• TABLE OF CONTENTS
HIDE
 Abstract
 Main














Group Title: interdependence of economic and hydrologic criteria in planning water resources development
Title: The interdependence of economic and hydrologic criteria in planning water resources development
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00075686/00001
 Material Information
Title: The interdependence of economic and hydrologic criteria in planning water resources development
Physical Description: 14 leaves : ill. ; 22 cm.
Language: English
Creator: Mao, S. W
Hildebrand, Peter E
Crain, C. N
Publication Date: 1979
Copyright Date: 1979
 Subjects
Subject: Water-supply -- Management -- Pakistan   ( lcsh )
Water resources development -- Planning -- Pakistan   ( lcsh )
Water-supply -- Economic aspects -- Pakistan   ( lcsh )
Genre: non-fiction   ( marcgt )
Spatial Coverage: Pakistan
 Notes
Statement of Responsibility: S.W. Mao, P. E. Hildebrand, and C.N. Crain.
General Note: "Abstract."
General Note: Typescript.
 Record Information
Bibliographic ID: UF00075686
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 82367581

Table of Contents
    Abstract
        Abstract
    Main
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
Full Text




Abstract


The Interdependence of Economic and Hydrologic Criteria
in Planning Water Resources Development

S.W. Mao, P. E. Hildebrand, and C. N. Crain


Hydrologic and economic criteria figure in many obvious ways in
water resources development, but they are rarely linked quantitatively,
and most of the applications are pertinent only to the case-in-point.

In the preparation of the Regional Plan for the massive program
of ground water development in the Punjab, situations have arisen where
the choice among two or more alternatives was made by analyzing the
economic implications of critical hydrologic parameters. Two of these
situations which are of more-or-less general interest are described herein.

The first involves basic development policy -- whether ground
water development should be based on the demand of the economy, or the
conservation concept of safe-yield. The analysis demonstrated that
mining of ground water is clearly indicated for areas where alternative
supplies are unavailable.

The second problem involves integrating hydrologic data into an
economic model to determine optimum well design. From an analysis
of all pertinent factors, a simple nomograph was derived which can be
used to determine well specifications on-site during construction. The
nomograph can be modified for use in any' alluvial terrain...








.,.
.i1.
It~~~ ~ ~ ~~~~~ 4 .i +' + ,: + '. +4 +++ '++ ,+++ "+ + 1I k ,+ : ' + '+++ '+






The Interdependence of Economic and Hydrologic Criteria
in Planning Water Resources Development

S, W. Mao, P. E. Hildebrand, and C, N. Crain(1)


Introduction

In the Northern Zone of the Indus Plains -- the region commonly
known as the Punjab -- Pakistan is proceeding with integrated develop-
ment of water and land resources on a scale never before contemplated.
The heart of the development plan is a massive program of ground water
development to supplement canal irrigation supplies and to provide drain-
age relief to the irrigated lands. By the close of 1966 about 2500 high-
capacity wells were in operation and furnishing the water supply and
drainage requirements for nearly two million irrigated acres. The
development plan calls for the construction by 1980 of a total of nearly
30, 000 wells with an aggregate capacity of 120, 000 cubic feet per second.
This is part of an integrated program eventually affecting over 20 million
acres of irrigated lands. (2)

Pakistan is proceeding with this large scale development of land
and water resources at a time when she is at the threshold of almost
calamitous population growth. Furthermore, she is attempting this de-
velopment at a time when most of her usable land already is occupied and
her food demands exceed current production. Here, in the Northern Zone,
is one of the largest contiguous areas of irrigated land in the world, easily
irrigated, with a warm dry climate and ample potential supplies of surface
and ground water. Except for an abundance of land and labor, other pri-
mary resources are lacking or are in very limited supply. The agriculture
sector so dominates the economy that only by means of an effective develop-
ment of the agricultural resources can other sectors grow and contribute
.to a viable economy. Because of inadequate irrigation supplies, agriculture
here has not effectively participated in the twentieth century revolution in
agriculture, and only by means of massive development of potential supplies
can Pakistan catch up. Capital for any kind of mass development must
come from the outside. But the overriding restraint to development of the
land and labor resources is the availability of water.

