Irrigation and Drainage Systems 10:297-317, 1996.
(~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.
A water balance model to estimate groundwater
recharge in Rechna doab, Pakistan
G H U L A M Z A K I R H A S S A N & M U H A M M A D NAWAZ B H U T T A
International Waterlogging and Salinity Research Institute, 13-West Wood Colony, Thokar
Niaz Baig, Lahore, Pakistan
Accepted 27 June 1995
Key words: groundwater recharge, Rechna doab, Water balance
Abstract. The main objective of this study was to develop a procedure to evaluate various
recharge components of a groundwater reservoir to estimate the long term average seasonal
groundwater recharge in Rechna doab in the Punjab province of Pakistan. A regional lumped
water balance model for the Rechna doab was developed and applied to estimate the long term
a seasonal recharge to groundwater reservoir.For comparison, recharge was also estimated by a
specific yield method from observed groundwater levels. A water balance study was conducted
on seasonal basis (6 months) for a period of 31 years (1960-1990). Recharge estimated by the
two methods was found to be in good agreement.The average value of net groundwater recharge
during Kharif (April-September) season was found to be some 60 mm. No recharge occurred
during Rabi (October-March), rather there was a depletion of the groundwater reservoir during
the winter months. Long term average annual depletion of a groundwater reservoir was found
to be greater than corresponding value of annual recharge. It was concluded that on a regional
basis the groundwater reservoir was being depleted resulting in an average groundwater table
of Rechna doab about 2.3 m fall over the 1960-1990 period.
Background
Pakistan's e c o n o m y is predominantly agricultural and the agricultural sector
provides more than 30 percent of gross domestic product (GDP). Agriculture
is the source o f subsistence for more than 72 percent o f population. Punjab
province o f Pakistan is triangular in shape with an area of about 127,000
k m 2 and has been known as "The Land of Five Rivers". F r o m east to west
these rivers are Sutlej, Ravi, Chenab, Jhelum and Indus. The flat stretches
o f land between two rivers have been called "doab", meaning land between
two waters. T h e s e doabs are Bari (between rivers Beas and Ravi), Rechna
(between rivers Ravi and Chenab), Chaj (between rivers Chenab and Jhelum)
and Thai (between rivers Jhelum and Indus). The plains of these doabs have
been formed by alluvial deposits brought by these rivers and are very fertile.
In order to develop the barren land and to utilize the water of these rivers,
an extensive and intricate network o f canals was constructed by the British
during the 1900s, during their rule over the Subcontinent. The fertile soils and
298
64 °
68 °
72 °
76°
N
/ iA ~'5~,.'I ~
!RABII
Trim~
H/~
Fig. 1.
Map of Pakistan showinglocationand details of study area.
irrigation water caused the area to blossom into lush green fields and millions
of rupees were invested in the construction of canals and headworks were
paid off within a few years. In Rechna doab, the Lower Chenab Canal (LCC)
was converted to weir control and extended during the years 1892-1900, with
a discharge capacity of 368 m3/s, and covered an area of 1.13 million hectare
(MH).
Unfortunately the period of prosperity following the canal irrigation was
rather short-lived. The intensive application of irrigation water coupled with
inadequate subsurface drainage resulted in a gradual rise of the water table. In
some areas the water table rise was 24.4 m or even more with an average rate
of rise of 0.46 m/year. Soomro (1975) reported that waterlogging was first
noticed in the upper regions of Rechna doab after a few years of opening of
Lower Chenab Canal. Water and Power Development Authority (WAPDA)
of Pakistan has installed public tubewells through the execution of various
299
Salinity Control and Reclamation Projects (SCARP) in different parts of
Rechna doab. These tubewells have been installed to serve a dual purpose: to
control waterlogging and to supplement the irrigation supplies. For planning
and management of available groundwater resource as well as for the subsurface drainage requirements, it is deemed imperative to estimate the long
term net seasonal recharge to groundwater reservoir.
