Prepared in cooperation with the Idaho Department of Water Resources

An Evaluation of Seepage Gains and Losses in Indian Creek 

Reservoir, Ada County, Idaho, April 2010–November 2011

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5047

An Evaluation of Seepage Gains and Losses in 

Indian Creek Reservoir, Ada County, Idaho, 

April 2010–November 2011

By Marshall L. Williams and Alexandra B. Etheridge

Prepared in cooperation with the Idaho Department of Water Resources

Scientific Investigations Report 2013–5047

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Williams, M.L., and Etheridge, A.B., 2013, An evaluation of seepage gains and losses in Indian Creek Reservoir, Ada 

County, Idaho, April 2010–November 2011: U.S. Geological Survey Scientific Investigations Report 2013-5047, 26 p. 
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Study Area Description........................................................................................................................3

Methods...........................................................................................................................................................4

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

Streamflow and Precipitation.............................................................................................................4

Change in Reservoir Storage..............................................................................................................4

Evaporation............................................................................................................................................5

Evaporation Rates........................................................................................................................8

Evaporation Uncertainty...........................................................................................................11

Seepage Meter Method ....................................................................................................................12

Segmented Darcy Method ................................................................................................................13

Reservoir Seepage ......................................................................................................................................14

Conceptual Model of Reservoir Seepage.......................................................................................14

Reservoir Seepage Results...............................................................................................................16

Sources of Error..................................................................................................................................20

Summary........................................................................................................................................................21

Acknowledgments.......................................................................................................................................22

References Cited .........................................................................................................................................22

Appendixes ...................................................................................................................................................25

Appendix A. Indian Creek Reservoir Stage, Elevation, Capacity, and Area, Ada County, 

Idaho, 2011..............................................................................................................................25

Appendix B. Bathymetric Survey of Indian Creek Reservoir, Ada County, Idaho, October 

2009..........................................................................................................................................25

Figures

1. Map showing Indian Creek Reservoir study area, Idaho ......................................................2

2. Map showing locations of sites in and near Indian Creek Reservoir, Ada County, 

Idaho ...............................................................................................................................................3

3. Graph showing relation between available energy and evaporation rates for 

Indian Creek Reservoir, Ada County, Idaho, 2010–11............................................................10

4. Graph showing evaporation rates for energy-budget periods, Indian Creek

Reservoir, Ada County, Idaho, 2010–11 ...................................................................................10

5. Graph showing relation between total solar radiation at Mountain Home Remote

Automated Weather Station (RAWS) and net solar radiation at Indian Creek

Reservoir, Ada County, Idaho, 2010–11 ...................................................................................12

6. Photograph showing a seepage meter constructed from a 55-gallon plastic drum,

connection hose, and measurement chamber used for bag deployment, Indian

Creek Reservoir, Ada County, Idaho........................................................................................12

7. Graphs showing comparison of groundwater elevations in shallow well D-0020068

to reservoir elevation, for Indian Creek Reservoir, Ada County, Idaho..............................15

8. Graph showing summary of seepage estimates derived from direct measurements,

estimated using the segmented-Darcy method, and those calculated using the

reservoir energy-budget for Indian Creek Reservoir, Ada County, Idaho, 2010–11.........20
iv

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3/s)

foot per day (ft/d) 0.3048 meter per day (m/d)

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

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

Energy

calorie per second per square foot 

(cal/ft2)

45.04 watts per square meter (W/m2)

Tables

1. Heat storage energy flux and other computation terms for seasonal energy budget

periods, Indian Creek Reservoir, Ada County, Idaho, June 2010–November 2011 ...........7

2. Summary of energy-budget components and calculated evaporation rates for

seasonal energy-budget periods, Indian Creek Reservoir, Ada County, Idaho,

seasonally June 2010–November 2011 ....................................................................................9

3. Monthly mean evaporation, area, volume, and evaporative losses from Indian

Creek Reservoir, Ada County, Idaho, 2010–11........................................................................11

4. Water budget by energy-budget period for Indian Creek Reservoir, Ada County,

Idaho, 2010–11 ............................................................................................................................17

5. Monthly water budget for Indian Creek Reservoir, Ada County, Idaho, 2010–11 ............18

6. Measured and estimated seepages and comparisons using various methods,

Indian Creek Reservoir, Ada County, Idaho, 2010–11 ...........................................................18

7. Mini-piezometer water levels, vertical hydraulic gradients, and calculated

hydraulic conductivities, Indian Creek Reservoir, Ada County, Idaho, 2011 ....................19
v

Conversion Factors, Datums, and Abbreviations and 

Acronyms

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

Volume

liter (L) 0.2642 gallon (gal)

liter (L) 61.02 cubic inch (in3) 

milliliter (mL) 0.03381 ounce (oz.)

Flow rate

centimeter per day (cm/d) 0.3937 inch (in.)

meter per second (m/s) 3.281 foot per second (ft/s) 

millimeter per day (mm/d) 0.03937 inch per day (in/d) 

milliliter per day (mL/d) 0.03381 ounce per day (oz/d)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

Density

kilogram per cubic meter (kg/m3) 0.06242 pound per cubic foot 

(lb/ft3) 

Energy

joule (J) 0.02390 calorie

joule per cubic meter (J/m3) 0.006767 calorie per cubic foot 

(cal/ft3)

watts per square meter (W/m2) 0.022204 calories per second per 

square foot [(cal/s)/ft2]

Specific Heat

joule per kilogram (J/kg) 0.002388 calories per gram (cal/g)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32.

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), 

Universal Transverse Mercator (UTM) zone 11.

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

E evaporation

ESRI Environmental Systems Research Institute

GIS geographic information system

GPS global positioning system

GRSAT Graphical Rating and Shift Application Tool

IDWR Idaho Department of Water Resources

NGS National Geodetic Survey

OPUS Online Position Users Service

RAWS Remote Automated Weather Station

RTK real-time kinematic

TIN Triangulated Irregular Network

USGS U.S. Geological Survey

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Abbreviations and Acronyms
Abstract 

The U.S. Geological Survey, in cooperation with 

the Idaho Department of Water Resources, conducted an 

investigation on Indian Creek Reservoir, a small impoundment 

in east Ada County, Idaho, to quantify groundwater seepage 

into and out of the reservoir. Data from the study will assist the 

Idaho Water Resources Department’s Comprehensive Aquifer 

Management Planning effort to estimate available water 

resources in Ada County. Three independent methods were 

utilized to estimate groundwater seepage: (1) the water- budget 

method; (2) the seepage-meter method; and (3) the segmented 

Darcy method. Reservoir seepage was quantified during the 

periods of April through August 2010 and February through 

November 2011. 

With the water-budget method, all measureable sources of 

inflow to and outflow from the reservoir were quantified, with 

the exception of groundwater; the water-budget equation was 

solved for groundwater inflow to or outflow from the reservoir. 

The seepage-meter method relies on the placement of seepage 

meters into the bottom sediments of the reservoir for the 

direct measurement of water flux across the sediment- water 

interface. The segmented-Darcy method utilizes a combination 

of water-level measurements in the reservoir and in adjacent 

near-shore wells to calculate water-table gradients between 

the wells and the reservoir within defined segments of the 

reservoir shoreline. The Darcy equation was used to calculate 

groundwater inflow to and outflow from the reservoir. Waterbudget results provided continuous, daily estimates of seepage 

over the full period of data collection, while the seepagemeter and segmented Darcy methods provided instantaneous 

estimates of seepage. As a result of these and other difference 

in methodologies, comparisons of seepage estimates provided 

by the three methods are considered semi-quantitative.

The results of the water-budget derived estimates 

of seepage indicate seepage to be seasonally variable in 

terms of the direction and magnitude of flow. The reservoir 

tended to gain water from seepage of groundwater in the 

early spring months (March–May), while seepage losses to 

groundwater from the reservoir occurred in the drier months 

(June–October). Net monthly seepage rates, as computed by 

the water-budget method, varied greatly. Reservoir gains from 

seepage ranged from 0.2 to 59.4 acre-feet per month, while 

reservoir losses to seepage ranged from 1.6 and 26.8 acrefeet per month. An analysis of seepage meter estimates and 

segmented-Darcy estimates qualitatively supports the seasonal 

patterns in seepage provided by the water-budget calculations, 

except that they tended to be much smaller in magnitude. 

This suggests that actual seepage might be smaller than those 

estimates made by the water-budget method.

Although the results of all three methods indicate that 

there is some water loss from the reservoir to groundwater, 

the seepage losses may be due to rewetting of unsaturated 

near-shore soils, possible replenishment of a perched aquifer, 

or both, rather than through percolation to the local aquifer 

that lies 130 feet below the reservoir. A lithologic log from an 

adjacent well indicates the existence of a clay lithology that 

is well correlated to the original reservoir’s base elevation. 

If the clay lithologic unit extends beneath the reservoir basin 

underlying the fine-grain reservoir bed sediments, the clay 

layer should act as an effective barrier to reservoir seepage to 

the local aquifer, which would explain the low seepage loss 

estimates calculated in this study.

Introduction

The U.S. Geological Survey (USGS) in cooperation with 

the Idaho Department of Water Resources (IDWR) conducted 

an investigation to evaluate seepage gains and losses for 

Indian Creek Reservoir (fig. 1), a small impoundment 

about 16 mi southeast of Boise, Idaho. The central issue 

addressed by the study is the nature of groundwater/surfacewater interaction between the local aquifer system and the 

reservoir. This information is vital to the understanding of 

the availability of water-resources in Ada County, Idaho. 

Data from the study will assist the Idaho Water Resources 

Department’s Comprehensive Aquifer Management Planning 

(CAMP) effort to estimate available water resources in Ada 

County. 

An Evaluation of Seepage Gains and Losses in Indian Creek 

Reservoir, Ada County, Idaho, April 2010–November 2011

By Marshall L. Williams and Alexandra B. Etheridge
2 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

tac13-0814_fig01

Dry Creek

Indian Creek Bowns Creek

Blacks Creek

Tenmile Creek

Boise River

Indian Creek

Reservoir

Spring Valley 

Creek

Boise River

Indian Creek

IDARNG1 RG2 RAWS

Mountain Home RAWS

Kuna

Boise

Eagle

Pearl

Oreana

Owyhee

Barber

Bruneau

Orchard

Montour

Mayfield

Meridian

Grand View

Idaho City

Garden City

Mountain Home

Horseshoe Bend

ADA

OWYHEE

ELMORE

BOISE

OWYHEE

GEM

Bogus Basin SNOTEL

13211131

13211130

84

184

84

30

20

26

78

21

67

51

55

69

44

52

44

55

52

116°15'W 115°50'W

43°45'N

43°20'N

42°55' N

0 20 MILES

0

10

10 20 KILOMETERS

Created from Environmental Systems Research Institute base map 

(multiple sources), Web Mercator Auxiliary Sphere, December 2009.

Study Area

IDAHO

EXPLANATION

 Indian Creek gaging station

Bogus Basin Snowpack Telemetry (SNOTEL)

Remote automated weather station (RAWS)

13211130

Figure 1. Indian Creek Reservoir study area, Idaho.
Introduction 3

Purpose and Scope

This report provides an evaluation of three independent 

methods used to estimate seepage from and into Indian Creek 

Reservoir. The three methods used to estimate seepage are 

the water-budget method, the seepage-meter method, and 

the segmented Darcy method. Seepage was estimated for the 

periods April through August 2010 and February through 

November 2011. 

Study Area Description

Ada County is in southwestern Idaho (fig. 1) and, 

according to U.S. Census Bureau statistics (U.S. Census 

Bureau, 2010), is the most populous county in the state 

(392,000 people), and has a land surface area of 1,052 mi2. 

The Boise River runs along the base of and roughly parallel to 

the foothills and mountains at the northern part of the county. 

