WATER BUDGET AND THE

EFFECTS OF LAND-USE CHANGES ON

GROUND-WATER RECHARGE, OAHU, HAWAII

Kauai

HAWAII

Niihau <^^ ^J>

Oahu

Molokai

,£> Lanai 

Kahoolawe

Maui

Hawaii

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1412-C
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Water Budget and the 

Effects of Land-Use Changes 

on Ground-Water Recharge, 

Oahu, Hawaii

By PATRICIA J. SHADE and WILLIAM D. NICHOLS

REGIONAL AQUIFER-SYSTEM ANALYSIS OAHU, HAWAII

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1412-C
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government.

Library of Congress Cataloging in Publication Data

Shade, Patricia J.

Water budget and the effects of land-use changes on ground-water recharge, Oahu, Hawaii / by Patricia J. Shade and 

William D. Nichols.

p. cm. - (Regional aquifer-system analysis-Oahu, Hawaii) (U.S. Geological Survey Professional Paper ; 1412-C)

Includes bibliographic references.

Supt. of Docs. no. : I 19.16:1412-0

1. Water balance (Hydrology)-Hawaii-Oahu. 2. Ground-water flow-Hawaii-Oahu

I. Nichols, William D. II. Title. III. Series. IV. Series: U.S. Geological Survey professional paper ; 1412-C. 

GB1025.H3S53 1996

551.49'09969'3-dc20 96-27761

CIP

For sale by the U.S. Geological Survey, Branch of Information Services, Box 25286,

Federal Center, Denver, CO 80225

ISBN 0-607-86103-7
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program represents a 

systematic effort to study a number of the Nation's most important aquifer 

systems, which, in aggregate, underlie much of the country and which represent an important component of the Nation's total water supply. In general, 

the boundaries of these studies are identified by the hydrologic extent of each 

system and, accordingly, transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for 

each study is to assemble geologic, hydrologic, and geochemical information; 

to analyze and develop an understanding of the system; and to develop predictive capabilities that will contribute to the effective management of the 

system. The use of computer simulation is an important element of the RASA 

studies to develop an understanding of the natural, undisturbed hydrologic 

system and the changes brought about in it by human activities and to provide a means of predicting the regional effects of future pumping or other 

stresses.

The final interpretive results of the RASA Program are presented in a 

series of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number 

beginning with Professional Paper 1400.

Gordon P. Eaton 

Director

CONTENTS

Page

Foreword ......................................................................................... Ill

Abstract.......................................................................................... Cl

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

Purpose and Scope .................................................................... 1

Study Area ................................................................................. 1

Previous Studies........................................................................ 2

Predevelopment Water Budget...................................................... 6

Southern Oahu .......................................................................... 6

Southeastern Oahu ................................................................. 10

Windward Oahu ...................................................................... 13

North-Central Oahu ............................................................... 17

Page

Predevelopment Water Budget Continued

Waianae Area .......................................................................... 19

Island of Oahu ......................................................................... 21

Distribution of Predevelopment Ground-Water

Recharge ............................................................................ 22

Effects of Land-Use Changes on Ground-Water Recharge...... 27

Southern Oahu ........................................................................ 27

North-Central Oahu ............................................................... 32

Distribution of Mid-1980's Ground-Water Recharge......... 33

Summary and Conclusions ........................................................... 37

References Cited ............................................................................ 37

ILLUSTRATIONS

Page

FIGURES 1-4. Maps showing:

1. Study area and generalized geology, island of Oahu.............................................................................................. 3

2. Mean annual precipitation, 1916-83, island of Oahu ............................................................................................4

3. Study areas, island of Oahu. ...................................................................................................................................... 5

4. Mid-1980's land use and mean annual precipitation, southern Oahu ................................................................. 7

5. Graph showing relation between precipitation and estimated ground-water recharge for

predevelopment land use, southern Oahu................................................................................................................................. 9

6. Graph showing relation between precipitation and estimated runoff for predevelopment land use,

southern Oahu............................................................................................................................................................................. 10

7. Map showing mean annual precipitation and areas of relatively low and high soil permeability,

southeastern Oahu...................................................................................................................................................................... 11

8. Graph showing relation between precipitation and estimated ground-water recharge based

on relative soil permeability, southeastern Oahu.................................................................................................................. 12

9. Graph showing relation between precipitation and estimated runoff, southeastern Oahu............................................. 13

10-17. Maps showing:

10. Drainage basins and mean annual precipitation, windward Oahu ................................................................... 14

11. Drainage basins and mean annual pan evaporation, windward Oahu.............................................................. 15

12. Mid-1980's land use and mean annual precipitation, north-central Oahu....................................................... 18

13. Mean annual precipitation and areas of phreatophytes and of relatively low and

high soil permeability, Waianae area, Oahu......................................................................................................... 20

14. Distribution of ground-water recharge for predevelopment land use, island of Oahu ...................................23

15. Ground-water recharge areas, island of Oahu ......................................................................................................24

16. Distribution of predevelopment recharge to and discharge from the noncaprock areas,

southern and north-central Oahu ...........................................................................................................................25

17. Distribution of predevelopment recharge to and discharge from windward Oahu basins,

Waianae area, and southeastern Oahu .................................................................................................................. 26

18-22. Graphs showing:

18. Relation (general trend) between precipitation and estimated ground-water recharge

for natural and urban land uses during the mid-1980's, southern Oahu........................................................ 27

19. Relation (general trend) between precipitation and estimated ground-water recharge

in areas of drip-irrigated sugarcane during the mid-1980's, southern Oahu ................................................. 29

20. Relation (general trend) between precipitation and estimated ground-water recharge

in areas of pineapple cultivation during the mid-1980's, southern Oahu ........................................................29

21. Estimated ground-water recharge for predevelopment and mid-1980's land use, southern Oahu.............. 30

22. Relation (general trends) between precipitation and estimated runoff for natural and

urban land uses during the mid-1980's, southern Oahu.................................................................................... 31

23. Map showing distribution of ground-water recharge for mid-1980's land use, island of Oahu .....................................34

24. Map showing distribution of recharge to and discharge from the noncaprock areas for mid-1980's land use,

southern and north-central Oahu, ............................................................................................................................................35
VI

	CONTENTS

	TABLES

	Page

TABLE 1. Land-use categories, southern Oahu .............................................................................................................................................. 6

2. Caprock and noncaprock areas for indicated mean annual precipitation, southern Oahu.................................................... 8

3. Regression equation coefficients for ground-water recharge for predevelopment land use, southern Oahu...................... 8

4. Regression equation coefficients for runoff for predevelopment land use, southern Oahu ................................................. 10

5. Estimated predevelopment water budget, southern Oahu ....................................................................................................... 10

6. Predevelopment water budget for the area defined by a geographic information system, southern Oahu ...................... 10

7. Regression equation coefficients for ground-water recharge, southeastern Oahu................................................................ 12

8. Areas for given relative soil permeability and mean annual precipitation, southeastern Oahu........................................ 12

9. Regression equation coefficients for runoff, southeastern Oahu ............................................................................................. 12

10. Predevelopment water budget, southeastern Oahu ................................................................................................................... 13

11. Runoff-precipitation ratios, windward Oahu basins .................................................................................................................. 16

12. Areas for indicated mean annual precipitation, windward Oahu basins ............................................................................... 16

13. Areas for indicated mean annual pan evaporation, windward Oahu basins ......................................................................... 17

14. Predevelopment water budgets, windward Oahu basins .......................................................................................................... 17

15. Estimated water budgets for selected windward Oahu basins................................................................................................. 17

16. Caprock and noncaprock areas for indicated mean annual precipitation, north-central Oahu.......................................... 19

17. Predevelopment water budget, north-central Oahu................................................................................................................... 19

18. Areas of phreatophyte and non-phreatophyte land cover and relative soil permeability

	for each precipitation zone, Waianae area, Oahu ......................................................................................................................21

19. Predevelopment water budget, Waianae area, Oahu.................................................................................................................21

20. Predevelopment water budget, island of Oahu ........................................................................................................................... 21

21. Water budgets, island of Oahu ......................................................................................................................................................21

22. Distribution of ground-water recharge for predevelopment land use, island of Oahu ......................................................... 22

23. Regression equation coefficients for ground-water recharge for mid-1980's nonagricultural land use,

	southern Oahu .................................................................................................................................................................................28

24. Areas of agricultural and nonagricultural land use during the mid-1980's, southern Oahu ..............................................28

25. Predevelopment and mid-1980's recharge to indicated areas of mid-1980's land use, southern Oahu ............................. 30

26. Regression equation coefficients for runoff for mid-1980's land use, southern Oahu ..........................................................31

27. Water budget for mid-1980's land use, southern Oahu .............................................................................................................31

28. Areas of agricultural and nonagricultural land use during the mid-1980's, north-central Oahu ......................................32

29. Estimated water budget for mid-1980's land use, north-central Oahu................................................................................... 33

30. Distribution of ground-water recharge for mid-1980's land use, island of Oahu .................................................................. 33

31. Estimated predevelopment and mid-1980's ground-water recharge to noncaprock areas

	and to the total areas, island of Oahu .........................................................................................................................................36

32. Predevelopment and mid-1980's water budgets for southern and north-central Oahu........................................................ 36

33. Mid-1980's water budget, island of Oahu ....................................................................................................................................36

CONVERSION FACTORS

Multiply By To obtain

acre 

foot (ft)

foot per year (ft/yr)

gallon (gal)

inch (in.)

inch per year (in/yr)

mile (mi)

square mile (mi2)

million gallons per day (Mgal/d)

0.4047

0.3048

0.3048

0.003785

25.4

25.4

1.609

2.590

0.04381

square hectometer (hm2) 

meter (m)

meter per year (m/yr) 

cubic meter (m3) 

millimeter (mm) 

millimeter per year (mm/yr) 

kilometer (km) 

square kilometer (km2) 

cubic meter per second (m3/s)
REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

WATER BUDGET AND THE EFFECTS OF LAND-USE CHANGES ON 

GROUND-WATER RECHARGE, OAHU, HAWAII

By RJ. SHADE and W.D. NICHOLS

ABSTRACT

Detailed water budgets calculated for southern and southeastern 

Oahu are used with a geographic information system to develop simplified methods for estimating areal water budgets for predevelopment and mid-1980's land use. The methods were applied to estimate water budgets for the Waianae area of western Oahu, and for 

north-central, southern, and southeastern Oahu. A water budget was 

calculated for windward Oahu by developing a separate geographic 

information system model of the area. The water budgets for these 

areas were combined into a single water budget for the entire island. 

The geographic information system model was used to calculate mid1980's ground-water recharge to small areas of specific interest and 

to determine the distribution of recharge by geologic formation.

The most significant changes in the water budget and ground-water recharge have occurred in north-central and southern Oahu as 

a result of large-scale agricultural development and urbanization by 

the mid-1980's. Runoff increased by 23 million gallons per day in 

southern Oahu where extensive urban areas had been developed. 

Evapotranspiration increased by 8 million gallons per day in southern Oahu and 28 million gallons per day in north-central Oahu as 

a result of the 146 million gallons per day of agricultural irrigation. 

Ground-water recharge increased in both areas by about 56 million 

gallons per day in southern Oahu and by about 32 million gallons 

per day in north-central Oahu. Predevelopment ground-water recharge to Oahu was an estimated 792 million gallons per day. 

Changes in land-use practices in the mid-1980's resulted in an estimated islandwide recharge of 880 million gallons per day.

INTRODUCTION

The water supply for the population of the island of 

Oahu, Hawaii, is obtained primarily from ground-water 

sources that are recharged by infiltration of precipitation and applied irrigation water. The importance and 

limited nature of these recharge sources to the groundwater system led to the inclusion of the island of Oahu, 

together with 27 other regions of the United States, in 

the U.S. Geological Survey's Regional Aquifer-System 

Analysis (RASA) Program.