Study area
The geographical location of study area with details is shown in Fig. 1. In the
present study, the area of Rechna doab between Qadirabad-Balloki (Q-B) and
Trimmu-Sidhnai (T-S) link canals, i.e. between longitudes 72015 ' to 74°E
and latitude 30°3ff to 32°22.5~N was considered. The total area of this doab
is about 28 500 km 2 with a length of 403 km and maximum width of about
113 km. An area of 1.622 million hectares (16220 km 2) was considered in
the present study. Administratively it is divided into five districts of Sialkot,
Gujranwala, Sheikhupura, Faisalabad and Jhang.
The climate of this area is characterized by large seasonal fluctuations of
temperature and rainfall. Broadly speaking the whole year can be divided
into two seasons. The summer (Kharif) is hot lasting from April through
September with a temperature range of 49 to 21 °C. During winter (Rabi),
October to March, day time temperature ranges from 15 to 27 °C and night
time temperatures are normally in the range of 7 to 20 °C. About 75 percent of
total annual rainfall occurs during the months of June through September.
The soils of this area are capable of maintaining high fertility levels.
These soils are predominantly medium to moderately fine textured, easily
drainable, low in organic matter, and adaptable to a wide variety of crops.
Major crops grown in this area are cotton, rice, wheat, sugarcane, pulses, oil
seeds, vegetables, orchards and fodder.
Development of the water balance model
Water balance of a basin is the accounting of water gains and losses for a
certain period of time. Generally speaking the overall hydrologic system can
be divided into three sub-systems: surface water sub-system, soil-water subsystem and groundwater sub-system (Fig. 2). The general mass conservation
equation for any hydrologic system over certain time period can be written
as follows:
Inflow - Outflow = AS
(1)
300
where AS is the change in storage.
Surface water budget
The mass balance equation for the surface-water system can be written as:
{ ARF + ACS + RES + IR } { RO +IS + CL + DS + E + I } = A SWS
(2)
where ARF is the actual rainfall, ACS is the actual canal supply to the
system, RES is the input from surface water reservoir, IR is the irrigation
water supplied by tubewells, RO is the runoff, IS is the interception storage,
CL is the canal water leaving the area, DS is the depression storage, E is
the evaporation from surface water bodies (canals and watercourses), I is the
infiltration from soil surface across the lower boundary of system which is
input for lower system and ASWS is the change in surface water storage.
A runoff coefficient (ROC) was used to account for the surface runoff and
to calculate the effective rainfall. Runoff coefficient is the ratio between the
volume of runoff and volume of actual rainfall (ARF) and effective rainfall
(ERF) is the difference between the actual rainfall and runoff. Therefore ERF
= (1 - ROC)ARF. Interception was included in the evaporation while the
contributions of surface water reservoirs and depression storage were assumed
to be insignificant. No irrigation canals are leaving the study area.
Evaporation losses occur from the surface of water bodies, i.e. the irrigation canals and water courses. These losses occur from both the water
supplied by canals and by the tubewells. A coefficient called the canal loss
coefficient (CLC) was introduced in the model to accommodate these losses.
This coefficient is actually the ratio of total volume of water lost through
evaporation and total volume of water available in canals and water courses.
For long term calculations no significant change in surface water storage was
assumed. Equation 3 then becomes:
ERF + { 1 - CLC) {ACS + IR} = I
(3)
Soil water budget
The mass balance equation for the soil water sub-system which consists of
the root zone and unsaturated zone can be written as follows:
I - EP - T - DP = A SMS
(4)
301
ARF
AGWP
E
~'.:
H
A
~o
S3
Figure 2. Schematic representation of components of water balance model.
$1 = Surface water system, $2 = Soil water system, $3 = Groundwater system, ACS
Actual canal supply, IR = Irrigation water supplied by tubewells, ARF = Actual rainfall, I
Infiltration from surface water system to soil water system, RLC = Recharge from link canal,
RR - Recharge from river, T * Transpiration through plants, E - Evaporation from surface
of water bodies, EP * Evaporation losses from soil water system, RO = Surface runoff, ET =
Evapotranspiration losses directly from groundwater table, DP - Deep percolation from soil
water system into groundwater system, GFD = Groundwater flow towards drains.
where I is the infiltration f r o m u p p e r sub-system, EP is the evaporation f r o m
this sub-system, T is the transpiration through plants, D P is the deep percolation which enters the groundwater system and A S M S is the change in soil
moisture storage.