The annual precipitation recorded at the Mountain Home 

Remote Automated Weather Station (RAWS) was 10 in. in 

2010, and 10.4 in. in 2011 (Western Regional Climate Center, 

2012b). The Bogus Basin Snowpack Telemetry (SNOTEL) 

site, operated by the U.S. Department of Agriculture’s Natural 

Resources Conservation Service in Boise County, is just 

north of the Ada County border, and recorded snow- water 

equivalencies of 91 percent of average in May 2010, and 

155 percent of average in May 2011, as compared to the 

monthly averages from May for 1971–2000 (Natural 

Resources Conservation Service, 2012).

Indian Creek Reservoir (formerly known as Orchard 

Reservoir), is a small impoundment near the eastern border 

of Ada County. It was created in 1892 with an originally 

estimated surface area of 125 acres and storage capacity 

of 4,737 acre-ft at the base of a 43-mi2 drainage (Soil 

Conservation Service, 1950). The drainage area was revised 

by the U.S. Geological Survey and is now estimated at 

51.5 mi2 (U.S. Geological Survey, 2009). Indian Creek is the 

primary contributor to the reservoir; there is no surface- water 

outlet, and only one unmeasured seep was observed in 

October 2011 on the downgradient side of the reservoir 

(fig. 2). The reservoir is southwest of Interstate 84 (fig. 2), 

and is surrounded by low-lying hills with grasses, sagebrush, 

and herbaceous vegetation. The reservoir is at the base of 

the foothills and mountains to the north-northwest, with the 

topography sloping off gradually to the south and southwest. 

Indian Creek

Indian Creek 

Reservoir

Indian Creek

Inlet gage (13211130) Stage gage (13211131)

Weather raft

Dam spillway

Control tower

Probable seep

S piezometer and seepage meter

NE piezometer

NGS benchmark R-84

IDWR well D-0020068

Observed seep

NW seepage meter

Ungaged intermittent inlet flow

Base map from National Agriculture Imagery Program (NAIP), 2011 

natural color 1-meter infrared ortho imagery from U.S. Department of 

Agriculture, Farm Service Agency.

116°02'W 116°01'W 116°W 115°59'W

43°24'N

43°23'N

0 1,000 2,000 FEET

0 1,000 2,000 METERS

84

Figure 2. Locations of sites in and near Indian Creek Reservoir, Ada County, Idaho. (IDWR, Idaho Department of 

Water Resources; NGS, National Geodetic Survey; NW, northwest; NE, northeast; S, south.)
4 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

The maximum daily mean temperature measured during the 

study was 27.4°C in July 2011, and the minimum daily mean 

temperature was 1.1°C in November 2011. The reservoir 

periodically freezes in the winter. 

Methods

Methods are presented for the three seepage estimation 

methods used in this study, the water-budget method, the 

seepage-meter method, and the segmented-Darcy method. 

Each method requires different instrumentation and techniques 

to provide seepage estimates for the reservoir.

Water Budget Method

Water budgets are important for evaluating availability 

of water resources and the potential sustainability of water 

supplies for current users and future development. A water 

budget is area specific, the results of which depend on climate, 

water resources, human activities, geology, vegetation, and 

land use (Healy and others, 2007). Analysis of the Indian 

Creek Reservoir water budget provides an understanding 

of which water budget components are important to the 

water balance of the reservoir and can be used to better 

understand the interaction of the reservoir and groundwater. 

The water- budget equation used to calculate Indian Creek 

Reservoir gains or losses to groundwater is as follows:

∆S Q− − sw P E + = Qgw (1)

where

∆S is the change in reservoir storage (positive 

values are an increase in storage),

Qsw is the surface water inflow to the reservoir,

P is precipitation to the reservoir,

E is evaporation from the reservoir (evaporation is 

an outflow component), and

Qgw is reservoir seepage gains and losses (positive 

values represent reservoir gains from 

groundwater inflow) to the reservoir.

Streamflow and Precipitation

Surface-water inflow from Indian Creek enters the 

reservoir from the northeast through an existing channel 

that was modified in the late 1960s to construct Interstate 84 

(fig. 2). The former channel of Indian Creek extends 

several hundred yards toward the center of the reservoir. An 

additional intermittent drainage to the northwest also may 

provide surface-water inflow, but this drainage area was not 

considered to be a significant contributor during the periods 

of study because the drainage basin is small (2.8-mi2). There 

is no surface-water outflow from the reservoir, and only one 

unmeasured seep was first observed in October 2011 on the 

downgradient side of the reservoir (fig. 2). The seep consisted 

of 50 m of saturated soil at the bottom of a narrow drainage 

channel with no discernible flow, and only a small amount of 

ponding at the beginning of the seep.

The Indian Creek above Indian Creek Reservoir 

streamgage (USGS station 13211130) was operated seasonally 

from March 2010 to October 2011 and has a 46.3-mi2 drainage 

area. This streamgage was installed to quantify surface-water 

inflow to Indian Creek Reservoir. Daily mean streamflow 

ranged from 0 to 79 ft3/s during the study period, with the 

highest daily streamflow of 79 ft3/s on January 17, 2011. 

Runoff to the reservoir ranged from 0 acre-ft in the 2010 study 

period to 416 acre-ft in the 2011 study period.

Precipitation values for the 2010 study period were 

obtained from a Remote Automated Weather Station (RAWS; 

fig. 1) in Mountain Home, Idaho, about 26 mi south-southeast 

of the reservoir. Precipitation values for 2011 were obtained 

from the IDARNG1 RG2 RAWS (fig. 1), about 10.6 mi 

southwest of the reservoir (Western Regional Climate 

Center, 2012b).

Change in Reservoir Storage

In October 2009, when the reservoir was nearly 

dry (3,314 ft), a bathymetric survey was completed. The 

bathymetric survey has a horizontal and vertical accuracy of 

1 ft. Survey data were collected using real-time kinematic 

(RTK) global positioning system (GPS) equipment. A base 

station was set up over an arbitrary point and a first-order 

vertical benchmark known to the National Geodetic Survey as 

R-84 was used as a quality-control point (fig. 2). The Online 

Position Users Service (OPUS) was used to acquire a precise 

position for the base station on each day of surveying. 

Bathymetric survey points were collected using roving 

GPS receivers and differential GPS to compute the height 

and location of each point in real time. Survey points were 

collected in three ways: (1) on foot using a 2-m survey rod 

equipped with a bipod, (2) on an all-terrain vehicle with the 

roving receiver affixed to the vehicle and measured at a known 

height above ground, and (3) from a kayak using a 2-m rod 

equipped with a bipod. An attempt was made to collect most 

GPS data on a 30-m grid, but GPS data in some of the flatter 

areas near the highway were collected on a 100-m grid.

After all survey data were assembled, OPUS solutions 

for the base station during each day of data collection were 

applied to correct all differential GPS survey data. Detailed 

descriptions of survey methods and metadata are available in 

appendix A and in Wilson and Richards (2006).

After bathymetric survey data were post-processed, 

quality-assured, and approved, a Triangulated Irregular 

Network (TIN) model of the bathymetric surface was created. 

The TIN model was used to compute a stage-capacity rating 

and a stage-area rating for the reservoir according to methods 

outlined in Wilson and Richards (2006). Both ratings were 

extended to the maximum capacity of the reservoir, which 

is the elevation of the spillway at 3,339.88 ft. The ratings 

are computed at stage increments of 1 ft and are provided in 

appendix A.
Methods 5

Area and capacity ratings also were computed at 

0.01- ft stage increments using the Aquatic Informatics 

Graphical Rating and Shift Application Tool (GRSAT) 

software, which is used for many calculations provided in this 

report. Elevations and corresponding surface area or capacity 

values from the TIN model were used as input points for 

the GRSAT ratings. Each rating was computed in log space 

and extended over the range of conditions observed. The 

0.01- ft rating tables were used for computational purposes 

only and were not provided in this report because the 

bathymetric survey and the TIN model are accurate only to the 

nearest foot.

Reference points used to establish the reservoir stage 

datum at the reservoir stage gage were surveyed using 

RTK GPS in October 2011. The surveyed elevations were 

referenced to the North American Vertical Datum of 1988 

(NAVD 88) for each reference point and mini-piezometer. 

This allowed the reservoir stage record to be referenced to the 

same vertical datum. Previously, the reservoir stage record was 

referenced to an arbitrary vertical datum and the new survey 

improved the accuracy of the existing stage datum of the 

reservoir. The base station was set up over NGS benchmark 

R-84 (fig. 2) during the 2011 survey, and the spillway 

elevation served as a quality-assurance check to ensure that 

bathymetric survey data agreed with the new survey data. 

The base station was set up for more than 4 hours during the 

October 2011 survey because the longer duration base station 

data collection period has been shown to drastically reduce the 

error associated with survey data, especially when it exceeds 

4 hours (Soler and others, 2006). Increased vertical accuracy 

was imperative because the gage-height record is reported to 

within 0.01 ft; elevations obtained for the NGS benchmark 

and the dam face during the 2011 survey were within 0.05 ft 

of survey results from 2009. Each point was surveyed with 

a roving GPS receiver on a 2-m survey rod equipped with 

a bipod and each point was occupied for 180 epochs to 

maximize vertical accuracy.

The overall vertical accuracy of the control points 

surveyed in October 2011 was 0.024 m, and the horizontal 

accuracy was 0.021 m. Accuracy was computed using overall 

root mean square error provided by the OPUS solution 

and methods described by the Federal Geographic Data 

Committee (1998).

To determine water volume and surface area during 

the study, a stage gage was installed on the south end of the 

reservoir at the deepest point and used in conjunction with the 

stage-capacity rating and a stage-area rating for water budget 

calculations. The Indian Creek Reservoir stage gage (USGS 

station 13211131) has a period of record from March 2010 

to August 2010, and from February 2011 to October 2011. 

The reservoir stage ranged from 4.13 to 0 ft during 2010, 

and 14.67 to 10.11 ft during 2011. Maximum surface area 

was 23.4 acres during the 2010 study period and 121 acres 

during the 2011 study period. In 2010, the maximum reservoir 

volume was 28.5 acre-ft at the end of March and decreased 

to desiccation in August. In 2011, reservoir volume increased 

from 320 acre-ft in late February to 760 acre-ft at the end of 

April, which was followed by a monthly decrease to a volume 

of 361 acre-ft by the end of October.

Evaporation

Reservoir evaporation was calculated using the Bowen 

ratio energy-budget method. The meterologic and hydrologic 

data used in the energy-budget method was collected with an 

instrumentation cluster installed on a raft positioned on the 

reservoir (fig. 2). The raft was initially installed in May 2010, 

and was operational from June 2010 through the end of 

August 2010. In March 2011, the raft was reinstalled on the 

reservoir and continued in operation until the end of the study 

period in November. The methods described in this report 

follow methods used for an open water evaporation study of 

Walker Lake, Nevada (Allander and others, 2009).

The Bowen ratio energy-budget method of estimating 

evaporation is considered to be one of the most rigorous and 

accurate methods to determine evaporation (Harbeck and 

others, 1958; Winter, 1981). The energy-budget equation for 

open water is as follows (Sturrock and others, 1992; Allander 

and others, 2009):

Q Q s r − + Q Q a a − −r b Q Q s v + − Q Q e h − − Q Q w b + = Qx (2)

where

Qs is incoming short-wave radiation,

Qr is reflected short-wave radiation, 

Qa is incoming long-wave radiation, 

Qar is reflected long-wave radiation, 

Qbs is long-wave radiation emitted from the body of 

water, 

Qv is net energy advected to the reservoir, 

Qe is energy used for evaporation, 

Qh is energy conducted from the water body as 

sensible heat, 

Qw is energy advected from the water body by the 

evaporated water,

Qb is energy transferred from reservoir bed 

sediments to the reservoir, and 

Qx is change in energy stored in the reservoir, or 

called heat-storage energy flux.