An important factor in the study of aquifer systems is 

the quantity of water that recharges the ground-water 

system. Recharge to the ground-water system can be 

estimated from water-budget calculations by subtracting 

runoff and evapotranspiration from precipitation and 

applied irrigation water. The quantification and distribution of the water-budget elements are important in 

water-resource appraisal studies and in the numerical 

analysis of the ground-water flow systems.

Water budgets were calculated for each of the five 

study areas defined for Oahu as part of the RASA 

project (Eyre and others, 1986). Two of these study area 

boundaries are topographic divides that broadly coincide 

with less well-defined ground-water flow boundaries. 

The Waialua-Wahiawa district boundary is used simply as a convenient north-south division across the 

Schofield Plateau. The Kaau Rift Zone is a geohydrologic divide. Previous water-budget studies commonly 

encompassed smaller subsets of these areas. Because 

boundaries were not standardized, the previous study 

areas are not comparable to the RASA study areas, and 

the results of previous studies are difficult to compare 

with one another and with the RASA study results.

PURPOSE AND SCOPE

The purpose of this report is to describe the calculation of a predevelopment and a mid-1980's water budget 

for five major study areas of the island of Oahu, and the 

spatial distribution and temporal change in groundwater recharge caused by changes in land use. Which 

of several methods that were used to calculate the elements of the water budget and to estimate ground-water recharge depended on the area being analyzed and 

on the availability of previous data. The calculation of 

water budgets and display of the resulting recharge 

distributions were aided by the use of a geographic information system (GIS) that combines spatial and 

numerical data for a particular area in a digital form. 

As a result, each location on a map has various attribute data precipitation, ground-water recharge, 

and geologic unit characterizing it. The water budgets for the individual areas are combined into a water budget for the entire island.

STUDY AREA

The study area includes the entire island of Oahu (fig. 

1), which encompasses about 596 mi2 of land area. The 

Koolau Range extends from the southeastern tip of the 

island northwestward to the northern coast. These

Cl
C2 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

mountains reach altitudes of 2,000 ft1 along most of the 

crest, and several peaks are more than 2,400 ft. The 

Waianae Range is located on the western side of the island and reaches altitudes of 2,000 ft along much of the 

crest of the range; several peaks reach altitudes of 

3,500 and 4,000 ft.

Oahu is underlain by two major aquifers. The Waianae aquifer, which is within the Waianae Volcanics of 

Pliocene age (fig. 1), underlies the western one-quarter 

of the island and extends eastward an unknown distance 

in the subsurface; and the Koolau aquifer, which is 

within the Koolau Basalt of Pliocene and Pleistocene 

age, underlies the eastern three-quarters of the island. 

The Koolau aquifer is the main source of water for southern Oahu and, hence, for the city of Honolulu. It also is 

the main source of water for irrigation of agricultural 

crops in southern Oahu, which included about 15,750 

acres of irrigated sugarcane and about 11,670 acres of 

pineapple in the mid-1980's.

Bordering the island along the southern and northern 

coastal areas are coastal-plain deposits locally known as 

caprock (fig. 1). These deposits include marine and terrestrial sediments, limestone, and reef deposits. In 

southern and north-central Oahu, the caprock confines 

the ground water in the underlying basalt aquifer. This 

impedes discharge of ground water to the sea and, especially in southern Oahu, raises inland ground-water 

levels to higher altitudes than would otherwise be the 

case. The impoundment of water by the caprock increases the volume of freshwater stored in the aquifers. 

Recharge from precipitation and irrigation-return flow 

in the caprock areas, however, does not reach the underlying basalt aquifers and does not contribute usable 

recharge to the ground-water system. In the following 

analysis of water budgets and ground-water recharge for 

southern and north-central Oahu, recharge to the caprock and noncaprock areas is considered separately.

An extreme range in annual precipitation extends 

over Oahu (fig. 2). Mean annual precipitation ranges 

from less than 25 in/yr in the western and southern 

parts of the island to more than 275 in/yr over the highest part of the Koolau Range. Figure 2 is based on 

records from a network of 13 base stations that have 

data from 1916 to 1983 (Giambelluca and others, 1986).

Water budgets were estimated for the five major study 

areas on Oahu (Eyre and others, 1986) (fig. 3A):

1. Southern Oahu (260.5 mi2), which includes the main 

population centers of Honolulu and the Pearl Harbor 

area and extensive agricultural areas;

2. Southeastern Oahu (28.4 mi2 ), which is largely a residential area with large tracts of conservation land 

along the ridges and steep slopes of the Koolau Range 

and along much of the coast;

3. Windward Oahu (124.5 mi2), which includes military, 

agricultural, conservation, and low-density urban 

land use;

4. North-central Oahu (121.4 mi2), which is predominantly an agricultural area with smaller areas of military, recreational, and low-density urban land use; 

and the

5. Waianae area (60.9 mi2), which includes a large 

tract of military reservation land, agricultural, 

conservation, and a small area of low-density urban land use.

Geohydrologic barriers internally divide the study 

areas into smaller ground-water areas (fig. 3B) that 

have been referred to by Stearns and Vaksvik (1935), 

Visher and Mink (1964), Takasaki and Mink (1985), 

and by the authors of ground-water studies cited in 

the next section of this report.

PREVIOUS STUDIES

A number of previous studies have addressed the water budget or estimated ground-water recharge in the 

study areas of the island (fig. 3A), but only one previous investigation has attempted to develop a water budget for the entire island (Takasaki, 1978). Southern 

Oahu has been the most intensely studied area because 

it contains the principal source of water supply for the 

most heavily populated part of the island, and it is the 

most intensely irrigated agricultural area of the island. 

Most of the studies in southern Oahu have concentrated 

on making estimates of ground-water recharge. Wentworth (1951) presented one of the earliest estimates of 

a water budget for various parts of the southern Oahu 

area. Visher and Mink (1964) studied the ground-water 

resources of southern Oahu, but did not directly address the issues of a water budget or ground-water 

recharge. Dale (1967) discussed the effects of land use, 

specifically irrigated agriculture, on recharge to the 

ground-water system in part of the southern Oahu 

area. Mink (1980) estimated a predevelopment water 

budget for much of the southern Oahu area, and also 

estimated irrigation-return flow for land-use conditions 

during an undefined period before 1972. The most detailed water-budget study for southern Oahu was made 

by Giambelluca (1983,1986). His studies constitute the 

basis for the southern Oahu water-budget estimate presented in this report.

The rest of Oahu has been studied less intensely Eyre 

and others (1986) developed a water budget for southeastern Oahu for use in a numerical analysis of groundwater flow in that area. Their budget provides the basis for the water budget for southeastern Oahu in the 

present study. Takasaki and others (1969) presented wa1 Above sea level.
INTRODUCTION C3

158°

Kahuku Point

24 MILES

i i i

21° 20'

Barbers' 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24.000.1983. Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59' 

Contours from U.S. Geological Survey 

digital elevation model data, 1:250,000,1986

EXPLANATION

COASTAL-PLAIN DEPOSITS (caprock)

WAIANAE WINDWARD SEDIMENT

KOOLAU CALDERA

SEDIMENT (Holocene)

KOOLAU BASALT (Pleistocene and Pliocene)

WAIANAE VOLCANICS (Pliocene)

GEOLOGIC CONTACT

 7,000- TOPOGRAPHIC CONTOUR Contour interval 

1,000 feet. Datum is sea level

160° 158° 156°

22°

20°

HAWAII

Molokai

Maui

FIGURE 1. Study area and generalized geology, island of Oahu (modified from Stearns, 1939).
C4 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

ter budgets for several of the windward Oahu drainage 

basins. Takasaki and Valenciano (1969) estimated a water budget for the Kahuku area in northeastern Oahu 

that corresponds approximately to the Kahuku basin of 

windward Oahu in the present study. Rosenau and others (1971) presented a water budget for part of northcentral Oahu, but their study area did not have the same

boundaries as the north-central Oahu area of the 

present study Dale (1978) and Dale and Takasaki (1976) 

estimated ground-water recharge to part of the northcentral Oahu area. Takasaki (1971) discussed components of a water budget for the Waianae area, but did not 

estimate ground-water recharge.

158°

Kahuku Point

2 4 MILES

i i i

21° 30'

21° 20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data. 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

EXPLANATION

-40 LINE OF EQUAL MEAN ANNUAL PRECIPITATIONInterval, in inches, is variable

MAJOR STUDY-AREA BOUNDARY

FIGURE 2. Mean annual precipitation, 1916-83, island of Oahu (modified from Giambelluca and others, 1986).
INTRODUCTION C5

A. 158° Kahuku Point

NORTH-CENTRAL OAHU

.^/SOUTHEASTERN' Point 

OAHU

Base modified from U S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

Barbers 

Point

B. 158° Kahuku Point

EXPLANATION

MAJOR GEOHYDROLOGIC 

BOUNDARY

SUBORDINATE GEOHYDROLOGIC 

BOUNDARY Queried where 

uncertain

21° 30'

158° 10'

 .j - 

Mokuleia\ Waialua

' *'''///''

/ £/ .$ i

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

21° 20'

Barbers 

Point

MAMAIA BAY

Head

FIGURE 3. Study areas, island of Oahu. A, Major study areas; topographic divides. 8, Relation among topographic

divides, geohydrologic barriers, and ground-water areas.
C6 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

PREDEVELOPMENT WATER BUDGET

A water budget for natural conditions is presented for 

each of the five study areas of the island of Oahu (fig. 

3A). Each budget was developed by a somewhat different method, but each is designed to simplify earlier or 

more complicated methods to provide an easily applied 

and consistent approach for subsequent investigators. 

The methods utilize the capabilities of the GIS model. 

All the component data, including areas and mean annual precipitation, are included in this report. These 

data, together with equations presented in this report, 

can be used to calculate various elements of the water 

budget for any of the five study areas by using standard 

mathematical methods. Because of the gridlike nature 

of the GIS data bases used in the computational approach for this study, other methods that apply single 

values of various elements of the budget to large areas 

(that is, an entire study area) may yield somewhat different results.

SOUTHERN OAHU

A detailed monthly water budget calculated by Giambelluca (1983) for most of the southern Oahu area provided the basis for the method developed in the present 

study to estimate the water budget for the entire southern Oahu area before development. Giambelluca (1983) calculated each element of the water budget (precipitation, evapotranspiration, runoff, and 

ground-water recharge) for the different land-use categories (table 1) in the southern Oahu area. Giambelluca (Water Resources Research Center, University 

of Hawaii, written commun., 1985; 1986) prepared a 

similarly detailed water budget based on his 1983 methodology for about the same area of southern Oahu that 

is encompassed by the present study as part of the 

Hawaii RASA (fig. 4). However, neither the area encompassed by the 1983 study nor the area used in the 1985

TABLE 1. Land-use categories, southern Oahu

Category Land use

1 Furrow-irrigated sugarcane

2 Pineapple

3 Urban low density

4 Urban medium density

5 Urban high density

6 Mixed (medium density urban, vacant)

7 Mixed (high and medium density urban)

8 Mixed (high density urban, vacant)

9 Parks, golf courses

10 Natural, includes vacant, forest, pasture

11 Drip-irrigated sugarcane

water budget model is totally coincident with the study 

area that is defined in the RASA project and used for 

the GIS analysis.

The 1983 study area did not extend as far north as the 

northern boundary of the southern Oahu area used in 

the present study, and the 1985 study area was not coincident with the crest of the Koolau Range. The difference between the area used in the 1985 study and this 

study is small, about 3 mi2 . However, because the excluded area is in the high precipitation zones, and, 

therefore high recharge zones, of the Koolau Range, the 

results of the regression model that uses the GlS-defined boundaries do not agree well with the results of 

Giambelluca (1986; Water Resources Research Center, 

University of Hawaii, written commun., 1985). To demonstrate the validity of the regression model, recharge 

must be estimated for the same area that Giambelluca 

used. Accordingly, to compare the two methods, the GIS 

area was reduced slightly to simulate Giambelluca's 

area for model calibration. The reduced area is referred 

to in the tables and the text as the "adjusted area." The 

total GIS-defined area is used for estimating the components of the islandwide water budget. The adjusted 

and total areas are given in table 2. Additionally, the 

areas are differentiated between caprock and noncaprock (fig. ]).