Evaporation and transpiration losses were c o m b i n e d as evapotranspiration
(ETP) which included evapotranspiration f r o m cultivated area, evaporation
losses f r o m fallow land and evapotranspiration f r o m the non-cultivated area.
It was a s s u m e d that moisture contents o f soil at the beginning and end o f
the cropping season are the s a m e and hence no significant net change in soil
moisture storage o v e r a season for long term calculations. Equation 5 then
becomes:
DP -- I - E T P
(5)
302
Groundwater budget
The hydrologic equation for the groundwater sub-system which includes the
saturated zone can be written as:
{GWI + AR + RR + RLC + DP} {GWO + AGWP + GFD + SPD + ET} = A GWS
(6)
where GWI is the regional groundwater inflow, AR is the artificial recharge,
RR is the recharge from rivers, RLC is the recharge from link canals, DP is the
deep percolation from the upper system, GWO is the regional groundwater
outflow, AGWP is the actual groundwater pumpage, GFD is the groundwater
flow toward the surface and sub-surface drains, SPD is the spring discharge,
ET is the evapotranspiration losses directly from the groundwater table and
AGWS is the change in groundwater storage.
For the regional water balance study the lateral inflows and out flows to and
from the system were considered non-significant as in WAPDA (1988). There
is no artificial recharge or spring discharge. Equation (6) then becomes:
RR + RLC + D P - AGWP - GFD - ET = AGWS
(7)
Combining the three sub-systems
The boundaries of the three sub-systems could not be defined absolutely as
detailed information regarding soil moisture conditions before and after every
season and contribution of groundwater to crop water requirements could not
be available. Also for the sake of accuracy and to minimize the assumptions,
the three sub-systems were coupled together. It was assumed that if surface
water was not sufficient to meet the crop water requirements the remaining
was contributed by groundwater. Schematic representation of the components
of the water balance model is shown in Fig. 2.
Putting Equations (5) and (3) in Equation (7) we get:
AGWS = [RR + RLC - GFD - ET - AGWP - ETP]
+ ERF + { 1 - CLC} { ACS + IR}
(8)
For simplification, in Equation (8), ET and ETP were combined together
as actual evapotranspiration (AET). A saline water coefficient (SWC) was
introduced to accommodate the percentage of pumpage which was saline and
hence was not being reused for irrigation. Then IR = [1 - SWC]AGWP. After
rearranging and simplification Equation (8) becomes:
{RR + RLC + ERF + ECS} {AET
+ EGWP + GFD} = AGWS
(9)
303
Altitude
210
200
(m) arnsl
-
~
L920 w a t e r t a b l e
-
~
1960 w a t e r t a b l e
~L992water~ble
~
~
190
180
i70
160
N ~ = Na~ttralland ~ur~c~
~
150
140
130
IIlilllJlilllJIllJllllll
4
7
10
13
Identification
16
19
22
No. o f O b s e r v a t i o n
ilr
25
Wells
28
[r i l l
31
34
Figure 3. Longitudinalpatternof groundwaterflow.
where ERF = { 1 - R O C ) A R F is the effective rainfall, ECS = {1-CLC}ACS
is the effective canal supplies and EGWP = {SWC + CLC - SWC*CLC}
AGWP is the effective groundwater pumpage.
Two major cropping seasons are Rabi (October-March) and Kharif (AprilSeptember). Crop water requirement is one of the major components of the
water balance. Moreover, in Pakistan the groundwater table for such large
areas is generally observed twice in a year viz: in May/June and in September/October. One objective of this study was to estimate the groundwater
recharge from observed groundwater levels for comparison with the recharge
estimated by the water balance approach. Therefor, these two seasons were
selected as the time step for more reliable calculations of crop water requirements and estimation of recharge from groundwater levels.