All of these energy terms are in watts per square meter 

(W/m2). The solar radiation terms Qs, Qr, Qa, and the sum 

of Qar, and Qbs were measured quantities using precision 

pyrgeometers (long-wave radiation) and pyranometers 

(short- wave radiation) suspended over the water, the values of 

which were used to calculate net radiation Qn:

Q Q n s = − Q Q r a + − Q Q ar bs − (3)

Net energy advected (Qv) into Indian Creek Reservoir 

comes from the surface-water inflows from Indian Creek, 

groundwater inflows, and precipitation. Energy advected away 
6 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

from the reservoir occurs through evaporation because there 

is no surface-water outflow. Net energy advected into the 

reservoir is calculated as follows:

Q c v x = − p F ∑ T T x b ( ) (4)

where

c is the specific heat of water (4,190 J/kg),

ρ is the density of evaporated water (1,000 kg/m3),

Fx is the flux of inflow component (m/s),

Tx is temperature of inflowing water (°C), and 

Tb is an arbitrary base temperature (0°C).

There are three primary sources of energy advection 

to the reservoir: surface water, precipitation directly on the 

reservoir, and groundwater. Of these sources, surface-water 

inflow is considered significant, and the others are low to 

negligible contributors. Heat advection into the reservoir due 

to precipitation was calculated for each energy-budget period, 

where the temperature of the precipitation was assumed the 

same as the mean air temperature for the energy- budget 

period. Results from the calculation indicate that heat 

advection owing to precipitation was fairly negligible when 

values for all but two energy-budget periods were less than 

1 W/m2, and for two other periods values were less than 

2 W/ m2. Although the resulting values were small, they were 

summed with the surface-water heat advection results by 

energy-budget period for a net heat advection value. Heat 

advection due to groundwater is assumed negligible. This 

assumption is based on a hypothetical exercise using real and 

estimated values to calculate heat advection from groundwater 

inflow. For example, the reservoir at a maximum capacity, 

with a surface area of 217 acres, would need to receive 

357 acre-ft of groundwater inflow during the year at 15°C 

to gain 1 W/m2. Because the 357 acre-ft groundwater inflow 

is three times the net groundwater inflow of 99.6 acre-ft in 

2011 (snowpack snow-water equivalent was 155 percent of 

average), heat advection from groundwater inflow probably 

is negligible. Heat advection between the reservoir sediment 

(Qb) and the water also is assumed to be negligible and usually 

is not measured (Winter, 1981; Rosenberry and others, 2007; 

Allander and others, 2009).

The Qe, Qh, and Qw terms in equation (2) were a function 

of evaporation rate (E) and were not directly measured. These 

terms are all related to E as follows:

Q p e = EL, (5)

Q R h e = Q , (6)

Q c w e = − pE T Tb ( ), (7)

where

E is the energy-budget evaporation rate (m/s),

L is the latent heat of vaporization of water 

(2,450,000 [J/kg]/°C),

R is the Bowen ratio (dimensionless), and 

Te is the temperature of the evaporated water (taken 

as the surface temperature, T0, °C).

Heat storage in the reservoir (Qx) is computed using the 

following equation (Allander and others, 2009): 

Q c d T

t x w e = ∆ , (8)

where

Qx is heat storage energy flux (W/m2),

Cw is the volumetric heat capacity of water, a 

constant (4.187 M [J/m3]/°C),

de is the mean reservoir depth (m),

	 ∆T is the change in volume-weighted temperature 

(°C), and

t is the time span of the energy-budget period 

(seconds).

The mean reservoir depth (de) at the end of each 

energy- budget period was determined by dividing the total 

reservoir volume by the reservoir surface area. The change in 

volume-weighted temperature was calculated by determining 

the thermal profile of the reservoir using a precision thermistor 

chain continuously suspended in the water column, and 

measuring temperatures from the sediment surface to the 

water surface in 1-ft increments. The temperature of each 1-ft 

horizontal layer was multiplied by the water layer volume, and 

their products were summed. A volume-weighted temperature 

was then computed by dividing that sum by the total reservoir 

volume. Vertical profile temperature data for energy-budget 

periods 13 and 14 were not available because of equipment 

failure. The mean reservoir temperature was computed using 

the mean surface-water and reservoir-bottom temperatures for 

the respective energy-budget periods, and was based on the 

assumption that the reservoir was well mixed. Additionally, 

a data gap occurred in the reservoir-stage gage record from 

July 28 to August 4, 2010. A linear interpolation was used 

for the data gap to estimate daily mean stage values based 

on a linear regression model for the reservoir stage. This 

method was determined to be sufficient for this short data 

gap because water-budget components were stable from 

July 23 to August 9, 2010 (including the 5 days before and 

after the data gap) with no surface-water inflow, only 0.03 in. 

of precipitation was recorded at the Mountain Home RAWS, 

and seepage estimates from the meters indicated negligible 

loss. The 2009 bathymetric survey provided the total reservoir 

volume, volumes by layer, and surface area by stage elevation 

needed for the calculations (appendix A). A summary of terms 

used to calculate the heat-storage energy flux is provided in

table 1. The heat-storage energy flux ranged from a minimum 

of -50.8 W/m2 during energy-budget period 41, October 7–13, 

2011, to a maximum of 41.8 W/m2 during energy-budget 

period 19, May 6–12, 2011.
Methods 7

Table 1. Heat storage energy flux and other computation terms for seasonal energy budget periods, Indian Creek Reservoir, Ada 

County, Idaho, June 2010–November 2011.

[All periods start at 0000 hours and end at 2359 hours on the corresponding end dates. Reservoir stage, volume, and surface area are from observation made 

on the end date for each budget period. Reservoir stage in feet above National Geodetic Vertical Datum of 1929. Reservoir surface area interpolated from 2009 

bathymetric survey stage-area relation table. Abbreviations: ft, foot; acre-ft, acre-foot; T, volume weighted reservoir temperature, in degrees Celsius (°C); ΔT, 

change in volume weighted reservoir temperature, in degrees Celsius (°C); de, mean reservoir depth (reservoir volume/reservoir area) in meters (m); J/m2, joules 

per square meter; Qx, heat-storage energy flux, in watts per square meter (W/m2); e, estimate; –, data not available]

Energybudget 

period

Start date End date Days in 

period

Reservoir

stage

(ft)

Reservoir

volume 

(acre-ft)

Reservoir

surface area 

(acre)

T

(°C)

ΔT 

(°C)

de

(m)

Change in 

heat storage 

(×104 J/m2)

Qx(W/m2)

0 06-04-10 06-10-10 7 3,314.73 26.3 19.3 17.4 – 0.439 – –

1 06-11-10 06-17-10 7 3,314.53 22.4 17.3 19.2 1.8 0.398 315 5.20

2 06-18-10 06-24-10 7 3,314.32 19.1 15.3 19.6 0.3 0.384 50 0.83

3 06-25-10 07-01-10 7 3,314.11 16.0 13.4 21.6 2.1 0.365 324 5.36

4 07-02-10 07-08-10 7 3,313.84 12.7 11.4 19.0 -2.6 0.342 -383 -6.33

5 07-09-10 07-15-10 7 3,313.56 9.83 9.52 22.6 3.6 0.318 496 8.20

6 07-16-10 07-22-10 7 3,313.21 7.02 7.42 22.4 -0.2 0.289 -21.9 -0.36

7 07-23-10 07-29-10 7 3,312.88 5.24 5.73 22.7 0.2 0.491 39.4 0.65

8 07-30-10 08-05-10 7 3,312.57 3.09 4.54 22.6 -0.0 0.207 -5.86 -0.10

9 08-06-10 08-12-10 7 3,312.23 1.77 3.29 20.8 -1.9 0.164 -147 -2.42

10 08-13-10 08-19-10 7 3,311.85 0.91 2.06 20.3 -0.5 0.135 -30.7 -0.51

11 03-09-11 03-15-11 7 3,321.13 320 73.2 5.4 – 1.330 – –

12 03-16-11 03-22-11 7 3,321.22 327 74.2 6.2 0.9 1.340 484 8.01

13 03-23-11 03-29-11 7 3,321.97 386 82.2 1

5.9 -0.3 1.360 -189 -3.13

14 03-30-11 04-07-11 9 3,324.07 581 104.0 1

8.3 2.7 1.540 1,480 19.0

15 04-08-11 04-14-11 7 3,324.65 644 111.0 8.9 0.3 1.750 388 6.41

16 04-15-11 04-21-11 7 3,325.12 697 116.0 10.5 1.6 1.770 1,190 19.7

17 04-22-11 04-28-11 7 3,325.63 757 121.0 10.2 -0.3 1.910 -226 -3.73

18 04-29-11 05-05-11 7 3,325.59 752 121.0 10.1 -0.1 1.900 -90.4 -1.50

19 05-06-11 05-12-11 7 3,325.63 757 121.0 13.3 3.2 1.910 2,530 41.8

20 05-13-11 05-19-11 7 3,325.54 746 120.0 14.8 1.6 1.890 1,240 20.6

21 05-20-11 05-26-11 7 3,325.42 732 119.0 14.7 -0.1 1.870 -101 -1.67

22 05-27-11 06-02-11 7 3,325.32 720 118.0 13.2 -1.5 1.860 -1,140 -18.9

23 06-03-11 06-09-11 7 3,325.26 713 118.0 15.6 2.3 1.850 1,800 29.7

24 06-10-11 06-16-11 7 3,325.09 693 116.0 16.8 1.2 1.820 951 15.7

25 06-17-11 06-23-11 7 3,324.93 675 114.0 17.9 1.1 1.800 858 14.2

26 06-24-11 06-30-11 7 3,324.71 650 112.0 19.1 1.2 1.770 898 14.8

27 07-01-11 07-07-11 7 3,324.51 628 109.0 20.8 1.7 1.750 1,230 20.4

28 07-08-11 07-14-11 7 3,324.29 604 107.0 21.2 0.4 1.730 304 5.03

29 07-15-11 07-21-11 7 3,324.05 579 104.0 21.4 0.2 1.700 112 1.86

30 07-22-11 07-28-11 7 3,323.80 553 101.0 21.3 -0.1 1.660 -75 -1.23

31 07-29-11 08-04-11 7 3,323.58 531 98.8 22.5 1.2 1.650 826 13.7

32 08-05-11 08-11-11 7 3,323.35 509 96.4 22.5 0.0 1.610 0.810 0.01

33 08-12-11 08-18-11 7 3,323.12 487 93.9 21.1 -1.4 1.590 -933 -15.4

34 08-19-11 08-25-11 7 3,322.90 467 91.6 21.0 -0.1 1.550 -50.2 -0.83

35 08-26-11 09-01-11 7 3,322.67 446 89.3 21.2 0.2 1.520 125 2.06

36 09-02-11 09-08-11 7 3,322.48 429 87.4 18.5 -2.7 1.500 -1,700 -28.1

37 09-09-11 09-15-11 7 3,322.32 415 85.6 20.0 1.5 1.480 917 15.2

38 09-16-11 09-22-11 7 3,322.16 402 84.1 18.4 -1.6 1.450 -991 -16.4

39 09-23-11 09-29-11 7 3,322.00 388 82.4 18.1 -0.3 1.440 -167 -2.75

40 09-30-11 10-06-11 7 3,321.92 382 81.6 17.1 -1.0 1.420 -616 -10.2

41 10-07-11 10-13-11 7 3,321.87 378 81.1 11.9 -5.2 1.420 -3,070 -50.8

42 10-14-11 10-20-11 7 3,321.81 373 80.5 12.7 0.8 1.410 495 8.19

43 10-21-11 10-27-11 7 3,321.71 365 79.4 11.0 -1.7 1.400 -997 -16.5

44 10-28-11 11-03-11 7 3,321.66 361 78.8 8.0 -3.1 1.380 -1,790 -29.6

45 11-04-11 11-10-11 7 3,321.66 361 78.8 3.9 -4.1 1.360 -2,340 -38.7

46 11-11-11 11-17-11 7 3,321.66 361 78.8 3.8 -0.1 1.360 -38.7 -0.64

1

Volume weighted reservoir temperature calculated from average of surface and bottom temperatures, based on the assumption that the reservoir was well 

mixed with no temperature stratification; the mean temperature represents the entire water column.
8 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

Substituting appropriate terms from the previous 

equations, the following equation is used to determine 

evaporation in this study: 

E Q Q Q

p L R cT

n v x

o

= + −

[ ( + +) ], 1

(9)

The Bowen ratio, R, is a unitless value for the ratio of 

sensible heat flux (Qh) to latent heat flux (Qe) at the air-water 

interface. It is computed from field-measured data as follows: 

R c P T T

e e

b = −

−

( ) , 1 2

1 2

(10)

where

cb is an empirical constant (0.00061°C-1),

P is the atmospheric pressure at Indian Creek 

Reservoir (89900 Pa),

T1 – T2 is the air temperature difference (°C) at the

reservoir surface and 1.5 m above the 

reservoir surface, and

e1 –e2 is the vapor pressure difference (Pa) between 

the reservoir surface and 1.5 m above the 

reservoir surface.

