simulation of an irrigation tank for modernization

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Simulation of an irrigation tank for modernizationRamu Govindasamy aa Research Assistant in the Department of Economics , Iowa State University , Ames, Iowa,50011, USAPublished online: 02 May 2007.

To cite this article: Ramu Govindasamy (1991) Simulation of an irrigation tank for modernization, International Journal ofWater Resources Development, 7:2, 97-106, DOI: 10.1080/07900629108722500

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Simulation of an irrigation tankfor modernization

Ramu Govindasamy

A digital simulation model of a tank irrigation system is used to quantify the effectsof adopting various modernization scenarios available for improved watermanagement. Out of ten different modernization scenarios considered, thescenario incorporating modifications to the sill level of the sluices, and in the con-veyance efficiency with improved catchment, yields the greatest water savingswhen compared with the baseline conditions. The policy options therefore suggestthat lining of the main canals is one of the modernization options which offers afairly large scope. There is also potential for substantially augmenting the carryingcapacity of the tank by desilting it.

Water has become an increasingly important ingre-dient in the development process of all countries(Biswas, 1983). It is a key resource in agriculturalproduction and is also required for domestic,industrial, navigation and other purposes. The impor-tance of irrigation water lies not only in its own pro-ductivity but also in its ability to increase theproductivity of other crop production inputs such asfertilizer (Eswaramoorthy, Govindasamy and Singh,1989). Thus there is an imperative need for the effi-cient use of this scarce resource, to increaseagricultural production to meet the demand for foodfor the growing population (Govindasamy andBalasubramanian, 1990). In general irrigatedagriculture plays a major role in regional economics(Mann et al, 1987). It involves fairly large invest-ments to render the water available for specific pur-poses such as irrigation. It has been found that in1981, out of the total cropped area of 173 million hain India, 57.01 million ha only were irrigated (Sundarand Rao, 1981). It has been estimated that theultimate feasible irrigation potential in India is 113

Ramu Govindasamy is Research Assistant in the Departmentof Economics, Iowa State University, Ames, Iowa 50011,USA.

The author is indebted to Dr K. Palanisami for helpful comments.

million ha (Govindasamy and Palanisami, 1990). Sofar 93 billion rupees have been invested in major,medium and minor irrigation projects, of whichminor irrigation has been given most importancebecause it involves lower cost, besides benefitingmore farmers (Palanisami and Easter, 1983).

Up to the end of 1980, the total cropped area whichbenefited from minor irrigation is assessed at 30million ha. Over a period of 30 years, the cumulativegrowth in minor irrigation from surface water sourceshas been about 1.60 million ha. Tank irrigation is anold established practice in most of the semi-aridtropical parts of India. It constitutes the most impor-tant minor irrigation source from surface water.Tanks have long existed in India, constituting 11.6%of the total irrigated area. Andhra Pradesh, TamilNadu and Karnataka have substantially large areasirrigated by tanks. Tamil Nadu has the maximumtank-irrigated area accounting for about 32% of thetotal irrigated area. With the utilization of more than92% of all available surface water and about 70% ofthe groundwater resources in the State, the per capitawater resources of Tamil Nadu are only 0.17 hectaremetres as compared to the overall Indian average of0.40 hectare metres (Sakthivadivel et al, 1982).

There is little scope for further harnessing of sur-face sources of water in the State. New investment is

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Simulation of an irrigation tank for modernization: Ramu Govindasamy

difficult due to the high cost of large projects and,moreover, electricity is a constraint on large-scalewell irrigation. Hence the emphasis should be onimproving existing systems such as tanks. The tankirrigation system thus offers significant scope forimprovement, being locally based and managed bylocal people.

Tank irrigation is currently considered a neglectedopportunity and irrigation tanks are under-utilizeddue to mismanagement. However, by suitable man-agement strategies it is possible to improve the effi-ciency of the tanks. This study is an attempt in thedirection of identifying suitable management strate-gies. The object of the article is to develop a simula-tion model quantifying the effects of adopting thevarious modernization options available for betterwater management in a tank irrigation system.

The article first gives an exposition of the selectedtank, the methodology of the simulation analysis isthen discussed, and the study results are presented interms of ten scenarios. The study concludes by con-sidering the policy options and implications suggestedby the results.

Description of the tank

The Srivilliputhur Big Tank is one of the most impor-tant non-system major irrigation tanks inSrivilliputhur taluk of the previously undividedRamanathapuram district. Srivilliputhur, InamNachiar Kovil and Padikasuvaithan are the threevillages which are irrigated by Srivilliputhur BigTank. The location of the tank is given in Figure 1.The registered command area of this tank irrigationsystem is 401.87 ha.

