20 in/yr in adjacent areas 

where Group C soils with comparatively low infiltration 

capacities occurred. The main Chehalis River valleys had 

relatively high recharge rates, locally exceeding 35 in/yr 

despite lower precipitation rates than the Willapa Hills where 

hydrologic soil groups A and B with relatively high infiltration 

capacities occurred. In areas of the valleys of the Chehalis 

River and its main tributaries, where hydrologic soil groups A 

and B with high infiltration capacities predominated, recharge 

was relatively high and locally exceeded 30 in/yr. Where 

hydrologic soil groups C and D (which have low infiltration 

capacities and high surface-runoff potentials) coincided with 

areas of low precipitation such as the uplands of the central 

part of the upper Chehalis River Basin, mean annual recharge 

was limited to less than 10 in/yr. 

A comparison of recharge estimated during WY 2015 

and WYs 2001–15 was made to examine the effect of drought 

conditions during WY 2015 on recharge. The difference in 

precipitation between WY 2015 and the average of WYs 

2001–15 was not uniform, with 2015 being driest in the 

southwestern and central part of the upper Chehalis River 

Basin including the main population centers of Centralia, 

Chehalis, and Napavine, but wetter in the northeastern and 

northwestern parts of the basin (fig. 11). Basin-averaged 

groundwater recharge during the drought of WY 2015 

was 17.2 in., or 21.8 percent less than the average of 

WYs 2001– 15. Following changes in precipitation patterns 

between WY 2015 and the average of WYs 2001–15, 

groundwater recharge decreased over most of the upper 

Chehalis River Basin, but increased slightly in the northeastern 

and northwestern parts of the basin (fig. 12).
14 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

tac18-1224_fig11

Skookumchuck River

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

CentraliaCentralia

NapavineNapavine

ChehalisChehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

Estimated difference in 

annual precipitation, 

in inches

>0

-5 to 0

-10 to -5

−15 to −10

−20 to −15

< −20

EXPLANATION

Figure 11. Difference in precipitation in the upper Chehalis River Basin, southwestern Washington, between water year 2015 

and the average of water years 2001–15.
Discussion 15

tac18-1224_fig12

Skookumchuck River

LEWIS

PACIFIC

WAHKIAKUM COWLITZ

THURSTON

CentraliaCentralia

NapavineNapavine

ChehalisChehalis

USGS 

streamgage 

12027500

Grand Mound

Newaukum River

Chehalis River

SouthFork

Chehalis River

Pe Ell

Boistfort

Valley

0 5 10 MILES

0 5 10 KILOMETERS

123°20' 122°50'

46°

50'

46°

20'

Base map modified from U.S. Geological Survey and other digital data, various scales. 

Coordinate Reference System: State Plane Washington South. 

Horizontal datum is North American Datum of 1983.

EXPLANATION

0 to 4

5 to 9

10 to 14

15 to 19

20 to 24

25 to 29

30 to 34

35 to 39

40 to 44

45 to 50

> 50

Estimated mean annual recharge, 

in inches

Figure 12. Mean annual recharge estimated by the Soil-Water-Balance model for the upper Chehalis River Basin, 

southwestern Washington, water year 2015.
16 Water Budget of the Upper Chehalis River Basin, Southwestern Washington

Summary

Water budget components of the upper Chehalis River 

Basin in southwestern Washington averaged for water years 

2001–15 were estimated by Soil-Water-Balance (SWB); 

base‐flow separation of the hydrograph recorded by the 

streamgage at the outlet of the basin; and groundwater 

pumpage from public water purveyor records, census 

population data, and water-use estimates. In the absence of 

groundwater or surface-water inputs to the basin, mean annual 

precipitation (estimated at 72.6 inches) was the single input 

of water into the basin. Nearly equal parts of water that fell as 

precipitation left the basin as evapotranspiration (35 percent), 

surface runoff (30 percent), or recharged groundwater 

(30 percent). An additional 5 percent did not reach the 

ground because of interception. The spatial distribution of 

groundwater recharge was controlled by soil properties and 

precipitation patterns, with the highest rates of recharge 

occurring in areas of high precipitation with relatively 

permeable soil types. Groundwater recharge estimated 

by SWB was within 17 percent of base flow estimated by 

hydrograph separation. Groundwater pumpage by Group A 

and B public-supply systems, self-supply domestic wells, and 

irrigation wells was estimated at 0.21 inches or about 1 percent 

of estimated recharge. Estimated water-budget components 

may be used to inform the future development of numerical 

groundwater-flow models to support decisions by managers of 

water resources in the upper Chehalis River Basin.

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Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Washington Water Science Center

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Tacoma, Washington 98402

https://wa.water.usgs.gov/
Gendaszek and Welch—Water Budget of the Upper Chehalis River Basin, Southwestern Washington—Scientific Investigations Report 2018–5084

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185084