Title: Groundwater Safe Yield Using Low Flow Frequency Analysis
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Permanent Link: http://ufdc.ufl.edu/WL00001858/00001
 Material Information
Title: Groundwater Safe Yield Using Low Flow Frequency Analysis
Physical Description: Book
Language: English
Spatial Coverage: North America -- United States of America -- Florida
Abstract: Groundwater Safe Yield Using Low Flow Frequency Analysis By: Katherine A. Popko and Young S. Yoon -Boyle Engineering Company Presented at 1983 ASCE National Specialty Conference March 14-16, 1983 Tampa, Florida and National Water Well Association Western Regional Conference
General Note: Box 9, Folder 7 ( SF-Safe Yield - 1956-1995 ), Item 11
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
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Bibliographic ID: WL00001858
Volume ID: VID00001
Source Institution: Levin College of Law, University of Florida
Holding Location: Levin College of Law, University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Full Text




Katherine A. Popko
Young S. Yoon
Boyle Engineering Corporation

Presented at the
1983 ASCE National Specialty Conference
Tampa, Florida
March 14-16, 1983
National Water Well Association Western Regional Conference
San Diego, California
October 24-26, 1983


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Katherine Ann Popkoi and Young Sub Yoon2

This analysis method was developed for a groundwater availability study in a

rural San Diego County, California, water district. Majestic Pines is a water

district governed by the San Diego County Board of Supervisors. The district

serves Kentwood-in-the-Pines and Whispering Pines, quiet mountain communities

located approximately one mile east of Julian. The customers served by the

district are completely dependent upon groundwater for their water supply.

Lengthy droughts common to most of Southern California occur in this area. If

the basin were pumped at a rate based on standard methods of estimating safe

yield, the groundwater supply could be severely depleted under drought

conditions. Subsequently, groundwater levels could decline below economic

pumping depths. Therefore, a low-flow frequency analysis method was developed

to estimate annual pumpage that would not result in excessive water level

declines during selected droughts. This paper addresses five questions:

What is safe yield?

How is it usually estimated?

Why use low-flow frequency analysis?

How is this method applied?

-- Who should use. this analysis?

IAssociate Civil Engineer, Boyle Engineering Corporation
San Diego, California

2Director of Hydrologic Services, Boyle Engineering Corporation
Golden, Colorado


Safe yield is the quantity of naturally occurring groundwater that can be

withdrawn over the long term without causing undesirable effects on a basin.

These effects are the direct result of lowered groundwater levels caused by

depleting groundwater in storage. Impacts to be avoided can be economic, such

as higher pumping costs and the need to deepen wells or replace pumps. They

can be legal or political, for example, interference with existing water

rights or modification of the groundwater regime in a neighboring basin. And

they can be environmental, such as sinkhole formation, baseflow depletion,

land subsidence, and salt water intrusion.

The term safe yield is currently falling into disfavor because "a never-

changing quantity of available water depending solely on natural water sources

and a specified configuration of wells is essentially meaningless from a

hydrologic standpoint" (Todd, 1980). In other words, there is no number that

will always prevent depletion of groundwater in storage, since hydrologic

conditions and basin management practices continually change. "Safe yield"

recently has been replaced by terms such as "perennial yield," "potential

sustained yield," and "maximum basin yield." But whatever the term, the

concept of a maximum withdrawal allowable without causing undesirable effects

remains the same.

There are many ways to estimate safe yield, but most require a substantial

amount of data. Estimates of safe yield are often based on long-term average

annual recharge if data necessary for other methods are not available.

Recharge can be computed using stream baseflow when suitable records exist.

Where precipitation is the principal source, recharge can be calculated by

considering the natural processes that divert the rainfall before it reaches

*W/. t

the aquifer system. The most important of these are soil moisture retention,

surface runoff, and evapotranspiration. This is a common method of computing

recharge in San Diego County, where few streams are perennial, few streamflow

records are lengthy, and relatively little hydrologic data on individual

basins are available.

When estimating recharge, it must be remembered that all of the water

recharging a basin is not available for pumping. Before it can be tapped by

wells, some groundwater will leave the basin. It will be consumed by

phreatophytes, seep to a river, or flow downgradient to another groundwater

basin. However, in the typical long-term average annual recharge method of

computing safe yield, these losses are not considered. Furthermore, the

effects of drought conditions are not accounted for in this safe yield

analysis method. But drought effects can be significant, because when

recharge becomes negligible, pumpage at rates equivalent to average annual

recharge can severely deplete a basin. Our modified method of estimating safe

yield using low-flow frequency analysis was developed to overcome these

deficiencies in the average annual recharge method.

The first step in this analysis was to calculate recharge expected during

droughts with various return periods and durations. In our study, annual

recharge was calculated for each water year with historical precipitation

records (1906 to 1980). This time span included a lengthy dry period (1959 to

1977) when average annual precipitation was 21 inches, and a wet period (1978

to 1980) when average annual precipitation exceeded 41 inches. Monthly

infiltration was calculated by subtracting monthly runoff and evapotran-

spiration from the sum of monthly precipitation and antecedent soil moisture.

