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HomeMy WebLinkAbout4 Nimbus Engineering Agenda Item # y 1 DONNER 94 Public Utility Distr ic .. . .... Memorandum To: Board of Directors From: Peter Holzmeister Date: March 16, 2001 Subject: Presentation by Nimbus Engineering Representatives from Nimbus Engineering will be present at the Board meeting to give brief presentations on two matters: a) They will give us an update on the development of the new Mantis Valley well. The developmental and test pumping have been underway all this week. We should have an idea of the projected yield of the finished well by Wednesday evening. b) They will give us a status report on the evaluation of the availability of water in the ground water basin. I have had discussions with Nimbus staff during the past week regarding the term safe yield. It is a term that is in some disfavor in the hydrogeology field. Hydrogeologiss now use terms such as sustainable yield or managed yield. I have attached a ouple of technical papers that discuss these concepts. I would like us to spend a few finutes on Wednesday evening discussing this matter, since it has an impact on how we interpret and use the ground water study. 3 1 WATER-RESOURCES ENGINEERING a THIRD EDITION i Bering Ray K. Linsley tors Professor Emeritus of Hydraulic Engineering Stanford University Partner, Linsley, Kraeger Associates Joseph B. Franzini Professor Of Civil Engineering Associate Chairman, Department of Civil Engineering Optimization Stanford University i - :use 'nciptes and McGraw-Hill Book Company a ` New York SC Louis San Francisco k Johannesbur Auckland Bogota D eldort S London Madrid Mexico Montreal New Delhi Panama Paris Sao Paulo Singapore Sydney Tokyo Toronto )UT THE book repre: from the u: :e manages ly with appl offers a k ng, Plannin, uter resourc Phasis is pi ns for exec r manner, ; e between of engineer Chapter 21) 3, 4, 5 and ngineering as often tre; here—are u: rs understat Of their pre WATER-RESOURCES ENGINEERING nentals. In : rojects are I Copyright © 1979 1972 1964 by McGraw-Hill, Inc All rights reserved. new editiot Printed in the United States of America No part of this publication Previous ec may be reproduced,stored in a retrieval system,or transmitted,in any orates upd; form or by any means,electronic,mechanical,photocopying, recording,or 1 data or a otherwise, without the prior written permission of the publisher. I hydrology ad. Extensiv 234567890 DODO 7832109 of digital c, This book was set in Times Roman. The editor was S; this empf Julienne V. Brown, the production supervisors f the chapb were Gayle Angelson and Leroy A. Young. 'hapter 21 c R. R. Donnelley& Sons Co,was the printer and binder.been extt better cot in a Singh 'r Supply S. Library of Congress Cataloging in Publication Data Je and V Linsley, Ray K, comph chobanoc Water-resources engineering. ' opts in y (McGraw-Hill series in water resources and environ- ar tr6atrn mental engineering) Originally published in 1955 under title: Elements of and SI U hydraulic engineering. .e aPPro.. Bibliography: P. ts, and to Includes indexes, its. Num, I. Hydraulic engineering. resoures S are inc development. I. Franzim,Joseph Bv,jo joint author. H. Title TC145155 1979 627 78-4498 ISBN 0-07-037965-3 i 'MOM GROCNDwdTER I01 4 ed. remote from the shot point.The velocity of the shock wave depends on the type of ell. formation and the presence of water. From the differences in the indicated veloci- I aid -es to the several geophones it may be possible to estimate the depth to the water us- cle or to the interface between formations. Resisriciry siirceys make use of the ent that the depth of penetration of current between two electrodes m the soil l .Urfj— increases as the electrode.spacing mereases. It is possible to estimate the { _ to alive resistivity of formations at different depths by measuring the current flow ac- i4ith various electrode spacings. Since water increases the conductivity of soil or indicated by a decrease it, resistivity. a it rock, the presence of groundwater may be the Both seismic and resistivity surveys should be made and interpreted by persons trained in the work. Neither method specifically locates groundwater but merely indicates discontinuities which may bound an aquifer. With a few test holes as ite- control points, large areas may be surveyed rapidly by seismic or resistivity eve- ;nethods. ins Geophysical methods are also useful in logo ng finished wells. Electrical logs hly iticlude measurement of resistivity between a pair of electrodes lowered into the of urcased well and the measurement of the self-potential(existing potential field)in ear die well. These data are useful in relating strata penetrated by one well with the ;rly same strata in another well. Resistivity data also give an indication of the chemical nce quality of the groundwater since dissolved salts reduce the resistivity of the water. ion Electrical logs in oil wells are often useful in studies of groundwater. _ing .ith YIELD OF GROUNDWATER rial Flo- 417 Safe yield Withdrawal of water from the ground at rates greater than those iter at which it is replenished results in lowering the water table and an increase in ake pumping cost. In coastal areas an overdraft may reverse the normal seaward t to gradient of the water table and permit salt water to move inland and contaminate the aquifer. An aquifer undisturbed by pumping is in approximate equilibrium. in, Water is added by natural recharge and removed by natural discharge.In years of ing aoundant water the water table rises and in years of drought the water level he iechnes, but rates of recharge and discharge tend to remain in approximate it if oalance.When a well is put into operation,new conditions are created.Water may 'oe removed from storage in the aquifer or mined in the sense that other minerals are mined. The deoression in the water table caused by the well may induce it is Increased recharge or may decrease natural discharge. The concept of safe yiei( t. It nas been used to express the quantity of groundwater which can be withdrawn ndi- without impairing the aquifer as a water source, causing contamination, or creat- cv a ne economic problems from increased pumping lift Actually,safe yield cannot be eter 'ginned in [Wily practical and general terms. The location of wells with respect to for Teas of recharge and discharge, the character of the aquifer, the potential sources test � pollution, and many other factors are involved in estimates of the maximum asibie withdrawal from an aquifer. A number of closely spaced wells will cause and 'such more rapid decline of local water levels than the same number of wells more Ines Videly dispersed. 102 WATER RESOURCES ENGINEERING Determination of safe yield is a complex problem in hydrology, geology, and economics for which each aquifer requires a unique solution. The general type cases are: — — sc.