(1) S.W. Mao is a hydrologist and P. E. Hildebrand is an economist.
Both are with Tipton and Kalmbach, Inc., Denver, Colorado. C.N. Crain
is Professor of Geography and Regional Planning, University of Denver,
and consultant to Tipton and Kalmbach, Inc.
(2) This development program is administered by the West Pakistan
Water and Power Development Authority (WAPDA) for which the firm of
Tipton and Kalmbach, Inc. serves as regional planning consultant for the
Northern Zone. The material presented herein is derived from planning
studies which have been made in connection with the preparation of a
regional development plan for the Northern Zone.







Economics of Development


The decision-making process in planning regional development
becomes more complex as the various sectors of the economy become
more highly developed. Where, as in this case, one sector dominates
the economy and its development can be considered the key to develop-
ment of other sectors, the decision-making process is simplified and
it is easier to identify alternatives and to trace the probable consequences
of each possible choice.

Since the development alternatives are dictated by economic
resource analysis on the one hand and by the technical ability to provide
tools on the other hand, the engineer and the economist must work
together to enumerate the parameters which will provide both the
identification of alternative development choices and the basis for
scientific choice among these alternatives.

A case in point is the regional planning of the Northern Zone of
West Pakistan, where engineers and economists, working together, have
evolved a frame of reference which permits the design of water develop-
ment within optimum limits.

Two of the many problems which had to be met were: (1) whether
ground water development can be regulated by the demands of the economy,
or must the economy be controlled by concepts of "safe yield", and (2)
how to integrate hydrologic data into an economic model to provide the
necessary parameters for determining optimum well design. The logic
used in providing answers to these two questions involves very basic con-
cepts of regional planning and development.

In this case, analysis of the area indicates the following conditions
which provide the framework for decision-making in development of the
regional plan: '

1. High population density
2. Very high rate of population growth
3. Food deficits
4. Rate of growth of demand greater than rate of
growth of production
5. Basic lack of primary non-agricultural resources
6. Lack of capital
7. Extensive areas of potentially very fertile land
8. Terrain which favors irrigation
9. Warm dry climate which permits year-round
growth when water is available
10. Ample potential supplies of water
11. Population of competent, intelligent farmers
12. Adequate power for pumping




-3-


13. Sufficient technical skills available to achieve
potential water supplies
14. Population increases in Asia alone are in excess
of 50 million per year, which will equate to about
8 million tons of wheat per year increase in demand --
or at least 80 million tons per year more demand
in the next decade
15. U.S. grain surpluses are fully committed and Canada's
are contracted for by Russia for the next five years
16. Thus the assumption that during the plan period
there will be a market for surplus agricultural goods
close to home (India and China, for example)

The only means by which dependence upon imports and foreign aid
can be reduced or eliminated is the development of the agricultural re-
sources to the point of providing capital accumulation for use in develop-
ing other sectors of the economy. In view of the magnitude of the area
and population involved, it is obvious that only a radical, massive and rapid
expansion of agriculture will be effective. The ultimate purpose of such
development is less the establishment of long range agricultural capability
than to promote capital formation and to generate a long range balance of
development among the economic sectors.

Thus the demand for resources in agriculture is immediate and
urgent; later, the results of this development will initiate shifts in resource
use to other sectors of the economy.

In Figure 1, two available alternatives are illustrated. The hori-
zontal axis is time and the vertical axis is volume of production. Starting
in 1960, the curve DL projects the lowest probable estimate of internal
demand and DH a higher probable estimate of demand. PL is a projection
of production under the decision not to develop reserves of water to the
maximum capability and PH illustrates the other alternative of developing
all available agricultural resources up to their maximum utility in the
shortest possible period of time. In particular, it implies use of ground
water resources in excess of that implied by the usual "safe yield" concept.