The capacity of the tank is 14 160 hectare cen-timetres and the annual number of fillings is three.The total length of the tank bund is 3640 m and it isconstructed with gravelly soil. The tank has a freecatchment area of 1535.87 ha, a combined catchmentof 4923.59 ha and an intercepted catchment of3387.72 ha. The Srivilliputhur Big Tank has foursluices. The sill level of the first sluice is 0.96 mfrom tank bed level and irrigates an area of105.22 ha. The sill level of the second sluice is1.38 m from tank bed level and irrigates an area of56.66 ha. The sill level of the third sluice is 0.04 mfrom tank bed level and irrigates an area of161.88 ha. The sill level of the fourth sluice is0.75 m from tank bed level and irrigates an area of78.11 ha. The mean length of the main canal is0.84 km. The tank receives surplus from Valaikulamand the surplus from this tank flows to Sholankulamtank.

Methodology

Srivilliputhur Big Tank was selected purposelybecause the data were readily available and it has areasonable command area for modernization. For thepresent study, the entire condition of the tank systemis simulated and the model works on a daily basis(Govindasamy, 1987). The inputs to the model aregiven in Table 1.

The model computes daily inflows into the tank byadding inflow through supply channel and inflowthrough catchment. The total storage at the beginningof each day is calculated by adding the previous day'sstorage to the inflow on that particular day. Thestorage at the end of each day is calculated by sub-

south India

PUDUKKOTTAt

HANJAVUR

Table 1. Input to the simulation model

Number Input

Figure 1. Location of the selected tank.

123456789

1011

121314

Number of sluicesNumber of cropsSill level of the sluices (m)Area of each sluice (ha)Conveyance efficiency (%)Number of crops for each sluiceSill level of the surplus weir (m)Full tank capacity (100 m3)Fortnightly evaporation rates (mm/day)Fortnightly seepage rates (mm/day)Fortnightly values of crop water requirement at the

sluice point for each crop (mm/day)Values on elevation (m) v capacity (100 m3)Values on elevation (m) v surface area (100 m2)Inflow into the tank on a daily basis

(million m3/day)

98 WATER RESOURCES DEVELOPMENT Volume 7 Number 2 June 1991

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trading evaporation, seepage, and outflow throughsluices from storage at the beginning of that day asfollows:

where,

S, = initial storage on (i)th day,5j_, = storage at the beginning of (i — l)th day,Aj_, = change of storage at the end of (i - l)th

day.The order and time of operation of each step has beenindicated in Figure 2 as a flow chart. The PublicWorks Department office has different tank storagecapacities and corresponding values on elevation.From this an elevation v capacity curve was drawn asshown in Figure 3. The values on elevation v surfacearea (Figure 4) were derived from the values onelevation v capacity using the following formula.

AV=[(A2+Al)/2](Z2-Z])

where,

AV = change in volume (100 m3)Ay = surface area at a height of Zl (100 m2)

A2 = surface area at a height of Z2 (100 m2)Z\ = initial depth of water with surface area A{

(m)Zj = final depth of water with surface area A2 (m)

Since the tank cannot hold more than its capacity,provisions are made to indicate the quantity of surplusand the day of surplus if the inflow exceeds storagecapacity. Provisions are also made to indicate theemptiness of the tank so that negative storage will notoccur if the water level goes below the deep bed level.

Earthen channels have a conveyance efficiency of60% in alluvial soils and 70% in black soils (Sally,1965). Since the command area has black soil, theconveyance efficiency is assumed to be 70%. Thereis a 15% increase in the supply of water when themain canal is lined. Since the study assumes only thelining of the main canals, the conveyance efficiencyafter lining is taken as 70 + 15, ie 85%.