+ 1. Aquifers in which safe yield is limited by the availability of water for recharge 2. Aquifers in which safe yield is limited by the transmissibility of the aquifer 3. Aquifers in which safe yield is limited by potential contamination - The first case is commonly encountered in and regions.The groundwater` may be visualized as a large reservoir which is drawn down to supply during periods of low recharge. Lowering of the water table during dry periods is not evidence that the safe yield has been exceeded,but a continuing decline during rainy periods warns of excessive withdrawals. The safe withdrawal from such a groundwater reservoir is equal to the annual recharge less the unavoidable natural discharge. Thus, Safe yield = P — R— (421) � r i where P and E,,, are the mean annual precipitation and evapotranspiration, f respectively, from the area tributary to the aquifer, R is the mean annual runoff from the tributary area, and G,is the net mean annual subsurface discharge from drat it the aquifer. The transmissivity of aquifers may be so low that although ad equate water is rescn i undo' f available this water does not move toward the wells fast enough to permit its full Surt;Ic utilization. Lowering the water table may increase the gradient from the recharge as a r t area and permit greater flow to the wells. The safe yield of such an aquifer is t determined not by the availability of water but by the rate at which water can be A t elide problem, nae delivered to the well.This problem is sometimes referred to as a pip p t the w r since it is analogous to a city supplied by a large reservoir but with an inadequate wclls t, pipeline. the must be planned in such a groundwater is possible, the layout a way that this the well rivers r, Where contamination of e field, the rates of use, and the types of wells , F conditions permitting contamination cannot develop. All three cases offer several a�h:.'j. t possible values of safe yield depending upon the physical situation and the atrcx: (d methods used to collect the groundwater. Safe yield is a concept which can be h given quantitative significance only when all controlling conditions are defined. whck G he u e w 418 Artificial recharge If the rate of recharge of a reservoir-type aquifer is in- �n ar creased, the safe yield is also increased. If an aquifer of low transmissibility can be In A o recharged close to the point of withdrawal, the safe yield may also be increased. tkm ti There are several advantages in storing water underground.`The cost of recharge the I sI may be less than the cost of equivalent surface reservoirs.The aquifer serves as a rr tii � 4 ' O. E. Meinzer, General Principles of Artificial Ground-water Recharge, Econ. Geol., Vol.41, C apl. pp. 191-20i, May, 1946. it �I mcGRAW-HII..L BOOK COIMPANY j New York St. Louis San Francisco Dusseldorf Johannesburg Kuala Lumpur London Mexico Montreal New Delhi Panama Paris Sao Paulo Singapore Sydney Tokyo Toronto RAY K. LP7NSLEY, JR. Professor of Hydraulic Engineering Stanford University Chairman,Hydrocomp, Inc. MAX A. KOHLER Consulting Hydrologist Formerly Associate Director,Hydrology U.S. National Weather Service JOSEPH L. H. PAULHUS Consulting Hydrometeorologist Formerly Chief, Water Management Information Division U.S. National Weather Service Hydrology for EngMeerS SECOND EDITIOti This book was set in Times New Roman. The editors were B.J.Clark and M. E. Margolies; the cover was designed by Pencils Portfolio,Inc.; the production supervisor was Dennis J. Conroy. New drawings were done by J&R Services,Inc. Kingsport Press,Inc.,was printer and binder. Library of Congress Cataloging in Publication Data Linsley,Ray K. Hydrology for engineers. (McGraw-Hill series in water resources and environ- mental engineering) Includes bibliographies. 1. Hydrology. I. Kohler, Max Adam,date joint author. if. Paulhus,Joseph L.H.,joint author. III. Title. GB661.1,53 1975 551.4'8 74-9922 ISBN 0-07-037967-X HYDROLOGY FOR ENGINEERS Copyright© 1958, 1975 by McGraw-Hill,Inc.All rights reserved. Printed in the United States of America.No part of this publication may be reproduced, stored in a retrieval system,or transmitted,in any form or by any means, electronic,mechanical,photocopying,recording,or otherwise, without the prior written permission of the publisher. 78910 KPKP 7832109 - SUBSURFACE WATER 213 Y ne that adds ^--o—� .t of the real r t for the real ell from that 1 water table without the 2 ual and the T— ons first set oss which no i .nner. More - FIGURE 613 ultiple image Unless the Grid notation for Eq. (6 Ir) . X j . the assump- P i where a is the grid size. This can also be solved with a digital computer [21] and without constructing an elaborate analog. If repeated solutions are III f single welts desired over a long period of time, the analog may be the most effective -quires more device I£ a relatively limited analysts is expected to suffice, the digital consisting of computer will generally prove more efficient. Grid sizes for both analog and F is often con- digital solutions may vary from 100 ft(30 m)to 10,000 ft(3000 m), depending on the nature of the problem. The solutions can be expected to be as precise ms [I]. The rr the viscous as the aquifer description used. Aquifer properties are required at each grid point (thickness, permeability, and coefficient of storage). I£ they are not v i a digital or accurately defined, the solution will be in error. ) , 'resistors and ntial: perme- ate analogs can indicate POTENTIAL OF A GROUNDWATER RESERVOIR the effect of A basic Problem in engineering groundwater studies is the question of the permissible rate of withdrawal from a groundwater basin. This quantity. ;n of ground- commonly called the safe yield, is defined by Meinzer F141 as "the rate at which water can be withdrawn for human use without depleting the supply to such an extent that withdrawal at this rate is no longer economically (6-16) feasible." Many other definitions of safe yield have been suggested, and 't a c-native terms such as sustained yield, feasible rate of withdrawal, and 3 this becomes 'iptimum yield have been proposed. The concept of safe yield has received onsiderable criticism. Kazmann [22] has suggested that it be abandoned because of its frequent interpretation as a permanent limitation on the permissible withdrawal. Safe yield must be recognized as a quantity deter - (6 17) 'dined for a specific set of controlling conditions and subject to change as a suit of changing economic or physical conditions. It should also be 214 HYDROLOGY FOR ENGINEERS recognized that the concept can be applied only to a complete groundwater ination of the groundwater by inf unit. The possible withdrawal from a single well or group of wells in a field especially prevalent near seacoasts-<' is affected by a variety of factors such as size, construction, and spacing of occur. A similar problem may de�<' wells as well as by any controls on the flow of groundwater toward the source of saline groundwater. particular field. Transmissibility of an aquif- Although Eq. (6-18) may indicat' 6 15 Safe Yield realized only if the aquifer is cap; source area to the wells at a rate`( The safe yield of a groundwater basin is governed by many factors, one of problem is especially likely to deve:` the most important being the quantity of water available. This hydrologic limitation is often expressed by the equation 6-16 Seawater Intrusion G = P — Qs — ET + Q, — AS, — AS, (6-18) Since seawater (specific gravity abe where G is safe yield, P is precipitation on the area tributary to the aquifer, groundwater under a uniformly pe Qy is surface streamflow from the same area,ET is evapotranspiration, Q,is shown in Fig. 6-14. The lens of fres.