It can be seen that even with a low demand curve the deficit con-
tinues to increase through the plan period in the absence of intensive
development. The consequence is an increasing dependence upon imports
and thus upon foreign aid. Furthermore, the widening gap between demand
and supply will adversely affect any future development program.

In contrast, if the utilization of water resources is developed at
the highest rate possible, the production curve exceeds the demand curve
by about 1970. Then dependence on food imports ceases. Subsequently,





-4-


exports replace imports and the capital earned becomes available for
decision-making without the constraint of having to use it to supplement
deficit food supplies.

By the end of the plan period, gross national product, and other
values will reflect a viable economy and water values will reflect com-
peting demands for water. At this time decisions can be made relative
to other alternatives available for further development or uses of water
resources.


Feasibility of Ground Water Development


Ground water development beyond that implied by the usual "safe
yield" concept is necessary if full economic development for the Northern
Zone of West Pakistan is to be achieved. But in order to demonstrate
the feasibility of the development plan it is necessary to establish the
economic feasibility of the high level of ground water development which
\will create a viable economy. If this level of development is not economi-
cally feasible, agricultural development would either have to be restricted
or subsidized, and full economic development would be jeopardized.

The history of the irrigated areas of the western United States
indicates that rapid and intensive development of ground water resources
can provide the impetus for overall economic development. In these
areas the value of water always has exceeded cost of development. It
is tempting to conclude that intensive ground water development in any
favorable agricultural climate always will lead to the same successful
results. But in planning the development of a large region, requiring
coordinated, long range decisions by government and directly affecting
all segments of the economy, it is necessary to demonstrate the feasibility
of intensive development for the particular case in point. And for the
Northern Zone of West Pakistan, both economic and technical consider-
ations have been used to show the feasibility of the plan evolved.

To establish that a low level of ground water development is
economic is a trivial case. Various estimates of the value of water under
present conditions range from about 50 to over 100 rupees per acre foot
annually at the heads of water courses. The cost of pumping ground
water from present depths and with present equipment range from about
15 to 30 rupees per acre foot. Obviously, the use of ground water under
these conditions is a profitable venture.

But as the rate of pumping increases, the water table will be
lowered and the cost of water will increase. Simultaneously, however,
the impact of the added water on agriculture will enhance the value of water.





-5-


The significant questions are: (1) at what rate can the water table be
lowered, and (2) how far can it be lowered before the limit of economic
feasibility is reached. This is analogous to asking what is the upper
economic limit of ground water development.

Consider first the case where the pumping equipment is a fixed
investment, adequate to pump from any required depth. The added cost
of pumping from greater depths is then primarily a function of the added
power cost.

If the storage coefficient of the aquifer is 0.25, the water table
will be lowered four feet minus recharge f6r every acre foot pumped.
If recharge from pumped water is 22 percent of the volume pumped (Vp),
the net decline in the water table is 3. 12 Vp.per gross acre. In the
Punjab, recharge from surface deliveries is approximately 54 percent
of the volume delivered to the heads of the water courses where the well
water is discharged. Recharge from canal deliveries per gross acre
(Dc) is 0. 54 Dc and the rise of the water table is 0. 54 Dc/0. 25 or 1. 52 Dc.
Minimum annual recharge from other sources is estimated at 0. 2 feet
per gross acre (GA). The rise of the water table from these sources is
0. 8 feet per year per GA and is considered constant. Hence, the annual
drop of the water table in a contiguous pumping area in terms of acre feet
per GA is:

Wd = 3. 12 Vp 52 Dc 0.8 (1)

Total pumping head, H, in any year, n, is the sum of (1) initial
depth to water, (2) dynamic head, and (3) the accumulated drop of the
SWd
water table, t = 1 Dynamic head is considered to be 30 feet and
initial depth to water, 10 feet. Hence, pumping head in year n when Vp
is an annual constant is:

Hn = 40 + n (3.12 Vp 1. 52 D 0.8) (2)

At a cost of 0. 07 rupees per KWH, the power cost per acre foot per foot
of lift is 0. 12 rupees, and the annual cost of power for any year n is:

C = 0. 12 Vp (Hn) (3)

Using dt as the annual .discount factor, the present worth of power costs
over a time span of n years is:

Cp = 0. 12Vp [dt (40+n(3. 12Vp 52Dc- 0.8)} (4)




-6-


The annual marginal value of water, MV, measured at the heads of the
water courses has been estimated elsewhere. The present marginal
value, in a moderately productive area, in terms of total volume of
irrigation water in acre feet per gross acre, V, is:

MV = 104 19.6 V (5)

As V = Vp + Dc, the present worth of the marginal value of water is:

MVYp = d [04 19. 6 (Vp + Dc) (6)

where d is the discount factor for a uniform series specified by Vp and DC.

For any selected volume of surface deliveries, an optimum annual
pumping volume which maximizes present worth of net return can be found
by equating the present worth of marginal value with the present worth of
marginal costs. The latter is defined as the change in present worth of
costs Cp, as Vp is changed or:

MC = (7)

The optimum pumping rate using (1) present water value, (2) a
planning span of 50 years, and (3) discounting at 5 percent, is 2. 94 acre
feet per gross acre annually when annual canal deliveries are 1.42 acre
feet per gross acre, which is the estimated future depth of canal supplies
to the development areas. This optimum, or upper limit of ground water
development compares with an annual ground water requirement of about
2 acre feet per gross acre projected in the development plan. Hence,
the projected level of ground water development for the development plan
falls well within the bounds of economic feasibility.

It should be noted that the optimum pumping rate increases with a
higher discount rate and with smaller canal deliveries. The optimum pump-
ing rate declines with periods of analysis longer than 50 years, but the
decrease is not significant. Hence, the upper limit found above can be
considered a conservative estimate.

It should also be noted that after pumping at the optimum rate for
about half the period of analysis, the marginal cost will exceed the present
marginal value of the water. This results from the choice criterion which
specifies a maximum present worth of net return. However, the higher



Regional Development Plan for the Northern Zone of the Indus
Plain, Tipton and Kalmbach, Inc., in process.





-7-


future costs, when discounted, are offset by the higher return from pump-
ing early in the period of analysis. Moreover, by 1990, it is estimated
that the marginal value of irrigation Water at the combined volume of 4. 36
acre feet per gross acre annually, will have risen to nearly double its
present marginal value. Even if the marginal value of irrigation water
does not rise above its 1990 level, the marginal cost will not exceed
marginal value during the 50 year period of analysis.

As it is certain that the marginal value of water will increase
through time, the above analysis of the upper economic limit of ground
water development produces a definitely conservative estimate.

The feasibility criterion of the preceding section omits some
important factors involved in lowering the ground water table. The model
assumed a fixed pumping plant capable of pumping from any depth. Obvious-
ly, if future depth to water and annual pumping volume are known at the
time of tubewell construction, the capacity and depth of the tubewell will
be affected. A tubewell designed to pump greater volumes will be more
expensive both because the capacity will be greater and it will have to be
deeper. But the greater initial expense will be spread over a larger
volume of water. Generally, within the relevant range of development,
the greater volume more than offsets the higher fixed costs, resulting in
lower fixed costs per acre foot pumped. Estimated costs for three different
cases are shown in the following table.


.,(
i




P


-8-


Comparison of Cost of Tubewell Water
for Various Rates of Pumpage-and
Depths to Ground Water Table


Item Unit Case A Case B C

Canal supplies @ HWC ft/yr/GA 1.42 1.42

Tubewell supplies @ HWC ft/yr/GA 0.79 1.24

Total supplies @ HWC ft/yr/GA 2.21 2.66

Tubewell capacity cusecs 3. 0 3.5

Depth to water table after 50 yrs, feet 15 50

Maximum pump lift feet 36 75

Annual pumpage acre feet 670 1,050

Annual utilization factor percent 31 41

Capital costs of tubewell rupees 44,000 54,100 6'

Annual costs

Amortization @ 5% for 15 yrs. rupees 4,240 5,210

0 & V rupees 2,000 2,000

Power costs rupees 2,890 9,450 2

Total annual costs rupees 9,.130 16,660 2

Cost of tubewell water

Fixed costs Rs/AF 9 30 6.85

Power costs Rs/AF 4. 30 9. 00

Total cost Rs/AF 13.60 15.85

Although the total cost of water does increase with depth, the costs
are well below the value of the water even after 50 years of pumping. The
costs are compared with the value of water in the following table.


ase C

1.42

1.69

3. 11

4. 0

90

118

1,440

49

9,400


6,690.