All values in Equation (9) are in volume units, i.e. million hectare meters
(MHM). This is the main equation upon which the water balance model was
based. The change in reservoir storage (volume of porous matrix) (ARVS)
for the ith season was calculated as follows:
(ARVS)i-
AGWS)i
(10)
Sy
where Sy is the specific yield. This change in reservoir was divided by the
gross area (1.622 MH) to obtain the rise or fall in the groundwater table during
a particular season. Long term seasonal average recharge to groundwater
reservoir (GWR) was calculated by averaging the recharge in individual
304
Discharge
(000 M^3/day)
16
"-~
14
1967--68
1968--69
1969--70
12
-'0-10
19B8-89
Aveg. of 4 y e a r s
8
6
2
,]'AN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
JAN
Fig. 4. Fluctuationsin dischargeof riverChenabbelowQadirabadheadworks.
seasons. Recharge for Rabi seasons was considered zero as change in storage
was negative for these seasons. The following equation was used:
GWR = _1~ (AGWS)j
n j=l
GA
(11)
where n is the number of recharge season (Kharif seasons) and GA is the
gross area (MH).
Calculation of water balance components
Different components of the water balance model were conceptualized and
estimated as follows.
Recharge from rivers
A general map of Rechna doab providing basic information about the study
area is shown in Fig. 1. Longitudinal pattern of groundwater flow in 34
observation wells in Rechna doab and of natural land surface (NLS) is shown
in Fig. 3. The identification number of observation wells increases from
the River Ravi to the River Chenab. This figure indicates that gradients of
groundwater flow and NLS are from the River Chenab towards the River Ravi
305
and from Q-B link canal to T-S link canal. From this it was concluded that
the River Ravi will not contribute any recharge to the aquifer underlying the
Rechna doab.
Hassan (1993), analyzed the water levels in the River Ravi at two headworks (Balloki and Sidhnai) and in observation wells along the bank of river,
and concluded that the water level in the River Ravi and adjacent observation
wells were more or less the same. Therefore it was concluded that there was
no significant flow to or from the the River Ravi. Similar analysis was conducted for the River Chenab by Hassan (1993) and results revealed that the
fiver contributes recharge to the aquifer.
Monthly fluctuations in discharge of the River Chenab below Qadirabad
headworks as depicted in Fig. 4 indicate very low discharge in the river
during the winter months (Rabi season). From this figure it was concluded
that recharge from river Chenab will occur only during Kharif season (April
to September).
In summary, in the present study, recharge from only the River Chenab
during the Kharif season was considered and was calculated by the following
equation (Darcy Law):
RR = 10-7*T*i*L*t
(12)
where RR is the recharge from fiver in million hectare meter (MHM) per
season, L is the length of fiver contributing for recharge in km, i is the
hydraulic gradient, T is the aquifer transmissivity in m2/day, t is the time in
days per season for which recharge from the fiver will occur and 10 -7 is the
conversion factor.
Recharge from link canals
Both link canals (Q-B and T-S) are un-lined and dug-in and serve the purpose
of supplying water from the River Chenab to the River Ravi. Both link
canals are very big (Q-B = 13416 ft3/s, T-S = 11009 ft3/sec), intercept the
groundwater table and act as groundwater divide. Hence no groundwater flow
across these canals was considered. Trimmu Sidhnai (T-S) link canal will not
contribute any recharge to the aquifer of the study area due to gradient of
groundwater and natural land surface (Fig. 3). Qadirabad-Balloki (Q-B) link
canal is a very big perennial canal and recharge from this canal was considered
constant irrespective of season. The following equation was used to estimate
the recharge from Q-B link canal:
RLC = 4.5* 10-5*Q*SLD
(13)
where RLC is recharge from link canal in MHM per season, Q is the discharge
capacity of Q-B link canal at head in (13416 ft3/s), SLD is the link canal loss
coefficient (5.8) and 4.5* 10 -5 is the conversion factor.
306
Rainfall(ram)
160 ,
"
140
[ Average from
) @
196t to 1970
/.
120
/
,,,,
/g,\
/ Z
"°' ,o ,o,o
,o ,08o
\
//_\\
I00
80
60
40
20
0
JAN
i
r
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
JAN
Months
Fig. 5. Meanmonthlyrainfall in Rechnadoab (averageof three stations).
Rainfall
Rainfall data of three stations (Gojra, Faisalabad and Chuharkana) for a
period of 31 years were collected from Pakistan Meteorological Department,
which was averaged arithmetically to obtain the average seasonal rainfall.
This rainfall (ARF) was utilized in Equation (9) to calculate the effective
rainfall as follows:
E R F = ( 1 - ROC)ARF
(14)
where ERF is the effective rainfall, ROC is runoff coefficient and ARF is the
actual rainfall.