`: net groundwater inflow to the area, AS, is change in groundwater storage, a Ghyben-Herzberg lens, after the cc' and AS,is change in surface storage. If the equation is evaluated on a mean 40 ft of fresh water is required beft. annual basis, AS,will usually be zero. above sea level to maintain hyd All terms of Eq. (6-18) are subject to artificial change, and G can be equilibrium does not exist with a sl, computed only by assuming the conditions regarding each item. Artificial- Thus, there is likely to be a seepage recharge operations can reduce Qs. Irrigation diversion from influent a zone of mixing along the saltwate streams may increase evapotranspiration. Lowering the water table by recharge, pumping of wells, and ti,> pumping may increase groundwater inflow (or reduce groundwater outflow) A hydrodynamic balance governs th and may make otherwise effluent streams into influent streams. low, the 1/40 ratio may be a rea`I The permanent withdrawal of groundwater from storage is called adequate methods of analysis are av( mining, the term being used in the same sense as for mineral resources. If the When a cone of depression is storage in the aquifer is small, excessive mining may be disastrous to any water,an inverted cone of salt water i economy dependent on the aquifer for water. On the other hand, many large A saltwater rise of approximately 40 i groundwater basins contain vast reserves of water, and planned withdrawal drawdown may occur, depending or of this water at a rate that can be sustained over a long period may be a wise ming wells are commonly used to av use of this resource. The annual increment of mined water,AS,of Eq. (6-18), Salt water sometimes enters an increases the yield. Thus Eq. (6-18) cannot properly be considered an if a well passes through an aquifer c equilibrium equation or solved in terms of mean annual values. It can be salt water into an underlying aquifei solved correctly only on the basis of specified assumptions for a stated casing and drop down to the fresh wa period of years. aquifers may exist at a coastline (F The factors which control the assumptions on which Eq. (6-18)is solved permitted in the upper aquifer, the d are primarily economic. The feasibility of artificial recharge or surface leakage, diversion is usually determined by economics. If water levels in the aquifer are lowered, pumping costs are increased. Theoretically, there is a water- 6-1'7 Artificial Recharge table elevation at which pumping costs equal the value of the water pumped g and below which water levels should not be lowered. Practically,the increased The yield of an aquifer may be increa; cost is often passed on to the ultimate consumer, and the minimum level is it. In most cases this is equivalent tc never attained. Excessive lowering of the water table may result in contain- area EQ., Eq. (6-18)]. The method., SUBSURFACE WATER 215 oration of the groundwater by inflow of undesirable waters. This hazard is especially prevalent near seacoasts, where seawater intrusion(Sec. 6-16) may occur. A similar problem may develop wherever an aquifer is adjacent to a source of saline groundwater. Transmissibility of an aquifer may also place a limit on safe yield. Although Eq. (6-18) may indicate a potentially large draft, this can be realized only if the aquifer is capable of transmitting the water from the source area to the wells at a rate high enough to sustain the draft. This problem is especially likely to develop in long artesian aquifers. 6-16 Seawater Intrusion Since seawater (specific gravity about 1,025) is heavier than fresh water, the groundwater under a uniformly permeable circular island would appear as shown in Fig. 6-14. The lens of fresh water floating on salt water is known as a Ghyben-Herzberg lens, after the codiscoverers [23] of the principle. About 40 ft of fresh water is required below sea level for each foot of fresh water above sea level to maintain hydrostatic equilibrium. True hydrostatic equilibrium does not exist with a sloping water table since flow must occur. Thus,there is likely to be a seepage face for freshwater flow to the ocean and a zone of mixing along the saltwater-freshwater interface. Areally variable recharge, pumping of wells, and tidal action also disturb the equilibrium. A hydrodynamic balance governs the form of the interface. If velocities are low, the 1/40 ratio may be a reasonable first approximation but more adequate methods of analysis are available [24, 25]. When a cone of depression is formed about a pumping well in fresh water,an inverted cone of salt water will rise into the fresh water(Fig. 6-14b). A saltwater rise of approximately 40 ft per foot(40 in per meter)of freshwater drawdown may occur, depending on the local situation. Horizontal skim- ming wells are commonly used to avoid this effect. Salt water sometimes enters an aquifer through damaged well casings. If a well passes through an aquifer containing undesirable water or through salt water into an underlying aquifer, salt water can enter through a leaky casing and drop down to the fresh water. Jacob [26]pointed out that several aquifers may exist at a coastline (Fig. 6-15), and if saltwater intrusion is permitted in the upper aquifer, the deeper aquifer may be contaminated by leakage. 4 6-17 Artificial Recharge The yield of an aquifer may be increased artificially by introducing water into it. In most cases this is equivalent to reducing the surface runoff from the area [Q„ Eq. (6-18)]. The methods employed for artificial recharge are : 3 3 Groundwater Hvdrology SECOND EDITION David Keith Todd ` UNIVERSITY OF CALIFORNIA, BERKELEY and DAVID KEITH TODD, CONSULTING ENGINEERS, INC. iohn Wilev & Sons New York Chichester Brisbane Toronto I Knowledge has grown r Certainly sind book appear) significance dI have occurrel N, example, lec, f.,J.! into water red ter pollutioi s stressing the'. resources, gl ( droughts,anC g..:! adequate wal J, impact on gi �:.: fact, the char) h. with the grog have been y C more nearly title than a re F Readers fam c> GROUNDWe note many i c . chapters hav ten and ex groundwater] Endpaper: Productive aquifers and withdrawals from wells in s : while the prE the United States after Dept. of Economic and Social Affairs, C has been orrj Ground water in the Western Hemisphere, United Nations, Y have been ri New York, 1976. been selecte e cance and g a` units have a' Copyright © 1959, 1980 by John Wiley & Sons, Inc. t because of +, c. tance. A cog All rights reserved. Published simultaneously in Canada. r:. units is incluc There are rs Reproduction or translation of any part of 1` groundwater this work beyond that permitted by Sections a<' groundwate 107 and 108 of the 1976 United States Copyright source, the 9 Act without the permission of the copyright 1 dents and owner is unlawful. Requests for permission t fields. Chief; or further information should be addressed to neers (incluc the Permissions Department, John Wiley & Sons. lies, hydroid soil mechani Library of Congress Cataloging in the Publication Data: j geologists, a Todd, David Keith, 1923- { engineers, ar Groundwater hydrology. sons indirect Includes bibliographical references and index. c can be found 1. Water, Underground. I. Title. mining, Petro GB1003.2.T6 1980 551.49 80-11831 , public health Although it is ISBN 0-471-87616-X {'. subject filled. ests, the cc Printed.in the United States of America understandin 10 9 8 7 6 5 4 3 2 1 i ciples, methc tered in the t book repres, make availat