If the infiltration was greater than the soil's water holding capacity,

recharge was calculated as infiltration minus water holding capacity. If

infiltration was less than water holding capacity, recharge was assumed to be

zero. A value of maximum probable monthly recharge was used so recharge would

not be overestimated when rainfall was very high. This value was based on

aquifer effective porosity and maximum probable monthly change in groundwater

levels. Annual recharge for each water year was computed as the sum of

recharge from October to September. A graphical low-flow frequency analysis

method described by Hudson and Roberts (1955) was then applied, using recharge

instead of streamflow. The return period for each value of annual recharge

was calculated using the Weibull formula:

T (n + 1)

T return period (years)

n number of years of record

m order number of events with the lowest event

in the array being order number 1.

Historical extreme values were plotted on extreme probability paper with

recharge on the logarithmic scale, and a straight line was drawn through the

points. As Figure 1 shows, in our results, cumulative groundwater recharge

during the 100-year drought would be about 10 inches for a drought lasting 10

years, and 44 inches for a drought lasting 20 years.

The next step of the analysis was to estimate groundwater level declines that

would occur during selected droughts if given quantities of water were


2 3 4 5

10 25 50


* .

withdrawn from the basin. The amount withdrawn could be current pumpage plus

natural outflow, long-term average annual recharge, or some future pumpage

plus natural outflow. In our case, the withdrawal quantity tested was a long-

term average annual recharge of 4.3 inches/year. The 100-year drought was

selected for our return period. A mass curve of recharge versus drought

duration was plotted. The points for that drought were obtained from the

recharge frequency curve. Cumulative withdrawals were then plotted as a

straight line on the mass curve. Figure 2 illustrates our results. The

greatest distance between the line and the curve represents the maximum water

deficiency in inches. This was converted to a groundwater level decline using

effective porosity of the aquifer. As shown in Figure 2, the maximum

groundwater deficiency was 41 inches. This is equivalent to a groundwater

level decline of 230 feet, using an effective porosity of 0.015.

The last step in this analysis was to evaluate the significance of the

resulting groundwater level decline. Of the undesirable economic, legal, and

environmental effects discussed previously, economic considerations were the

most significant in our relatively small and isolated inland basin. We found

a 230-foot decline would be excessive, considering power costs and existing

well depths. After testing other values, a safe yield of 550-acre-feet per

year was determined. This would result in a water level decline of approxi-

mately 120 feet for the 100-year drought. This safe yield was approximately

50Z less than long-term average annual recharge derived from 75 years of


The large difference in safe yield calculated with the average annual recharge

method versus low-flow frequency analysis illustrates the significant impact



80 -
0 41 1

0 60


Z 40 --



5 10 15 20 25


of drought conditions. Therefore, the low-flow frequency analysis method

should be considered for any basin subject to droughts. This method also has

a flexibility lacking in the average annual recharge method. Many withdrawal

rates can be evaluated quickly, so the impact of varying management practices

can be tested. Also, the hydrologist is not limited to one basin yield that

will supposedly avoid problems. Instead, a range of alternative withdrawal

rates that produce a range of groundwater level declines under a given set of

circumstances can be presented. Then the various members of a management team

can evaluate the impacts and decide on the best policy.


1. Hudson, H.E., Jr., and W.J. Roberts, 1955. 1952-1955 Illinois Drought
with Special Reference to Impounding Reservoir Design. Illinois State
Water Survey Bulletin 43, pp. 25-31.

2. Todd, D.K., 1980. Groundwater Hydrology. John Wiley & Sons, New York,
New York, p. 363.

woLile Enqlneewrn Corporatwon

1501 Quail Street consulting enaUneers I architects
P.O. Box 7350 714/476-3400
- ~Mmpet PasakONA 92658-7350 TeAlx 65561

June 7, 1988

Deborah A. Locklair
*Blain & Cone, P.A.
202 Madison St.
Tampa, FL 33602

Dear Ms. Locklair:

In response to your letter of May 31, enclosed are reprints of the following
technical articles:

Agricultural Master Planning and Water Quality
Cypress Wetlands for Tertiary Treatment
Evaluation of Agricultura Irrigation Projects Using Reclaimed
Municipal Wastewater
Modeling for Land Application of Wastewater
Water Conserv II Innovative Solution to Effluent Disposal by
Orlando and Orange County
Water Hyacinths for Wastewater Treatment
Water Resource Management in Orlando, Florida
9 Conjunctive Use of Surface and Ground Waters The California
e Financing of Water Projects
e Groundwater Safe Yield Using Low Flow Frequency Analysis
e Need for a Hydrogeologist During Well Construction
Regional Water Supply and Distribution Planning for Orange County
e Water Availability The Ultimate Development Limitation

It is our intent to offer pertinent technical article reprints in each issue
of Boyle's newsletter. New articles will be offered throughout the year, so
review the reprint list carefully.

Further information on the above subjects is available by contacting Boyle's
Orlando office. Thank you for your interest.


KathyO s s
Corporate Editor


cc: K. L. Prime, Boyle/OR

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