Giambelluca's (1983) approach was simplified by developing precipitation-recharge and precipitation-runoff relations for natural, predevelopment land use and 

for agricultural and urban land use presented later in 

this report using Giambelluca's data (Water Resources 

Research Center, University of Hawaii, written 

commun., 1985). A plot of estimated annual recharge 

in relation to mean annual precipitation is shown in figure 5. A single regression line does not adequately describe the relation, however, because it underestimates 

significant recharge in the high precipitation zones, as 

well as in those areas where annual precipitation 

ranges from 20 to about 35 in. Four regression equations were developed that more effectively describe the 

precipitation-recharge relation (fig. 5). The regression 

equation is of the general form

Recharge = a(Ppt) + b, (1)

where Recharge is the mean annual ground-water recharge rate, in inches; a is the slope of the regression 

line; Ppt is the mean annual precipitation, in inches; and 

b is the regression line intercept at the y axis. The equation coefficients are given in table 3. Predevelopment 

recharge to southern Oahu was calculated by selecting 

the areas in the GIS model in each precipitation range 

and then by applying the proper equation (tables 2, 3). 

The resulting ground-water recharge was converted to 

million gallons per day.
PREDEVELOPMENT WATER BUDGET C7

158°10' 158° 157° 50'

21°30'

21°20'

Barbers 

Point

PACIFIC OCEAN

Base modified from U.S. Geological Survey 

digital data, 1:24.000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

4 MILES

OAHU

Pacific 

Ocean

-25 

024 KILOMETERS

EXPLANATION

PINEAPPLE FIELDS

DRIP-IRRIGATED SUGARCANE FIELDS 

FURROW-IRRIGATED SUGARCANE FIELDS 

URBAN, NATURAL, PARK, AND MIXED LAND USE

LINE OF EQUAL MEAN ANNUAL PRECIPITATION 

Interval, in inches, is variable

FIGURE 4. Mid-1980's land use and mean annual precipitation, southern Oahu.
C8 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

TABLE 2. Caprock and noncaprock areas for indicated mean annual precipitation, southern Oahu 

[in., inch; mi , square mile; GIS, geographic information system; , no data or not applicable]

Mean annual

precipitation 

(in.)

21.6

23.6

27.6

35.4

49.2

68.9

78.7

98.4

137.8

157.5

177.2

216.5

255.9

275.6

Total...................

Caprock

area 

(mi2)

41.27

1.21

16.19

9.41

1.59

.91

.02

70.60

Noncaprock area (mi

Adjusted

5.60

16.25

23.33

51.93

18.51

.63

27.97

21.01

1.02

11.57

5.30

3.46

.20

186.78 1

2)

GIS

5.60

16.25

23.33

51.93

18.51

.63

27.97

21.01

1.17

13.22

6.06

3.95

.23

[89.86

Total area (mi2)

Adjusted

46.87

1.21

32.44

32.74

53.52

19.42

.63

27.99

21.01

1.02

11.57

5.30

3.46

.20

257.38

GIS

46.87

1.21

32.44

32.74

53.52

19.42

.63

27.99

21.01

1.17

13.22

6.06

3.95

.23

260.46

A similar analysis was made to determine the relation 

between precipitation and estimated runoff for natural, 

predevelopment land use calculated by T.W. Giambelluca (Water Resources Research Center, University of 

Hawaii, written commun., 1985). The data are shown 

in figure 6, and as with recharge, several regression 

equations were needed to describe the data adequately. 

The general form of the equation is

Runoff'= a(Ppt)b , (2)

where Runoff is the mean annual runoff, in inches; 

Ppt is the mean annual precipitation, in inches; and 

a and b are the regression coefficients. The regression 

coefficients to calculate runoff for ranges of mean 

annual precipitation are given in table 4. Runoff for 

each precipitation zone was calculated by applying 

equation 2 in the same manner as equation 1. Evapotranspiration for the southern Oahu area was calculated as the residual of precipitation minus estimated 

recharge and runoff.

The predevelopment water budget for the southern 

Oahu study area determined by using the simple regression equations is compared with the water budget 

determined by using the more detailed method of T.W. 

Giambelluca (Water Resources Research Center, University of Hawaii, written commun., 1985) in table 5. 

The difference of 4 Mgal/d in recharge estimated by the 

two methods represents less than 1.5 percent of the estimated recharge. Giambelluca (1986) estimated recharge to the noncaprock part of southern Oahu at 

about 313 Mgal/d; the regression model used in this

study estimated about 310 Mgal/d to the adjusted noncaprock area of southern Oahu. Recharge to the noncaprock area of southern Oahu is significant because 

the area is underlain by basalt aquifers from which the 

area's ground-water supply is obtained. The 3-Mgal/d 

difference between the two estimates of recharge to the 

noncaprock area represents a difference of less than 1.0 

percent. The close agreement among the elements of 

the budget calculated by each method (table 5) suggests 

that the regression model is an appropriate simplified 

method for estimating water budgets in southern Oahu. 

The water budget for the GIS-defined area of southern 

Oahu is given for caprock and noncaprock areas in 

table 6.

TABLE 3. Regression equation coefficients for groundwater recharge for predevelopment land use,

southern Oahu

[in., inch; a, slope of regression line; b, regression line

intercept at y axis; r, coefficient of determination;

>, actual value is greater than value shown]

Mean annual

precipitation

range (in.)

20-13

43-95

95-152

>152

a

0.06

.78

.6

.89

b

3.4

-28.0

-15.0

-56.0

r2

0.09

.93

.24

.91
PREDEVELOPMENT WATER BUDGET C9

250

200

150

100

CC 

< 

LLJ

ffi 5° 

Q_

OD

LU

I 

O

S o

XV"

.0* /*

,rf*

 This area enlarged ' i.

0

LU 

O 

CC

LLJ 

CC

<

LLJ

50 100 150 200 250

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

300 350

60

50

40

30

20

10

Z'J* %-

v' - 

,' »

Recharge = 0.06 (Ppt) + 3.4

 K>

10 20 30 40 50 60 70 80 

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

90 100

FIGURE 5. Relation between precipitation and estimated ground-water recharge for 

predevelopment land use, southern Oahu. Ppt, mean annual precipitation.
CIO REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

100

80

60

UJ

CC 

UJ 

D_

CO 

UJ

I

o

3 40

<UJ

20

50 100 150 200 250

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

300 350

FIGURE 6. Relation between precipitation and estimated runoff for predevelopment land use, southern Oahu.

Ppt, mean annual precipitation.

TABLE 4. Regression equation coefficients for runoff

for predevelopment land use, southern Oahu

[in., inch; a and b, regression coefficients; r2, coefficient of

determination; >, actual value is greater than value shown;

 , not applicable]

Mean annual 

precipitation 

range (in.)

20-175

1 175-200

>200

a

0.013

.018

b

1.536

1.445

r2

0.92

.91

Constant 37.5 in. to connect power function curves.

TABLE 5. Estimated predevelopment water budget,

southern Oahu 

[In million gallons per day]

Water-budget 

category

Giambelluca1 Adjusted 

area2

Precipitation.............................. 802

Evapotranspiration.................... 362

Runoff....................................... 117

Recharge................................... 323

808

363

118

327

T.W. Giambelluca, Water Resources Research Center, University 

of Hawaii, written commun., 1985. 

Estimated using regression model and adjusted area in table 2.

TABLE 6. Predevelopment water budget for the area 

defined by a geographic information system,

southern Oahu 

[In million gallons per day]

Water-budget 

category

Precipitation................

Evapotranspiration ......

Runoff.........................

Recharge .....................

Caprock 

area

......... 88

......... 63

......... 7

......... 18

Noncaprock 

area

754

293

120

341

Total

842

356

127

359

SOUTHEASTERN OAHU

A monthly water budget for part of the 28.4 mi2 southeastern Oahu area (fig. 7) was calculated by Eyre and 

others (1986) as part of a ground-water flow analysis of 

that area. They did not simulate ground-water flow in 

the 3.29-mi2 coastal-plain area southwest of Makapuu 

Head. Therefore, their water budget does not include the 

distribution of water in that part of southeastern Oahu. 

However, their budget provides the basis for the development of a simple estimation model for southeastern 

Oahu that is similar to the one developed for southern 

Oahu. Simple linear recharge-precipitation relations for
157°50'

PREDEVELOPMENT WATER BUDGET 

157° 45'

Gil

157° 40'

21°20'

21 0 15'

Diamond 

Head

Base modified from U.S. Geological Survey 

digital data. 1:24.000,1983, Albers equal area 

projection, standard parallels 21 C 15' and 21 C45', 

central meridian 157°59'

3 MILES

I

3 KILOMETERS

Pacific 

Ocean

 25 

EXPLANATION

SOIL PERMEABILITY

Rock land with particularly low permeability

Other soil types with higher permeability

LINE OF EQUAL MEAN ANNUAL PRECIPITATION 

Interval, in inches, is variable

FIGURE 7. Mean annual precipitation and areas of relatively low and high soil permeability, southeastern Oahu.

use in the GIS model were determined on the basis of 

the results of Eyre and others (1986). Two different relations were developed on the basis of relative soil permeabilities (fig. 8). Eyre and others (1986) distinguished 

10 different soil types, based on data from Foote and 

others (1972), to account for the effects of variable soil 

properties, including permeability, on estimated recharge. The GIS model reduces these data to two relative soil types with high and low permeabilities, and 

uses a separate regression equation for each type to estimate recharge. Both equations are in the form of

equation 1, and the coefficients for the equations are 

listed in table 7. Table 8 lists the areas for each relative permeability soil type for a given mean annual precipitation zone in southeastern Oahu. Estimated 

ground-water recharge (in million gallons per day) was 

then determined by multiplying the appropriate recharge rate calculated with the regression equation by 

the area over which that rate applies.

A runoff-precipitation relation developed by Eyre and 

others (1986) was modified slightly and used to calculate runoff for southeastern Oahu. The equation used
C12 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

is the same form as equation 2, and the coefficients are 

listed in table 9. The curve and data are shown in figure 9. Evapotranspiration was estimated to be the residual of precipitation minus runoff and recharge. The 

resulting predevelopment water budget determined by 

the simplified GIS model is compared with the results 

of Eyre and others (1986) in table 10. They estimated a 

total ground-water recharge of about 15 Mgal/d compared with about 19 Mgal/d estimated by using the GIS 

model in this study. The difference is attributed to the 

additional 4-Mgal/d mean annual precipitation estimated for the coastal-plain area by this study but that 

was not considered by Eyre and others (1986).

TABLE 8. Areas forgiven relative soil permeability and 

mean annual precipitation, southeastern Oahu 

[in., inch; mi2, square mile; , not applicable]

Mean annual 

precipitation 

(in.)

23.6 

27.6 

35.4 

49.2 

68.9 

98.4 

137.8 

Total....................

Area with indicated soil type (mi2)

High 

permeability

0.28 

7.1 

3.81 

5.17 

2.08 

1.65 

.35 

............ 20.44

Low 

permeability

0.57 

2.48 

4.59 

.36

8.00

TABLE 7. Regression equation coefficients for groundwater recharge, southeastern Oahu 

[a, slope of regression line; b, regression line intercept at y axis; 

j2, coefficient of determination]

TABLE 9. Regression equation coefficients for runoff,

southeastern Oahu

[in., inch; a and b, regression coefficients; r2, coefficient of 

determination; >, actual value greater than value shown]

Relative soil a b r2

permeability

Low............................... 0.501 -13.32 0.91 

High.............................. .927 -16.74 .85

50

cr

LU

ffi 40

Q_

CO

LU I

O

^^

~ 30

LU

CD

CC

I

O 20

LU

cr_j 

^

< 10

z

LU

0 

I I I

*

® HIGH PERMEABILITY /

/

 LOW PERMEABILITY

/

/

/

/

/

/

/ ® i

_ w / /

/

 f

/_/ 9

,' ®

A / -.