groundwater 912 UROLOGY MANAGEMENT OF GROUNDWATER 363 ory. For and subsurface drainage systems control groundwater levels, better esti- i 'ban and mates of subsurface flow are usually possible because more data are avail- ientative able. the unit to water Alternative Basin Yields )tat con- The maximum quantity of water that is actually available from a changes groundwater basin on a perennial basis is limited by the possible deleterious side effects that can be caused by pumping and by the be mea- operation of the basin.As a result,several concepts of basin yield are Chapter generally recognized.' These are briefly defined in the following i.me and subsections together with comments as to their consequences. cement. •.ange in Mining Yield. If groundwater is withdrawn at a rate exceeding and end the recharge, a mining yield exists." As a consequence, this yield ild cor- must be limited in time until the aquifer storage is depleted. Many ers and groundwater basins today are being mined; if mining continues, the can be local economy served by this pumping may change, evolving into ing rec- other forms that use less water or involve importations of water Specific into the basin. The Salt River Valley of southern Arizona and the samples High Plains of western Texas are classic examples of such situations. t deter- Various valid arguments,economic and other,have been advanced to justify mining of groundwater. One is that water in storage is of dement no value unless it is used46 In arid areas, such as the Sahara Desert, ,e mea-the where groundwater represents the only available water resource, al- wrcon- most any development of groundwater constitutes a mining yield, lers or But the needs are there and the benefits are great so that such ex- udy of ploitation will continue. With proper management plus water con- ,nge in servation, such groundwater resources can be made to last from r level several decades to a few centuries. ;forage Perennial Yield. The perennial yield of a groundwater basin -.re the defines the rate at which water can be withdrawn perennially under Often specified operating conditions without producing an undesired re- in the sult.* An undesired result is an adverse situation such as (1) pro- urface gressive reduction of the water resource, (2) development of uneco- urface,at the nomic pumping conditions, (3) degradation of groundwater quality, basin `In the past the term sale yield, implying a fixed quantity of extractable water r table basically limited to the average annual basin recharge, has been widely used. The term has now fallen into disfavor because a never-changing quantity of available vibes, water depending solely on natural water sources and a specified configuration of reams wells is essentially meaningless from a hydrologic standpoint. �+b 364 GROUNDWATER HYDRO h41NAG (4)interference with prior water rights,or(5)land subsidence ca by lowered groundwater levels.29"31,54 Evaluation of perennial yi is discussed in a subsequent section.Any draft in excess of pere yield is referred to as overdraft. Existence of overdraft implies continuation of present water management practices will result ,11 significant negative impacts on environmental, social, or econo sv conditions. A schematic diagram of a groundwater basin developed to I` than perennial yield is shown in Fig. 9.4a. Here a portion of the oral recharge is lost by subsurface outflow from the basin. But 9.4b suggests a minimum perennial yield situation in which extra tions balance recharge so that no groundwater is lost. Deferred Perennial Yield. The concept of a deferred pere Dial yield consists of two different pumping rates. The initial ra is larger and exceeds the perennial yield, thereby reducing groundwater level. This planned overdraft furnishes water fro n,< storage at low cost and without creating any undesirable effects. { fact, reducing storage eliminates wasteful subsurface outflow groundwater and losses to the atmosphere by evapotranspiratio — from high water table areas. After the groundwater level has bee lowered to a predetermined depth, a second rate, comparable to th of perennial yield, is established so that a balance of water enteri and leaving the basin is maintained thereafter. With a larger ava' able storage volume, more water can be recharged and a larger p rennial yield can be obtained. Figure 9.4c indicates this situatio schematically. Maximum Perennial Yield. The maximum perennial yield, as o o. the name suggests, means the maximum quantity of groundwateP perennially available if all possible methods and sources are de- veloped for recharging the basin. In effect, this quantity depends air_ the amount of water economically, legally, and politically availabl to the organization or agency managing the basin. Clearly, the more' water that can be recharged both naturally and artificially to a basin; the greater the yield. To achieve the maximum perennial yield the aquifer should be"- managed as a unit.Thus, efficient and economic production of water requires that all pumping, importations, and distributions of water,;, be done for the benefit of the largest manageable system. Where sur- Fig C face water is available in addition to groundwater, these two sources grour are operated conjunctively. Such a conjunctive use scheme provides perer a larger and more economic yield of water than can be obtained perer a MANAGEMENT OF GROUNDWATER 365 f Extraction Recharge s Ground surface 77 it Spill p r (a) Extraction Recharge 9 I. Ground surface (e) I Extraction Recharge ' Ground surface i i Legend Storage capacity Storage needed to 0 regulate recharge j ®Water in storage p;p 6 IB 1 Fig. 9.4 Schematic diagram showing storage relations in a groundwater basin for three stages of development. (a) Less than perennial yield. (b) Minimum perennial yield. (c) Increased perennial yield (after Peters"). a 366 GROUNDWATER HYDROLOGY MANAGEMENT OF from the two sources operated independently. The limit to such an nomic limit for operation is governed by the ability to import and distribute water ca support e hi s and also by the storage available for surface water and groundwater. duces grounds pumping in a F Evaluation of Perennial Yield the basin(see C Consideration of the above definitions of perennial yield reveals to pumping of that there can be more than one "undesired result" from pumping nearby areas a groundwater basin, that perennial yield may be limited to an limitation on amount less than the net amount of water supplied to the basin, and standard of we that perennial yield can vary with different patterns of recharge, the pumped v development, and use of water in a basin. the perennial If groundwater is regarded as a renewable natural resource, then Legal consi only a certain quantity of water may be withdrawn annually from a with prior wa groundwater basin. The maximum quantity of water that can be subsidence, a extracted from an underground reservoir, and still maintain that is supply unimpaired, depends on the perennial yield. Overdraft areas Calculatior constitute the largest potential groundwater problem in the United criterion will States.49 Until overdrafts are reduced to perennial yields in these one or more t basins, permanent damage or depletion of groundwater supplies pumpage excs must be anticipated. vial yield wh specified con } Factors Governing Perennial