Daily rainfall was taken at Tamil Nadu AgriculturalUniversity Research Station at Srivilliputhur, whichis about 2 km from the Srivilliputhur Big Tank.Based on Strange's table (Ellis, 1963) of runoff fromdaily rainfall for an average catchment depending on

Declarations:Number of last day in each of the 21 fortnights, starting from January

[ Input (as in Table i"j~|

Compute storage at beginning of starting day |

For each day over the simulation period 1

yes[ Is inflow from feeder channel non-zero? Update the counter for number of such days

Is inflow from local catchment non-zero? \-no L »

->- | Update the counter for number of such days |

Calculate the sum of storage at beginning of day and the inflow

I Is this value equal to zero? I— yes

| Compute the level for this storage |

M Update counter for number of days tank is empty and print message

< = 3

Is this level higher than full tank levellh

Determine surface area for that level- * -

Update counter for number of days of surplus, compute the excess overfull tank capacity, print the messages and set the storage to the

full tank capacity and the level to full tank level

Compute evaporation volume and seepage for that day. Subtract these two from storage

Over

|

1

the sluice |

Co

Is

npute release required from a

Ythe water level below the sill

Compute cumulative release

sluice based on crop-water requirement on

level

1Sum releases from sluices for

yesof the sluice? | Update number of d

that day 1

tha

sys

t day and the

of deficiency.

area covered by each crop under

compute cumulative deficiency and

that sluice

duration 1

| Compute storage at the end of day |

| Compute corresponding water level

Compute cumulative values of inflow from supply channels, from catchment, evaporation losses etc.

Printout: Deficient volume in each sluice, no. of deficient days, total inflow from supply channel and catchment,total evaporation volume, total outflow from all sluices, total surplus quantity from tank, total outflow from tank,

details of closing error, total deficient quantity computed and number of days tank remains empty.

Figure 2. Flow chart of the program.

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0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 1 . 0 1 . 5 5 . 0 5 . 5 6 . 0 6 . 5

E l e v a t i o n ( m )

Figure 3. Elevation v capacity curve.

12

0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 U . 5 5 . 0 5 . 5 6 . 0 6 . 5E l e v a t i o n ( m )

Figure 4. Elevation v surface area curve.

the amount of rainfall, percentage of runoff and yieldwere computed.

Once the crop water requirement (Elango et al,1981) is calculated, the model then works on a dailybasis to calculate the deficit or surplus quantities. Thetotal water drawn in all four sluices was calculated forthe entire season and from this the available water per

hectare of the command area is reached. Ten differentscenarios were considered for the purpose ofmodernization, as shown in Table 2.

Results and discussion

Results and discussion are mainly dealt with underthree different inflows from the catchment.

The first inflow deals with the existing inflow fromthe free catchment area. The second inflow deals withinflow from the equivalent catchment after moder-nization work, namely clearing of the channel whichcollects runoff from the combined catchment. Thethird inflow deals with inflow after modernization,namely improvement of the equivalent catchment toa good quality. The tract of land draining into anystream or reservoir is termed its catchment basin andthe area of this tract is its catchment area. When theentire rain that falls in a catchment runs into a singletank, then that catchment is called a free catchmentarea. On the other hand, when the rain that falls in acatchment runs into more than one tank, that catch-ment is called a combined catchment. If the channelcollecting the runoff from the combined catchment isclear without any obstacles, then one fifth of the com-bined catchment can be taken as an effective catch-ment. By adding the effective catchment to the freecatchment, the equivalent catchment is arrived at. Ifthe combined catchment is not properly maintainedthe yield from the free catchment alone acts as inflowinto the tank. This is the present condition ofSrivilliputhur Big Tank.

Simulation of the tank system with free catchment(SI).The first inflow was calculated using a free catchmentarea of 1535.87 ha. Under this the total inflow intothe tank was 3 006 700 m3, in 26 days during the

Table 2. Water management structure in each scenario

Equivalent catchment Improved catchment

FreeScenario catchment No modification Sill level Conveyance efficiency Capacity No modification Sill level Conveyance efficiency

I X2 X3 X4 X5 X X6 X X7 X8 X9 X

10 X X


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study year (1980). The maximum inflow was on the317th and 342nd days amounting to 8486.40 ha cmand 8352.00 ha cm respectively, which is about 50%of the total annual inflow.

The first sluice has a deficiency of 675 041 m3 ofwater in 78 days. The second sluice has a deficiencyof 392 124 m3 of water in 84 days. The third sluicehas a deficiency of 852 821 m3 of water in 68 days.The fourth sluice has a deficiency of 501 116 m3 ofwater in 78 days. The total deficient quantity was2 421 162 m3. The tank was empty on the 365th dayand there was no surplus.

The first and last sluices have more or less the samesill level, so the days of occurrence of deficiency arethe same for both sluices. However, the deficientquantity under these sluices differs due to the dif-ference in the command area of the sluices. Thesecond sluice is at the highest sill level so it has moredeficient days. This sluice has the least deficit volumebecause it supports a minimum command area.