* ' 0

 v / ® »*x

'v/ <0>V

<0* / 9 v\ft x A

/ / Cke ''' «0 ; . v:''; '

~ *$*/ * * fff *

ft / 8 « ''* e> ' x*

<Q>/ *" 

&i / \ \

0 20 40 60

Mean annual

precipitation a b r

(in.)

>0 0.0011 2.15 0.99

1 ' ' /

*

 '''

,'' 

 *,'''

ff

ff

 _^'

y x

\i'x* 

^ ''

^^ xX

\ ''*

<b^x''

f

* 

-

1 1 1

80 100 120 140

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

FIGURE 8. Relation between precipitation and estimated ground-water recharge based on relative

soil permeability, southeastern Oahu (data from Eyre and others, 1986).

Ppt, mean annual precipitation.
PREDEVELOPMENT WATER BUDGET CIS

HI >- 

cc

HI 

D_

CO 

HI

I

o

50

40

30

g 20

10

< 

HI

20 40 60 80 100

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

120 140

FIGURE 9. Relation between precipitation and estimated runoff, southeastern Oahu.

TABLE 10. Predevelopment water budget,

southeastern Oahu 

[All values in million gallons per day]

Water-budget 

category

Precipitation .................

Evapotranspiration .......

Runoff..........................

Recharge..... ..................

This 

study

....... 62

....... 36

....... 7

....... 19

Eyre and 

others (1986)

57

36

6

15

WINDWARD OAHU

Windward Oahu covers a little more than 124 mi2 of the 

northeastern part of the island, east of the crest of the 

Koolau Range. Predevelopment water budgets for the 11 

drainage basins of windward Oahu (fig. 10) were calculated by using methods similar to those of Takasaki and 

others (1969). The annual water budget is given by

Ppt-R-ET=GW, (3)

where Ppt is the mean annual precipitation, R is mean 

annual direct runoff, ET is mean annual evapotranspiration, and GWis mean annual ground-water recharge.

Figure 10 shows the 11 drainage basins of windward 

Oahu and the mean annual precipitation (Giambelluca 

and others, 1986). Figure 11 shows the estimated mean 

annual evapotranspiration that was based on pan-evaporation data (Ekern and Chang, 1985). The name of each 

basin is given in table 11. Pan coefficients used to estimate actual evapotranspiration vary above and below 

1.0, depending on the type of vegetation and time of year. 

For this study, which considers only mean annual conditions, the pan coefficient is assumed to be equal to 1.0, 

and pan evaporation is assumed to equal mean annual 

evapotranspiration. Direct runoff is calculated as the 

difference between mean basin discharge and base flow 

and is expressed as a percentage of precipitation based 

on data from Takasaki and others (1969), Takasaki and 

Valenciano (1969), and the U.S. Geological Survey 

(1970, 1971, 1972). The ratios are given in table 11.

The mean annual volume of precipitation for each basin was determined by multiplying the area by the mean 

annual precipitation that applies for that area (table 12). 

Evapotranspiration was estimated by multiplying the 

mean annual pan-evaporation rate by the area for which 

that rate applies (table 13). Direct runoff was determined by multiplying the volume of precipitation for the 

basin by the appropriate ratio from table 11. Groundwater recharge was determined to be the residual of
C14 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158° 157°50' 157° 40'

Kahuku Point

OCEAN

21

EXPLANATION

I ~ 2 ~ j NUMBERED DRAINAGE BASIN 

Names of basins are given in table 11

 60 LINE OF EQUAL MEAN ANNUAL

PRECIPITATION Interval, in inches, 

is variable

21°30'

21°20'

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

4 MILES

4 KILOMETERS

Pacific 

Ocean

FIGURE 10. Drainage basins and mean annual precipitation, windward Oahu.
PREDEVELOPMENT WATER BUDGET C15

158° 157°50' 157°40'

21=40'

21°30r

21°20'

EXPLANATION

NUMBERED DRAINAGE BASIN 

Names of basins are given in table 1 1

 60 LINE OF EQUAL MEAN ANNUAL 

PAN EVAPORATION Interval 10 

inches

waia/ua district boundary! ^'

Base modified from U.S. Geological Survey 

digital data. 1:24,000,1983. Albers equal area 

projection, standard parallels 21°15' and 21°45'. 

central meridian 157°59'

2 4 MILES

j_____I

2 4 KILOMETERS

OAHU

Pacific 

Ocean

FIGURE 11. Drainage basins and mean annual pan evaporation, windward Oahu (modified from Ekern and Chang, 1985).
C16 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

these three budget elements. The resulting water budgets for each basin are listed in table 14.

The values for estimated evapotranspiration and recharge in table 14 derived from the GIS model cannot 

be duplicated for all basins with the data in tables 12 

and 13. The GIS model makes use of smaller elements 

within each drainage basin. When evapotranspiration 

for any element of the GIS model equals or exceeds precipitation minus runoff, evapotranspiration cannot occur at the maximum rate and thus is set equal to precipitation minus runoff, and recharge is equal to zero.

TABLE 11. Runoff-precipitation ratios, windward 

Oahu basins

Basin

no. 

(fig. 10)

10

11

Basin 

name

Runoffprecipitation 

ratio

Kahana........................0.26

Kaluanui........................ .23

Punaluu........................ .16

Waikane........................ .17

Waiahole........................ .24

Maunawili ..................... .21

Kailua........................ .11

Waimanalo..................... .11

Kaneohe........................ .17

Kahuku........................ .20

Kaaawa/Hakipuu.................. .09

Because the GIS model calculates the water budget for 

the smaller elements that compose each basin, using the 

total area given in tables 12 and 13 to estimate the total volume of precipitation and evapotranspiration yields 

no recharge for some basins; however, when using the 

smaller elements of the GIS model, some recharge is 

estimated.

Comparison of the GIS budgets with water budgets 

determined from previous studies is difficult because 

different boundaries were used to define study areas. 

Takasaki and others (1969, p. 85, 93, and 97) estimated 

water budgets for three basins included in their study 

of windward Oahu; these budgets are given in table 15. 

The major difference between the previous estimates 

and the budgets calculated for this study is the estimated volume of precipitation. Part of the reason for this 

may be the different base period for mean annual precipitation used in the two studies; the base period for 

precipitation data used by Takasaki and others (1969) 

ranged from 1933 to 1957 and for the present study it 

ranged from 1916 to 1983 (Giambelluca and others, 

1986). Similar values for evapotranspiration and runoff 

were obtained by both studies. The difference in precipitation estimated by each study results in similar differences in estimated ground-water recharge, which is to 

be expected because each study estimated recharge as 

a residual based on precipitation.

TABLE 12. Areas for indicated mean annual precipitation, windward Oahu basins 

[Basins shown in fig. 10. in., inch; mi2, square mile; , not applicable]

Basin no. ........

Mean annual

precipitation

(in.)

27.6

35.4

49.2

68.9

98.4

137.8

177.1

216.5

255.9

275.6

Total. ...........

1

_

0.08

2.41

1.57

1.02

1.00

1.23

1.03

8.34

2

_

0.71

.66

.42

.35

.40

.26

2.80

3

_

1.44

1.12

1.05

.99

1.22

.91

.10

6.83

4

_

0.64

1.50

.91

.68

.36

4.09

5

Area

(mi2)

_

0.34

.64

.97

1.51

.45

3.91

6

_

0.29

3.73

3.26

7.28

7 8

 0.11

 .33

16.97 8.08

.38 1.86

 .18

 

 

 

 

 

17.35 10.56

9

_

9.40

9.93

1.94

.65

21.92

10

_

15.22

8.78

3.62

2.55

1.77

1.96

1.23

.25

35.38

11

_

3.12

2.89

6.01
PREDEVELOPMENT WATER BUDGET C17

TABLE 13. Areas for indicated mean annual pan evaporation, windward Oahu basins 

[Basins shown in fig. 10. in, inch; mi2, square mile; , not applicable]

Basin no.

Pan

evaporation

(in.)

18

25

35

45

55

65

75

82

Total............

1

1.83

1.15

1.43

1.57

1.81

.55

8.34

2

0.42

.44

.64

.61

.68

.01

2.80

3

1.52

1.34

1.41

1.06

1.18

.32

6.83

4

0.38

.79

1.48

1.03

.41

4.09

5 6

Area

(mi2)

1.54 

1.17 

.69 

.31 4.95

.20 2.33

 

 

 

3.91 7.28

7

_

1.50

4.06

8.76

3.03

17.35

__

1.66

3.74

4.08

1.08

10.56

9

_

3.04

4.69

8.62

5.06

.51

21.92

10

2.92

3.57

5.27

6.86

5.16

5.05

4.52

2.03

35.38

11

_

0.22

1.66

2.74

1.39

6.01

TABLE 14. Predevelopment water budgets, windward Oahu basins 

[Basins shown in fig. 10. Mgal/d, million gallons per day; mi2, square mile]

Precipitation (Mgal/d) .................

Runoff (Mgal/d)... .......................

Evapotranspiration (Mgal/d).......

Recharge (Mgal/d) ......................

Area (mi2) ...................................

1

... 56

... 15

... 14

... 27

8.34

2

18

4

5

9

2.80

3

50

8

11

31

6.83

4

24

4

7

13

4.09

5

28

7

5

16

3.91

6

28

6

15

7

7.28

7

41

5

36

0

17.35

8

27

3

23

1

10.56

9 10

96 151

16 30

45 63

35 58

21.92 35.38

11

17

2

14

1

6.01

Total

536

100

238

198

124.47

NORTH-CENTRAL OAHU

The predevelopment water budget for north-central 

Oahu (fig. 12), which covers an area of slightly more 

than 121 mi2, was modeled as an extension of southern 

Oahu because both areas have similar climate, topography, and soil types. Both have areas of caprock that affect recharge to the basalt aquifers, and both have large 

areas of irrigated agriculture that have altered the predevelopment ground-water recharge. The total volume of 

precipitation for caprock and noncaprock areas (fig. 1) 

of north-central Oahu was determined by multiplying 

the areas in table 16 by the appropriate value of annual 

precipitation.

Equation 1 was used with the coefficients in table 3 

and the areas in table 16 to estimate ground-water 

recharge for predevelopment conditions in northcentral Oahu. Similarly, equation 2 and the coefficients in table 4 were used to calculate estimated 

runoff. Finally, as in southern Oahu, evapotranspiration was estimated to be the residual of precipitation 

minus runoff and recharge. The predevelopment water budget for caprock and noncaprock areas of northcentral Oahu is given in table 17.

The water budget for north-central Oahu, calculated 

with the GIS model for this study, is difficult to compare 

with budgets estimated by previous studies because of 

the difference in the boundaries of the study areas. The 

study by Rosenau and others (1971) covered 152 mi2 of 

the north-central part of Oahu, but it excluded a small 

area west of the crest of the Koolau Range along the 

northern coast. They appear to have included some of 

the southern Oahu area (figs. 3A, 4); their location map 

is not detailed enough to ascertain the exact location 

of these boundaries.

TABLE 15. Estimated water budgets for selected windward

Oahu basins (from Takasaki and others, 1969)

[Basins shown in fig. 10. Mgal/d, million gallons per day;

mi , square mile]

Precipitation (Mgal/d)............... ......

Runoff (Mgal/d) ..............................

Evapotranspiration (Mgal/d) ...........

Recharge (Mgal/d) ..........................

Area (mi2) .......................................

1

....... 60

....... 15

....... 12

....... 33

....... 8.38

2

22

5

4

13

3.5

3

57

9

8

40

65
C18 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158°10' 158°

21°30'

21°20'

KahukuPprrrt!