Yield. Determination of the on- basin is avail ' rennial yield of a groundwater basin requires analysis of the on- equilibrium c - desired results that may accrue if the extraction rate is exceeded. perennial yH ; The recharge'criterion(progressive reduction of the water resource) can be witht is the most important because exceeding this factor is normally without pro responsible for introducing other undesired results. Water supplied to a basin may be limited either by the storage volume of the under- Variabilit ground basin or by the rate of water movement through the basin yii from the recharge area to the withdrawal area.The quantity concept perennial yi tive is usually applicable to u nconfined aquifers where supply and dis- quantiexisting or a posal areas are near, whereas the rate concept applies more to con- the ng or < fined aquifers where supply and disposal areas are widely separated. re erennh' dwate Economic considerations can govern perennial yield in basins gactors that where the cost of pumping groundwater becomes excessive. Ex- Itor sthat cessive costs may be associated with lowered groundwater levels, are t icgat necessitating deepening wells, lowering pump bowls, and installing are Y' larger pumps. Where pumpage is largely for irrigation, power costs, estimate for crop prices, or government farm subsidies may establish an eco- between nE where absence of 'Recharge here refers to water reaching the saturated zone of an aquifer, ment inves' it is available for extraction. F 4 MANAGEMENT OF GROUNDWATER 367 comic limit for pumping groundwater; alternatively, other uses that can support higher pumping costs may evolve. Water quality can govern perennial yield if draft on a basin pro- daces groundwater of inferior quality. Possibilities include: (1) pumping in a coastal aquifer could induce seawater intrusion into the basin(see Chapter 14); (Z) lowered groundwater levels could lead to pumping of underlying connate brines: (3) polluted water from nearby areas might be drawn into a pumped aquifer. A quality limitation on perennial yield depends on the minimum acceptable standard of water quality, which in turn depends on the use made of the pumped water. Therefore, by lowering the auality requirement, the perennial yield can be increased. Legal considerations affect perennial yield if pumpage interferes with prior water rights.'' Finally. if pumpage is responsible for land subsidence, a limitation on perennial yield can result. Calculation of Perennial Yield. In general, the basin recharge criterion will govern perennial yield because, as mentioned earlier, one or more of the other undesired results will often be induced by pumpage exceeding this rate. Quantitative determination of peren- nial yield where recharge is the limiting factor can be made under specified conditions if adequate knowledge of the hydrology of the basin is available. Methods are based on the equation of hydrologic equilibrium or approximations thereto.' Basically, this implies that perennial yield is defined in terms of a rate at which groundwater can be withdrawn from a basin over a representative time period �,vithout producing a significant change in groundwater storage. Variability of Perennial Yield. It is important to recognize that perennial yield of a groundwater basin tends to vary with time. Any auantitative determination is based on specified conditions, either existing or assumed, and any changes in these conditions will modifc the perennial yield. This fact applies to the degree and pattern of groundwater development within a basin as well as to the other factors that govern safe yield Investigations of the availability of groundwater within a basin are typically not initiated until basin development has produced an overdraft. Yet this is almost necessary in order to obtain a reasonable estimate For perennial yield. in a virgin basin,where a balance exists between natural inflow and outflow and there is no pumping, the absence of hydrogeologic data may not justify the cost of a manage- ment investigation. Simiia estimating future perennial yield of a $GS GROUNDWATER HYDROLOGY MANAGEM basin under greater development than at present requires careful table slo evaluation of all items in the equation of hydrologic equilibrium. results. Perennial yield may vary with the level of groundwater within a For a e' basin.Thus,if levels are lowered,subsurface inflow will be increased from the and subsurface outflow will be decreased, recharge from losing govern t'' streams will be increased and discharge from gaining streams will water frc z be decreased, and uneconomic evapotranspiration losses will be lishing a ' reduced. Conversely, a rise in water levels will have the opposite the piezc effects. Therefore, where recharge is sufficient, the greater the utili. is seldon` zation of underground water, the larger the perennial yield. The Beside maximum perennial yield will be controlled by economic or legal gradual constraints. in veget An unconfined basin fed by an adequate recharge source can affected.' increase its perennial yield,not only by increasing pumpage but also tion to t by rearrangement of the pumping pattern. If the concentration of greater wells is shifted to near the recharge source, greater inflow can be expectec induced. The rearrangement has the additional advantage that a groundv ' greater supply may be obtained without necessarily increasing may—frc ' f pumping lifts. For example, in the cross section shown in Fig. 9.5a, consequ'; it is assumed that the stream is the principal recharge source. By factors i moving the well field nearer to the stream as in Fig. 9.5b, the water increase revised t Losing Old well field Ground stream surface Maint ' salinity i it destrE; Water table salts enz, basin Unconfined aquifer Fig. 9.5 Example of increased groundwater Impermeable where (t yield for same (a) flow co pumping depths Losing seldom< obtained by shifting stream New well field Ground surface water, v{ wells nearer to a Salt zr recharge source. \'+ from re' Water table circumE: tion rer' <-- Unconfined mestic Luau conditic / Salts le' Impermeable (b) pumpe( M,VNAGEMENT OF GROUNDWATER 369 table slope is increased and a greater yield for equal pumping depths results. For a confined aquifer with its recharge area located some distance from the pumping area, the rate of flow through the aquifer will govern the perennial yield. In large confined aquifers, pumpage of water from storage can be carried on for many years without estab- lishing an equilibrium with basin recharge. Although the slope of the piezometric surface wdl increase, the permeability of the aquifer is seldom sufficient to maintain a compensating flow into the basin.'