As indicated in Fig. 5, the maximum proportion of annual rainfall occurs
during the months of July and August (Kharif season) which indicates no
runoffduring Rabi season. Most of the study area is under irrigated agriculture
and runoff is not so significant due to "bunds" around the fields. Most of the
rainfall is consumed where it falls except during some rainy months.
Canal water supply
Recharge from irrigation canals is the significant component of the water
balance model. Historical record of water supplied by Lower Chenab Canal
307
Thousands
of t u b w e l l s
300
PAKIBTAN
280
j
PUNJAB
f
200
160
100
50
0
1960
1965
1970
1975
Years
1980
1985
1990
Fig. 6. Developmentof privatetubewells.
(LCC) was obtained from Faisalabad Irrigation and Power Department and
was utilized in Equation (9) to calculate effective canal supply as follows:
ECS = { 1 - CLC}ACS
(15)
where ECS is the effective canal supply, CLC is canal loss coefficient and
ACS is the actual canal supply.
Groundwater pumpage
Total pumpage from the groundwater reservoir includes the pumpage by
private and by SCARP tubewells. The number of private tubwells developed
in the study area from 1960 to 1990 was collected from different statistical
reports, e.g. Bureau of Statistics (1991, 1992) and Vander Velde and Kijne
(1992) and is shown in Fig. 6.
Hassan (1993) compiled the results of various studies carried out for
the estimation of discharge and utilization factor of private tubewells. The
utilization factor was in the range of 15.6% to 26.8% and tubewell discharge
ranged from 0.89 to 1.33 s2. Pumpage from groundwater reservoir has not
much effect on the regional water budget of the system as most of the pumped
water is again diverted for irrigation purposes. Pumpage by private tubewells
was calculated by following equation:
PPTW = 8.9* 10-5*N*UF*Qp
(16)
308
where PPTW is the pumpage by private tubewells in MHM per season, N is
the number of private tubewells for a season, UF is the utilization factor of
private tubewells, Qp is the discharge of private tubewell in ft3/s and 8.9* 10 -5
is the conversion factor.
Data on SCARP tubewell pumpage were obtained from SCARP Monitoring Organization (SMO) of WAPDA. Calculated pumpage by private
tubewells and collected data on SCARP pumpage were added to obtain the
actual groundwater pumpage (AGWP), which was utilized in Equation (9) to
calculate the effective groundwater pumpage (EGWP) as follows:
EGWP = {SWC + CLC - SWC*CLC}AGWP
(17)
where all symbols are as already defined.
Drainage
Hassan (1993) analyzed the contours of depth to water table which indicated
the location of sources (irrigation canals) and sinks (drains) causing the local
flow of groundwater and no significant sub-surface flow out of the study
area was considered. Large areal extent and mild slope of the basin are also
the factors which support the non-significant subsurface outflow from the
system. Up to the year 1990 there were no significant sub-surface horizontal
drainage projects in the study area. Only some surface drains were present
which carried the surface runoff and groundwater seepage. The groundwater
flow to surface drains (GFD) was calculated by following equation after Awan
(1983):
GFD = SMAX (1 - DWT/DC) if DWT < DC
[ = 0 if DWT > DC]
(18)
where SMAX is the maximum seepage to drains (0.012 m/season, Awan
1983), DWT is the weighted average depth to water table m, and DC is the
critical depth of drains (3.5 m).
Weighted average depth to water table was calculated from the maps of
depth to water table obtained from SCARP Monitoring Organization (SMO)
of WAPDA. The area under a particular range of depth to water table during
a particular year was measured by planimeter and then was multiplied with
the average depth to water table of that range. Then sum of all such products
was divided by the gross area to calculate average depth to water table.
Actual evapotranspiration
Doorenbos and Pruitt (1977) recommended that the Modified Penman Method
is the most accurate method to calculate reference crop evapotranspiration
309
Kc v a l u e s
1.4
I
0.8
~
q
,
0.6
0.4
0.2
o-~-
I
I
[
I
[
I
I
I
I
I
San
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Months
Dec
of year
Figure 7. Weighted average values of Kc for three cropping patterns.
(ETO), and should be used whenever the data are available. The data needed
for this method include temperature, wind, sunshine and humidity. As these
data were available from the Meteorological Department of Pakistan, this
method was used to calculate ETO on monthly basis.
The study area was divided into cultivated and uncultivated areas. The
uncultivated area was further divided into two sub-areas; uncultivated area
but having vegetation and uncultivated bare soil. Doorenbos and Pruitt (1977)
described the crop coefficient values for different weeds under different climatic conditions. For uncultivated area with vegetation, the values of crop
coefficient (Kc) were selected from this paper and this area was considered as
cropped area to calculate the evapotranspiration values. The following equation was used to calculate the evaporation losses from uncultivated bare area
(Awan 1983):
F o1_68
EPBS = EPAN*NCBA [(0.201 + DWT) + 0.009
]
(19)
where EPBS is the evaporation from bare soil in M H M per season, EPAN is
the pan evaporation (m/season), DWT is the average depth to water table (m),
NCBA is the never cultivated bare area (MH).
Data on pan evaporation were obtained from Meteorological Department
of Pakistan.
The cultivated area was divided into cropped and fallow areas. Doorenbos
and Pruitt (1977) recommended a method for selection of values of crop coef-
310
ficient for fallow land keeping in view the ETO and irrigation or significant
rainfall. It was assumed that evaporation from fallow land follows the pattern
of evapotranspiration from cropped fields during the initial stage of crop. Kc
values to calculate evapotranspiration from fallow area were selected from
this publication and the area was included in the cultivated area with these
values of Kc. Crop water requirements of cultivated area were calculated by
the following equation.
ETCROP = ETO * Kc
(20)
During a particular season, there was overlap between different grown
crops with different values of crop coefficients. Therefore one value of Kc
was required for each month which could take into account all the crops
grown during a particular month. The study period was divided into three
periods and cropping pattern for each period was determined from the data
collected from Irrigation and Power Department. Hassan (1993) compiled
the values of Kc for different crops grown in Rechna doab, which were used
in calculation of weighted average value of Kc. Weighted average value of
Kc for every month for a particular cropping pattern was calculated from Kc
values of different crops grown in that month as follows:
in
E (Kcij*Aij
K c j - i=l
Aj
(21)
where Kcj is the weighted average crop coefficient for jth month, Kci,j is the
crop coefficient for ith crop grown in jth month, Ai,j is the area under ith crop
in jth month in percent of culturable commanded area (CCA), m is the total
number of crops grown in jth month and Aj is the total area under different
crops in jth month (percent of CCA).
Explanation of procedure for calculations and calculated values of weighted average crop coefficients are given in Hassan (1993). Values of weighted
average crop coefficients for three cropping patterns are plotted in Fig. 7.
After calculating the weighted average values of crop coefficient for different months of year the area was considered under one crop with weighted average value of Kc. It was assumed that vegetables and miscellaneous
crops consume water equal to the weighted average crop water requirement
(ETCROP) of all the crops grown. Hence weighted average value of Kc was
used to calculate the ETCROP of vegetables and miscellaneous crops. The
total ETCROP for a season was the calculated by the following equations:
ETCROPs
A.~
j=l
(ETOj • Kcj • Aj • NODj)
1000
(22)
311
N
o
~_
o
32 ° 22"5*
'O° 30' N
Figure 8. Thiessenpolygonshowingeffectivearea of observationwells.
where ETOj is the reference crop evapotranspiration (ram/d) for jth month,
ETCROPs is the crop water requirement for a season (MHM), n is the number
of months in a season, Kcj is the weighted average crop coefficient for jth
month, NODj is the number of days in jth month, Aj is the total cropped area
under different crops in jth month (MH).
This area also included the area under vegetables, miscellaneous crops,
the uncultivated area covered with weeds and fallow area. This ETCROP
and evaporation from bare soil (EPBS) were added to get the total actual
evapotranspiration as follows.
A E T = ETCROP+EPBS
This value of AET was used in Equation (9).
(23)
312
Results of water balance calculations
An interactive computer program in FORTRAN 77 was written to facilitate
the computations and to permit the changes in different coefficients. The
coefficients could be changed for every run and output was calculated in
the form of change in groundwater reservoir and recharge to groundwater
reservoir on seasonal basis.
Results from the water balance model indicated a net recharge to groundwater reservoir during Kharif seasons and depletion of groundwater reservoir
during Rabi seasons. Seasonal net recharge to groundwater reservoir was in
the range of 14 mm to 123 mm for a period of 31 years (1960-1990). Long
term average seasonal net recharge was found to be 57 mm per season. Results
indicated that long term seasonal positive change in storage was 0.309 MHM
and corresponding values for reservoir depletion was 0.435 MHM, which
indicated the overall depletion of groundwater reservoir over a period of 31
years.
Groundwater recharge estimated by the specific yield method
Thirty four observation wells as shown in Fig. 8 in Rechna doab were selected,
with records of observed seasonal water levels. The rise or fall for every
observation well was calculated for every season by taking the difference
of observed water levels at the end and beginning of that season. By the
Thiessen Polygon Method every observation well was assigned an effective
area as shown in Fig. 8. Here it was assumed that the rise or fall in water level
in a particular observation well represents the effective area of that observation
well. Then this rise or fall at an observation well was multiplied by effective
area of that observation well to calculate the change in reservoir storage
(volume of porous matrix). This process was repeated for all observation
wells for a particular season.
Most of the observation wells indicated a negative change (depletion) in
reservoir for Rabi seasons and a positive change (recharge) for Kharif seasons.
However, some observation wells indicated reverse response which may be
due to local sources and sinks consequent upon irrigation canals and pumping
by SCARP tubewells. Net change in reservoir storage (ARVS) for a particular
season was obtained by summing algebraically changes in reservoir storage
at all the observation wells for that season. The long term seasonal recharge
(GWR) was calculated by following equation:
1 n (ARVS)j*Sy
GWR = n j~1=
GA
(24)
313
Change in S t o r ~ e
(MHM)
1
0.8
0.6
7
0.4
0.2
0
-0.2
-0.4
-0.6
1 < Water Balance
-0.8
-I
0
IgSO
r~ Specific Yield
i
i
i
i
i
i
i
i
i
I
i
5
I0
15
20
25
30
3,5
40
4,5
50
,55
Seasons
60
1990
Fig. 9. Comparisonof changesin reservoirstorage calculatedby two methods.
where n is the number of recharge (Kharif) seasons, S v is the specific yield
of aquifer and GA is the gross area (MH).
The results obtained from this method were similar to those obtained by
the water balance model, i.e. on average, recharge during Kharif seasons
and depletion during Rabi seasons. Seasonal net recharge estimated by this
method for the thirty years period was in the range of 17 mm to 119 mm with
a long term seasonal average of 58 mm. Long term seasonal average positive
change in reservoir storage was 0.302 MHM and the long term seasonal
average negative change in storage was 0.438 MHM which also indicates a
continuous depletion of groundwater reservoir and matches with the observed
2.32 meter fall in average groundwater table of Rechna doab over a period of
31 years.
Results
A linear regression analysis was conducted to evaluate the relationship between
the recharge estimated by water balance and specific yield methods. The
results obtained by first model run did not indicate good comparison as shown
in Table 1 and the adjustment in the coefficients of the water balance model
was necessary to obtain a reasonable matching between the results obtained
by two methods. The coefficients to be adjusted in the computer model with
their values after final adjustment are given in Table 2. Based upon the review
314
Table 1. Results of linear regression of recharge estimated by two methods.
Parameter
Initial run of model
Final run of model
X Coefficient
Constant
Standard Error of Y Est
R Squared
No. of observations
Degree of freedom
Standard error of coefficient
Correlation coefficient
0.07
47.45
28.04
0.01
30
28
0.18
0.08
0.98
1.55
3.15
0.98
30
28
0.03
0.99
Table 2. Coefficients used in computer program with the final adjusted values.
Coefficient
Description of coefficient
Final value
ROC
CLC
Runoff coefficient
Canal loss coefficient, i.e. loss of canal water
from water surface and bank vegetation
Link canal loss coefficient
Discharge of private tubewell
Utilization factor of private tubewells
Aquifer storage coefficient or specific yield
Aquifer transmissivity
Saline water coefficient, i.e. percentage
of pumpage which is saline
10%
SLD
QI'
UF
SC
T
SWC
12%
5.8%
1.0 ft3/s
16%
30%
5818 m2/days
10%
of literature, ranges of adjustment for different coefficients were prepared and
coefficients were adjusted within those ranges.
R u n o f f coefficient for the Rabi season (winter) was considered zero due
to low rainfall. Loss of water from water surface of irrigation canals and
water courses in the Rabi season was taken 50 percent of the corresponding
value for the Kharif season due to low evaporation losses. Utilization factor
of private tubewells was decreased as the number of tubewells increased as
in W A P D A (1988). Recharge from the fiver was changed by changing the
aquifer transmissivity, time period o f recharge and length of river contributing
recharge. Seepage from link canal was also adjusted by changing the link canal
loss coefficient. Groundwater flow towards drains was adjusted by changing
the depth of drains.
315
Recharge
by Specific Yield (ram)
140
Corr. coef. O.@g
120
1:1 Line
100
60
[]
60
40
20
0
I
1
I
10
20
SO
Fig. 10.
~<
Rainfall
I
I
40
50
Recharge
I
I
I
I
I
60
70
flO
90
100
by Water Balance (ram)
I
I
t
110
120
130
140
C o m p a r i s o n of r e c h a r g e e s t i m a t e d by two methods.
~
Canal supply
~
Recharge
. 0
AET
mm
1000
800
600
400
200
0
1960
I
I
I
|
I
1965
1970
1975
Years
1960
1985
I
I
I
1990
Fig. 11. Majorseasonal components of water balance model.
Changes in reservoir storage over every season were estimated by both
methods and are plotted in Fig. 9. This figure indicates a very close match
between results obtained by two methods.
The results also indicated that by both methods a long term average depletion of reservoir is higher than the long term average recharge to reservoir.
316
It implies the continuous depletion of overall reservoir over a period of 31
years. Reasons for this are the continuous increase in cropping area, cropping
intensity, public and private tubewells. Scatter plot of recharge estimated by
two methods is presented in Fig. 10. Scattering of points around a line of
45 ° provides a proof of agreement between the results obtained by the two
methods.
The temporal changes in major water balance components is depicted in
Fig. 11. From the water balance study it was concluded that the major inputs to
the hydrologic system are rainfall and canal water supply, followed at a large
distance by the recharge from Qadirabad-Balloki link canal. Recharge from
the River Chenab seemed to be not significant. The major out component of
hydrologic system is evapotranspiration, with the next significant component
discharge to drains. Pumpage by tubewells is not significant for the regional
water balance of the entire basin as most of the water pumped by tubewells
in reused for irrigation.
Conclusions
In the present study recharge to groundwater reservoir has been estimated
by two methods, viz: water balance approach and specific yield method.
Efforts have been made to avoid the assumptions and sources of error in the
methodology. For this purpose the sub-components of the hydrologic cycle
have been lumped into a single system to minimize the errors associated with
the calculations of different components of the individual sub-systems. The
time step for calculations in both methods has been kept the same (Rabi and
Kharif), which depended upon the available observed groundwater level data
and the cropping seasons in Pakistan.
In the specific yield methods the results depended upon the observed water
levels and the aquifer specific yield. The intensity of observation wells was
only 34 wells for an area of 1.622 million hectares. The results from both
methods were compared to come up with a better value of the long term
recharge to the groundwater reservoir. The major objective of the study was
to evaluate the general response of the whole reservoir of Rechna doab and
to estimate a value of net recharge to the reservoir on regional basis; the local
response of the reservoir may be different from this overall response. The
results from this study indicated that the reservoir was being depleted on a
regional basis and the long term annual recharge to the groundwater reservoir
is about 60 mm.
317
References
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Excellence in Water Resources Engineering, University of Engineering and Technology,
Lahore, Pakistan.
Bureau of Statistics (1991). Punjab Development Statistics. Government of The Punjab,
Lahore, Pakistan.
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Lahore, Pakistan.
Doorenbos, J. & Pruitt, W.O. (1977). Guidelines for Predicting Crop Water Requirements.
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Rome.
Hassan, G.Z. (1993). Evaluation of the Groundwater Resources of Faisalabad, Pakistan.
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12-13, 1992, New Dehli, India.
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