2, 000

0,400

9,090



6.05

14. 15

20. 20





-9-


Cost and Value of Water
Per Acre Foot After
50 Years of Pumping


Case A

Case B


Case C


Total.
Water
Supplies
ft/yr/GA.
@ HWC

2.21

2.66


3.11


Average
Cost of
Tubewell Water
Rs/AF @ HWC

13.60

15.85


Present Future
Average Average
. Value of Water Value of Water
Rs/AF @ HWC Rs/AF @ HWC


77

73


20.20


201

205

202


Of more significance is the total
as shown below:


net value of the water in each case


Net Value of Water
Per Gross Acre
After 50 Years


Total
Water
Supplie s
ft/yr/GA
@ HWC


Tubewell
Water
Supplies
ft/yr/GA
@ HWC


Total
Cost of
Ground
Water
Rs/GA


Total
SCost of
Irrigation
Supplies
Rs/GA*


Total Net
Present Present
Value of Value of
Water Water
Rs/GA Rs/GA
..- -- -w- *


Total
Future
Value of
Water
Rs/GA


Net
Future
Value of.
Water
Rs/GA


.10.70 16.40

19.70 25.40

34.20 39.90


.170


194

218


154

169

178


445


429


545 520


628


588


* Including charge of 4 rupees per acre foot for canal supplies


Hence, in Case B, an additional 15 rupees per gross acre is generated
annually with present values by the additional pumping and in Case C, an
additional 24 rupees per gross acre is forthcoming annually from ground
water development. For the 16. 7 million acres underlain by non-saline
ground water, this.involves an annual gain to the economy of 250 million
rupees and 400 million rupees annually for Case B and Case C, respectively.
Using future values of water, the annual gain to the economy is 1520 million
rupees and nearly 2700 million rupees annually for Case B and C, res-
pectively, over Case Ab


Case A

Case B

Case C


2.21

2.66

3.11


0.79


1.24

1.69





-10-


An additional factor, often overlooked, but of paramount importance,
is the creation of additional water generated by pumping from greater depths.
Inflow to the aquifer in one area (recharge) cannot be withdrawn from another
area (pumpage) unless a gradient or differential in elevation exists between
the two areas. In the Northern Zone the tubewells will be arranged in a
relatively uniform density pattern and will withdraw water at an essentially
uniform rate over large areas. Although recharge originating from water
applied to the lands will also be relatively uniform, the other major com-
ponent of recharge consisting of leakage from canals, links, rivers and
similar line sources is not uniformly distributed. These latter sources
account for 40 to 60 percent of the total recharge. If the water table is
maintained at shallow depths throughout the area, much of the recharge
from these line sources is lost non-beneficially through evaporation near
the source of recharge. But by establishing a gradient between these
sources of recharge and the areas of pumping, non-beneficial losses are
reduced and the recharge, when used for irrigation, enhances the results
of development.

For the cases described above, total recharge has been conserva-
tively estimated as 1. 14 feet per year per gross acre for Case A, 1. 24
feet for Case B and 1. 34 feet for Case C. The additional 0. 10 acre foot,
in each case, is worth more than 5 rupees annually at present water values.
In Case A, recharge exceeds-pumping by 0. 35 ft/yr/GA and in Case C,
pumpage exceeds recharge by a like amount. In Case B, pumpage equals
recharge.

Thus ground water development to the high levels required to
generate a viable economy in the Northern Zone is economically feasible.
That is, ground water development need not be restricted to so-called
"safe yield" levels but can proceed as required by the development demand
of the economy. It remains to determine the technical design criteria for
the tubewells to provide the necessary quantities of ground water.