° Besides operational changes perennial yield can also vary due to gradual and subtle modifications occurring within a basin. Changes in vegetation and even in crops, particularly where root depth is affected, may influence surface infiltration and subsequent percola- tion to the water table. Urbanization of an area, accompanied by greater surface runoff and installation of sewer systems, can be expected to reduce recharge. Changes in the purpose of pumping groundwater, such as from irrigation to municipal or industrial use, may—from an economic viewpoint—permit greater pumping lifts; consequently, perennial vield can be increased. Other economic factors include, among others, changes in value of irrigated crops, increased efficiency of new well's and pumps, treatment to meet revised water quality standards, and power costs. Salt Balance Maintenance of a usable groundwater basin requires that the salinity of the groundwater not increase with time to a point where it destroys the value of the resource. A dynamic balance of total salts entering and leaving a basin is desired, so that on a long-term basin 0 where (CQ), is the salt concentration times the discharge of one of n ;low components to or from the basin. In practice this condition seldom exists because most uses of water add dissolved solids to water, which is subsequently recharged to groundwater. Salt may be added to groundwater by solution of aquifer materials, from rainfall and surface and subsurface inflows, and in special circumstances from connate brines and seawater. Evapotranspira- tion removes water, leaving higher salt concentrations behind. Do- mestic and industrial uses of water add salts, as do fertilizers, soil conditioners, pesticides, and other chemicals in agricultural areas. Salts leave a groundwater basin by natural outflow. drainage. and pumped extractions." E= 370 GROUNDWATER HYDROLOGY MANAGE The salt problem becomes most important for irrigated land in net inp and and semiarid regions.13,37 If a high water table persists with in salt inadequate drainage, evapotranspiration of irrigation water and cipal a( groundwater gradually increases the salt content of the soil, leading 4 mg/1 I to destruction of the land for agricultural purposes. The solution 30 mg/1 depends on local conditions, but, in general, the requirements are that the water table be lowered, that soil salinity be reduced by leaching, and that a drainage system to transport saline water out of In ba the basin be constructed. timal bE It should be recognized that excellent groundwater can be found volves t= in a basin with an adverse salt balance, and vice versa. Groundwater and gro'r' is rarely uniformly mixed.Typically,good-and poor-quality ground- whereb 11 waters are segregated both horizontally and vertically within a basin. usual sr Thus, an unfavorable salt balance poses a serious long-term threat develop but seldom concerns the current usability of groundwater. a undv w An illustration of salt balance for a semiarid region in the Central ; grorocan be � Vallev of California is shown in Table 9.2. Under 1970 conditions its the latte , can be seen that input of salt exceeds output by 1629 tons. During The cr the decade 1970-1980 an increased volume of imported water en- is predi tered the basin, newly irrigated lands were leached, and a drainage :. is then t t system was started.The effect of these changes by 1980 is a projected .3. -...- face stos p. tf requirer TABLE 9.2 Calculated and Projected Salt Balances marily f for the Tulare Lake Basin, California ,. Thus, gr (after Schmidt47) (values in 1000 tons) ,,,- cycle of g Figure 9 ' Item 1970 1980 Fi g system c Input _ Durin:' Precipitation 23 25 utilized Sireamflow 357 357 _ Imported water 326 846 charged. Soil amendments 476 543 groundv Fertilizers 176 190 periods Animal waste 51 55 ing groL Leaching new lands 0 771 bility of Municipal wastewater 38 44 groundv: Urban runoff 7 8 be space. Industrial waste 28 30 water in Oil field waste 182 5 Mana€} Subtotal 1664 2874 water d: Output procedu Streamflow and groundwater 35 102 Subsurface drainage export 0 545 Subtotal 35 647 'Coordir. Net accretion 1629 2227 water, as r: water ecoa. )GY MANAGEMENT OF GROUNDWATER 371 of in net input of 2227 tons of salt, representing an increase Of 73 percent of :th in salt accumulation. Assuming this salt is mixed within the prin- �d cipal aquifers, the salinity of the groundwater will increase 1 to Is rig 4 mg/1 per year in the eastern zone of higher precipitation and 10 to n 30 mg/I per year in the arid western zone. G re e. of Basin Management by Conjunctive Use e In basins approaching full development of water resources, op- a- of d timal benefical use can be obtained by conjunctive use, which in- or r volves the coordinated and planned operation of both surface water he 1- and groundwater resources to meet water requirements in a manner at whereby water is conserved.` The basic difference between the t usual surface water development with its associated groundwater development and a conjunctive operation of surface water and 1 groundwater resources is that the separate firm yields of the former t _ can be replaced by the larger and more economic joint yields of the latter. The concept of conjunctive use of surface water and groundwater is predicated on surface reservoirs impounding streamflow, which is then transferred at an optimum rate to groundwater storage. Sur- face storage in reservoirs behind dams supplies most annual water requirements, while the groundwater storage can be retained pri- marily for cyclic storage to cover years of subnormal precipitation. Thus, groundwater levels would fluctuate, being lowered during a cycle of dry years and being raised during an ensuing wet period. Figure 9.6 depicts how groundwater levels might vary under such a system of conjunctive use. During periods of above-normal precipitation, surface water is utilized to the maximum extent possible and also artificially re- charged into the ground to augment groundwater storage and raise groundwater levels (see Chapter 13). Conversely, during drought periods limited surface water resources are supplemented by pump- ing groundwater, thereby lowering groundwater levels. The feasi- bility of the conjunctive-use approach depends on operating a groundwater basin over a range of water levels; that is, there must be space to store recharged water, and, in addition, there must be water in storage for pumping when needed. Management by conjunctive use requires physical facilities for water distribution, for artificial recharge, and for pumping. The procedure does require careful planning to optimize use of avail- 'Coordinated use of surface water and groundwater does not preclude importing water, as required, to meet growing needs. In fact, to store and distribute additional water economically may require more intensive use of groundwater storage space. 372 GROUNDWATER HYDROLOpy - htA`AGEME' 0 c n Q Mean precipitation 0 0 5 10 15 20 Time,years e t. a C 7 O V 0 5 10 15 20 Time,years Fig. 9.6 Illustrative example of variation in groundwater levels in relation to annual precipitation under conjunctive use management. Fig. 9.7 _ studyin. Burges2- able surface-water and groundwater resources. Such operations can AssociG be complex and highly technical; they require competent personnel, copyrig detailed knowledge of the hydrogeology of the basin, records of 6666 WE pumping and recharge rates, and continually updated information on groundwater levels and quality. A schematic diagram of a sys- data on watei tematic approach for a conjunctive use analysis is illustrated in ' posal are also Fig. 9.7. of the various A conjunctive use management study requires data on surface water resources, groundwater resources, and geologic conditions; study in Calif g g g required in or rDRCLocy 373 ytAV:yCEMcNT OF GROUNDWATER Iceny y "cre of the rob;em +� I fde,t fy level o`the problem i.. --i lcenti ty all physical aconomic,l and egal var ad'ee �Determ ne he stgeificant elemna ! of[,he system Define thwe�y),"[— —' System dynamics; - i mathemaG� Dl ata, ibiodel veri ficatior. C:item nGude spa ai �---I preferences Y DeCisior, '_eammg kmg d J cpn optimal polic�entatton �C Fig. 9.7 Schematic diagram of a systematic approach to' studying conjunctive use problems (after Maknoon and Surges"; reprinted from journal American Water Works , can Association. Vol. 70, by permission of the Association; :nel, copyright 1978 by American Water Works Association. s of 6666 West Quincy avenue. Denver, Col. 80286). [ion sys data on water distribution systems, water use, and wastevvater dis- t° posal are also necessary.'