Temperature and humidity were measured directly at 

1.5 m above the water surface; the temperature of the water 

surface was measured using a precision thermistor suspended 

in the top 1 cm of the reservoir water. Vapor pressures 

were calculated using temperature and humidity data, and 

calculations assumed vapor at the air-water boundary layer 

was 100 percent saturated. The typical temperature humidity 

sensor height for calculating the Bowen ratio is 2 m, except in 

circumstances where the fetch is short, as is the case on Indian 

Creek Reservoir. When the fetch is short, the air moving from 

one surface type to another (land to water) has insufficient 

time to reflect the changes in latent and sensible heat, and, 

in turn, can create higher uncertainty (Stannard and others, 

2004). Heilman and others (1989) determined that in situations 

where the fetch was short and the Bowen ratio was small, 

a fetch-to-height ratio of as little as 20-to-1 could be used 

compared to the typical 100-to-1 ratio. Stannard and others 

(2004) also determined that fetch-induced errors are reduced 

by placing the lower sensor at the water surface when using 

the Bowen ratio energy-budget method over short fetches. 

Meteorological data were collected every 10 seconds and 

averaged for 15-minute intervals for 96 values during each 

24-hour period. Energy-budget and subsequent-water budget 

calculations were computed for periods of not less than 1 week 

because of increased error in the Qx values (energy storage in 

the reservoir) for shorter intervals (Winter, 1981; Swancar and 

others, 2000). 

Evaporation Rates

Evaporation rate estimates were made during two study 

periods, March–August 2010, and March–November 2011, 

for Indian Creek Reservoir for 47 energy-budget periods with 

duration of at least 7 days. A summary of the evaporation rates 

by energy-budget period and the energy-budget components 

used to compute them are listed in table 2. The maximum 

evaporation rate was 9.2 mm/d during energy-budget 

period 6, July 16–22, 2010; the minimum evaporation rate 

was 0.8 mm/d during energy-budget period 46, November 

11–17, 2011. Evaporation rates during the study were highest 

in July and lowest during the early spring and late autumn 

periods. A correlation of evaporation in relation to available 

energy was statistically significant with a coefficient of 

determination (R2) value of 0.92 (fig. 3), and Spearman’s 

rank correlation coefficient of 0.94. Energy-budget period 14, 

March 30– April 7, 2011, had significant heat flux because 

of surface-water inflow. These large fluctuations are typical 

when short energy-budget periods are calculated, but tend 

to average out when energy-budget periods of 2 weeks or 

longer are used (Tanny and others, 2008). The evaporation rate 

distribution (fig. 4) indicates that the highest evaporation rates 

tend to occur in June and July of each year. Evaporation rates 

from the energy-budget periods form the basis for estimating 

monthly evaporation.

To estimate monthly evaporation, the daily mean 

evaporation rate from the energy-budget period was applied to 

the applicable day of the month and the results were summed 

for total evaporation in inches for each month. The discharge 

volumes from evaporation were calculated by multiplying 

the monthly evaporation total by the monthly mean surface 

area. During the study, the surface area of the reservoir varied 

from totally desiccated in August 2010, to a maximum of 

120 acres in May and June 2011. For April and May 2010, 

direct measurements to calculate evaporation were not 

available, so the daily mean evaporation rates from April and 

May 2011 were used to estimate evaporation. The estimates 

seem reasonable because seepage meters that were placed 

on the reservoir during these months measured reservoir 

gains from groundwater of 0.014 to 0.29 acre-ft/d (mean of 

0.15 acre- ft/d), and the water-budget calculations indicate 

groundwater seepage to the reservoir in April and May at 0.16 

and 0.10 acre-ft/d, respectively.

Evaporation estimates during this study indicate that 

evaporation from the reservoir exceeds the average annual 

precipitation. In 2011, estimated evaporation from the 

reservoir during the study period was 50.2 in, with a total 

discharge from evaporation of 428 acre-ft. The annual 

precipitation average for Boise is 11.8 in.(National Weather 

Service, 2012) and for Mountain Home is 10.0 in. (Western 
Methods 9

Table 2. Summary of energy-budget components and calculated evaporation rates for seasonal energy-budget periods, Indian Creek 

Reservoir, Ada County, Idaho, seasonally June 2010–November 2011.

[All periods start at 0000 hours and end at 2359 hours on indicated dates; all units are daily mean values and are in watts per square meter (W/m2) unless 

otherwise noted. Qn: net radiation. Qv: net energy advected to the lake. Qx: heat storage energy flux; measured independently and therefore did not need to be 

estimated. R: Bowen ratio. T0: water-surface temperature, in degrees Celsius, (°C). E: energy budget period evaporation rate, in millimeters per day (mm/d). 

Total E: total evaporation during energy budget period, in inches (in.). –, insufficient data to calculate]

Energybudget 

period

Start date End date Days in 

period Qn Qv Qx

R

(unitless)

T0

(°C)

E

(mm/d)

Total E

(in.)

0 06-04-10 06-10-10 7 134 1 – 0.17 18.2 4.0 1.1

1 06-11-10 06-17-10 7 246 0 5 0.15 17.7 7.2 2.0

2 06-18-10 06-24-10 7 238 0 1 0.08 19.4 7.5 2.1

3 06-25-10 07-01-10 7 226 0 5 0.00 21.7 7.5 2.1

4 07-02-10 07-08-10 7 254 0 -6 0.06 19.5 8.4 2.3

5 07-09-10 07-15-10 7 247 0 8 0.00 22.0 8.1 2.2

6 07-16-10 07-22-10 7 253 0 0 -0.06 22.2 9.2 2.5

7 07-23-10 07-29-10 7 208 0 1 -0.03 23.0 7.3 2.0

8 07-30-10 08-05-10 7 219 0 0 -0.06 22.9 7.9 2.2

9 08-06-10 08-12-10 7 175 0 -2 -0.08 19.6 6.6 1.8

10 08-13-10 08-19-10 7 205 0 -1 -0.09 20.1 7.6 2.1

11 03-09-11 03-15-11 7 64 0 – -0.10 5.3 2.5 0.7

12 03-16-11 03-22-11 7 64 0 8 0.24 6.1 1.6 0.4

13 03-23-11 03-29-11 7 89 37 -3 0.34 6.4 3.4 0.9

14 03-30-11 04-07-11 9 115 220 19 0.25 9.1 8.8 3.1

15 04-08-11 04-14-11 7 125 66 6 0.36 8.8 4.7 1.3

16 04-15-11 04-21-11 7 131 50 20 0.30 10.4 4.3 1.2

17 04-22-11 04-28-11 7 149 57 -4 0.35 10.1 5.4 1.5

18 04-29-11 05-05-11 7 192 8 -1 0.25 10.1 5.6 1.6

19 05-06-11 05-12-11 7 192 19 42 0.14 13.6 5.1 1.4

20 05-13-11 05-19-11 7 161 2 21 0.21 14.9 4.1 1.1

21 05-20-11 05-26-11 7 206 1 -2 0.19 14.6 6.0 1.7

22 05-27-11 06-02-11 7 154 1 -19 0.29 13.1 4.7 1.3

23 06-03-11 06-09-11 7 191 1 30 0.16 16.4 4.8 1.3

24 06-10-11 06-16-11 7 223 0 16 0.15 17.0 6.2 1.7

25 06-17-11 06-23-11 7 232 1 14 0.02 18.8 7.3 2.0

26 06-24-11 06-30-11 7 244 0 15 0.03 19.3 7.6 2.1

27 07-01-11 07-07-11 7 242 0 20 -0.03 21.5 7.7 2.1

28 07-08-11 07-14-11 7 241 0 5 0.02 21.7 7.9 2.2

29 07-15-11 07-21-11 7 242 0 2 0.00 21.6 8.2 2.3

30 07-22-11 07-28-11 7 248 0 -1 -0.02 21.7 8.6 2.4

31 07-29-11 08-04-11 7 201 0 14 -0.09 22.8 7.0 1.9

32 08-05-11 08-11-11 7 210 0 0 -0.02 22.7 7.3 2.0

33 08-12-11 08-18-11 7 190 0 -15 -0.04 21.4 7.2 2.0

34 08-19-11 08-25-11 7 205 0 -1 -0.14 20.5 8.1 2.2

35 08-26-11 09-01-11 7 159 0 2 -0.12 19.5 6.1 1.7

36 09-02-11 09-08-11 7 155 0 -28 -0.12 17.4 7.1 1.9

37 09-09-11 09-15-11 7 133 0 15 -0.12 19.6 4.6 1.3

38 09-16-11 09-22-11 7 124 0 -16 0.06 18.3 4.6 1.3

39 09-23-11 09-29-11 7 108 0 -3 -0.05 18.3 4.0 1.1

40 09-30-11 10-06-11 7 50 2 -10 0.04 17.2 2.0 0.6

41 10-07-11 10-13-11 7 58 1 -51 0.14 11.8 3.3 0.9

42 10-14-11 10-20-11 7 46 1 8 0.16 12.7 1.2 0.3

43 10-21-11 10-27-11 7 23 0 -16 0.30 10.9 1.1 0.3

44 10-28-11 11-03-11 7 27 0 -30 0.11 7.9 1.8 0.5

45 11-04-11 11-10-11 7 16 0 -39 0.53 3.7 1.3 0.3

46 11-11-11 11-17-11 7 25 0 -1 0.11 3.7 0.8 0.2
10 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

Figure 3. Relation between available energy and evaporation rates for Indian Creek Reservoir, Ada County, 

Idaho, 2010–11.