Base modified from U.S. Geological Survey 

digital data. 1:24,000.1983. Albers equal area 

projection, standard parallels 21°15' and 21 C45', 

central meridian 157°59'

4 MILES

I

4 KILOMETERS

Pacific 

Ocean

OAHU

 60 

EXPLANATION

LOW-DENSITY URBAN LAND USE 

DRIP-IRRIGATED SUGARCANE FIELDS 

FURROW-IRRIGATED SUGARCANE FIELDS 

NATURAL LAND USE

LINE OF EQUAL MEAN ANNUAL PRECIPITATION 

Interval, in inches, is variable

FIGURE 12. Mid-1980's land use and mean annual precipitation, north-central Oahu.
PREDEVELOPMENT WATER BUDGET C19

TABLE 16. Caprock and noncaprock areas for indicated

mean annual precipitation, north-central Oahu

[in., inch; mi2, square mile; , not applicable]

Mean annual 

precipitation 

(in.)

31.5

35.43

49.21

68.9

78.74

98.42

137.8

177.2

216.5

255.9

275.6 

Total....................

Caprock 

area 

(mi2)

1.03

10.92

1.34

13.29

Noncaprock 

area 

(mi2)

_

16.72

37.52

26.75

.56

9.92

4.75

3.77

2.63

3.32

2.12 

108.06

Total 

area 

(mi2)

1.03

27.64

38.86

26.75

.56

9.92

4.75

3.77

2.63

3.32

2.12 

121.35

TABLE 17. Predevelopment water budget, north-central Oahu 

[In million gallons per day]

Water-budget 

category

Precipitation... ........... ...

Runoff..........................

Recharge ......................

Caprock 

area

........... 23

........... 2

........... 17

........... 4

Noncaprock 

area

411

63

168

180

Total 

area

434

65

185

184

WAIANAE AREA

The Waianae area covers nearly 61 mi2 of the western 

part of Oahu, west of the crest of the Waianae Range 

(figs. 3A and 13). Climate, land cover, and soil types in 

this dry part of Oahu are similar to those of southeastern Oahu (fig. 7). Given the similarity of conditions, the 

ground-water recharge-precipitation and runoff-precipitation relations that were developed for southeastern 

Oahu were assumed to apply to the Waianae area as well. 

In addition to areas of low and high relative soil permeability, the Waianae area has extensive areas of algaroba 

(Prosopis chilensis), also called kiawe, which is a close 

relative of the mesquite of the Southwestern United 

States (Zones, 1963), and koa haole (Leucaena glauca), 

both of which are phreatophytes (fig. 13).

Little information is available on evapotranspiration 

rates for algaroba and koa haole. It is uncertain to what 

extent rates determined for similar plants in the Southwestern United States can be applied to the Waianae 

area. Zones (1963) estimated a ground-water discharge 

rate for these plants of from 3 to 6 ft/yr. Examination of 

figure 13 indicates that most of these plants grow in 

areas that have a mean annual precipitation of less than 

about 35 in. The depth to ground water in these areas

may be several hundred feet below land surface 

(Takasaki, 1971). Estimated annual runoff for this annual precipitation is about 2 in. On this basis, it is assumed that the evapotranspiration rate for these trees 

is about 33 in/yr and that, in areas of phreatophytes 

having a mean annual precipitation of 35 in. or less, no 

ground-water recharge occurs.

The volume of mean annual precipitation in the Waianae area was calculated by using the precipitation 

and area data in table 18. Equations 1 and 2, together 

with the regression coefficients given in tables 7 and 

9, were used to calculate ground-water recharge and 

runoff. Evapotranspiration was not allowed to exceed 

precipitation in any area and was estimated to be the 

residual of precipitation minus runoff and groundwater recharge. For phreatophyte areas (fig. 13) having a mean annual precipitation of less than 35 in., calculated recharge was assumed to be zero, and the precipitation minus runoff in these areas was distributed 

as evapotranspiration. The resulting predevelopment 

water budget for the Waianae area is given in table 19.

A complete water budget has not been estimated previously for the Waianae area. Takasaki (1971) estimated 

annual precipitation to be about 98 Mgal/d; evapotranspiration, about 77 Mgal/d; and runoff, about 15 Mgal/d. 

An earlier study by Zones (1963) suggested that surfacewater discharge from the Waianae area was negligible 

(this study estimated 8 Mgal/d), the consumptive use 

of ground water by phreatophytes ranged from about 5 

to 10 Mgal/d, and the total ground-water discharge by 

all means ranged from about 10 to 20 Mgal/d. The 

present study estimated ground-water recharge, and, 

therefore, discharge, to be about 32 Mgal/d.

The water budget developed by this study can be used 

to evaluate further the consumptive use of ground water 

by phreatophytes in the Waianae area and to estimate 

ground water that may discharge as underflow to the 

ocean. The assumptions made about evapotranspiration 

by phreatophytes at altitudes above about 100 ft above 

sea level in the zone of 35 to 40 in. of mean annual precipitation imply that the phreatophytes growing there 

use between 33 and 38 in. of water per year. If about 35 

in./yr is assumed to be a reasonable estimate for mean 

annual evapotranspiration by these phreatophytes, then 

phreatophytes growing in areas of lower mean annual 

precipitation, but in the presence of shallow ground 

water, will transpire at this same rate. The area covered 

by phreatophytes where mean annual precipitation is 

less than about 35 in/yr is about 11.5 mi2 (table 18). (The 

estimated depth to ground water in higher mean annual 

precipitation zones appears to preclude these plants 

from using ground water.) The estimated phreatophyte 

evapotranspiration in this area is about 19 Mgal/d. The 

calculated evapotranspiration in this area from the GIS
C20 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158°15' 158°10' 158°5'

21°35'

21°30'

21°25'

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21 C 15' and 21 C45', 

central meridian 157°59'

4 MILES

4 KILOMETERS

 25 

EXPLANATION 

PHREATOPHYTE AREA 

SOILS WITH LOW PERMEABILITY 

SOILS WITH HIGH PERMEABILITY

LINE OF EQUAL MEAN ANNUAL PRECIPITATIONInterval, in inches, is variable

Pacific 

Ocean

FIGURE 13. Mean annual precipitation and areas of phreatophytes and of relatively low 

and high soil permeability, Waianae area, Oahu.
PREDEVELOPMENT WATER BUDGET C21

TABLE 18. Areas of phreatophyte and nonphreatophyte land cover and relative soil permeability for each precipitation zone,

Waianae area, Oahu

[in., inch; mi , square mile; , not applicable]

Mean annual 

precipitation 

(in.)

21.6

27.6

35.4

49.2

68.9

78.7 

Total ...................

Low-permeability 

soils area (mi2)

Nonphreatophytes

6.26

2.16

1.71

3.49

2.50

.01 

...... 16.13

Phreatophytes

2.94

4.91

1.75

.13

9.73

Subtotal

9.20

7.07

3.46

3.62

2.50

.01 

25.86

High-permeability 

soils area (mi2)

Nonphreatophytes

3.30

5.96

10.20

9.79

1.03

.11 

30.39

Phreatophytes

2.33

1.28

.87

.18

4.66

Subtotal

5.63

7.24

11.07

9.97

1.03

.11

35.05

Total 

(mi2)

14.83

14.31

14.53

13.59

3.53

.12 

60.91

TABLE 19. Predevelopment water budget, 

Waianae area, Oahu

[Mgal/d, million gallons per day]

Water-budget category Mgal/d

Precipitation...........

Runoff....................

Evapotranspiration. 

Recharge................

103

8

63

32

model is about 13 Mgal/d. Therefore, the estimated 

ground-water consumption by phreatophytes that grow 

in these areas was about 6 Mgal/d. Given the calculated 

recharge for the Waianae area of 32 Mgal/d, the discharge of ground water by underflow to the ocean was 

then estimated to be about 26 Mgal/d.

ISLAND OF OAHU

A summary predevelopment water budget for the island 

of Oahu was constructed from the budgets for the individual areas. This budget is given in table 20. The only 

previous study that attempted to estimate a water budget for Oahu is that of Takasaki (1978). However, as with 

other previous studies, the boundaries of the various areas are different from those of the present study, so individual area budgets cannot be compared. Additionally, 

Takasaki's (1978) estimate of ground-water recharge in 

southern Oahu may have addressed only the noncaprock 

area and the 1950's or 1960's land-use conditions, which 

included significant irrigation-return flow components. 

The estimate of 6 Mgal/d for ground-water recharge in his 

budget for the Waianae area was based on estimated 

ground-water pumpage in that area rather than estimated recharge. For comparison, however, the island budget of Takasaki (1978) is given in table 21.

TABLE 20. Predevelopment water budget, island of Oahu 

[In million gallons per day]

Water-budget 

category

Precipitation...... .. ..............

Runoff ...............................

Evapotranspiration ............

Recharge...........................

Southern 

Oahu

.......... 842

.......... 127

.......... 356

.......... 359

Southeastern 

Oahu

62

7

36

19

Windward 

Oahu

536

100

238

198

NorthCentral Oahu

434

65

185

184

Waianae

103

8

63

32

Oahu

1,977

307

878

792

TABLE 21. Water budgets, island of Oahu

[In million gallons per day]

Water-budget 

category

Precipitation... ......... ...

Runoff ........................

Evapotranspiration 

Recharge ....................

This 

study

1 977

... 307

... 878 

... 792

Takasaki 1

1,800

430

715 

655

1978, p. M12.
C22 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

DISTRIBUTION OF PREDEVELOPMENT 

GROUND-WATER RECHARGE

Estimates of predevelopment ground-water recharge 

commonly are useful in studies of ground-water flow and 

development. The distribution of this recharge, as estimated by the GIS model, is given in table 22 and is 

shown in figure 14. This distribution may be analyzed 

further by developing a more detailed set of boundaries 

than have been used for estimating water budgets for 

the various study areas (fig. 15). The area boundaries 

shown in figure 15 are based on the boundaries shown 

in figure 3B. Some boundaries, notably the northern 

and southern Schofield ground-water barrier boundaries, have been extended to intersect topographic divides. Several small areas are defined by the intersections of the southwestern boundary of the Koolau Rift 

Zone and the topographic divide marking the crest of 

the Koolau Range. By using the GIS water-budget 

model, the recharge estimates made in this study can 

be aggregated by the areas shown in figure 15 (table 

22), which, when combined in various patterns, may be 

comparable to areas described in previous investigations.

The distribution of predevelopment ground-water recharge to the noncaprock areas and ground-water underflow to the noncaprock basalt aquifers of southern and 

north-central Oahu are given in table 22 and are shown 

in figure 16. Of the 521-Mgal/d recharge to the noncaprock areas of southern and north-central Oahu, about 

199 Mgal/d is derived from the Schofield ground-water 

area and those parts of the Koolau and the Waianae 

Rift Zones adjacent to the Schofield area (figs. 3B, 16). 

An estimated 111 Mgal/d moves into the Schofield area 

as underflow from the Koolau Rift Zone, and about 9 

Mgal/d moves as underflow from the Waianae Rift Zone. 

About 79 Mgal/d recharges the Schofield area directly 

from precipitation. About 103 Mgal/d is estimated to 

move as underflow from the southern Schofield area into 

the Pearl Harbor and the Ewa ground-water areas in 

southern Oahu; about 96 Mgal/d moves as underflow 

from the northern Schofield area into the Waialua 

ground-water area in north-central Oahu (figs. 3B, 16).

Predevelopment recharge and ground-water underflow 

to the Pearl Harbor ground-water area is estimated to 

total about 261 Mgal/d (fig. 10). This estimate assumes 

that 103 Mgal/d from the southern Schofield area enters the Pearl Harbor area; some water flows into the 

adjacent Ewa ground-water area to the west, but the 

flow is probably less than 1 Mgal/d. Recharge and 

underflow to the Ewa area is estimated to be about 9

Mgal/d. Predevelopment recharge to the remaining areas of the southern Oahu study area totals about 71 

Mgal/d. In north-central Oahu, predevelopment recharge and underflow to the Waialua ground-water area 

is estimated to total about 104 Mgal/d; to the Kawailoa 

area, about 62 Mgal/d; and to the Mokuleia area, about 

14 Mgal/d (figs. 3B, 16). The distribution of predevelopment recharge to windward Oahu basins (fig. 3A), the 

Waianae area, and southeastern Oahu is shown in figure 17.