=' " Figure 9.8 shows a simplified locvchart of the various phases and steps involved For a basin management -'ace study in California. This suggests the diversity of data and effort 3 ns: ement plan. required in order to determine an optimal basin mar w ,'-"�"'�t'.�a"s+Tmtf:.Mttrrw H=ua#:::mw..a�urr+zuawaNNnvas�wwrr+*Nrtau. W V Hydrologic phase g' Development Analyses Survey of of primary of capacities Hydrologic and analyses 90 pipeline a primary locations network pipeline of existing models networks facilities Number of facilities and grswety of water supply of Simulation each plan of DHFuture Formulation of operation Cost water of coordinated comparison al demand alternative operation of of and plans of surface and al temative supply operation subsurface plans facilities Cost of facilities, energy,and water supply Analyses of of each er plan Development of opation O and test of plans et Unit cost operation on analyses of 0 Geologic u,mmonatieai [ analyses fl model of 9toon ties, bassinin energy,and p groundwater mathematical water supply basin model 9 M m Geologic phase Operational-economic phase K O Pig. 9.9 Flow diagram of a management study for the San Gabriel Valley, California, 0 groundwater basin (after Amer. Sec. Civil Engrs.e}. o K is W 'Z O v+. tS Q u, to ;a ca N ,-+ o co Co rn cn p m ti '0 to 5 M C 0 n ox� ,� (D (9 t0 O' O D N m 7� r G't o Wq cn m 7d � � m y to to q. o"_� t't r too n m m me ., rn... . " x m n m q '„'j .. 375 of 7MANAGEMENT OF GROUNDWATER of be noted from Figs. 9.7 and 9.8 that mathematical models s are usually incorporated in such studies (see Chapter 10). A basin is model simulates the responses of a basin to variations in variables d such as natural and artificial recharge and pumping so that the 9 best operating procedures for basin management can be practiced. d s. In effect, this will optimize the water supply obtained from the e basin.16,22,44 9, Because every water development project is unique, it is impos- _ sible to present economic considerations generally for conjunctive or operations and have them apply specifically to any given situation. he Nevertheless, the advantages and disadvantages, mostly economic, at are summarized in Table 9.3.The tabulation compares a conjunctive- use operation relative to development of surface-water resources only, assuming irrigation to be the principal water use in a semiarid region. Total usable water supply can be increased by coordinated opera- tion of surface and underground water resources. With an optimum coordinated operation the unit cost of water supply storage and distribution can be minimized.The basic principles of groundwater I i TABLE 9.3 Conjunctive Use of Surface Water and Groundwater Resources (after Clendenen13) 1 Advantages Disadvantages 1. Greater water conservation 1. Less hydroelectric power 2. Smaller surface storage 2. Greater power consumption i 3. Smaller surface distribution 3. Decreased pumping efficiency t system 4. Greater water salination 4. Smaller drainage system 5. More complex project operation 5. Reduced canal lining 6. More difficult cost allocation 6. Greater flood control 7. Artificial recharge is required - 7. Ready integration with 8. Danger of land subsidence existing development 8. Stage development facilitated 9. Smaller evapotranspiration ' losses 10. Greater control over outflow 11. Improvement of power load and pumping plant use factors 12. Less danger from dam failure 13, Reduction in weed seed distribution 14. Better timing of water distribution 376 GROUNDWATER HYDROLOGY MANAGEMEN basin operation that will produce an optimum water resources management scheme include, as reported by Fowler:" 1. The surface and underground storage capacities must be integrated to : obtain the most economical utilization of the local storage resources and { the optimum amount of water conservation. ° 2. The surface distribution system must be integrated with the ground- water basin transmission characteristics to provide the minimum cost distribution system. J. An operating agency must be available with adequate power to manage surface-water resources, groundwater recharge sites, surface-water distri- bution facilities, and groundwater extractions. The procedure for developing a sound conjunctive-use operation '. within a basin requires estimation of the various elements of water supply and distribution. The optimum use of surface-water and groundwater resources is determined for assumed conditions, usu- ally those during the most critical drought period of record. Ex- amples of coordinated basin management include studies for basins in California,5,33,38s' Colorado,6.3a }8 Idaho,42 Maryland,'-1 New York,20 England,ts and India:" Examples of Groundwater Management Los Angeles Coastal Plain, California. This 1240 km2 basin supplies approximately one-half of the water supply for the Los Angeles metropolitan area. In the recent past the basin has been critically overdrawn, resulting in declining groundwater levels and seawater intrusion. Detailed management studies1o.12 were under- taken to formulate the most economic plan for operating the ground- water basin in coordination with surface-water storage and trans- mission facilities to: (1) meet the growing and fluctuating water ; demands of the area, (2) conserve the maximum amount of locally available water,and(3)minimize the undesirable effects of overdraft. A schematic representation of the coordinated use of surface-water and groundwater resources in shown in Fig. 9.9. Because of the vast increases in imported water to the Los Angeles area,' the study concentrated, first, on evaluating the dynamic re- sponse of the basin to recharging and pumping so that maximum use _ of the underground reservoir could be made, and, second, on deter- mining the most economic plan for operating the basin—taking into x 'It should be noted that the Los Angeles coastal plain is served by a network of water sources,including local surface water and groundwater, reclaimed wastewater, and imported water from the Owens, Colorado, and Feather rivers. Applied Hydrogeology Third Edition C. W. Fetter University of Wisconsin— Oshkosh i i RENTICE HALL, Upper Saddle River, New Jersey 07458 Applied h Edition g examinai Library of Congress Cataloging-in-Publication hydrogec Fetter,C.W. (Charles Willard), 1942— geologic Applied hydrogeology I C.W. Fetter.—3rd ed. advance( p. cm. beginnin Includes bibliographical references and index. ISBN 0-02-336490-4 it offers 1. Hydrogeology. 