Figure 4. Evaporation rates for energy-budget periods, Indian Creek Reservoir, Ada County, Idaho, 

2010–11.

tac13-0814_fig03

0

2

4

6

8

10

12

0 50 100 150 200 250 300 350 

Energy-budget period

14 outlier 

Available energy, in watts per square meter

Evaporation rate, in millimeters per day

y = 0.0330 * (available energy) - 0.0780 

 R2 = 0.92

SE = 0.28 

0

1

2

3

4

5

6

7

8

9

10

06-10-2010 06-17-2010 06-24-2010 07-01-2010 07-08-2010 07-15-2010 07-22-2010 07-29-2010 08-05-2010 08-12-2010 08-19-2010 

03-15-2011 03-22-2011 03-29-2011 04-07-2011 04-14-2011 04-21-2011 04-28-2011 05-05-2011 05-12-2011 05-19-2011 05-26-2011 06-02-2011 06-09-2011 06-16-2011 06-23-2011 06-30-2011 07-07-2011 07-14-2011 07-21-2011 07-28-2011 08-04-2011 08-11-2011 08-18-2011 08-25-2011 09-01-2011 09-08-2011 09-15-2011 09-22-2011 09-29-2011 10-06-2011 10-13-2011 10-20-2011 10-27-2011 11-03-2011 11-10-2011 11-17-2011 

Energy-budget period end dates

Evaporation rate, in millimeters per day

0 

1 

2 3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 23 

24 

25 

26 27 28 

29 

30 

31 

32 33 

34 

35 

36 

37 38 

39 

40 

41 

42 43 

44 

45 

46 

14 Energy-budget period

EXPLANATION
Methods 11

Regional Climate Center, 2012a; precipitation for Mountain 

Home RAWS in 2010 was 10.1 in., and in 2011 was 10.4 in., 

indicating that local precipitation during the study was 

close to normal in both years (Western Regional Climate 

Center, 2012b). Therefore, surface-water and groundwater 

contributions to the reservoir are essential for it to maintain 

observed water levels. The monthly mean (March–October) 

estimates of reservoir characteristics for 2010 and 2011 are 

shown in table 3. Monthly evaporation estimates were lowest 

in March (2.8 in.) and October (2.3 in.), and were highest 

during June (mean of 7.6 in.) and July (mean of 9.9 in.). Mean 

reservoir surface area varied during the first study year, with a 

mean reservoir surface area of 22.2 acres in April 2010, which 

then decreased until the reservoir desiccated in August 2010. 

The mean reservoir surface area in 2011 ranged from a 

minimum of 74.8 acres in March to a maximum of 120 acres 

in May.

A correlation of the net energy at the reservoir (as 

compared to total energy measured at the Mountain Home 

RAWS over the two seasons) was done as a quality-assurance 

evaluation of the solar radiation equipment placed on Indian 

Creek Reservoir. The results were statistically significant, 

with an R2 of 0.96 (fig. 5), and a Spearman’s rank correlation 

coefficient of 0.98.

Evaporation Uncertainty

Although the Bowen ratio energy-budget method is 

considered the most accurate method to estimate evaporation, 

it is not without limits and uncertainty. Winter (1981) did a 

comparison of uncertainty from other energy-budget studies 

and determined that monthly uncertainty was about 10 percent 

in the summer. Gunaji (1967) determined that energy-budget 

periods with a duration of about 2 weeks had a computed error 

of 4.4–27.8 percent, with a mean study error of 10.5 percent. 

Uncertainty also is introduced by the accuracy of the 

bathymetric survey, a short fetch, and other instrumentation 

used to calculate volume and discharge. 

Finally, the reservoir is partially covered in emergent 

hydrophytes that vary in their areal coverage, depending on 

the time of year and the depth of water. Information on how 

much vegetation affects evaporation varies by plant species, 

area coverage, plant density, and other factors (Mitsch and 

Gosselink, 2007). 

Table 3. Monthly mean evaporation, area, volume, and evaporative losses from Indian Creek Reservoir, 

Ada County, Idaho, 2010–11.

[Evaporation: Monthly evaporation estimated from evaporation rates in the energy-budget periods. The 2009 Indian Creek 

Reservoir bathymetric survey provided the data for computing the stage-area and stage-volume relation. Volume evaporated:

Determined by multiplying monthly mean evaporation by monthly mean reservoir area. –, not estimated]

Year March April May June July August September October

Evaporation, in inches

2010 – 16.9 16.2 7.7 10 – – –

2011 2.8 6.9 6.2 7.5 9.8 8.8 5.9 2.3

Mean reservoir area, in acres

2010 – 22.2 19.0 17.5 9.6 – – –

2011 74.8 111 120 116 106 94.7 85.6 81.0

Mean reservoir volume, in acre-feet

2010 – 31.4 25.5 22.8 9.8 – – –

2011 332 641 746 692 595 494 413 374

Volume evaporated, in acre-feet

2010 – 12.2 9.8 11.2 7.7 – – –

2011 17.5 63.3 62.4 72.6 86.0 69.2 42.0 15.4

1

Monthly evaporation rate estimated using 2011 daily mean evaporation for respective months.
12 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

Seepage Meter Method

Seepage meters are commonly used to observe and to 

measure exchange of water between surface water (lakes, 

reservoirs, and streams) and groundwater at a point location. 

For this study, the seepage meter method used two seepage 

meters constructed of rigid plastic 55-gal drums cut in 

half (fig. 6), and deployed according to methods described 

in Rosenberry and LaBaugh (2008). Seepage meter bags 

were attached to the drums and were used to measure 

actual seepage that occurs in the area of sediment covered 

by the drum, during a specific period. With the seepage 

meter method, it was assumed that the measured seepage 

occurred in 5–20 percent of the reservoir surface area. This 

assumption was made because seepage rates commonly 

decrease significantly with distance from shore (McBride and 

Pfannkuch, 1975). The seepage meter method also assumed 

that all subsurface flow is isotropic and occurs in the vertical 

direction.

The bag used to estimate seepage into or out of the 

reservoir was attached using a brass ball valve, two barb 

fittings, vinyl tubing, and hose clamps. The smallest inside 

diameter of tubes and fittings used in the seepage meter 

construction was 6.35 mm. Two-liter, thin-walled bags were 

used to measure volume change over time during a seepage 

measurement. The seepage bags had a nominal thickness of 

0.025 mm. The bag thickness and minimum inside diameter 

of the seepage meter were selected to minimize potential 

measurement error. 

A total of 23 seepage measurements were collected 

in 553 days. The first seepage measurement was made on 

April 23, 2010, and the last seepage measurement was made 

on October 28, 2011. For each measurement, the bag was 

filled with about 1,000 mL of water and the bag and water 

were weighed. The bag was deployed during a known period, 

Figure 5. Relation between total solar radiation at Mountain Home Remote Automated Weather Station 

(RAWS) and net solar radiation at Indian Creek Reservoir, Ada County, Idaho, 2010–11.

tac13-0814_fig05

0 

50 

100 

150 

200 

250 

300 

0 50 100 150 200 250 300 350 400 450 

Total solar radiation, in watts per square meter

Net solar radiation, in watts per square meter

Net solar radiation = 0.8261 * (total solar radiation) - 70.081

 

 

R2 = 0.96

SE = 7.28 

Figure 6. Seepage meter constructed from a 55-gallon plastic 

drum, connection hose, and measurement chamber used for bag 

deployment, Indian Creek Reservoir, Ada County, Idaho
Methods 13

retrieved, and weighed again. The weight difference in grams 

was equivalent to volume change in milliliters (equation 11). 

An increase in weight indicated a seepage gain to the reservoir 

from perched groundwater, and a weight loss indicated 

seepage loss from reservoir to perched groundwater or 

underlying sediments.

Q w w

t = 2 1 − (11)

where

Q is seepage through seepage chamber (mL/d; 

positive values represent reservoir gains 

from groundwater inflow), 

w1 is initial weight of seepage bag (gram [g]),

w2 is final weight of seepage bag (g),

1 g of water is 1 mL of water, and

t is time in days.

Generally, seepage meters were placed in two locations 

and moved as the reservoir stage increased or decreased. One 

seepage meter was placed at the base of the rock face north 

of the reservoir stage gage. The other seepage meter was 

placed at the south-central edge of the reservoir east of the 

control tower (fig 2). During the first year of measurements, 

the reservoir was relatively low and seepage meters were 

deployed at the reservoir edge and then moved inward toward 

the center of the reservoir pool as it became smaller. During 

February of the second year, the reservoir stage was 7 ft higher 

than the peak stage from the prior year. One seepage meter 

was deployed in February 2011 on the northwest side of the 

reservoir. Placing the seepage meter was difficult because the 

reservoir was frozen and finding a suitable location free of 

basalt riprap to deploy the meter was a trial-and-error process. 

A second meter was deployed at the southern edge of the 

thawed reservoir in March 2011. Between late March and late 

April 2011, the reservoir stage rose more than 4 ft and both 

seepage meters became inaccessible and were not used for 

measurements thereafter. Two new meters were deployed in 

accessible locations in May 2011.

Seepage measurements were evaluated for potential 

errors prior to being used to estimate seepage into or out of 

the reservoir. Two types of errors were observed during the 

study that prevented use of some seepage measurements. 

Both sources of error were related to seepage bag condition 

upon retrieval. If the seepage bag was leaking or contained 

air bubbles, the seepage measurement was omitted from the 

dataset. Seepage meters have many additional sources of error 

and those are discussed in the section “Sources of Error.” 

Segmented Darcy Method

The segmented Darcy method utilizes a combination 

of water-level measurements in the reservoir and in adjacent 

near-shore wells to calculate water-table gradients between 

the wells and the reservoir; the Darcy equation was used 

to calculate groundwater inflow to and outflow from the 

reservoir within discrete segments of the reservoir shoreline 

(Rosenberry and LaBaugh, 2008). With this method, the 

length of the shoreline segment, m, is multiplied by the 

effective thickness of the aquifer, b, to determine the area, A, 

of a vertical plane at the shoreline through which water passes 

to enter or leave the surface-water body. The reservoir-bed 

sediment thickness was assumed to be 10 ft, or the thickness 

of the clay layer observed in the nearby shallow well and 

correlated to the bottom of the reservoir. The Darcy equation 

commonly is used to calculate the flow of water that passes 

through the vertical plane associated with each segment. 

Flows to or from the surface-water body for each segment are 

summed to calculate net flow for the entire surface-water body 

and are computed using the following equation:

Q K= − A h (( h L ) / ) 1 2 (12)

where

Q is flow through a vertical plane that extends 

beneath the shoreline of a surface-water 

body in (ft3/d);

K is the vertical hydraulic conductivity (ft/d);

A is the area of the plane through which all water 

must pass, depending on the direction 

of flow [shoreline length (m) × effective 

thickness of the aquifer (b)] (ft2);

h1 is the hydraulic head in the well of interest (ft);

h2 is the surface-water stage (ft); and

L is distance from the well to the shoreline (ft).

Three mini-piezometers were installed in the reservoir 

bed sediment in May 2011 to provide information for the 

segmented-Darcy calculations. The mini-piezometers 

were used to: (1) calculate the hydraulic gradient in 

reservoir sediments; (2) use Darcy’s law to back-calculate 

hydraulic conductivity at the locations of the seepage meter 

measurements; and (3) calculate seepage gains and losses 

for the reservoir using the segmented-Darcy approach. 

Mini-piezometers were completed at depths of 1.2–1.3 ft 

in the reservoir-bed sediments. Each mini-piezometer was 

constructed of 1-in. galvanized pipe with 1/8-in. holes drilled 

in the bottom 6 in. of the pipe. The bottom of the pipe was 

then crimped and filled with coarse sand before driving it 

into the reservoir-bed sediments. One mini-piezometer was 

installed near the inflow at the highway and was used for 

observing the groundwater, one was installed next to the south 

seepage meter, and one was installed next to the northwest 

seepage meter (fig. 2). The elevations of the mini- piezometers 

were surveyed utilizing high-precision RTK GPS, as 

previously described in the “Change in Reservoir Storage”

section of this report. Hydraulic gradients between perched 

groundwater and the reservoir were calculated for three field 

visits between June 15 and August 19, 2011, using water 

levels in each mini-piezometer and reservoir stage and the 

depth of the mini-piezometer below the reservoir-bed surface.
14 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

A triangular irregular network (TIN) model for reservoir 

bathymetry was used to compute A and L in equation 12. 

The shoreline length varied during the study and contours 

generated using the TIN model were used to adjust the 

shoreline lengths and distance from each mini-piezometer to 

the shoreline based on the reservoir stage. The survey points 

used to generate the TIN model are attached to this report as a 

geographic information system (GIS) shapefile with methods 

presented and described within the metadata. All survey points 

used to generate the TIN model had a vertical accuracy of 1 ft.