TABLE 22. Distribution of ground-water recharge for 

predevelopment land use, island of Oahu

[In million gallons per day; , no data or not applicable]

Area

(fig. 15)

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

Total..........

Caprock

area

_

0.15

.78

.86

1.84

.16

.25

2.71

6.59

3.60

.75

2.70

1.20

.10

.40

22.09

Noncaprock

area

3.77

193.67

35.98

25.67

8.36

5.12

7.34

1.31

7.76

7.69

31.70

8.94

16.48

64.09

44.53

33.14

1.94

.37

.23

9.78

6.13

6.76

135.02

22.79

24.76

15.97

17.84

10.40

7.10

8.90

2.40

.51

.24

1.68

.30

0.0

768.67

Total

3.77

193.67

36.13

26.45

9.22

6.96

7.50

1.31

7.76

7.69

31.70

8.94

16.48

64.09

44.53

33.14

2.19

.37

.23

9.78

6.13

9.47

141.61

22.79

28.36

16.72

20.54

11.60

7.20

9.30

2.40

.51

.24

1.68

.30

0.0

790.76
PREDEVELOPMENT WATER BUDGET C23

158°

Kahuku Point

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

EXPLANATION

GROUND-WATER RECHARGE, IN INCHES PER YEAR 

Greater than or equal to 150 

Greater than or equal to 100 and less than 150 

Greater than or equal to 50 and less than 100 

Greater than or equal to 25 and less than 50 

Greater than or equal to 10 and less than 25 

Less than 10

FIGURE 14. Distribution of ground-water recharge for predevelopment land use, island of Oahu.
C24 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

Kahuku Point

21° 30'

slrict boundary xje \ 4

21° 20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

EXPLANATION

17 i GROUND-WATER RECHARGE AREA AND NUMBER 

~ ~ ~ ~ ~ Numbers are area designations referenced in tables 22 and 30

COASTAL PLAIN (caprock)

FIGURE 15. Ground-water recharge areas, island of Oahu.
PREDEVELOPMENT WATER BUDGET C25

158°

Kahuku Point

24 MILES

21° 20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data. 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

23

25

EXPLANATION

COASTAL PLAIN (caprock)

MAJOR GEOHYDROLOGIC BOUNDARY

SUBORDINATE GEOHYDROLOGIC BOUNDARY

DIRECTION OF UNDERFLOW

UNDERFLOW, IN MILLION GALLONS PER DAY

RECHARGE BY INFILTRATION OF PRECIPITATION, 

IN MILLION GALLONS PER DAY

FIGURE 16. Distribution of predevelopment recharge to and discharge from the noncaprock 

areas, southern and north-central Oahu.
C26 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158°

Kahuku Point

4 MILES

4 KILOMETERS

Barbers' 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21°45', 

central meridian 157°59'

EXPLANATION

19

19

COASTAL PLAIN (caprock)

MAJOR GEOHYDROLOGIC BOUNDARY

WINDWARD BASIN BOUNDARY

DIRECTION OF UNDERFLOW

UNDERFLOW, IN MILLION GALLONS PER DAY

RECHARGE BY INFILTRATION OF PRECIPITATION, 

IN MILLION GALLONS PER DAY

FIGURE 17. Distribution of predevelopment recharge to and discharge from windward Oahu basins,

Waianae area, and southeastern Oahu.
EFFECTS OF LAND-USE CHANGES ON GROUND-WATER RECHARGE C27

EFFECTS OF LAND-USE CHANGES ON 

GROUND-WATER RECHARGE

Land-use changes affect the water budget through 

changes in evapotranspiration, runoff, and ground-water recharge. Low- to medium-density urbanization includes irrigation of lawns, gardens, and parks, which, in 

turn, increase water available at the land surface for 

increased evapotranspiration. Runoff increases and recharge in wet areas may be reduced, whereas recharge 

in dry areas may increase because of irrigation. Higher 

density urbanization enhances runoff, decreases evapotranspiration, and may increase recharge. Irrigated 

agriculture, however, has the greatest effect on the water 

budget. Applied irrigation water increases evapotranspiration, but irrigation practices seldom are efficient 

enough for all applied water to be consumed, and a significant percentage of the applied water recharges the 

underlying ground-water system.

The southern and north-central areas of Oahu have 

experienced the greatest changes in land use since the 

turn of the century. Although parts of southeastern and 

windward Oahu have undergone urbanization and 

small areas of windward Oahu have been cultivated, 

changes in these areas have affected the water budget

less than the changes in the southern and north-central areas. The following discussion will concentrate on 

the changes in southern and north-central Oahu and 

the effect on ground-water recharge in these areas.

SOUTHERN OAHU

The effects of urbanization on ground-water recharge 

in southern Oahu can be evaluated by using the results 

of the detailed water-budget study by T.W. Giambelluca 

(1986; Water Resources Research Center, University of 

Hawaii, written commun., 1985). Recharge as a function of precipitation for natural and urban land uses 

during the mid-1980's is shown in figure 18. The major change in the relation is in areas that have less than 

about 60 in/yr of precipitation; these are the areas that 

have undergone the most urbanization. Consequently, 

a new set of coefficients for equation 1 was developed 

to reflect mid-1980's land-use practices. The coefficients 

given in table 23 and the areas listed in table 24 for the 

appropriate precipitation zone are used to calculate 

ground-water recharge for nonagricultural areas.

Ground-water recharge for mid-1980's agricultural 

land use also was estimated on the basis of the detailed

60

<

LU

DC

<

LU

LU 

CL

CO

LU

g 40

LU

CTS 30

DC

<

I 

O 

LU

20

 Natural land use 

o Urban land use

Natural land-use trend line

10

Urban land-use trend line °

°Cb

10 20 30 40 50 60 70 80 

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

90 100

FIGURE 18. Relation (general trend) between precipitation and estimated ground-water recharge 

for natural and urban land use during the mid-1980's, southern Oahu.
C28 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

studies of southern Oahu by Giambelluca (1983,1986). 

Agricultural land use was subdivided into furrow-irrigated sugarcane, drip-irrigated sugarcane, and pineapple. Giambelluca (1983, 1986) suggested that an 

average of about 120 in/yr was applied in furrow-irrigated fields, and that recharge from furrow-irrigated 

sugarcane averaged 87.3 in/yr and accounted for 72.9 

percent of the applied irrigation water (Giambelluca, 

1983, p. 105). By using data from T.W. Giambelluca 

(Water Resources Research Center, University of Hawaii, written commun., 1985), the present study developed a linear relation between precipitation and recharge in areas of drip-irrigated sugarcane (fig. 19). 

This relation is given by

RechDS = 0.64(Ppt) + 11.1 (r* = 0.89),

TABLE 23. Regression equation coefficients for groundwater recharge for mid-1980's nonagricultural land use,

southern Oahu

[in., inch; a, slope of regression line; b, regression line intercept

atyaxis; r2, coefficient of determination; >, actual value is greater

than value shown]

(4)

Mean annual

precipitation 

range (in.)

20-60

60-95

95-152

>152

a

0.41

.78

.6

.89

b

-5.7

-28.0

-15.

-56.0

r2

0.57

.93

.24

.91

where Rechnc, is the mean annual recharge, in inches,

from drip-irrigated sugarcane; Ppt is the mean annual 

precipitation, in inches; and r2 is the coefficient of determination. The average applied water in drip-irrigated 

fields was about 65 in/yr (Giambelluca, 1983; 1986).

A similar relation was developed for areas of pineapple 

cultivation (fig. 20), given by

Rechp = 0.935(Ppt) -13.54 (r2 = 0.98), (5)

where Rechp is the mean annual recharge, in inches, 

from pineapple.

Ground-water recharge for mid-1980's land use is compared with recharge for predevelopment conditions in 

table 25 and figure 21. Recharge in noncaprock agricultural areas, which includes large amounts of irrigationreturn flow, increased 33 Mgal/d in the mid-1980's over 

predevelopment recharge to these same areas (table 25), 

and recharge to noncaprock, nonagricultural areas decreased about 1 Mgal/d from predevelopment to about 

327 Mgal/d. Total noncaprock recharge in the mid-1980's 

increased about 32 Mgal/d above predevelopment recharge.

The runoff-precipitation relation changed for mid1980's land use in areas that had a mean annual precipitation of less than about 130 in. (fig. 22). Runoff in 

these areas is calculated by using the linear regression 

equation 1 and the coefficients in table 26. The coefficients for the 20- to 130-in/yr areas are based on a regression that uses runoff data for urban and natural

TABLE 24. Areas of agricultural and nonagricultural land use during the mid-1980's, southern Oahu

[in., inch; mi2, square mile; , not applicable]

Caprock area Noncaprock area

Irrigated sugarcane 

Mean annual Nonaeriprecipitation Drip 

(in.) (mi2")

21.65 11.40

23.62 

27.56 1.70

35.43 

49.21 

68.90 

78.74 

98.42 

137.80 

157.50 _

177.20 

216.50 

255.90 

275.60 

Total 13.10

Furrow cultural 

(mi2) (mi2)

2.25 27.62

 1.21

 14.49

 9.41

 1.59

 .91

 

 .02

 

 

 

 

 

2.25 55.25

Total 

(mi2)

41.27

1.21

16.19

9.41

1.59

.91

.02

70.60

Irrigated agriculture

Drip 

(mi1)

0.06

3.91

3.56

.20

7.73

Furrow Pineapple 

(mi2) (mi2)

0.07 

 

1.46 0.69

 3.81

 10.31

 3.11

 

 .31

 

 

 

 

 

1.53 18.23

Nonagricultural 

(mi2)

5.47

10.19

15.96

41.42

15.40

.63

27.66

21.01

1.17

13.22

6.06

3.95

.23 

162.37

Total 

(mi2)

5.60

16.25

23.33

51.93

18.51

.63

27.97

21.01

1.17

13.22

6.06

3.95

.23 

189.86
EFFECTS OF LAND-USE CHANGES ON GROUND-WATER RECHARGE C29

60

50

40

cc

LU

CC 

LU 

Q_

C/D 

LU

X 

O

g 30

X 

O 

LU 

CC 

_i 20

10

20 25 30 35 

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

40

FIGURE 19. Relation (general trend) between precipitation and estimated ground-water recharge 

in areas of drip-irrigated sugarcane during the mid-1980's, southern Oahu.

cc

LU

>

CC 

LU 

D_

C/D 

LU

X 

O

_ 

<

<

z 

<

70

60

50

- 40

LU

(D 

CC

X 30 

O

LU 

CC.

20

10

10 20 30 40 50 60 70 

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

80 90

FIGURE 20. Relation (general trend) between precipitation and estimated ground-water recharge 

in areas of pineapple cultivation during the mid-1980's, southern Oahu.
C30 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

land uses (fig. 22). For areas having more than 130 

in/yr of precipitation, runoff is calculated by using the 

power regression equation 2 and the appropriate coefficients in table 26.

To estimate a mid-1980's water budget for southern 

Oahu, applied irrigation water reported by T.W. Giambelluca (Water Resources Research Center, University 

of Hawaii, written commun., 1985) was added to precipitation, and then recharge and runoff were subtracted to estimate evapotranspiration as a residual. 

Estimated runoff increased by about 18 percent above 

predevelopment runoff rates to about 150 Mgal/d. Estimated recharge increased by about 56 Mgal/d (15 

percent above predevelopment rates) to about 415 

Mgal/d, but only by about 32 MgaVd (9 percent) to about 

373 Mgal/d in the hydrologically important noncaprock 

area. Evapotranspiration increased by about 8 Mgal/d 

above predevelopment rates. The water budget is given 

in table 27.

