Assessment of groundwater potential for conjunctive water use in a large irrigation project in India

Journal of Hydrology, 107 (1989) 283-295 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

283

A S S E S S M E N T OF G R O U N D W A T E R POTENTIAL FOR CONJUNCTIVE W A T E R USE IN A LARGE IRRIGATION PROJECT IN INDIA

S.K. SONDHI', N.H. RAO 2 and P.B.S. SARMA 2

1Department of Soil and Water Conservation Engineering, Punjab Agricultural University, Ludhiana (India) 2 Water Technology Centre, Indian Agricultural Research Institute, New Delhi .110 012 (India) (Received July 15, 1988; accepted for publication August 8, 1988)

ABSTRACT Sondhi, S.K., Rao, N.H. and Sarma, P.B.S., 1989. Assessment of groundwater potential for conjunctive water use in a large irrigation project in India. J. Hydrol., 107: 283-295. In many large-surface irrigation projects, the potential for groundwater development has increased significantly. The additional potential can be used to develop conjunctive water management plans for augmenting canal water supplies and increasing agricultural productivity in the project area, if its spatial distribution is also known. A methodology for determining the available additional groundwater potential and its distribution in the Mahi Right Bank Canal Project in Gujarat is presented. The procedure is based on the use of specific empirical constants for estimating groundwater recharge from the water conveyance and distribution system and the annual water balance of the project. The spatial distribution of groundwater potential is determined by "recharge distribution coefficients" derived from a digital simulation model of the groundwater basin of the project area.

INTRODUCTION

In India, canal irrigation by major and medium projects accounts for about 30.5 x 10Sha of the total irrigation potential. Of this about 21 x 10Sha was created after 1950-51 during the successive five year plans (Planning Commission, Government of India, 1985). Groundwater was the major source of irrigation in many areas prior to the introduction of canal irrigation. It continues to be so in several major surface irrigation project areas, even though this factor is not explicitly considered in their design and operation. In fact, the potential for groundwater development has increased because of recharge of aquifers from water conveyance and distribution systems of the surface water projects. In many projects, the water table is rising and some areas are threatened by water logging (Water Technology Centre, 1983). The Groundwater Estimation Committee (Ministry of Irrigation, 1984) recognised the availability of additional groundwater potential in major irrigation

0022-1694/89/$03.50

(~ 1989 Elsevier Science Publishers B.V.

284

projects and recommended specific empirical constants for estimating groundwater recharge from the water conveyance and distribution systems of these projects. These constants are now routinely applied by the various government agencies to estimate the groundwater potential of a region (Raju, 1987). Coupled with this recognition of the increasing groundwater potential, is the recent emphasis on conjunctive use of canal water and groundwater to: (1) augment canal supplies; (2) prevent land deterioration; (3) increase water use efficiency; and (4) ensure reliable and equitable distribution of water resources (Planning Commission, Government of India, 1985). The main difficulty in translating such policy imperatives into operational conjunctive water management plans lies in assessing the spatial distribution of recharge over the groundwater basin. This is controlled by local variations in aquifer characteristics, rainfall, water resources and land use. The empirical constants recommended by the Groundwater Estimation Committee provide only a lumped value of regional groundwater recharge for the project. Delineation of the spatial distribution of this recharge is possible either by costly and timeconsuming field experiments at a large number of locations in a project area, or analytically by using groundwater models. The specific objectives of this study are to first assess the net regional recharge of the groundwater basin underlying a major irrigation project in India. Next, a procedure for determining the spatial distribution of net recharge is developed. The procedure is based on a simulation model of the groundwater basin. The model results are validated by comparison with data of observed groundwater levels. Finally, the spatial distribution of the potential for groundwater development is also assessed. THE STUDYAREA The Mahi Right Bank Canal (MRBC) Project in Gujarat is selected for this study. The geographical area covered by this project is 316,000 ha of which about 213,000ha are suitable for cultivation. The climate of the area is arid to semi-arid with an average annual rainfall of 823 ram. About 96% of this rain occurs in the monsoon season (June-Sept.) and there is substantial variation in monthly and annual rainfall. Medium to coarse textured soils cover the major portion of the area. The soils are deep ( > 90 cm) with moisture holding capacity in the range 38-50%. The irrigation project is operative since 1958. The specific area selected for this study (Fig. 1) is the region within the project area bounded by the rivers Mahi and Shedi, and the Alang drain. The selected region has an area of about 295,200 ha or about 93% of the geographical area covered by the project. The data of crop years (June-May) 1976-77 to 1979-80 are used.

Canal system The Kadana Reservoir and Wanakbori Weir, both located on the Mahi River, constitute the headworks of the irrigation system (Fig. 1). The water

285

/

f" L"~

SCALE O._J_~O Km

-

.J" ~

f.~ c~ c

~. J

q" : • .,

7 ~

~ -'~

Canatsystem

~

Project area bou,~ry River

~

Drain

,o,,o,.o

GULFOF CAHBAY ~

~

j

Fig. 1. Area selected for conjunctive water use studies in the MRBC command area.

distribution system comprises a main canal, six branch canals, and a number of distributaries and outlets. The main canal and branch canals are lined. Below the outlet the water distribution is mainly field-to-field. The irrigation system has a conveyance efficiency of 63% (Water Technology Centre, 1983). Grour,~dwater The aquifers are mainly gravel and sand with an average thickness of 30-60 m. There is a distinct water table aquifer of 10-15 m thickness overlying a main semiconfined aquifer. In the northeastern portion, and in some areas to the east of Cambay, the aquifer is unconfined. Over the remaining portion, leaky confined conditions are common. The groundwater quality is good in a major portion of the area. The aquifer transmissivity ranges from 196 to 6830m 2 d -~ (Fig.2) with the higher values occurring in the eastern portion. The storage coefficient varies between 0.00015 and 0.00417 and the leakage coefficient between 0.0016 and 0.006 d-!. The estimated annual groundwater drafts during 1976-77 to ]979-80, based on the number of different types of wells operating in the area, are (Sondhi, 1984} 148, 156, 168, 192 × 106 m 3. Need for conjuctive water use In an earlier study (Rao and Sarma, 1988) on water resources utilization in the Project, it was observed that conjunctive use of canal and groundwater was necessary for the following reasons:

286

--" 12/*2 .... 1552"5

N -- ~ 6m'°-

t

b

.12 ?e k.,~

-931"5

~ ' ~

PROJECT

AREA BOUNDARY

RIVER ------.- CONTOURS(mZ/~y)

,.% Fig. 2. Transmissivity map of confined aquifers (MRBC command area).

(1) The distribution of cana! water over the project area is not uniform and the canal releases are not adequate to meet the water requirements of the existing cropping pattern. (2) When augmented with groundwater (at the current level of development) and after making better use of rainfall and river flows, the total water resources are adequate to meet the crop water requirements, except in the summer season. (3) Since the introduction of canal irrigation, there has been a gradual rise in the water table. In recent years, the average rate of rise has been about 1 m v r - 1. ASSESSMENT OF NET REGIONAL GROUNDWATER RECHARGE

The net regional recharge (Rn) to groundwater during any time period is estimated as:

R,~ = Rg + Q g - Qp

(1)

in which: Rg = gross recharge to groundwater basin; Qg = groundwater inflow/outflow to neighbouring areas; and Qp = groundwater extraction through wells. The gross recharge (Rg) to the groundwater basin is given by: Rg = Rp + Rc + Rd +

Rci

+ Rwi

(2)

287 where: Rp --- recharge due to rainfall; Rc = rechsrge due to seepage from main canal and branches; Rd = recharge due to seepage from distributaries; Rci --- recharge due to percolation losses from areas irrigated by canal water; and Rwi = recharge due to percolation losses from areas irrigated by wells. All the terms on the right hand side of eqn. (2) can be estimated using the recommendations of the Groundwater Estimation Committee of the Ministry of Irrigation (1984), with some modifications based on field observations. The specific percentage allowances used for the various recharge components are as follows (Rao and Sarma, 1988): (1) Recharge from rainfall (Rp) is estimated at 18% of the annual rainfall. (2) Recharge due to seepage losses from the lined main and branch canals (Re) are calculated as 0 . 6 m 3 s -1 10 e m -2. (The recommended value is 0 . 3 m 3 s -l 106 m-2 but field experience indicated that actual losses may be nearly twice this value). This is 2.5% of the average annual canal release at headworks. (3) Recharge due to seepage from distributaries (Rd) is 7%. (4) Recharge due to return seepage from canal irrigated fields (Rci) is 30%. (5) Recharge due to return seepage from well-irrigated areas (Rwi) is 15%. (This relatively low value was adopted as in many fields an underground pipe line system is used.) (6) In addition to above, seepage losses from paddy fields contribute 3 m m d - ' for 100 days. In eqn. (1) the volume of ground water extracted each y e a r (QD) is estimated from data of number and types of wells operating in the region (Sondhi, 1984). The subsurface inflow or outflow of groundwater across the aquifer boundaries (Qg) c a n n o t be directly estimated. It is, therefore, calculated as a residue from the annual (crop year) water balance equation: (P + I )

= ( E T + E + Qg + QD + Q~: +_ AS8 +_ ASm +_ ASg)

(3)

where: P = precipitation over the region; I = sum of irrigation water applied over the region by the canal system (Qc) and ground w a t e r (Qp) from the cultivated areas of the region (I = Q~ + Qp); E ~ ' = evapotranspiration; E = soil evaporation from uncultivated areas; Qs = direct surface runoff from thc area; ASm = change in soil moisture storage; ASs = change in surface water storage; and ASg = change in groundwater storage. All the above components are expressed in volume units (10e m s) for the area of 295,200 ha. Average monthly rainfall data of eight raingauge stations located within the region are used to determine P for the years 1976-77 to 1979-80. Data of the monthly canal water releases at the headworks and groundwater extraction during 1976-80 are used to obtain I. For the annual water balance (for the crop year June-May) both ASs and ASm may reasonably be assumed to be zero. ASg is calculated from data of premonsoon (June) water levels recorded at a number of observation wells distributed over the project areas. The volume of water corresponding tn this rise is computed by using a value of 0.15 for the specific yield of the aquiIer.

288

Evapotranspiration (ET) is estimated from monthly data of potential evapotranspiration and the area under various crops. A distinction is made between evapotranspiration from irrigated and unirrigated areas for each crop. It is presumed that in the monsoon season, evapotranspiration from both irrigated and unirrigated cropped areas occurs at potential rate. In the nonmonsoon season, evapotranspiration from the irrigated crops is at potential rate. For the unirrigated crops, E T is limited by the maximum available soil moisture at the beginning of the season, in the effective root zone of crops. The effective root zone depths for various crops are obtained from Water Technology Centre (1983). Evaporation from uncultivated areas (E) is calculated using Ritchie's equation (Ritchie, 1972):

E = ETo t-1/2

(4)

in which ETo is the reference evapotranspiration, and t is time after the last rain, in days. In the monsoon season, when the soil is frequently wetted, E is estimated to be about 50% of ETo by this formula. In other seasons it is about 16% (Oct.-Feb.) and 9% (March-June) of ETo. The values are modified suitably if there is rain in nonmonsoon months. Surface runoff (Qs) in the project area was estimated using historical data of monsoon seasonal runoff at the K a d a n a Reservoir site and the weighted seasonal rainfall (R) in the catchment of this reservoir. Based on this data (Water Technology Centre, 1983; P a r t h a s a r a t h y et al., 1987) a linear relationship given by: Qs =

- 285 + 0.76 R (106 m s )

(5)

was obtained. Qs for the project area was determined using this equation and values of observed monsoon rainfall in the area during the year 1976-77 to 1979-80. Now all the terms of the annual water balance equation except Qg are known, this quantity can be determined as a residual of eqn. (3) (Table 1). The net annual recharge and the corresponding water table rise can therefore be estimated using eqns. (1) and (2) (Table 2). Comparisons with the observed water table rise during the four years 1976-77 to 1979-80 show (Table 2) t h a t the estimates of net regional groundwater recharge are realistic. A S S E S S M E N T OF R E G I O N A L G R O U N D W A T E R

POTENTIAL

The average net annual groundwater recharge for the period 1976-77 to 1979-80 is 485 x 106 m 3. The average rainfall during this period is 1053mm. However, the long-term average rainfall for the region is only 823 mm. When the net recharge is normalized for the long-term mean rainfall, the average annual groundwater recharge is about 380 × 106 m 3. About 70% of this value is considered utilizable (Ministry of Irrigation, 1984). Hence, the additional groundwater potential of the region (Rn) is estimated at 265 x 106 m 3. How this

289 TABLE 1

Annual water balance of the project area (106 m3) Water balance component

Rainfall (P) Canal supplies (Qc) Groundwater (Qp) Evapotranspiration (ET) Evaporation (E) Change in groundwater storage (ASg) Surface runoff (Qs) Groundwater outflow (Qs) +

Year

Average

1976-77

1977-78

1978-79

1979-80

4440 849 148 1180 615 806

3041 1244 156 1169 553 593

2453 1470 168 1282 487 156

2497 1448 192 1463 466 425

3198 1253 166 1274 530 495

2412

1461

1003

941

1454

276

509

995

650

608

net potential is distributed spatially over the groundwater basin will determine the specific program of groundwater development for conjunctive water use in the irrigation project. SPATIAL DISTRIBUTION O F G R O U N D W A T E R

RECHARGE

The spatial distribution of the net groundwater recharge obtained above is sought by using a simulation model of the groundwater basin underlying the region. The model is based on a digital computer model developed by Prickett and Lonnquist (1971). It allows for simulation of both leaky confined and unconfined zones of the aquifer. Essentially, the procedure consists of numerical solution of the two-dimensional groundwater flow equation by a finite difference approach. The aquifer is discretized by superimposing a twodimensional rectangular grid over the region with 143 block centered nodes in ~,h;rteen columns (i) and eleven rows (j). A constant grid spacing of G0(10 × 6000 m is used. In all 82 nodes lie in the study area (Fig.3).At each node, ~he data of initial water table levels, transmissivity, storage coefficients and average annual pumping periods at nodes with wells, are fed into the model. A detailed discription of the model is given by Sondhi (1984). The model is firstrun with net annual recharge at each node (i,j)set to zero. rhe groundwater levels computed by the model at each node (hi#)are compared wi~h the corresponding observed water levels (Hij).Before doing so, the latter are corrected for the effects of groundwater pumping (ifany) at the node. The difference between the corrected observed water levelsand the computed levels

290 TABLE 2 Net annual groundwater recharge in the project area (106 m 3) Serial no.

1 2

Source of recharge

Rainfall (Rp) Seepage from main canal and branches (Re) Seepage from distributaries (Rd) Returnflow from canal irrigation (Rci) Return flow from well irrigation (Rwi) Groundwater extraction (Qp) Groundwater outflow (Qg) Net recharge (R n) --- (1) + (2) + (3) +

3 4 5 6 7 8

(4) + (5) -

9

(6) -

1976--77

1977-78

1978-79

1979-80

799 33

547 33

442 31

450 39

560 34

57

85

101

99

85

393

525

661

640

555

22

24

25

29

25

148

156

168

192

166

276

509

995

650

608

880

549

97

415

485

(7)

Equivalent water table rise (m) Observed water table rise (m)

10

Average

Recharge

1.99

1.24

0.22

0.94

1.10

1.82

1.34

0.35

0.96

1.12

at each node is ascribed to the effects of groundwater recharge (r~j). If Qo is the groundwater draft: ro

Hij --h~ 1 + v~j/Ao

(6)

A is the area represented by the node (i, j) and S the specific yield of the aquifer. The recharge distribution coefficient at node (i, j) is defined as. rd °

=

1

rij M N

(7)

i=l j=l

M N

The term i,~lj=j Z Z rij represents the total net regional groundwater recharge of the basin. Thus, the denominater of eqn. (7) represents the average recharge per node, if the total net regional recharge is distributed uniform] ~, over the entire region. Values of rd o were obtained for all nodes for each of the years 1976-77 to

291 --------~.

1

!

1

2

0

0

3

S

4

6

N

"

3

&

•

S

•

®

•

•

• I I

•

inUili

.

m

_

•

~ • I

•

•

•

•

•

•

•

~*~

• ;-T--

•

•

• i

•

"/ 8

~

9

0

,,

o

k

•

•

~ , ~

•

•

®

~

•

•

o

o

o

0

0

0~ ~ 0

12km

|

I 6

•

•

J----

I

i

•

•

•

•

•

•

•

•

•

I 0

O

0

e

0

O

O

•

•

e

i • !

•

•

•

•

o

o

o

•

•

iI •

•

•

0

0 I re

0

0

0

o

o

•

Boundary nodes Unconfined nodls

• e

Co,fined nodpe No~s outside the equifer . . . . Approximated ~ . boundnris - - ' - - ProJacf erie beucdary River Drain

ooooo

Fig. 3. Fixed, block-centered grid configuration for the MRBC command area aquifer model.

| ~'~-"~"

1

2

~

i

3 I

4 I

5 I

"

6 i

7 i

0.456 o.6st, o.31,. _

11

!

"" ~ - . . . ~ c ~

~ : ro.ol~

12

I

N

. o.o6o . . .

1.361 1.610 1.653 1"037 1"429 1"369 0"5~ 0"255

).127 0"221 1"676 0.700 1'939 1'339 o~

10

9 i

t 0.753 0'.52

5 ¸

0 i

0.665 o639 1.0,,4 2.3~

2"593 2'623

d 2'10S 0"660 1"029/ f~

2.936 1.7s5 2.07,. i . ~

0.9,.7

O

A

8

12km

~.~

0.300 0.4/,3 1.200 1.463 1.603 0.722 1.341 1.6S5 0.O t

"310

1.219 1.619 1"973 1''.70

0.59, 0"9,5 0 , , , 1 i ] . ~

601 1.261 0,69e 1,005 o.e9~ 0,490 0.e14 1.131 Il ;:~~i:: 1,092 0.569 0.969 0.530 0.742

:t

"

--

Prol

~

ict erea boundary

1.265 1367

0.974 0.9Z6 0"229 I0.726 :'~'I ~ j .. ~-..~.~ '

Fig. 4. Recharge distribution coefficient for distribution of net recharge in the MRBC command area.

292 m

1979-80. At each node, the average value for the 4 years (rdij) is computed (Fig. 4). They represent the average recharge distribution coefficients of the project area.

Validation of the procedure The groundwater model is run with all the required input data including groundwater draft and recharge at each node for each of the 4 years. The recharge at each node is given by:

RD~j = ~d o Rn

(8)

Values of rd---~ i are taken from Fig.'4 and Rn for each year from Table 2. The comparisons between observed and computed water table levels are given in Figs. 5 and 6. The percentage of area under different ranges of water table depth predicted by using the model is also comparable to the corresponding observed values (Table 3). It is clear that the above procedure of spatial distribution of net groundwater recharge reproduces the observed groundwater conditions adequately. The recharge distribution coefficient may therefore, be directly employed in future development of groundwater resources for conjunctive water use. SPATIAL DISTRIBUTIONOF GROUNDWATERPOTENTIAL It was shown that about 265 × 108 m 3 of additional groundwater resources can be developed in the project area as a whole. The availability of additional I

1

2

]

/~

5

6

7

8

9

10

11

"

'1.

1 J z

N

i

31

t

!

(, /

-.20

t

:;% River oreQ boundory Observed contour(m) *,'..":, Pmdid~d contour(m) Drain

--.-1 II It~

"*'~.

t J I ~

•

Project

Fig. 5. Map of observed and predicted isobaths (premonsoon 1978) of ~he MRBC command area.

16.26 (480) 25.58 (755) 18.63 (550) 18.60 (549) 15.10 (446) 5.83 (172)

11.58 (342) 28.38 (838) 16.36 (483) 22.53 (665) 16.06 (474) 5.08 (150)

23.58 (696) 25.37 (749) 17.14 (50~,) 17.31 (51!) 16.06 (474) 0.54 (1~.~)

Observed

Observed

Predicted

1978

1977

Figures in parenthesis refer to area in square kilometres.

> 25

20-25

15-20

10-15

5-10

<5

Depth range (m) 22.39 (661) 26.93 (795) 17.45 (515) 17.78 (525) 12.47 (368) 2.98 (88)

Predicted 18.70 (552) 28.86 (852) 17.51 (517) 18.77 (554) 12.05 (355) 4.13 (122)

Observed

1979

Percentage of area under different depths to water table (range) in the month of June

TABLE 3

21.21 (626) 26.12 (771) 16.36 (483) 19.95 (589) 12.16 (359) 4.20 (124)

Predicted

24.49 (723) 28.83 (851) 17.51 (517) 17.07 (504) 12.10 (357) -

Observed

1980

28.96 (855) 25.75 (760) 16o63 (491) 15.99 (472) 12.67 (374) -

Predicted

t~

294 1 979

60

1980

60

SO

50

40

40

"_.o 30

~0

~D U :3

.~_ 2O

20

10

TO

o0 •

10

20

30 40 Observed (m)

;

50

60

!

C

10

20

30 40 Observed (m)

50

60

Fig. 6. Comparison of observed and predicted water levels (MRBC command area).

resources at each node ( R D P i j ) can be estimated using the recharge distribution coefficient (rdii) as: ×

RDPi~

-

""~

MN

265

i

=

1, M , j

=

1, N

(9)

The additional potential available for development is not uniform at all nodes. It varies from 0.04 × 106 I~3 in the northeast near the Shedi River, to 9.5 × 10~m3 in the central portio:l of the project area. In general, the potential for groundwater development is higher in the central and southern portions of the project area than in the western and northern portions. CONCLUSIONS

The available annual grour~dwater potential in the Mahi Right Bank Canal project in India is about 265 ": 106 m 3. This is about 1.6 times the existing level of groundwater abstraction. ~rhe spatial distribution of this potential over the project area is not uniform. It is relatively high in the central and southern portions of the project area m d is low at the northern and western boundaries. REFERENCES Ministry of Irrigation, 1984. K, port of the Groundwater Estimation Committee - - groundwater estimation methodology, G,)v. India, New Delhi. Parthasarathy, B., Sontakke, ~I.A., Monot, A.A. and Kothawale, D.R., 1987. Droughts and floods in summer monsoon seaso'~l over different meteorological subdivisions of India for the period 1871-1984. J. Climatol., 7" 57-70. t ) l a n n ~ g Commission, Government of India, 1985. Seventh Five Year Plan, 1985-90, Vol. II. Gov. India, New Delhi. Prickett, T.A. and Lonnqv~st, C.G., 1971. Selected digital computer techniques in groundwater resource evaluation. I]~. State Water Surv., Bull., 55.

295 Raju, K.C.B., 1987. Groundwater-assessment and monitoring - - a review. Proc. First Natl. Water Cony., Vol. II, New Delhi. Rao, N.H. and Sarma, P.B.S., 1988. Water resources utilization in an irrigation project in India. Int. J. Water Resour. Dev, 4(3): 200-207. Ritchie, J.T., 1972. A model for predicting evaporation from a row crop with incomplete cover. Water Resour. Res., 8(5): !204-1213. Sondhi, S.K., 1984. Groundwater basin simulation for conjunctive use in canal command areas. Ph.D. Thesis, Div. Agric. Eng., Indian Agric. Res. Inst., New Delhi. Water Technology Centre, 1983. Resources analysis and plan for efficient water management - - a case study of Mahi Right Bank Canal Command Area, Gujarat. Indian Agric. Res. Inst., New Delhi, Res. Bull. 42.