Hydraulic conductivity was estimated based on literature 

values for the observed sediment matrix, which was organic 

clay to silt. Freeze and Cherry (1979) present a range from 

0.13 ft/d for silt to 1.34×10-5 ft/d for organic clay. Hydraulic 

conductivity also was calculated using gradient from head 

differences between the reservoir and the mini-piezometers 

as described in equation 13. Hydraulic conductivity was 

estimated based on the observed sediment matrix, which 

was organic clay to silt. Measured seepage rates were used 

to calculate the hydraulic gradient at two mini-piezometers 

for three specific dates between June 15 and August 19, 2011 

(equation 13):

K q

i = (13)

where

K is hydraulic conductivity (ft/d);

q is the seepage rate (ft/d); and

i is hydraulic gradient (dimensionless) where i is 

the head difference between the reservoir 

and the mini-piezometer divided by the 

vertical distance between the reservoir bed 

and the middle of the screened interval 

(0.9–1.1 ft).

The segmented-Darcy approach assumes several 

conditions: 

1. All water that exchanges with a surface-water body passes 

horizontally through a vertical plane positioned at the 

shoreline that extends to a finite depth (b) beneath the 

surface of the surface-water body. At depths greater than 

b, groundwater flows beneath the surface-water body and 

does not exchange with the surface-water body;

2. The direction of waterflow is horizontal and perpendicular 

to the shoreline as flow enters or leaves the vertical plane 

defining the shoreline of the surface-water body;

3. The hydraulic gradient between the well and the 

surface- water body is uniform; and

4. The aquifer is homogeneous and isotropic within the 

segment.

Reservoir Seepage

Conceptual Model of Reservoir Seepage

A conceptual understanding of the reservoir, its sources 

of inflow, and groundwater in the study area is necessary to 

provide context for the estimates of seepage made during 

this study. Surface-water inflow from Indian Creek enters 

the reservoir from the northeast through an existing channel 

that was modified in the late 1960s to construct Interstate 84 

(fig. 2). As a result of highway construction, surface-water 

has spread out and created a small alluvial plain on the north 

side of the highway. Hyporheic flow (subsurface streamflow 

through adjacent stream and bank sediments) likely enters the 

reservoir through the alluvium north of the highway during 

and shortly after runoff events in the intermittent channel. 

The former channel of Indian Creek extends several hundred 

yards toward the center of the reservoir. An additional 

intermittent drainage to the northwest also may have provided 

surface- water inflow, but because the drainage basin is small, 

it was not considered a significant contributor during the 

periods of the study. 

IDWR has measured water levels in a shallow well 

completed at 160 ft below land surface and located 600 ft 

south of the reservoir spillway (IDWR well log tag number 

D-0020068). Water levels in this well are understood to 

represent the water table in the local aquifer. The reservoir 

desiccates at 3,311 ft and water levels in the shallow well 

ranged between 3,180 and 3,172 ft during this study. The 

driller’s log for well D-0020068 shows a clay layer between 

3,310 and 3,300 ft, which is well correlated to the clay 

layer at the maximum depth of the reservoir at 3,311 ft. The 

well log also documents that the water-table elevation was 

about 3,270 ft when the well was drilled in November 2000, 

indicating that the water table has declined about 100 ft. The 

water table measured in well D-0020068 was about 40 ft 

deeper than the maximum depth of the reservoir in 2000, and 

was between 131 and 139 ft deeper than the maximum depth 

of the reservoir during the study. If the lithology reported in 

this well extends below the reservoir, the reservoir likely is 

underlain by at least 10 ft of clay and an unsaturated zone 

of about 130 ft exists below the bottom of the reservoir. The 

well log for well D-0020068 describes a more permeable 9-ft 

layer of gravel above the clay lithology. Following a period 

when the reservoir is dry (as it was in the year prior to the 

study and in August 2010), the clay layer below the bottom 

of the reservoir likely acts as a water sink and as the reservoir 

fills the clay becomes saturated. During wet years such as 

2011, perched groundwater likely accumulates over this 

impermeable clay layer. Seepage measurements may reflect 

gains from perched groundwater or losses to unsaturated nearshore or reservoir-bottom sediments. However, these gains 

and losses do not mean significant gains or losses to the local 
Reservoir Seepage 15

aquifer. Transient subsurface flow (that is, flow to unsaturated 

reservoir sediments) has been shown to cause erroneously 

large estimates of seepage loss in wetland environments 

(Gerla, 1992). Plant transpiration also may account for some 

observed seepage losses during peak plant growth in the 

summer. Figure 7 shows the attenuated, delayed response 

in the local aquifer recharge in relation to large changes in 

reservoir elevation. This response is likely due to groundwater 

percolation from areas outside of the reservoir, and not 

seepage from the reservoir itself.

Figure 7. Comparison of groundwater elevations in shallow well D-0020068 

to reservoir elevation, for Indian Creek Reservoir, Ada County, Idaho. (A) 

Reservoir elevation; and (B) groundwater elevation.

tac13-0814_fig07ab

3,310 

3,312 

3,314 

3,316 

3,318 

3,320 

3,322 

3,324 

3,326 

3,328 

12-2009 01-2010 02-2010 03-2010 04-2010 05-2010 06-2010 07-2010 08-2010 09-2010 10-2010 11-2010 12-2010 01-2011 02-2011 03-2011 04-2011 05-2011 06-2011 07-2011 08-2011 09-2011 10-2011 11-2011 

Reservoir elevation 

Data Gap 

Elevation, in feet above NAVD 88

Reservoir stage was not

recorded when reservoir

was dry or frozen. 

EXPLANATION

EXPLANATION

3,172 

3,173 

3,174 

3,175 

3,176 

3,177 

3,178 

3,179 

3,180 

3,181 

3,182 

3,183 

Groundwater elevation

A.

B.
16 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

Reservoir Seepage Results

Water-budget derived estimates of seepage followed a 

predictable pattern, with the reservoir gaining water from 

seepage during the wetter months of March–May; the reservoir 

consistently lost water to groundwater seepage during the drier 

months of June–October (table 4). The reservoir desiccated 

in August 2010, so there are no monthly evaporation or 

water- budget estimates for August–October 2010. 

A summary of Indian Creek’s monthly water budget 

is presented in table 5. Groundwater gains contributed 

71.2 percent (4.8 acre-ft) of the reservoir inflows 

(precipitation, surface water inflows, and groundwater 

seepage gains) in April 2010, and 47.7 percent (3.0 acre-ft) 

in May 2010. Groundwater gains contributed 29.4 percent 

(37.5 acre-ft) of the reservoir inflows in March 2011, and 

15.1 percent (59.4 acre-ft) in April 2011, and diminished 

to less than less than 1 percent (0.2 acre-ft) by May 2011. 

Groundwater losses constituted 12.6 percent (-1.6 acre-ft) 

of the total reservoir outflow (evaporation and groundwater 

seepage losses) in June 2010, increasing to 31.2 percent 

(-3.5 acre-ft) of reservoir outflows in July 2010. Reservoir 

losses to groundwater in June 2011 were 12.5 percent 

(-10.4 acre-ft), and increased incrementally each month 

to 59.1 percent (-22.3 acre-ft) by October 2011, although 

actual monthly volume losses from the reservoir remained 

relatively stable from July through October (ranging from 

-20.1 to -26.8 acre-ft per month). Overall reservoir losses 

to groundwater are less significant relative to losses from 

evaporation.

Direct seepage measurements and the seepage calculated 

using the segmented-Darcy method provide seepage 

estimates on specific dates when the measurements were 

made. Results for each seepage measurement and each 

reservoir seepage estimation method are shown in table 6

and figure 8. Seepage estimation methods are described as 

(1) estimated seepage describes a direct application of the 

seepage rate measured at each meter to a percentage of the 

reservoir-bed surface area at the time of the measurement, 

and (2) segmented- Darcy method describes the segmented 

approach using head differences and distances along the 

shoreline between mini-piezometers. The net reservoir seepage 

from water budget calculations also is provided in table 6 as a 

comparison. Mini-piezometer water levels, vertical hydraulic 

gradients, and calculated hydraulic conductivities used in the 

segmented- Darcy method (Rosenberry and LaBaugh, 2008, 

p. 22) are listed in table 7.

Losses estimated using measured seepage in 2010 and 

2011 might have been overestimated because of unsaturated 

flow conditions, whereas measured gains likely indicate 

interaction with perched groundwater (fig. 8). Direct seepage 

measurement results did not show significant gains or losses 

to the reservoir and likely did not exceed measurement 

uncertainty for this method. That a seepage loss was 

measured on June 15, 2011, in the northwest seepage meter 

while the water level in the mini-piezometer was greater 

than the reservoir stage elevation indicates that such small 

seepage rates do not always exceed measurement uncertainty 

(table 6). Quantification of measurement uncertainty was 

beyond the scope of this project. Qualitatively, direct seepage 

measurement results indicate that Indian Creek Reservoir most 

likely was gaining water in April and May of 2010 and 2011. 

In 2010, which was a much drier year than 2011, the reservoir 

began losing water to reservoir-bed sediments on June 9, 

although measurements in 2011 did not indicate water loss 

conditions until August 19.

Although seepage estimates made using the seepage 

meter method were all less than one-third of an acre-foot 

per day, seepage estimates made using the segmented-Darcy 

method between May and August 2011 were essentially zero 

(table 6). This method comparison suggests that minimum 

estimated seepage using measured flux greater than 5 percent 

of the reservoir surface area is still an overestimate of seepage 

in Indian Creek Reservoir. Vertical hydraulic conductivity 

calculated at the seepage meters ranged from 9.79×10-4

to 6.37×10-2 ft/d, which is within the range of hydraulic 

conductivities for silt and clay provided in Freeze and 

Cherry (1979).

Reservoir seepage measured using seepage meters and 

the segmented-Darcy approach was generally negligible 

relative to magnitude of losses from evaporation from Indian 

Creek Reservoir during this study. Seepage measurements 

and water levels in mini-piezometers provided insights to 

conditions within reservoir-bed sediments rather than the 

local aquifer, which was 131–139 ft below the bottom of the 

reservoir during this study. Estimated water gains are likely 

from seasonally perched groundwater, whereas estimated 

water losses are likely to unsaturated near-shore sediments or 

the 10-ft-thick clay layer underlying the reservoir. Given the 

error associated with methods used to estimate seepage in this 

study and the relatively small fluxes measured with seepage 

meters, seepage to the vertically distant local aquifer beneath 

Indian Creek Reservoir is likely negligible. 

An evaluation of the lithologic log in a nearby shallow 

well shows that the thickness of the fine-grained reservoir- bed 

sediments likely is a factor in the low seepage estimates. The 

lowest point in the reservoir bottom was measured at 3,311 ft 

when the reservoir desiccated in August 2010. A clay layer 

was observed in the lithologic log of the nearby shallow well 

(Idaho Department of Water Resources Well D-020068) at 

elevations of 3,300–3,010 ft. The potential existence of a 

10-ft-thick clay layer below the reservoir supports the low 

seepage estimates from the reservoir.
Reservoir Seepage 17

Table 4. Water budget by energy-budget period for Indian Creek Reservoir, Ada County, Idaho, 2010–11.

[Reservoir volume calculated from measured reservoir stage, and 2009 bathymetric survey. Evaporation was calculated using the energy-budget method. 