Optimum Tubewell Design.


In project planning, the magnitude of ground water development is
determined in detail by coordinating the availability of surface water
supplies, existing irrigation distribution systems, local effective pre-
cipitation, and irrigation requirements for the projected level of agri-
cultural development. This procedure generally results in a large number
of tubewells of varying capacities with various utilization factors. Many
physical, technical and economic factors affect the design of these tube-
wells. Engineering considerations and construction techniques restrict
the range of choice of components and physical conditions affect performance.





-11-


Still, within the restrictions imposed by these factors, a wide range of
design possibilities exists. The best or optimum design is one which
results in a minimum overall cost for the water pumped.

The cost of pumped water is derived from three cost components:
(1) initial construction cost, (2) energy consumption cost, and (3) operation
and maintenance cost. Length and diameter of the screen are the most
important determinants of the first two cost components. In the optimi-
zation of tubewell design, length of screen is the most important factor.

For a tubewell of specified capacity at a particular location, the
length and diameter of the screen are the primary factors governing
electric energy consumption owing to their effect on tubewell drawdown,
and thus, the dynamic head. In the absence of other considerations,
minimum drawdown would result in minimum cost of water. But to reduce
drawdown, either diameter of screen must be increased, or more impor-
tantly, length of screen must be increased. Both factors increase initial
construction cost of the tubewells. In order to determine optimum screen
dimensions a balance between added construction cost and reduced power
cost must be achieved.

Regardless of changes in screen length and diameter, many com-
ponents of construction do not change or are of minor influence. Only
those components whose cost change with screen dimensions need to be
considered in determining optimum tubewell design.

The construction components which vary with screen length or
diameter are:

1. Depth of tubewell
2. Bore hole diameter and amount of gravel shrouding
3. Length of housing as influenced by drawdown
4. Length of blank pipe as influenced by amount of
impervious layers in the aquifer and total well depth
5. Cost of screen

Figure 2 shows the general arrangement of a tubewell and the
components which govern both initial variable construction cost and
energy consumption cost. Operation and maintenance cost is treated
as constant for all the tubewells.

Total initial construction cost, CI, is computed by:


CI = Pdi (Dw,) + P, (Ls) + Pb(B) + Phg(Hg) Pg(G) + FC





-12-


Here the variables are:

Pdi = cost of drilling within interval "i" per linear foot

Ps = cost of screen per linear foot

Pb = cost of blank pipe per linear foot

Phg = cost of housing pipe per linear foot

P = cost-of gravel shrouding per linear foot

D = depth of drilling in feet within interval "i"

Ls = length of screen in feet

B = length of blank pipe in feet

H = length of housing pipe in feet
g
G = linear feet of gravel shrouding

FC = fixed cost items

Using the life expectancy of the tubewells and the interest rate on
investment, the initial construction cost can be converted to an annual
basis:

CIa = ACI (9)

where A is the appropriate amortization factor.

Electric energy consumption is a function of total pumping head,
H, which is the sum of static head, Wt, and dynamic head, Hd. Static
head is defined as the design depth from discharge pipe to the static ground
water level; it is assumed to be constant at a future depth in optimum
tubewell design analysis. The dynamic head is a variable consisting of
tubewell drawdown, aquifer and screen losses, pipe friction head and
velocity head.

The annual energy consumption cost, CE, can be reduced to:

CE = FQUH (10)

In this equation F is a derived factor consisting of wire to water efficiency
and the unit cost of electricity, Q is tubewell capacity, U is the annual
utilization factor, and H is the total pumping head.





-13-


Annual variable cost of a volume of water specified by U and Q is:

C = CE + Ca ()

This equation is a continuous, concave function of screen length; its mini-
mum can be found by differentiation with respect to screen length and
equating to zero. Figure 3 illustrates the shape of the cost curves and the
optimum design for a single well with current unit costs, rate of interest,
and other specified conditions shown.