It can be seen from the Figure 5 that the maximumdeficient quantity is on the 304th, 305th, 306th,313th, 314th, 315th and 316th days. The tank wasdeficient from the 244th to 268th day, 272nd to 306thday, 308th to 316th day, 333rd to 341st day and bet-ween 356th and 365th day. The tank was in a defi-cient condition for 83 out of 121 days, which is thetotal duration of the crop in this scenario.

380

360

310

320

300

280

260

210

220

200

180

160

110

120

100

80

60

10

20

0210 260 280 310300 320

Days

Figure 5. Simulation of tank system with free

360 380

catchment.

Simulation of the tank system with equivalentcatchment (S2).The second inflow was computed using an equivalentcatchment of 2520.07 hectares. If the channels collec-ting the runoff from the combined catchment areclean without any obstacles then one fifth of the com-bined catchment can be taken as an effective catch-ment. By adding the effective catchment (985hectares) to the free catchment (1535.87 hectares),the equivalent catchment is arrived at. So if themodernization option of clearing the channel and col-lecting combined catchment runoff is carried out, theinflow increases by 60% of the free catchment.

The inflow on 317th day was 1 392 300 m3 andon 342nd day was 1 370 200 m3 of water. Thesewere the days on which the maximum inflow occur-red amounting to 56% of the total inflow in that year.As a result of modernization, namely clearing of thechannel which collects runoff from the combined catch-ment, the inflow has increased by 1 925 000 m3.Even after increased inflow, the second sluice has themaximum number of deficient days and minimumdeficient quantity due to the high level of the sill andminimum command area respectively.

With the second inflow, there was a deficiency of456 218 m3 of water in 53 days in the first sluice.The second sluice had a deficiency of 259 033 m3 ofwater in 56 days. The third sluice had a deficiency of595 546 m3 of water in 48 days. The fourth sluicehad a deficiency of 338 673 m3 of water in 53 days.The total deficient volume under all the sluicesamounted to 1 649 470 m3. Thus there was a reduc-tion in deficient volume by 771 692 m3 of water.The total surplus quantity from the tank was44 800 m3 of water in one day. The tank remainedempty for 20 days (five times). The balance of waterin the tank on 365th day was 30 949 m3. About 50%of the total inflow was used for crop production. Thethird sluice has the maximum deficient volume withleast number of deficient days due to maximum com-mand area and lowest sill level.

It may be seen from the Figure 6 that on 304th,305th, 306th, 315th and 316th days the tank had max-imum deficiency. The tank was deficient from 244thto 268th, 278th to 306th and from 313th to 316th day.The tank was in a deficient condition for 55 days outof the total duration of the crop of 121 days in thisscenario.

Simulation of modification in the sill level of thesluices with equivalent catchment (S3).The study further analyses the effects of channelclearing and change in sill level of the sluices onwater storage; the equivalent catchment was used to


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380

360

340

320

300

280

• ^ 260

| 240

^ 220

3 200o

t. 180S 160u*£ 140Q

120

100

80

60

40

200

—1 n r

1-

i i l i

J

i i i i i i i

240 260 280 300 320 340 360 38Days

Figure 6 Simulation of tank system with equivalent catch-

ment.

simulate the tank system. The first and fourth sluicesill levels were modified (reduced) from the existing0.96 m and 0.95 m to 0.40 m and 0.50 m respec-tively. The sill level of the second and third sluiceswas kept at the same level because the second sluicesupports the minimum command area and the thirdsluice already had the lowest sill level of 0.04 m.

The total inflow into the tank after both modifica-tions was 4 931 700 m3 in 26 days. Since the secondsluice had the highest sill level, the deficient volumeand number of deficient days were not affected. Thedeficient volume of the third sluice had increased by19 632 m3 of water. The number of deficient daysremained the same, however, when compared withsimulation of the tank system with equivalent catch-ment because the amount of inflow and the date ofinflow remained the same. This shows that thedecrease in the sill level of the first and fourth sluicesaffected the volume of water available in the thirdsluice for crop production.

At the same time the deficient volume of the firstand the last sluices decreased by 30 552 m3 and2279 m3 respectively when compared with simula-tion of the tank system with equivalent catchment.After these modifications, the first sluice had a defi-ciency of 425 666 m3 of water in 50 days] Thesecond sluice had a deficiency of 259 033 m3 of

water in 56 days. The third sluice had a deficiency of615 178 m3 of water in 48 days and the fourth sluicea deficiency of 336 394 m3 of water in 53 days. Thetotal deficiency amounted to 1 636 271 m3. Therewas a saving of 13 200 m3 of water which amountedto 8% decrease in the deficient volume when com-pared with simulation of the system with equivalentcatchment.