The irrigation-return flow estimates for furrow-irrigated sugarcane by Giambelluca (1983, 1986) are considerably higher than those of previous investigators 

(Dale, 1967; Mink, 1980; Eyre and Nichols, in press). 

Giambelluca (1983, p. 105) estimated an irrigation-reTABLE 25. Predevelopment and mid-1980's recharge to indicated areas of mid-1980's

land use, southern Oahu 

[In million gallons per day; , not applicable]

Predevelopment1 Mid-1980's

Mid-1980's 

land use

Sugarcane ...........

Pineapple. ............

Nonagricultural . . . 

Total....................

Caprock

..... 4

..... 14 

..... 18

Noncaprock

2

11

328 

341

Total

6

11

342 

359

Caprock

27

15

42

Noncaprock

17

29

327 

373

Total

44

29

342 

415

1 Predevelopment recharge to indicated mid-1980's land-use area.

70

60

50

40

30

20

10

D Predevelopment 

Q Mid-1980's

LLJ <

cn Q

rr Z 

rrO

P II

O ^ rr ^

rj

Ir

21.65 23.62 27.56 35.43 49.21 68.90 78.74 98.42 137.80 157.50 177.20 216.50 255.90 275.60 

MEAN ANNUAL PRECIPITATION, IN INCHES

FIGURE 21. Estimated ground-water recharge for predevelopment and mid-1980's land use, southern Oahu.
EFFECTS OF LAND-USE CHANGES ON GROUND-WATER RECHARGE C31

turn flow of about 6.4 ft/yr. Dale (1967) estimated an 

annual return-flow rate of 3.5 ft/yr, and Mink [1980, 

cited by Giambelluca (1983, p. 104)1 estimated an annual rate of about 5.2 ft/yr. A numerical analysis of the 

southern Oahu ground-water flow system, which is part 

of the Hawaii RASA study, estimated an annual returnflow rate of about 3.6 ft/yr (Eyre and Nichols, in press). 

Giambelluca (1983, p. 105-106) discussed the differences from earlier studies and suggested that previous 

investigators may have overestimated evapotranspiration and, thereby, underestimated recharge.

The detailed studies of Giambelluca (1983,1986) represent a more rigorous approach to estimating evapotranspiration and irrigation-return flow for furrow-irrigated sugarcane than previous investigations, but the 

numerical analysis by Eyre and Nichols (in press) suggests that this more rigorous approach may not have 

yielded more accurate estimates. Giambelluca (1983, 

p. 103) concluded that evapotranspiration from sugarcane was about 4.4 ft/yr. Campbell and others (1959, 

p. 644) concluded from lysimeter studies in sugarcane

60

£ 50

QC 

01

CO

I 40 

O

£ 30 

O

 Natural land use 

o Urban land use

D 

QC

D 

Z

20

10

 Combined urban 

 and natural landuse trend line

» Urban land-use trend line

. V

° n6>

o8>

O 

O <D

Natural land-use trend line

o

20 40 60 80 100

MEAN ANNUAL PRECIPITATION, IN INCHES PER YEAR

120 140

FIGURE 22. Relation (general trends) between precipitation and estimated runoff for natural and 

urban land uses during the mid-1980's, southern Oahu.

TABLE 26. Regression equation coefficients for runoff 

for mid-1980's land use, southern Oahu

[in., inch; a and b, regression coefficients; r2, coefficient of

determination; , not applicable; >, actual value is greater

than value shown]

Mean annual

precipitation

range (in.)

1 20-130

2 130-175

3 175-200

2>200

a

0.185

.013

_

.018

b

-1.022

1.536

_

1.445

r2

0.59

.92

.91

Coefficients for equation 1. 

Coefficients for equation 2. 

3 Constant 37.5 in. used to connect power function curves.

TABLE 27. Water budget for mid-1980's land use,

southern Oahu 

[In million gallons per day]

Water-budget 

category

Applied irrigation.......

Runoff. .......................

Caprock 

area

...... 88

...... 54

...... 13

...... 87

...... 42

Noncaprock 

area

754

33

137

277

373

Total

842

87

150

364

415
C32 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

fields on the island of Maui that sugarcane evapotranspired about 6.8 ft/yr. This is 2.4 ft/yr more than estimated by Giambelluca (1983) and would reduce the estimated irrigation-return flow rate to about 4 ft/yr, 

which is close to the estimates made by previous investigators. This also would reduce recharge from furrow-irrigated sugarcane in southern Oahu by about 5 Mgal/d 

for mid-1980's acreages, of which about 2 Mgal/d was in 

the noncaprock area. For the present study, the estimates of return-flow recharge from furrow-irrigated 

sugarcane of 16 Mgal/d, of which 6 Mgal/d was to the 

noncaprock area, have a relatively small effect on the 

total mid-1980's recharge estimates for southern Oahu. 

The rates developed during previous studies would suggest recharge to caprock and noncaprock areas from furrow-irrigated sugarcane of about 8 Mgal/d.

Recharge from irrigation-return flow from drip-irrigated sugarcane cultivation similarly may be too high 

because of the estimated rate of sugarcane evapotranspiration used by Giambelluca (1983). Recharge during 

the mid-1980's in areas of drip-irrigated sugarcane cultivation in southern Oahu amounts to nearly 29 Mgal/d, 

which is an increase of 24 Mgal/d over predevelopment 

recharge rates to the same area; of this, 11 Mgal/d was 

in the noncaprock area and represents an increase of 9 

Mgal/d over predevelopment recharge rates to these areas. This is 60 percent of the estimated increase in recharge to the noncaprock area planted in sugarcane 

compared with predevelopment conditions. However, increasing sugarcane evapotranspiration by 2.4 ft/yr reduces recharge in drip-irrigated sugarcane areas by 

about 29 Mgal/d in southern Oahu and by about 11 

Mgal/d in the noncaprock area.

Total recharge for the agricultural areas of southern 

Oahu (caprock and noncaprock areas) in the mid-1980's 

was estimated to be about 73 Mgal/d (table 25). However, 

if estimated evapotranspiration by sugarcane is increased from 4.4 to 6.8 ft/yr, then recharge in these 

areas is reduced by about 34 Mgal/d to about 39 

Mgal/d. Mid-1980's recharge in sugarcane areas may be 

overestimated by a factor of two if the evapotranspiration rate for sugarcane is more accurately estimated to 

be 6.8 ft/yr. It is, however, beyond the scope of this 

study to estimate the appropriate evapotranspiration 

rate for sugarcane. This is a problem that deserves 

serious consideration because of the resulting effects on 

ground-water recharge estimates.

NORTH-CENTRAL OAHU

A water budget for mid-1980's conditions for north-central Oahu was estimated by using the equations and coefficients for recharge and runoff for southern Oahu 

with the appropriate precipitation zones and land-use 

areas given in table 28. An estimated rate of applied irrigation water for sugarcane was based on the rates of 

T.W. Giambelluca (Water Resources Research Center, 

University of Hawaii, written commun., 1985) for 

southern Oahu. No change was made to estimated recharge or runoff for nonagricultural areas because the 

only urbanized area in north-central Oahu has a mean 

annual precipitation of less than 30 in. Recharge in areas of irrigated sugarcane was estimated by using the 

same estimated recharge rates for southern Oahu. As

TABLE 28. Areas of agricultural and nonagricultural land use during the mid-1980's, north-central Oahu

[in., inch; mi", square mile; , not applicable]

Caprock area

Irrigated sugarcane

JVLCHn cinniicii 

precipitation Drip 

(in.) (mi2 )

31.50 0.11

35.43 .99

49.21 

68.90 

78.74 

98.42 

137.08 

177.20 

216.50 

255.90 

275.60 

Total ........... 1.10

Furrow

(mi2)

0.12

2.27

2.39

Nonagricultural 

(mi2)

0.80

7.66

1.34

9.80

Noncaprock area

Irrigated sugarcane

Total

(mi2)

1.03

10.92

1.34

13.29

Drip 

(mi2)

_

5.54

6.75

12.29

Furrow cultural 

(mi2) (mi2)

_ _

1.07 10.11

 30.77

 26.75

 .56

 9.92

 4.75

 3.77

 2.63

 3.32

 2.12 

1.07 94.70

Total

(mi2 )

_

16.72

37.52

26.75

.56

9.92

4.75

3.77

2.63

3.32

2.12 

108.06
EFFECTS OF LAND-USE CHANGES ON GROUND-WATER RECHARGE C33

with the estimates of recharge in irrigated sugarcane 

areas of southern Oahu, the estimates for north-central 

Oahu also may be too high. The estimated water budget given in table 29 can be compared with predevelopment recharge given in table 17.

TABLE 29. Estimated water budget for mid-1980's land use, 

north-central Oahu 

[In million gallons per day]

Water-budget Caprock Noncaprock Total 

category area area

Precipitation.......................... 23 411 434

Applied irrigation... ............... 12 47 59

Runoff................................... 2 62 64

Evapotranspiration................ 19 194 213

Recharge............................... 14 202 216

The total recharge for 1980's land use of 216 

Mgal/d represents an increase of 32 Mgal/d over predevelopment recharge, all of it from irrigation-return 

flow. Recharge to the hydrologically important noncaprock area during the mid-1980's was 202 Mgal/d compared with 180 Mgal/d for predevelopment conditions, 

which is an increase of 22 Mgal/d. Evapotranspiration 

increased a little more than 15 percent over predevelopment rates from 185 to about 213 Mgal/d. Increases 

in recharge and evapotranspiration account for all estimated applied irrigation water (tables 17, 29).

DISTRIBUTION OF MID-1980'S GROUND-WATER RECHARGE

The distribution of ground-water recharge for mid1980's land use for the island of Oahu is shown in figure 23 and given in table 30. The influence of irrigated 

agriculture is noticeable in southern and north-central 

Oahu, where areas of significantly higher recharge are 

surrounded by areas of low recharge. In the natural 

land-use areas along the Koolau Range, the distribution 

of recharge generally follows the pattern of rainfall distribution (fig. 2). 

The distribution of ground-water recharge and underflow for the southern and north-central areas of Oahu 

for mid-1980's land use is shown in figure 24. This distribution can be compared with the predevelopment distribution shown in figure 16. Recharge and underflow 

to the noncaprock areas of the Pearl Harbor groundwater area for mid-1980's land use is estimated to be

283 Mgal/d, which is an increase of about 22 Mgal/d

over predevelopment conditions and the result of irrigation-return flow. Recharge and underflow to the noncaprock section of the Ewa ground-water area increased 

by 9 Mgal/d from predevelopment conditions. Similarly, 

recharge and underflow to the noncaprock areas of 

north-central Oahu increased by 22 Mgal/d from predevelopment conditions. Recharge to the caprock areas 

of southern and north-central Oahu was increased by 

irrigation-return flow to about 57 Mgal/d, which is an 

increase of about 32 Mgal/d above predevelopment 

land-use conditions. Ground- water recharge to windward Oahu basins, the Waianae area, and southeastern Oahu was the same for mid-1980's and predevelopment land uses.