Precipitation for 2010 obtained from Mountain Home Remote Automated Weather Station (RAWS), and in 2011 from the IDARNG1 RG2 RAWS; surface-water 

inflow was measured at the mouth of the Indian Creek inlet. Negative groundwater seepage values indicate a reservoir loss, positive values indicate a reservoir 

gain. All units are in acre-feet, unless otherwise noted. Abbreviations: acre-ft/d, acre-foot per day; –, not estimated]

Energybudget 

period

Start date End date Number 

of days

 Mean 

reservoir 

volume 

Change in 

reservoir 

volume

Surface area 

at end period 

(acres)

Total 

evaporation

for period 

Total 

precipitation

Net surface 

water inflow 

for period

Calculated net 

groundwater 

seepage

Calculated net 

groundwater 

seepage rate 

(acre-ft/d)

– 06-03-10 – 27.7 – – – – – – –

0 06-04-10 06-10-10 7 26.3 -1.4 19 1.76 0.48 0.0 -0.1 -0.01

1 06-11-10 06-17-10 7 22.4 -3.9 17 2.86 0.00 0.0 -1.0 -0.14

2 06-18-10 06-24-10 7 19.1 -3.3 15 2.65 0.00 0.0 -0.7 -0.09

3 06-25-10 07-01-10 7 16.0 -3.1 13 2.30 0.01 0.0 -0.8 -0.11

4 07-02-10 07-08-10 7 12.7 -3.3 11 2.18 0.00 0.0 -1.1 -0.16

5 07-09-10 07-15-10 7 9.8 -2.9 10 1.78 0.02 0.0 -1.1 -0.16

6 07-16-10 07-22-10 7 7.0 -2.8 7 1.56 0.00 0.0 -1.2 -0.18

7 07-23-10 07-29-10 7 5.2 -1.8 6 0.96 0.00 0.0 -0.8 -0.12

8 07-30-10 08-05-10 7 3.1 -2.2 5 0.82 0.00 0.0 -1.3 -0.19

9 08-06-10 08-12-10 7 1.8 -1.3 3 0.50 0.01 0.0 -0.8 -0.12

10 08-13-10 08-19-10 7 0.9 -0.9 2 0.36 0.00 0.0 -0.5 -0.07

– 03-08-11 – 320 – – – – – – –

11 03-09-11 03-15-11 7 321 0.7 73 4.18 1.10 0.0 3.8 0.54

12 03-16-11 03-22-11 7 327 6.7 74 2.72 3.34 0.0 6.1 0.87

13 03-23-11 03-29-11 7 386 58.5 82 6.38 4.45 43.6 16.8 2.40

14 03-30-11 04-07-11 9 581 195.3 104 27.14 3.04 192 27.4 3.04

15 04-08-11 04-14-11 7 644 62.5 111 12.04 0.09 62.7 11.8 1.68

16 04-15-11 04-21-11 7 697 53.4 116 11.58 6.98 43.0 15.0 2.14

17 04-22-11 04-28-11 7 757 60.5 121 15.09 2.42 53.0 20.2 2.88

18 04-29-11 05-05-11 7 753 -4.9 121 15.61 0.10 7.5 3.1 0.44

19 05-06-11 05-12-11 7 757 4.9 121 14.28 4.94 11.7 2.5 0.36

20 05-13-11 05-19-11 7 747 -10.9 120 11.20 2.30 0.9 -2.9 -0.41

21 05-20-11 05-26-11 7 732 -14.4 119 16.53 2.78 0.6 -1.2 -0.18

22 05-27-11 06-02-11 7 720 -11.8 118 12.73 2.76 0.5 -2.3 -0.33

23 06-03-11 06-09-11 7 713 -7.1 118 13.05 3.72 0.4 1.8 0.26

24 06-10-11 06-16-11 7 693 -19.8 116 16.55 0.10 0.3 -3.7 -0.52

25 06-17-11 06-23-11 7 675 -18.3 114 19.16 2.29 0.2 -1.6 -0.23

26 06-24-11 06-30-11 7 650 -24.9 112 19.51 0.00 0.0 -5.4 -0.77

27 07-01-11 07-07-11 7 628 -22.1 109 19.40 0.55 0.0 -3.3 -0.46

28 07-08-11 07-14-11 7 604 -23.8 107 19.27 0.36 0.0 -4.9 -0.70

29 07-15-11 07-21-11 7 579 -25.4 104 19.59 0.09 0.0 -5.9 -0.84

30 07-22-11 07-28-11 7 553 -25.6 101 19.99 0.00 0.0 -5.6 -0.80

31 07-29-11 08-04-11 7 531 -22.0 99 15.81 0.08 0.0 -6.3 -0.90

32 08-05-11 08-11-11 7 509 -22.4 96 16.05 0.00 0.0 -6.3 -0.91

33 08-12-11 08-18-11 7 487 -21.9 94 15.63 0.00 0.0 -6.3 -0.90

34 08-19-11 08-25-11 7 467 -20.4 92 17.12 0.00 0.0 -3.3 -0.47

35 08-26-11 09-01-11 7 446 -20.8 89 12.44 0.07 0.0 -8.4 -1.20

36 09-02-11 09-08-11 7 429 -16.8 87 14.19 0.00 0.0 -2.6 -0.37

37 09-09-11 09-15-11 7 415 -13.9 86 8.99 0.36 0.0 -5.3 -0.75

38 09-16-11 09-22-11 7 402 -13.6 84 8.82 0.00 0.0 -4.8 -0.68

39 09-23-11 09-29-11 7 388 -13.3 82 7.50 0.00 0.0 -5.8 -0.83

40 09-30-11 10-06-11 7 382 -6.6 82 3.84 4.08 0.0 -6.8 -0.98

41 10-07-11 10-13-11 7 378 -4.0 81 6.23 4.46 0.0 -2.2 -0.32

42 10-14-11 10-20-11 7 373 -4.9 80 2.14 2.82 0.0 -5.6 -0.80

43 10-21-11 10-27-11 7 365 -7.9 79 1.94 0.00 0.0 -6.0 -0.85

44 10-28-11 11-03-11 7 361 -4.0 79 3.19 0.66 0.0 -1.5 -0.21

45 11-04-11 11-10-11 7 361 0.0 79 2.27 0.39 0.0 1.9 0.27

46 11-11-11 11-17-11 7 361 0.0 79 1.46 0.85 0.0 0.6 0.09
18 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

Table 5. Monthly water budget for Indian Creek Reservoir, Ada County, Idaho, 2010–11.

[Reservoir volume is calculated from measured reservoir stage at the end of the period, and 2009 bathymetric survey. Evaporation was calculated using the 

energy-budget method. Precipitation for 2010 obtained from Mountain Home Remote Automated Weather Station (RAWS) and for 2011 from the IDARNG1 

RG2 RAWS station; surface-water inflow was measured at the mouth of the Indian Creek inlet. Negative groundwater seepage values indicate a reservoir loss, 

positive values indicate a reservoir gain. All values in acre-feet, unless otherwise noted. Abbreviations: acre-ft/d, acre-feet per day; –, not estimated]

Start date End date Reservoir 

volume 

Change in 

reservoir 

volume

Total 

evaporation 

Total

precipitation

Surfacewater 

inflow

Calculated net 

groundwater 

seepage

Calculated net 

groundwater 

seepage rate 

(acre-ft/d)

Groundwater 

reservoir inflow

(percent)

Groundwater 

reservoir outflow

(percent)

03-01-10 03-30-10 28.5 – – – – – – – –

04-01-10 04-30-10 28.3 -0.2 1

6.9 1.9 0 4.8 0.16 71.2 –

05-01-10 05-30-10 28.3 0.0 1

6.2 3.3 0 3.0 0.10 47.7 –

06-01-10 06-30-10 16.4 -11.8 11.2 1.0 0 -1.6 -0.06 – 12.6

07-01-10 07-31-10 5.24 -11.2 7.7 0.0 0 -3.5 -0.12 – 31.2

– 02-28-11 320 – – – – – – –

03-01-11 03-31-11 430 110 17.5 12.5 77 37.5 1.25 29.4 –

04-01-11 04-30-11 760 330 63.3 11.8 322 59.4 2.05 15.1 –

05-01-11 05-31-11 725 -34.8 62.4 11.7 16 0.2 0.01 0.8 –

06-01-11 06-30-11 650 -74.8 72.6 7.1 1 -10.4 -0.36 – 12.5

07-01-11 07-31-11 545 -105 86.0 1.1 0 -20.1 -0.67 – 18.9

08-01-11 08-31-11 449 -95.9 69.2 0.1 0 -26.8 -0.89 – 27.9

09-01-11 09-30-11 386 -62.8 42.0 0.4 0 -21.2 -0.73 – 33.6

10-01-11 10-31-11 361 -25.7 15.4 12.0 0 -22.3 -0.74 – 59.1

1

Total evaporation estimated using 2011 daily mean evaporation rate for respective months.

Table 6. Measured and estimated seepages and comparisons using various methods, Indian Creek Reservoir, Ada County, 

Idaho, 2010–11.

[Direct measurement: Seepage rates were measured at northwest and south seepage meters. Estimated and calculated seepage rates: Estimated 

gain/loss to/from reservoir using seepage measurements and a range of reservoir surface-area percentages applicable to measured seepage flux. 

Estimated overall seepage and hydraulic conductivities using segmented Darcy method: Seepage estimated using the segmented-Darcy 

method. Seepage rates were measured from two seepage meters located on the southern edge and the northwestern edge of the reservoir. Overall 

gain or overall loss indicates that one seepage meter measured a gain and the other measured a loss. Net gain or loss was used in the calculations.. 

Abbreviations: acre-ft/d, acre-foot per day; ft/d, foot per day; K, hydraulic conductivity; E, exponent; na, not available]

Direct measurement

Date Seepage rate 

(ft/d) Date Seepage rate (ft/d)

Northwest seepage meter South seepage meter

04-23-10 1.32-02 04-23-10 6.75-02

06-09-10 -3.53-02 06-09-10 -2.13-02

07-16-10 -2.27-02 03-11-11 -1.78-05

02-04-11 3.29-03 05-04-11 3.21-03

03-11-11 -8.08-04 05-25-11 4.75-03

05-03-11 -1.12-04 06-15-11 2.66-03

06-15-11 -9.12-04 07-12-11 8.98-04

07-12-11 5.40-04 07-15-11 -1.21-04

08-19-11 1.05-03 08-19-11 -9.39-03

09-28-11 1.96-04 10-04-11 7.73-04

10-28-11 6.37-04
Reservoir Seepage 19

Estimated seepage rate using seepage flux 

and reservoir surface area Calculated seepage rate using the water budget method

Date Condition Total

(acre-ft/d)

20 percent

(acre-ft/d)

5 percent

(acre-ft/d) Water budget period Seepage rate(acre-ft/d)

04-23-10 Gain 1.47 0.29 0.01 na na

06-09-10 Loss -0.69 -0.14 -0.02 June 4–10, 2010 -0.01

07-16-10 Loss -0.40 -0.08 -0.02 July 9–22, 2010 -0.16

02-04-11 Gain 0.24 0.05 0.01 na na

03-11-11 Loss -0.06 -0.01 -0.00 March 9–15, 2011 0.54

05-03-11 Overall gain 0.38 0.08 0.02 April 29–May 5, 2011 0.44

05-25-11 Gain 0.57 0.11 0.03 May 20–26, 2011 -0.17

06-15-11 Overall gain 0.20 0.04 0.01 June 10–16, 2011 -0.52

07-12-11 Gain 0.10 0.02 0.00 July 8–14, 2011 -0.70

07-15-11 Loss -0.01 -0.00 -0.00 July 15–21, 2011 -0.84

08-19-11 Overall loss -0.78 -0.16 -0.04 August 19–25, 2011 -0.47

09-28-11 Gain 0.02 0.00 0.00 September 23–29, 2011 -0.83

10-04-11 Gain 0.06 0.01 0.00 September 30–October 6, 2011 -0.98

10-28-11 Gain 0.05 0.01 0.00 October 28–November 3, 2011 -0.21

Estimated overall seepage using segmented-Darcy method Hydraulic conductivities used for segmented-Darcy method

Date Median seepage

(acre-ft/d)

Hydraulic 

conductivity

(ft/d)

Source

05-25-11 5.68-05 2.52-02 Calculated at south meter 06-15-11

05-27-11 1.01-05 2.12-03 Calculated at northwest meter 06-15-11

06-15-11 1.41-04 9.79-04 Calculated at northwest meter 07-12-11

07-12-11 1.57-04 6.37-02 Calculated at south meter 08-19-11

07-15-11 1.42-04 2.08-03 Calculated at northwest meter 08-19-11

08-19-11 5.15-04 1.339-01 Freeze and Cherry, 1979. Typical K for silt.