Simulation of modification in the conveyanceefficiency with equivalent catchment (S4).Two further modifications, namely conveyance effi-ciency of the channels and clearing of the channel col-lecting the runoff from the combined catchment, werealso considered appropriate in working out the waterstorage in the tank. Lining of the main channels andother field channels can improve the conveyance effi-ciency by 15 to 25%. Since the modernization is thelining of main channels alone, 15% increase in theconveyance efficiency has been assumed (Sally,1965). The conveyance efficiency of the earthenchannels ranges from 50 to 75 %. Since the study areahas black soil, the conveyance efficiency is assumedto be 70%. After lining, the total conveyance effi-ciency of the channel will be 85% for all four sluices.

The total inflow into the tank after modificationwas 4 931 700 m3 in 26 days. The deficiency afterthe modifications of the first sluice was 354 838 m3

of water in 50 days. The deficiency for the secondsluice was 202 089 m3 in 53 days. The deficiency ofthe third sluice was 445 511 m3 in 43 days, and thedeficiency of the fourth sluice was 263 414 m3 in 50days. The total deficient volume was 1 265 852 m3.The total deficient volume decreased by 380 419 m3

amounting to about 23 % of the total deficient volumeof simulation of the system with equivalent catchmentwith changes in the sill level of the sluices. The totalloss from the tank was 2 530 700 m3 of water due toseepage and evaporation, accounting for about 51%of the total inflow. The deficient volume decreased by16.60%, 22.00%, 27.60% and 21.70% when com-pared with simulation of the system with equivalentcatchment with changes in the sill level of the first,second, third and fourth sluices respectively. There isa saving of 101 380 m \ 56 944 m3, 150 035 m3 and75 259 m3 of water for the first, second, third andfourth sluices when compared with simulation of thetank system with equivalent catchment. But whencompared with simulation of the tank system withequivalent catchment with changes in the sill levelof the sluices, there was a saving of 70 828 m3,56 944 m3, 169 667 m3 and 62 980 m3 of water forthe first, second, third and fourth sluices respec-tively. The tank remained empty for 16 days (fourtimes). The balance of water remaining in the tank on


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365th day was 131 294 m3. The total surplus was152 400 m3 in two days. It increased by 107 600 m3

when compared with simulation of the system withequivalent catchment with changes in the sill level ofthe sluices.

Simulation of modification in the sill level of thesluices; modification in the capacity of the tankwith equivalent catchment (S5).The sill levels of the first and the fourth sluice weremodified from the existing 0.99 m and 0.95 m to0.40 m and 0.50 m respectively. The sill levels of thesecond and third sluices were kept at the same level.The tank capacity increased from 1 416 000 m3 to2 100 000 m3 by increasing the height of the tankfrom the existing 5.00 m to 5.60 m. This increase inheight was a modification in which the height of thetank bund was assumed to be increased by 0.60 m.This can also be increased by desilting the tank by0.60 m in height.

The total inflow into the tank was 4 931 700 m3 ofwater in 26 days. When compared with simulation ofthe system with equivalent catchment there was asaving of 30 552 m3 of water in the first sluice, and2279 m3 of water in the fourth sluice. There was nochange in the quantity of deficient water in the secondsluice because it is at the highest sill level. But thequantity of deficient water increased in the thirdsluice by 19 632 m3 because it has the lowest silllevel of all the sluices.

The net saving of water due to these modificationswas 13 199 m3. The total surplus from the tank waszero due to increased capacity and the total outflowfrom all the sluices was 2 496 300 m3. The totaloutflow quantity from the tank was reduced by36 000 m3 when compared with simulation of thetank system with equivalent catchment with changesin the sill level of the sluices. The balance of waterremaining in the tank increased by 11 670 m3 on365th day when compared with simulation of the tanksystem with equivalent catchment with changes in thesill level of the sluices.

This modification will be very useful when there isheavy rainfall within a short time because the tankwill not achieve a surplus unless the quantity of waterstored exceeds 2 100 000 m3. The capacity of thetank has been increased by 684 000 m3, ie nearly50% of the original capacity, and the total capacityincreases to 150%.