2.Water-supply. I.Title. the elem GB1003.2.F47 1994 hydroloc 551.49—dc20 93 22893 of chem CIP 3 hydroge Cover Illustrator: T.S.Jobst the appl Cover Art:Sue Birch to probl, Editor: Robert A. McConnin derivatit Production Editor. Sheryl Glicker Langner numero Art Coordinator:Peter A.Robison Text Design Coordinator Jill E.Bonar student Cover Designer.Thomas Mack occurre Production Buyer: Patricia A.Tonneman ground geologi This book was set in Times Roman and Optima by The Clarinda Company Earlier editions copyright® 1988, 1980 by Merrill Publishing Company Well-de 01994 by Prentice-Hall,Inc. clearly A Simon&Schuster Company and the HUpper Saddle River,New Jersey 07458 tables 1 - - All rights reserved. No pan of this book may be unit co reproduced,in any form or by any means,without data fc permission in writing from the publisher. water t contan Printed in the United States of America This et 109876 proble the me ISBN 0-02-336490-4 using i updat( Prentice-Hall International(UK)Limited,London digits, Prentice-Hall of Australia Pry.Limited,Sydney the we Prentice-Hall Canada Inc.,Toronto field h Prentice-Hall Hispanoamericana,S.A.,Mexico accon Prentice-Hall of India Private Limited,New Delhi studei Prentice-Hall of Japan,Inc.,Tokyo Simon&Schuster Asia Pre.Ltd.,Singapore by grc Edirora Prentice-Hall do Brasil,Ltda.,Rio de Janeiro f 518 GROUND-WATER DEVELOPMENT AND MANAGEMENT ? avoided with ground-water storage; furthermore, there is no worry over dam safety. On the other side of the coin, a person cannot waterski in a ground-wa* ' reservoir! Aquifers are also used for the storage of natural gas. The aquifer must be . confined, and a structural or stratigraphic feature, such as an anticline, is required to hold the gas in place. Wells are drilled through the structure and the gas is pumped into the aquifer under pressure. It displaces water and forms a bubble in the aquifer. Fresh water could also be stored in salt-water aquifers, as the fottner is less dense and would float as a bubble in the saline water. 12.5 PARADOX OF SAFE YIELD It is a natural inclination of scientists to compare and classify phenomena in v. quantitative terms.Thus, it is to be expected that hydrogeologists have attempted to define the amount of water that could be developed from a ground-water reservoir. The term safe yield was apparently used in this regard as early as 1915 (Lee 1915).At that time,safe yield was regarded as the amount of water that could r be pumped "regularly and permanently without dangerous depletion of the storage reserve." Later, other factors that need to be considered were added, _ i such as economics of ground-water development (Meinzer 1923b), protection of the quality of the existing store of ground water (Conkling 1946), and protection of existing legal rights and potential environmental degradation (Banks 1953). Synonyms for safe yield appear in the literature, including "potential sustained 7 yield" (Fetter 1972a), "permissive sustained yield" (American Society of Civil ;#. Engineers 1961), and "maximum basin yield" (Freeze 1971). A composite deft- nition, based on the ideas of many authors, could be expressed as follows: Safe yield is the amount of naturally occurring ground water that can be withdrawn 1 from an aquifer on a sustained basis, economically and legally, without impairing the native ground-water quality or creating an undesirable effect such as environ- mental damage. The concept of ground-water withdrawals causing environmental damage warrants more than a mention. Many surface-water systems are dependent upon =- natural ground-water discharge. It has been shown by model studies that ground- water development may reduce streamflow and, as a consequence, lower lake levels and dry wetlands(Collins 1972). As these may be environmentally sensitive areas, the danger of environmental harm is real. Likewise, ground-water with- drawals have been linked to subsidence of the land surface (Bouwer 1977). This has resulted in land-surface cracking and damage to structures, highways, pipe- lines, dams, and tunnels. The gradients of irrigation canals have been changed— even reversed—and low areas have become flooded by sea water. In a broads sense,environmental impacts include ecological,economical, social,cultural,and political values (Fetter 1977b). Many authorities are uneasy with the concept of safe yield. For some,the term is too vague(Thomas 1951). Obviously, the amount of ground water that can be produced will vary under varied patterns of pumping and development. to i WATER LAW 519 addition, the question of what would constitute an undesirable result to be avoided is open to debate (Anderson & Berkebile 1977). The abandonment of the term safe yield has been proposed on the grounds that it does not take into account the interrelationship of ground water and surface water and may preclude the development of the storage functions of an aquifer (Kazmann 1956). However, in spite of the reservations of many hydrogeologists with regard to the concept of safe yield and its implications, the basic concept must be applied whenever the use of an aquifer is planned and managed. Ground-water management programs obviously imply that water must be pumped from the ground(Peters 1972). If there is no evaluation of the hydrologic and environmental impacts of various withdrawal programs, it is possible that uncontrolled with- i drawals will exceed prudent levels. uf� A single value for the safe yield of an aquifer cannot be provided in the same sense as a quantity such as mean annual precipitation. Safe-yield values are based on a number of constraints; such values must be determined by a team of professionals, in the same manner that an environmental impact statement is prepared. Economists, engineers, engineering geologists, plant and wildlife ecol- ogists, and lawyers might all participate with the hydrogeologist in preparing a safe-yield determination for an aquifer or a ground-water basin. The safe-yield evaluation should include a statement of the legal and economic constraints that were considered, as well as the limiting values of environmental damage that were considered. Indeed, such a study should provide a series of safe-yield values and the different factors that applied to each determination. This is obviously not a simple matter. Computer models of ground-water flow systems are ideal tools for estimating the series of values. All the hydraulic factors can be evaluated. R. A. Freeze has shown how a computer model can compute a "maximum basin yield" (Freeze 1971). The safe yield of an aquifer system is only one facet of a ground-water management program. Artificial augmentation of precipitation or recharge could increase the amount of water that can be withdrawn on a sustained basis. The use of ground-water reservoirs for cyclic storage means that in drought years it is necessary, and desirable, to pump water on a temporary basis far in excess of the safe yield. Under these conditions, the ground-water supply would replace surface-water supplies that might be critically low or be used to irrigate crops normally watered by rainfall. In wet years, the ground-water reservoir would be replenished by above-average recharge and pumping at rates below the safe yield. The underlying principle of ground-water development is that by with- drawing water from an aquifer, some of the natural discharge may be made available for use (Peters 1972). 12.6 WATER LAW 12.6.1 Legal Concepts The development and management of water resources must take place within a framework of legal obligations, rights. and constraints. Naturally these factors