TABLE 30. Distribution of ground-water recharge for

mid-1980's land use, island of Oahu 

[In million gallons per day; , no data or not applicable]

Area Caprock Noncaprock Total 

(fig. 15) area area

1 3.8 3.8 

2 193.7 193.7 

3 0.2 36.0 36.2 

4 .8 32.0 32.8 

5 4.4 20.0 24.4 

6 9.1 6.9 16.0 

7 .2 7.3 7.5 

8 1.3 1.3 

9 9.5 9.5 

10 7.7 7.7 

11 31.7 31.7 

12 11.5 11.5 

13 17.0 17.0 

14 64.1 64.1 

15 49.6 49.6 

16 31.3 31.3 

17 .2 2.4 2.6 

18 -5 .5 

19 .7 .7 

20 11.2 11.2 

21 5.8 5.8 

22 9.0 14.8 23.8 

23 21.7 152.2 173.9 

24 21.4 21.4 

25 4.0 25.2 29.2 

26 1.0 16.1 17.1 

27 3.8 17.4 21.2 

28 2.0 10.2 12.2 

29 .1 7.1 7.2 

30 .4 8.9 9.3 

31 2.4 2.4 

32 .5 .5 

33 .2 .2 

34 1.6 1.6 

35 .3 .3 

36 -0 .0 

Total............... 56.9 822.3 879.2
C34 REGIONAL AQUIFER-SYSTEM ANALYSIS-OAHU, HAWAII

158°

Kahuku Point

024 MILES

i i I i i

024 KILOMETERS

21 "SO1

21° 20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data. 1:24,000,1983, Albers equal area 

projection, standard parallels 21 C 15' and 21°45', 

central meridian 157°59'

EXPLANATION

GROUND-WATER RECHARGE, IN INCHES PER YEAR 

Greater than or equal to 150 

Greater than or equal to 100 and less than 150 

Greater than or equal to 50 and less than 100 

Greater than or equal to 25 and less than 50 

Greater than or equal to 10 and less than 25 

Less than 10

FIGURE 23. Distribution of ground-water recharge for mid-1980's land use, island of Oahu.
EFFECTS OF LAND-USE CHANGES ON GROUND-WATER RECHARGE C35

158°

Kahuku Point

4 MILES

024 KILOMETERS

21° 30'

21° 20'

Barbers 

Point

Base modified from U.S. Geological Survey 

digital data, 1:24,000,1983, Albers equal area 

projection, standard parallels 21°15' and 21 C45', 

central meridian 157°59'

68

25

EXPLANATION

COASTAL PLAIN (caprock)

MAJOR GEOHYDROLOGIC BOUNDARY

SUBORDINATE GEOHYDROLOGIC BOUNDARY

DIRECTION OF UNDERFLOW

UNDERFLOW, IN MILLION GALLONS PER DAY

RECHARGE BY INFILTRATION OF PRECIPITATION 

AND APPLIED IRRIGATION, IN MILLION 

GALLONS PER DAY

FIGURE 24. Distribution of recharge to and discharge from the noncaprock areas for mid-1980's land use,

southern and north-central Oahu.
C36 REGIONAL AQUIFER-SYSTEM ANALYSIS - OAHU, HAWAII

Estimated ground-water recharge for predevelopment 

and mid-1980's land uses for noncaprock areas and the 

total areas of the five major study areas of Oahu are 

summarized in table 31. Predevelopment and mid-1980's 

water budgets for southern and north-central Oahu are

summarized in table 32. These two areas are the only 

areas where the budgets were significantly affected by 

land-use changes. Table 33 summarizes the mid-1980's 

water budget for Oahu.

TABLE 31. Estimated predevelopment and mid-1980's ground-water recharge

to noncaprock areas and to the total areas, island of Oahu

[In million gallons per day]

Predevelopment

Study 

area

Southern Oahu1 ..........

Southeastern Oahu .....

Windward Oahu.........

North-central Oahu2 ...

Waianae area..............

Total ................... ......

Noncaprock 

area

........ 341

........ 19

........ 198

........ 180

........ 32

........ 770

Total 

area

359 

19 

198 

184 

32 

792

Mid-1980's

Noncaprock 

area

373 

19 

198 

202 

32 

3824

Total 

area

415 

19 

198 

216 

32 

3880

Includes southern Schofield area. 

Includes northern Schofield area 

Total does not agree with total given in table 30 because of rounding off the individual area totals.

TABLE 32. Predevelopment and mid-1980's water budgets for southern and north-central Oahu 

[In million gallons per day; , not applicable]

Water-budget 

category

Precipitation..................

Applied irrigation..........

Runoff...........................

Evapotranspiration ........

Recharge .......................

Southern Oahu

Predevelopment

842

............... 127

............... 356

359

Mid-1980's

842 

87 

150 

364 

415

North-Central Oahu

Predevelopment

434

65

185 

184

Mid-1980's

434 

59 

64 

213 

216

TABLE 33. Mid-1980's water budget, island of Oahu 

[In million gallons per day; , not applicable]

Water-budget 

category

Precipitation.....................

Applied irrigation. ........... .

Runoff..............................

Evapotranspiration ...........

Recharge ..........................

Southern 

Oahu

........... 842

........... 87

........... 150

........... 364

........... 415

Southeastern 

Oahu

62

7

36

19

Windward 

Oahu

536

100

238

198

NorthCentral Oahu

434

59

64

213

216

Waianae

103

8

63

32

Oahu

1,977

146

329

914

880
SUMMARY AND CONCLUSIONS C37

SUMMARY AND CONCLUSIONS

Detailed water budgets were developed by previous 

investigators for southern and southeastern Oahu and 

for parts of windward Oahu. The methods used for 

these budgets have been simplified, incorporated into 

a geographic information system, and applied in a consistent manner to all areas of the island to provide an 

estimated water budget for Oahu for predevelopment 

and mid-1980's land uses. Predevelopment recharge to 

the ground-water system of Oahu was estimated to be 

792 Mgal/d, including recharge to the noncaprock part 

of the island estimated to be 770 Mgal/d.

The effects of changes in land use on ground-water 

recharge were evaluated for southern and north-central 

Oahu, where significant changes have been caused by 

urbanization and agriculture. Urbanization in southern 

Oahu has led to an increase in estimated runoff of about 

18 percent, or 23 Mgal/d. The rate of predevelopment recharge to areas of southern Oahu having nonagricultural 

land uses during the 1980's would have been about 328 

Mgal/d; recharge to these areas during the mid-1980's 

was estimated to be about 327 Mgal/d. Irrigated agriculture has had a much greater effect on recharge, and 

has led to an estimated increase of about 56 Mgal/d for 

the entire area over predevelopment recharge. However, 

the estimated recharge from irrigated sugarcane determined by earlier studies may be overestimated by a factor of two; more detailed investigations are needed to resolve this issue.

A comparison of the predevelopment and the mid1980's water budgets for Oahu indicates an increase in 

runoff of 22 Mgal/d as a result of increased urbanization in the 1980's. The estimated increase in evapotranspiration of 36 Mgal/d in the 1980's is a result of 

irrigated agriculture in the southern and north-central 

areas. Islandwide, a substantial increase in estimated 

recharge of 88 Mgal/d in the 1980's was similarly the 

result of irrigated agriculture. The mid-1980's island 

ground-water recharge was estimated to be 880 

Mgal/d, of which 822 Mgal/d recharged the noncaprock 

area.

REFERENCES CITED

Campbell, R.B., Chang, J.H., and Cox, B.C., 1959, Evapotranspiration of sugarcane as measured by field lysimeters in relation 

to climate: International Society Sugarcane Technologists, 10th 

International Congress, Proceedings, p. 637-649.

Dale, R.H., 1967, Land use and its effects on the basal water supply, 

Pearl Harbor area, Oahu, Hawaii, 1931-65: U.S. Geological 

Survey Hydrologic Investigations Atlas HA-267, 2 sheets, scale 

1:62,500.

___1978, Aground-water inventory of the Waialua basal-water

body, Island of Oahu, Hawaii: U.S. Geological Survey Open-File

Report OF 78-24, 71 p. 

Dale, R.H., and Takasaki, K.J., 1976, Probable effects of increasing

pumpage from the Schofield ground-water body, Island of Oahu,

Hawaii: U.S. Geological Survey Water-Resources Investigations

Report 76-47, 45 p. 

Ekern, PC., and Chang, J.H., 1985, Pan evaporation in Hawaii, 1894-

1970: State of Hawaii, Department of Land and Natural

Resources Report R74, 171 p. 

Eyre, PR., Ewart, C.J., and Shade, P.J., 1986, Hydrology of the

leeward aquifers, southeast Oahu, Hawaii: U.S. Geological

Survey Water-Resources Investigations Report 85-4270, 75 p. 

Eyre, PR., and Nichols, W.D., in press, Regional analysis of the

southern Oahu ground-water flow system, Hawaii: U.S.

Geological Survey Water-Resources Investigations Report 95-

4256. 

Foote, D.E., Hill, E.L., Nakamura, Sakuichi, and Stephens, Floyd,

1972, Soil survey of islands of Kauai, Oahu, Maui, Molokai, and

Lanai, State of Hawaii: U.S. Department of Agriculture, Soil

Conservation Service, 232 p. 

Giambelluca, T.W., 1983, Water balance of the Pearl Harbor-Honolulu

basin, Hawaii, 1946-1975: University of Hawaii, Water

Resources Research Center Technical Report 151, 151 p. 

___1986, Land-use effects on the water balance of a tropical island:

National Geographic Research, v. 2, no. 2, p. 125-151. 

Giambelluca, T.W, Nullet, M.A., and Schroeder, T.A., 1986, Rainfall

atlas of Hawaii: State of Hawaii, Department of Land and Natural

Resources Report R76, 267 p. 

Mink, J.F., 1980, State of the groundwater resources of southern

Oahu: City and County of Honolulu, Board of Water Supply, 80 p. 

Rosenau, J.C., Lubke, E.R., and Nakahara, R.H., 1971, Water

resources of north-central Oahu, Hawaii: U.S. Geological Survey

Water-Supply Paper 1899-D, 40 p. 

Stearns, H.T., 1939, Geologic map and guide of the Island of Oahu,

Hawaii: Hawaii Division of Hydrography Bulletin 2, 75 p., map

scale 1:62,500. 

Stearns, H.T. and Vaksvik, K.N., 1935, Geology and ground-water

resources of the island of Oahu, Hawaii: Hawaii Division of

Hydrography Bulletin 1, 479 p. 

Takasaki, K.J., 1971, Ground water in the Waianae District, Oahu,

Hawaii: U.S. Geological Survey Hydrologic Investigations Atlas

HA-358, 2 sheets, scale 1:62,500. 

___1978, Summary appraisals of the Nation's ground-water

resources; Hawaii region: U.S. Geological Survey Professional

Paper 813-M, 29 p. 

Takasaki, K.J., Hirashima, G.T., and Lubke, E.R., 1969, Water

resources of windward Oahu, Hawaii: U.S. Geological Survey

Water-Supply Paper 1894,119 p.

Takasaki, K.J., and Mink, J.F., 1985, Evaluation of major dikeimpounded ground-water reservoirs, island of Oahu with a

section on Flowing hydraulics in dike tunnels in Hawaii, by J.F.

Mink: U.S. Geological Survey Water-Supply Paper 2217, 77 p. 

Takasaki, K.J., and Valenciano, Santos, 1969, Water in the Kahuku

area, Oahu, Hawaii: U.S. Geological Survey Water-Supply Paper

1874, 59 p. 

U.S. Geological Survey, 1970,1968 Water resources data for Hawaii

and other Pacific areas: U.S. Geological Survey, 296 p. 

___1971,1969 Water resources data for Hawaii and other Pacific

areas: U.S. Geological Survey, 289 p. 

___1972,1970 Water resources data for Hawaii and other Pacific

areas: U.S. Geological Survey, 299 p. 

Visher, F.N., and Mink, J.F., 1964, Ground-water resources in

southern Oahu, Hawaii: U.S. Geological Survey Water-Supply

Paper 1778, 133 p.
C38 REGIONAL AQUIFER-SYSTEM ANALYSIS - OAHU, HAWAII

Wentworth, C.K., 1951, Geology and ground-water resources of the 

Honolulu-Pearl Harbor area, Oahu, Hawaii: City and County 

of Honolulu, Board of Water Supply, 111 p.

Zones, C.P., 1963, Preliminary report on the ground-water resources 

of the Waianae area, Oahu, Hawaii: State of Hawaii, Department of Land and Natural Resources Circular C16, 12 p.

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the U.S. Geological Survey, 582 National Center, Reston, VA 

20192.

Note. Prices of Government publications listed in older 

catalogs, announcements, and publications may be incorrect. Therefore, the prices charged may differ from the prices in catalogs, 

announcements, and publications.
ISBN Q-bQ7-flblQ3-7

780607''861037l