Figure 4 is a composite of optimum design conditions for several
tubewell capacities; it demonstrates the influence on pumped water cost
as the utilization factor varies. If annual volume is fixed and capacity is
variable then smaller capacity wells pumping at a higher annual utilization
rate result in lower unit water costs. If, on the other hand, capacity is
specified, lower unit water costs result from higher annual utilization of
the tubewells, The influence on optimum design and cost of water of other
components, such as static water level, length of blank pipe, cost of screen
and electric energy, rate of interest and life of tubewell, can also be
determined.

Figure 5 demonstrates that economic decisions appear even in
selecting hydrologic parameters for tubewell design purposes. In esti-
mating tubewell dynamic head (more specifically, the aquifer loss), a
most important, yet most uncertain, hydrologic parameter is aquifer
permeability. As site by site estimation to determine the precise per-
meability for design of every tubewell is impractical, one value of aquifer
permeability normally has to be selected for designing all the tubewells
within one project area. From an engineering point of view, there is a
tendency to select a lbw design value of aquifer permeability as a con-
servative measure. However, from an economic point of view, a design
value of aquifer permeability should be selected which would result in
a minimum expected cost of water within the area.

Figure 5-a shows the influence of aquifer permeability on cost per
acre foot of water. The heavy dots represent the optimum design for each
of three permeability values. The deviation of the curves from these
points shows the effect on water cost when actual permeability conditions
differ from the assumed design value. When an incorrect permeability
value is assumed,' the cost of water is higher than when the correct value
is used. As can be seen in the figure, the magnitude of deviation from
optimum varies with the choice of the assumed permeability.

Figure 5-b shows the typical variation of aquifer permeability for
the unconfined alluvial sediments of unknown depth in the Punjab. Although
the range of variation is rather small from a geological point of view, it
is large enough to have a significant effect on water costs. A value for





-14-


use in design can be selected which results in minimum cost of water over
a broad area if distribution of permeability for the area is obtained in
advance from exploration toets. Knowing the frequency distribution of
permeability and the effect on water cost from design permeabilities, an
average cost for any design permeability value can be found and an optimum
design permeability value can be established. In the case of Figure 5, the
Minimum cost design permeability value appears to be near, but less than,
the mean permeability value.

A nomograph such as Figure 6 can be developed from the optimum
design concepts above. The nomograph can be used in conjunction with
the well log on site at time of drilling to determine optimum screen length
and concomitant lengths of housing and blank pipe as well as total depth of
Swell.

In actual practice, as the lenticular clay beds are randomly distri-
buted, the water-bearing formations cannot be pre-determined on site
before drilling. However, as drilling proceeds, the inspector can log
length of blank pipe required as shown by the dash line in the nomograph.
When the log intercepts the curve in the upper right hand quadrant corres-
ponding to the capacity of the well being drilled, approximate well depth
has been reached. A horizontal line drawn to the left from the point of
intersection to the appropriate curve in the upper left quadrant specifies
optimum length of screen and a vertical line from this point tohe appro-
priate curve in the lower left quadrant specifies proper lengh of housing
pipe. Rounded values of blank pipe, screen and housing w uld be entered
in the lower right corner and their sum would give actual well dep h.

Thus, embodied in Figure 6 are all the components of tubewell
design which can be measured and specified at time of tubewell construction.
The use of such a nomograph provides a simple process by which tubewells
of optimum dimensions considering geological, engineering, and economic
factors, can be constructed throughout the entire project area.


Summary


Well-planned water resource development hinges on a series of
\ joint decisions involving many disciplines. Selected examples have been
.used to show that the disciplines of engineering and economics'lend them-
selves to joint analysis. Furthermore, it has been demonstrated that
engineering and economic concepts are necessarily interdependent in the
optimum planning of water resources development.




















































































1975 1980 1985 1990


25










20



V)
z
O

Z

0 15
I-
-J
-J







10










5


20





Z
0


Z
15 0
_J
_1
-0


1960 1965 1970


1995 2000






TYPICAL TUBEWELL ARRANGEMENT


0-


S'-N.S.L.