1.339-05 Freeze and Cherry, 1979. Typical K for clay.

Table 7. Mini-piezometer water levels, vertical hydraulic gradients, and calculated hydraulic conductivities, Indian Creek Reservoir, 

Ada County, Idaho, 2011.

[Segmented-Darcy method used for calculations (Rosenberry and LaBaugh (2008, p. 22). Elevations are referenced to North American Vertical datum of 1988 

(NAVD 88). Abbreviations: ft, foot; ft, foot; ft/d, foot per day; ft3/d, cubic foot per day; E, exponent; na, not available]

Piezometer Date Time

Groundwater 

elevation at 

piezometer

(ft NAVD 88)

Reservoir 

elevation

(ft NAVD 88)

Head 

difference

(ft)

Depth to middle 

of screen from 

ground surface

(ft)

Vertical 

hydraulic 

gradient

Seepage

(ft3/d)

Seepage rate

(ft/d)

Hydraulic 

conductivity

(ft/d)

South 6-15-11 1034 3,325.25 3,325.15 0.10 0.95 1.05-01 7.33-03 2.66-03 2.52-02

Northwest 6-15-11 1100 3,325.61 3,325.15 0.46 1.07 4.30-01 -2.52-03 -9.12-04 2.12-03

South 7-12-11 0900 3,324.37 3,324.37 0.00 0.95 0.00+00 2.48-03 8.98-04 na

Northwest 7-12-11 1020 3,324.96 3,324.37 0.59 1.07 5.51-01 1.49-03 5.40-04 9.79-04

South 8-19-11 1030 3,322.99 3,323.13 -0.14 0.95 -1.47-01 -2.59-02 -9.39-03 6.37-02

Northwest 8-19-11 1205 3,323.67 3,323.13 0.54 1.07 5.05-01 2.89-03 1.05-03 2.08-03

Table 6. Measured and estimated seepages and comparisons using various methods, Indian Creek Reservoir, Ada County, 

Idaho, 2010–11. —Continued

[Direct measurement: Seepage rates were measured at northwest and south seepage meters. Estimated and calculated seepage rates: Estimated 

gain/loss to/from reservoir using seepage measurements and a range of reservoir surface-area percentages applicable to measured seepage flux. 

Estimated overall seepage and hydraulic conductivities using segmented Darcy method: Seepage estimated using the segmented-Darcy 

method. Seepage rates were measured from two seepage meters located on the southern edge and the northwestern edge of the reservoir. Overall 

gain or overall loss indicates that one seepage meter measured a gain and the other measured a loss. Net gain or loss was used in the calculations. 

Abbreviations: acre-ft/d, acre-foot per day; ft/d, foot per day; K, hydraulic conductivity; E, exponent; na, not available]
20 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

tac13-0814_fig08

3,310 

3,312 

3,314 

3,316 

3,318 

3,320 

3,322 

3,324 

3,326 

3,328 

-1

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

01-22-2010 05-02-2010 08-10-2010 11-18-2010 02-26-2011 06-06-2011 09-14-2011 12-23-2011 

Reservoir elevation, in feet above NAVD 88 

Seepage, in acre-feet per day 

Data gap

Losing

Gaining

Reservoir desiccation elevation 08-19-2010 

Reservoir elevation 

Maximum seepage meter estimate

Minimum seepage meter estimate 

Segmented-Darcy method

Seepage calculated from water budget

EXPLANATION

Reservoir stage was not

recorded when reservoir

was dry or frozen. 

Sources of Error

An extended discussion of error sources associated with 

seepage meter measurements is provided in Rosenberry and 

LaBaugh (2008). Of those listed, leaks and gas accumulation 

were the most common. No data affected by these sources of 

error were used to calculate seepage gain or loss. Determining 

the heterogeneity of seepage through different locations in the 

reservoir was beyond the scope of this study. The remaining 

sources of error were carefully worked around through 

seepage meter design or general field practices. During the 

first study year, seepage was measured over short periods; 

during the second study year, seepage was measured over 

several hours or days. All measurements displayed negligible 

groundwater/surface water interaction.

Significant measurement error can be associated with 

frictional flow loss within the seepage meter, restrictions to 

flow through the connector between the bag and the chamber, 

and any resistance to movement of the bag (Rosenberry and 

LaBaugh, 2008). Coefficients typically are applied to the 

indicated flux to correct for this problem. Rosenberry and 

LaBaugh (2008) suggest a seepage meter coefficient between 

1 and 1.1 if seepage meters are constructed according to their 

recommendations. The seepage meters constructed for this 

study met all the recommendations with the exception that the 

minimum diameter of tubes and fittings used in construction 

of the seepage meter was less than 9 mm. The tubes and 

fittings in the seepage meters constructed for this study had a 

minimum diameter of 6.35 mm at the bag connection. Because 

many measurements were made in the same location with 

Figure 8. Summary of seepage estimates derived from direct measurements, estimated using the 

segmented-Darcy method, and those calculated using the reservoir energy-budget for Indian Creek 

Reservoir, Ada County, Idaho, 2010–11.
Summary 21

each seepage meter, and all seepage rates were less than plus 

or minus 3 cm/d, applying a coefficient would have negligible 

effect on results with respect to other sources of error. A 

review of published experiments in the context of seepage 

rates measured in Indian Creek Reservoir suggests that no 

coefficient need be applied to the measurements made during 

this study. It was beyond the scope of the study to extensively 

analyze seepage measurement error; two seepage meters 

are insufficient to provide seepage estimates for an entire 

reservoir, and only allows for qualitative estimates of overall 

gain or loss to groundwater from the reservoir. Similarly, bag 

resistance was not directly measured as part of this study, but 

Rosenberry and Menheer (2006) list 0.93 as a bag correction 

factor for the type of bag used in this study.

A comparison of seepage meter results to the appropriate 

weekly water-budget calculations for net groundwater flux 

(table 6) are qualitatively similar in that gains and losses 

to groundwater from the reservoir are small; that they do 

not always quantitatively align is to be expected for several 

reasons. The range of uncertainty during short energy-budget 

periods is higher than the range calculated for long periods. 

In all but two periods (June and July 2010), net groundwater 

seepage calculated from the meters was within 10 percent of 

discharge variability expected from evaporation uncertainty. 

Alternatively, seepage meters are single points in position 

around the reservoir and provide results only at the time of 

the measurement. The evaporation estimates are based on 

measurements made over a minimum period of a week, and 

some variability is expected between the estimates. There also 

may be spatial variability such as heterogeneity of sediment 

type and permeability, fluctuation owing to evapotranspiration 

from nearshore vegetation, or topography around the reservoir 

that allows for more or less groundwater movement than is 

occurring at the seepage meter location.

Summary

The U.S. Geological Survey, in cooperation with 

the Idaho Department of Water Resources, conducted an 

investigation on Indian Creek Reservoir, a small impoundment 

in east Ada County, Idaho, to quantify groundwater seepage 

into and out of the reservoir. Data from the study will assist 

the Idaho Water Resources Department’s Comprehensive 

Aquifer Management Planning (CAMP) effort to estimate 

available water resources in Ada County. Three independent 

methods were utilized to estimate groundwater seepage: 

(1) the water-budget method; (2) the seepage-meter method; 

and (3) the segmented Darcy method. Reservoir seepage was 

quantified during the periods of April through August 2010 

and February through November 2011. 

With the water-budget method, all measureable sources 

of inflow to and outflow from the reservoir were quantified, 

with the exception of groundwater; the water-budget equation 

was solved for groundwater inflow to or outflow from the 

reservoir. For the reservoir water budget, surface-water inflow, 

precipitation falling directly on the reservoir, and change in 

reservoir storage were measured. Reservoir evaporation was 

estimated with the Bowen ratio energy-budget method. There 

is no measureable surface-water outflow from the reservoir.

The seepage-meter method relies on the placement of 

seepage meters into the bottom sediments of the reservoir for 

the direct measurement of water flux across the sedimentwater interface. Two seepage meters were utilized in the study 

and a total of 23 discrete seepage measurements were made 

during the study. Although seepage meters provide objective 

measurements of seepage, they are representative of single 

positions in the reservoir, which may not be representative 

of the reservoir as a whole. Furthermore, seepage-meter 

measurements are representative of the distinct time at which 

the measurements are made, whereas, the water-budget 

estimates of seepage provide continuous estimates of seepage 

over the study period. As a result of these differences in 

methodologies, comparisons between water-budget results and 

seepage-meter measurements are considered semi-quantitative.

The segmented-Darcy method utilizes a combination 

of water-level measurements in the reservoir and in adjacent 

near-shore wells to calculate water-table gradients between 

the wells and the reservoir. The Darcy equation was used to 

calculate groundwater inflow to and outflow from discrete 

segments of the reservoir shoreline. During the study, six 

seepage estimates were made with the segmented-Darcy 

method. As a result of the discrete nature of these estimates, 

comparisons of the segmented-Darcy estimates of seepage 

to the estimates derived from the water-budget and seepagemeter methods are considered semi-quantitative.

The results of the water-budget derived estimates of 

seepage indicate seepage to be seasonally variable in terms of 

the direction and magnitude of flow. The reservoir tended to 

gain water from seepage of groundwater in the wetter months 

(March-May), while seepage losses to groundwater from the 

reservoir occurred in the drier months (June-October). Net 

monthly seepage rates, as computed by the water-budget 

method, varied greatly. Reservoir gains from seepage ranged 

from 0.2 to 59.4 acre-feet/month, while reservoir losses to 

seepage ranged from 1.6 and 26.8 acre-ft/month. An analysis 

of seepage meter estimates and segmented-Darcy estimates 

qualitatively supports the seasonal patterns in seepage 

provided by the water-budget calculations, except that they 

tended to be much smaller in magnitude. This suggests that 

actual seepage might be smaller than those estimates made by 

the water-budget method.

Although the results of all three methods indicate that 

there is some water loss from the reservoir to groundwater, 

the seepage losses may be due to rewetting of unsaturated 

near-shore soils, possible replenishment of a perched aquifer, 

or both, rather than through percolation to the local aquifer 

that lies 130 feet below the reservoir. A lithologic log from an 

adjacent well indicates the existence of a clay lithology that 

is well correlated to the original reservoir’s base elevation. 

If the clay lithologic unit extends beneath the reservoir basin 
22 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

underlying the fine-grain reservoir bed sediments, the clay 

layer should act as an effective barrier to reservoir seepage to 

the local aquifer which would explain the low seepage loss 

estimates calculated in this study.

Acknowledgments

The authors would like to thank the Idaho Department 

of Water Resources for their support and participation in 

this investigation. We would also like to thank Donald O. 

Rosenberry, a research hydrologist with the National Research 

Program and Kip K. Allander, a hydrologist and groundwater 

specialist for the Nevada Water Science Center, for their 

valuable guidance, expertise, and insight during several phases 

of this investigation.

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24 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

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Appendix A 25

Appendixes

Appendix A is a Microsoft® Excel file and appendix B is an ASCII x, y, z and associated metadata. Appendixes are 

available for viewing or download at http://pubs.er.usgs.gov/publication/sir/2013/5047.

Appendix A. Indian Creek Reservoir Stage, Elevation, Capacity, and Area, Ada County, Idaho, 2011.

Appendix B. Bathymetric Survey of Indian Creek Reservoir, Ada County, Idaho, October 2009.
26 An Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho, April 2010–November 2011

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

Director, Idaho Water Science Center

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230 Collins Road

Boise, Idaho 83702

http://id.water.usgs.gov
Williams and Etheridge— Evaluation of Seepage Gains and Losses in Indian Creek Reservoir, Ada County, Idaho—Scientific Investigations Report 2013–5047