Simulation of modification in the sill level of thesluices; modification in conveyance efficiency withequivalent catchment (S6).Modifications were also attempted by changing boththe sill level of the sluices and the conveyance effi-

ciency using the equivalent catchment inflow. The silllevels of the first and fourth sluices were modifiedfrom the existing 0.96 m and 0.95 m to 0.40 m and0.50 m respectively. The sill levels of the second andthird sluices were kept at the original levels. In addi-tion to this modification, lining of the main channelswas also assumed. The conveyance efficiency of thechannels increased from 70 to 85 %. The total inflowinto the tank after the above modifications was4 931 700 m3 in 26 days.

The deficiency after these modifications of the firstsluice was 328 735 m3 in 48 days, and in the secondsluice it was 202 089 m3 in 53 days. The third sluicehad a deficiency of 467 938 m3 of water in 45 days;the fourth sluice a deficiency of 257 422 m3 of waterin 49 days. The total deficiency was 1 256 184 m3

of water.Thus, when compared with simulation of the

system with equivalent catchment, there is a saving of28% of water in the first sluice, 22% in the secondsluice, 21 % in the third sluice and 24% in the fourthsluice.

The tank remained empty for 19 days (four times).The surplus volume from the tank was 155 400 m3

and the balance of water remaining in the tank on365th day was 131 294 m3. This water can beutilized to grow the next crop.

Simulation of the tank system with improvedcatchment (S7).The third inflow was calculated using an equivalentcatchment of 2520.07 hectares, with the assumptionthat when the quality of the catchment is average, therunoff from the catchment can be increased by about25% by making it into an improved catchment. As aresult of this the catchment area will remain the samebut the total inflow into the tank will increase from4 931 700 m3 to 6 321 300 m \ an increase of.25%.

The total inflow in the catchment was thus6 321 300 m3 of water - 28% more than the inflowin simulation of the system with equivalent catch-ment. The inflow on 317th day was 1 784 500 m3

and on 342nd day was 1 756 300 m3, which con-stitutes about 56% of the total inflow. Due to thismodernization the inflow has increased by1 389 600 m3. The number of deficient days in thefirst, second and fourth sluices are now almost equal,but the deficient quantity differs under each sluice.

The first sluice has a deficiency of 413 304 m3 ofwater in 48 days. The second sluice has a deficiencyof 226 745 m3 of water in 49 days. The third sluicehas a deficiency of 524 827 m3 of water in 42 daysand the fourth sluice has a deficiency of 306 816 m3

of water in 48 days.When compared with the second inflow in simula-


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240 380

Figure 7. Simulation of tank system with improved catchment.

tion of the system with equivalent catchment, therewas a saving of 42 914 m3 of water in the firstsluice, 32 288 m3 of water in the second sluice,70 179 m3 of water in the third sluice and 31 857 m3

of water in the fourth sluice.When compared with the first inflow, namely

simulation of the tank system with free catchment,there was a saving of 39%, 42%, 38% and 39% ofwater in the first, second, third and fourth sluicesrespectively. The tank remained empty for 15 days(four times). The balance of water remaining in' thetank on the 365th day was 56 399 m3. The totalsurplus from the tank was 916 800 m3 in three days.Out of the total requirement of crops nearly 36% ofwater was deficient. The maximum number of defi-cient days and minimum deficient quantity were inthe second sluice and the minimum number of defi-cient days and maximum deficient quantity were inthe third sluice. I

It can be seen from the Figure 7 that the tank wasin maximum deficient condition on 304th, 305th and306th day. The tank was in deficient condition from244th to 266th, 280th to 291st, 292nd to 306th andfrom 315th to 316th day. The tank was in deficientcondition for 48 days out of the total duration of thecrop of 121 days in this scenario.

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Simulation of modification in the sill level of thesluices with improved catchment (S8).Modifications included with the inflow includechange in the sill level of the sluices. The first andfourth sluice sill levels were modified from 0.96 mand 0.95 m to 0.40 m and 0.50 m respectively andthe sill level of the second and third sluices was keptat the original level.

The total inflow into the tank after these modifica-tions was 6 321 300 m3 of water in 26 days. Afterthe introduction of these modifications the first sluicewas deficient by 379 624 m3 of water in 45 days.The second sluice was deficient by 226 745 m3 ofwater in 49 days. The third sluice was deficient by544 288 m3 of water in 43 days and the fourth sluicewas deficient by 290 875 m3 of water in 46 days.The total deficient volume was 1 441 532 m3 ofwater.

There is a saving of 33 680 m3 of water in thefirst sluice and 15 941 m3 of water in the fourthsluice when compared with simulation of the tanksystem with improved catchment. There was no dif-ference in the deficient quantity computed in thesecond sluice because it has the highest sill level. Thedeficient quantity of the third sluice increased by19 461 m3 of water when compared with simulationof the tank system with improved catchment and thereis a net saving of 30 160 m3 of water due to thismodification.

Simulation of modification in conveyance efficiencywith improved catchment (S9).With improved catchment, modification in the con-veyance efficiency of channels was taken into con-sideration in this scenario. The conveyance efficiencyof the channels increased from 70% to 85 %. The totalinflow into the tank after these modifications was6 321 300 m3 of water in 26 days. After thesemodifications there was a saving of 53 728 m3 ofwater in the first sluice, 51 253 m3 of water in thesecond sluice, (the conveyance efficiency can beincreased by 15 to 25% by lining the main and otherdistributing channels depending on soil conditions.Conveyance efficiency has been increased by anaverage of 15 % due to lining of the main channels)150 915 m3 of water in the third sluice, and 48 946m3 of water in the fourth sluice when compared withsimulation of the tank system with improved catch-ment, with modification in the sill level of the sluices.There was a total saving of 304 842 m3 of water dueto these modifications when compared with simula-tion of the tank system with improved catchment,with modification in the sill level of the sluices.

After the modifications, the first sluice had a defi-ciency of 325 896 m3 of water in 46 days. The se-

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cond sluice was deficient by 175 492 m3 of water in46 days. The third sluice was deficient by 393 373 m3

of water in 39 days and the fourth sluice was deficientby 241 929 m3 of water in 46 days. The total defi-cient volume was 1 136 690 m3.

Simulation of modification in the sill level of thesluices: modification in conveyance efficiency withimproved catchment (S10).

The sill level of the sluices and the conveyance effi-ciency of the main channels were modified to seetheir effects on water storage with improved catch-ment.

The first and fourth sluice sill levels were modifiedfrom 0.96 m and 0.95 m to 0.40 m and 0.50 mrespectively. The sill level of the second and thirdsluices was not modified. In addition to this modifica-tion, lining of the main channels was also assumed.The conveyance efficiency of the channels increasedfrom 70 to 85%. The total inflow into the tank was6 321 300 m3 of water.

After these modifications, there was a saving of35 786 m3 of water in the first sluice and 17 428 m3

of water in the fourth sluice, no change in the secondsluice and the deficient volume increased by 31 513m3 of water in the third sluice when compared withsimulation of the tank system with improved catch-ment, with modification in conveyance efficiency.

When compared with simulation of the tank sys-tem with improved catchment, there was a savingof 123 195 m3 of water, 51 253 m3 of water,99 941 m3 of water and 82 315 m3 of water in thefirst, second, third and fourth sluices respectively.

Comment

Out of the ten simulation scenarios, simulation ofmodification in the sill level of sluices and modifica-tion in conveyance efficiency with improved catch-ment, namely simulation scenario 10, gives the leastamount of deficient quantity of irrigation water viz1 114 988 m3. In this scenario, three modificationswere considered:

• modification in the inflow by improved catchment;• modification in the sill level of the first and fourth

sluices, and• modification in conveyance efficiency in all the

sluices.

Modification in the inflow by improved catchment ispossible by way of artificial treatment of the soil,afforestation and meadowing in the catchment area.Modification in the sill level of the first and fourthsluices will affect the command area of all the sluices.So, if the farmers are amenable to such an allocation

of activity, this can be carried out. The finalmodification, namely lining of the main channel, willnot adversely affect the groundwater recharge sincethe mean length of the main channel is only 0.84 km.

If these modifications are carried out, then there isa surplus (overflow) of 1 067 500 m3 of water in thetank. In order to use this water, the fifth simulation,namely modification in the sill level of the sluiceswith modification in the capacity of the tank, must becarried out along with the tenth modification.

Since modification in the sill level of the first andfourth sluices saves 21 702 m3 of water, if there isany social problem in the allocation of water throughdifferent sluices, simulation of modification in con-veyance efficiency with improved catchment can betaken up.

The simplest modification which can save morewater is the simulation of modification in conveyanceefficiency with equivalent catchment (S4). Thisinvolves two modernizations, namely cleaning thesupply channel which collects runoff from the catch-ment area, and modification in conveyance efficiencyof all the sluices by way of lining. The total deficientquantity after these two modifications was 1 265 852m3. Depending on the cost of modernization andsocial considerations, a suitable option can beselected. These are the modernization options cur-rently available. While technical feasibility indicatesthese possible solutions, the consideration of costsand benefits does tend to influence the decision onwhat is best.

Policy implications

The results show that there is a significant increase inwater availability as a consequence of modernization.The scenarios also help to locate the tanks which aremore responsive, in order to minimize water deficitsdue to modernization. Thus tanks can also be rankedfor modernization based on the above determinants.Once the tanks are selected for modernization, in aparticular tank there can be many modernizationoptions such as lining of the canals, deepening of thetank etc. The simulation models would help in identi-fying the modernization option which offers mostefficiency in water saving. The output of the varioussimulation scenarios does specify certain approachesand directions for policy considerations in embarkingupon modernization.

Lining is one of the main modernization optionswhich offers a fairly large scope. As indicated in oneof the scenarios, the potential exists for substantiallyaugmenting the carrying capacity of the tank byincreasing its depth. Implementing certain of themodernization options has clearly contributed by


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saving a sufficient quantity of water on the 365th dayadequate enough to introduce summer cropping withpulses, peanuts and so on. Since most of the supplychannels which collect runoff from the catchmentarea remain partially blocked, clearing them willresult in increased inflow into the tank. If there is agreater difference between the sill level of the lowestsluice and the deep bed level, then altering the silllevel of the sluices will result in more efficient use ofwater by a process of regulation. Since the catchmentarea has only sparse vegetation, soil treatment andafforestation will increase the runoff. Currently thetechniques of modernization of tanks have not givenserious consideration to the idea of afforestation ofthe catchment areas. Observations indicate a tendencyto encroach on catchment regions for residential andother purposes. Such an approach needs to bediscouraged and policy instruments designed to pre-vent encroachment, not to mention assignment ofsuch areas for other purposes. As far as SrivillipuihurBig Tank is concerned, lining of the main channelsand clearing the supply channel which collects runofffrom the catchment areas are the important moder-nization options. This would certainly improve theoperational efficiency of the tank irrigation system,besides ensuring much more stability and minimizingrisk.

ReferencesBiswas, Asit K. (1983). 'Major water problems facing the

world', Water Resources Development, Vol 1, No 1, pp1-13.

Elango, K., N. Indrasenan and S. Shanmuganathan (1981). 'A

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simulation model for a minor irrigation system: Pillaipak-kam Tank', Technical Session VIII, Planning and Manage-ment of Irrigation System, Hydraulic Engineering Division,Indian Institute of Technology, Madras.

Ellis, W.M. (1963). College of Engineering Manual Irriga-tion, Government of Madras, Madras.

Eswaramoorthy, K., Ramu Govindasamy and Ikbal Singh(1989). 'Integrated use of water resources in the LowerBhavani projects in India', Water Resources Development,Vol 5, No 4, pp 279-286.

Govindasamy, Ramu (1987). 'A simulation model to improvethe operational efficiency of a tank irrigation system inRamanathapuram District, Tamil Nadu', InternationalWorkshop on Rehabilitation of Tank Irrigation System forImproved Crop Production, Centre for Water Resources,Perarignar Anna University of Technology, Madras.

Govindasamy, Ramu and R. Balasubramanian (1990). 'Tankirrigation in India: problems and prospects', WaterResources Development, Vol 6, No 3, pp 211-217.

Govindasamy, Ramu and K. Palanisami (1990). 'Optimalmodernization for a tank irrigation system using a simula-tion model', Indian Journal of Agricultural Economics, Vol45, No 2, pp 141-149.

Mann, R., E. Sparling and A.R. Young (1987). 'Regionaleconomic growth from irrigation development: Evidencefrom northern high-plains Ogallala groundwater resource',Water Resources Research, Vol 23, No 9, pp 1711-1716.

Palanisami, K. and K. William Easter (1983). The Tanks ofSouth India (A Potential for future expansion in Irrigation),Economic Report ER 83 -4 , Department of Agriculturaland Applied Economics, University of Minnesota, St Paul.

SakthivadiVel, R., S. Savadamuthu, C.R. Shanmugam, P.Balakrishnan and C. Arputharaj (1982). 'A pilot projectstudy of modernization of tank irrigation in Tamil Nadu', inWorkshop on Modernization of Tank Irrigation: Problemsand Issues, Centre for Water Resources, Perarignar AnnaUniversity of Technology, Madras.

Sally, H.L. (1965). Lining of Earthern Irrigation Channels,Asia Publishing House, Bombay.

Sundar, A. and P.S. Rao (1981). 'Irrigation potential in India',WAMANA, Vol 1, No 4, p 10.

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simulation of an irrigation tank for modernization

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