- I----- --------~--
C!


I


Gravel pack


I













rn
I






---


I








Y_J


-Blank pipe as required
for clay formation


--Slotted casing


1--Centralizer


NOTATIONS:
0 : Pump discharge
Dw Depth of drilling
Hg : Length of housing pipe
Ls : Length of screen
B : Length of blank pipe
G : Length of gravel shrouding
Ds : Diameter of screen
DG : Diameter of bore hole
wtf Static head
Sw : Total tubewell drawdown
S'w : Aquifer loss
Sw : Screen loss
NOTE:
This sketch is not to scale.


i I
FG--DG --

FIGURE 2


I--l--Centralizer


"--'-, q {'{T'-',,I yTTr --










COST PER ACRE-FOOT OF WATER VS. TUBEWELL SCREEN LENGTH
100 200 300 400


TUBEWELL CAPACITY
SCREEN DIAMETER
DEPTH TO STATIC WATER LEVE
LIFE OF TUBEWELL
UTILIZATION FACTOR
INTEREST RATE


= 4 CUSECS
- 8 INCHES
- 20 FEET
= 15 YEARS
= 60%
= 4%


TOTAL VARIABLE COST C


U)
a:

a





0
0
0
ui

a-



C-)
a:


0
u
ui
I



co
m

a:


I I


z I -TSRUTO


ufl
'T UC O
Oal


20 300III


200
TUBEWELL SCREEN LENGTH,FT.


LI
u01
cl
I-,, POWER COST (CE)
II


300
















COST OF WATER VS TUBEWELL CAPACITY
(OPTIMUM SCREEN DIMENSIONS)


0 F-


9 -


8 I-


DEPTH TO STATIC WATER LEVEL=20 FT.
LIFE OF TUBEWELL = 15 YEARS
INTEREST RATE = 43/4%


40 60 80

UTILIZATION FACTOR, PERCENT







0.5 1.0 1.5 2.0 2.5
I I 1 I I


in

I-
C-
n.-


I-


0
0
I-

LL




Ia
n

I-

0

w 8-
-J
m

>


-30








-20








- 10


1.0 1.5 2.0
AQUIFER PERMEABILITY, 103 FT./SEC.

FIGURE 5-b


FIGURE 5


K = 1.50x10-- tt./sec.

--- Optimum screen length designed
on the basis of aquifer permeability



AQUIFER PERMEABILITY, 10IO FT./SEC.

FIGURE 5-a


0.5 1.0 1.5 2 .0 2.5
I I I I I


CHAJ DOAB AREA
TOTAL NUMBER OF SAMPLES=470

Kmean = 1.65 x 163ft./sec.


T E CAPACITY = 4 C S


I U0-VV L L r-1 I 1 -
SCREEN DIAMETER = 8 INCHES
DEPTH TO STATIC WATER LEVEL 20 FEET
UTILIZATION FACTOR = 60 %





S_ OPTIMUM DESIGN WITH CORRECTLY
ASSUMED AQUIFER PERMEABILITY



IN,

-- Optimum screen length designed
on the basis of aquifer permeability
K =0.75x 10-3 ft./sec. -

-- Optimum screen length designed
on the basis of aquifer permeability


0.
0.5


FIGURE 5


S




















25o --


200 -


ISO


o00 o-


250 225 ZO 75 I S0 125
LENGTH OF SCREEN -FT


,00 7S


SO '00 ISO

TOT AL


ZOO 250

DEPTH OF WELL FT


WELL NUMBER
DESIGN CAPACITY
AQuiFER PERMEABILITY
DEPTH TO STAT C WATrFi
UT LIZATION FACTOR


DIMENSION
BLANK
SCREEN
HOUSING

WELL DEPTH


EVE L


300 350 400


4 cusecs
I 50 10 '" SE
20 FT
6C %


50 FT
170 FT
45 FT

265 FT


NOMOGRAPH FOR OPTIMUM TUBEWELL DESIGN




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs