DEVELOPMENT OF A NEW PROCEDURE FOR THE DETERMINATION OF THE SY$TEM CAPACITY AND

MANAGEMENT PROGRAM : FOR IRRIGATION OF TRANSPLANTED RICE*

L.J. Wein Engineer of Irlgation, Engineering Division, Joint Commission

on Rural Reconstruction, Republic of China

'

In theirrigation of transplanted rice, land preparation requires amounts of water varying from 100 to 200 mm which is usually the maximum water requirement. In the planning and design of rice- irrigation, a conventional formula based on the water supply during land preparation has been used to determine the capacity of the canal or pump. This conventional formula- was derived with an assumption that the discharge varies with a straight line from a small amount of water at the beginning to the maximum amount of water at the end of land preparation. The defects in the formula are'that the irrigation water can hardly be controlled as assumed and that the maxi- mum discharge, as determined, will require a large capacity canal or pump. The reults are uneconomical use of water and high costs in construction of irrigation systems.

The objectives of this report are : , - +.

(1) To analyze the conventional formula used rbr . the determination ' I .

of the capacity of a canal or pump for the irrigation of transplan: ted rice.

(2) To modify the conventional formula. (3) To compare the modified and conventional formula. (4) To provide nornographs for application of the modified formula.,, In some regions, including Taiwan, two or more rice crops can be

grown each year. In Taiwan, the first crop of rice is transplanted from January to March and harvested from 'May to July. The 'second crop is transplanted from late June to August and harvested from lak September to early December.

Prior to transplanting, the paddy fields are plowed, supplied with water, puddled by harrowing severat-Pmes, and finally leveled. According to PDAF(S) , the purposes of land preparation are :

. . . . . (1) To soften the 'soil for transpl~mting.,., . . . . . .' - .. .. . . ... ..

* Etablissement d'qne nouyelie proddure pour Ia ddteirn.iaaiion do lri cap'aci t6 du syst6me et du prQgkamme d'aveaagement p d u ~ ltirriagti.on du tiz transplant&. . . .

i c l ~ TECHN~CAL M E M O ~ S NO. 1

(2) To mix fertilizers and organic composts uniformly into the soil. (3) To prevent excess percolation. -(4) To provide favorable conditions for the root growth of young

plants. . .

RESUME . . .

Dans le domaine d'irrigation du riz transplante, la d u sol exige une quantitd d'eau qui varie entre 100 et 200 mm, ce qui constitue normalement le maximum des besoins en. eau. Dans le planning et l'6tude d'irrigation du riz, on se sert d'une formule classique baste sur lapport d'eau pendant la prkparation du sol ppur dbterminer la capacitC du canal ou de pompe. Cette formule est obtenue en supposant que le d6bit varie en ligne droite A partir d'une petite quantite d'eau au depart atteignant le maximum d'eau vers la fin de..la preparation du sol. Les d6fauts de cette formule sont : l'eau &irrigation peut &re A peine contr6lCe, cornme on a pens6; le debit maximum ddtermint! exigera une grande capacite du canal ou de pompe. Les rbsultats sont : ernploi non tconomique d'eau .et prix Clevds dans.la construction de systkmes d'irrigation.

Ce rapport vise il .: r .

( I ) analyser la formule classique utilisee pour la determination de la capacitk d'un canal ou d'une pompe d'irrigation du riz trans- plantd;

(2) modifier la formule classique; (3) cornparer la formule classique et celle modifibe, et

(4) fournir nomographes pour application de la formule rnodifik.

Dans certaines r6gions y compris Taiwan, on ' peut cultiver deux cul- tures ou plus du riz dans une annee. A Taiwan, le premier riz est. transplant6 entre Janvier et Mars et rbcoltt! entre Mai et Juillet. Le deuxibme est trans- plant6 entre fin Juin et AoClt et ricol$6 entre fin Septembre et dkbut Dbcembre.

Avant la transplantation, les rizieres sont labour&es, arrostes et her- aCes 2 plusieurs reprises et finalement nivelets. Selon PDAF les buts de la prkparation du sol sont-; -

(1) adoucir le sol pour transplantation. (2) m61anger les engrais e't les funiures organiques dans le sol, (3) bite) I'exds d'irrigation, et (4) cr6er des conditions favorables pour la croissance des racines

des jeunes plants.

In practice, the land preparation over an area of paddy fields takes a.c~nsidelyab_le length d time, which ranges from two to four weeks depend- ing on the locality, climatic 'conditions, varieties, Cropping patterns;farm mechanization, and the avslability of water and labour. -

. .

Following the land preparation -of individual fields, seedlings are successively transplanted from the nursery, which is about 1/25 of the size of the fr_ansplanted field. The suitable transplanting age of the young plants is 20 to 40 days, depending on the variety and the locality. . ' The practice of rice transplantation is widely carried out in every

-country of the Far East. Fukuda and Tsutsui ('d) give the advantages of this practice as follows :

,

ti) Rice plants can be grownwith less. weeds, compared to the direct sowing method, provided. that the seedlings are transplanted at the proper time to the well puddled paddy field.

'

(2) Uniform and healthy seedings are produced since concentrated care is provided by the farmers in the nurseries.

(3) Damage by birds can be eliminated- as the seeds are grown under submergence.

(4) The length of time for growth of the second crop can be prolon- ged, since most paddy fields are kept free until transplantation. -

(5) The practice acts as a safeguard against the damage which occurs during the initial growing stage, since re-sowing is easily carried out in nurseries and importation of healthy seedlings -from non- damaged areas is possible.

The transplanting practice makes Taiwan capable of adopting the intensive-relay cropping system. By inter-planting and transplanting, two, three, and even four crops with growing seasons adding up to as much as 14 to 1 5 months can be grown on a single piece of land each year.

During the time of land preparation and transplanting, a large amount of water is usually required. The maximum water requirement for land pre- paration and the water requirement for transplanted rice fields, occurs in this period if there is no rainfall or the whole area is developed within a very limited period of time. In practice, some formulas are used in Taiwan and Japan for determining the capacity of a canal or pump for transplanted rice,

CONVENTIONAL FORMULA ,USED IN TAIWAN

'The formula of PWCB(4) conventionally used for the determination of canal or pump capacity can be writ ten as follows :

where Q=canal or pump capacity (maximum discharge),

A,i,, = area to .be irrigated in mZ,

96 . . . . . .- . ...a . . -. I C ~ D TECBNCAL ' MEMOIRS NO. - 1

. . .

, . '' .W(4. t, ri) =water requirements in the transplanted rice ' field in metres . .

- . Per day, w ( ~ t z m pud) =puddling water requirement for soaking the field in mete&,

. t(landp) =period of land preparation, in number of days, for the entire area, A(&

.. . 86,40O=number of seconds in one day, and '

. . . -

. . .. . (I,) = conveyame loss, in decimal s.

Chow(l) gave a Wised form of Equation (1) for 'rotational irrigation as follckws : .

Y ( R ~ . pud) + J'(tr. f i ) I ~ = m ( ' ~ ( ~ ~ ~ i . y( ir )

where . .

Q='the required canal capacity, in cubic.metres per second, .' .

, A(i,, =the .irrigated area, in hectares,

=the depth of water, required for soaking the field in metres, Y ( R ~ . pud)

yy, ji,=the depth of water for each application to the transplanted field in metres,

t(pr,a) =the period of soaking the field in days, . . .i

Yti,, = rotation interval in days, and 1

(led =canal conveyance loss in decimals.

. -

FORMULA USED IN JAPAN .. . .

. .' , .

According to Fukuda . and Tsutsui(2) daily water requirements for the 10th day from the beginning of land preparation is :

. . . ' : . . . .

lo A(pud) w ( ~ q . aud) da'yfO = " ' r o . o ( p u d ) + ( ! - J ) w ~ ( r y . f i ) day), - . - , .(3) .

. ..

and the maximum water requirement in the period is . . .

::

where . , . .

1 *.

W(Rg,fpud) &ite-,=daily water requirement for 10th day from the begin- ningofiandpreparationincubicmetresperday,

- . '

W(Rq, pud~max=maximum water requ.irement in, cubic . . metres per day,

A ( ~ 4 =area t v be puddled in hectares, . . . . , ., . . ,. .

m A G E M E N T PROORAM FOR IRRIGATION OF TRANSPLANTED RTCE 97

' f i =unit water requirement (mmlday) in the transplanted field.

y(pud, =puddling water depth, in millimetres, and n=mmber of days in land preparation.

ANALYSIS OF CONVENTIONAL PORMULA

Equation (1) or ( l a ) i s derived on the assumptions that the rate at; which an area is supplied with puddling water is constant through the whole period of Iand preparation and that the water requirement for trans-- planted rice WfRq) is supplied to an area which has been provided with the puddling water, and that the effective rainfall js negligible in the short period of N days.

Equation ( I ) can be analysed with Figure 1. Figure l(a) shows the .

relationship between time (in days) and the rate which an area is provided with puddling water, p ( j d p ) , which is equal to A(j=,,/n (in m8/day)1 through the whole period of land preparation, n. Figure l(6) shows the relationship between time and the discharge, Qtpud) , needed for puddling; the fleld, where

Q(piid)' land p) W(R~. prrd) =A(ir) lt(land p) W ( R ~ . pud)

(in mgjday). Figure I(c) shows the relationship between time 'and the dis-1 charge. Q(,. r i ) , needed in the transplanted rice field, where.

(in m3/day) at the point of n (in days) from the beginning ; and Figure l ( d ) shows relationship between time and the total discharge, QrotQl (in m3/day). which is equal to Q(pud) f QctK ,i), i.e.9

. When n= #(land p) , the discharge will be maximum. Therefore,

if conveyance loss of a canal is considered in addition to Equation (5 ) , Equatio.? (1) can be obtained.

From Figure 1, one can find the defects of the conventional formula as ' . folIows :

. (1) The formula shcws that the capacity of a c a v l or pumpisdesigned -

) , . for the maximum discharge, Qto,/ , at the end of land prepara- ..

. tion, whilc a small discharge is all that is required at the beginning of the period. Actually, water is not controlled daily with variable discharges. through the period as assuped in the furmulg,

IMD TECHNICAL MEMOIRS NO. 1

+ t l / o n d p ) c - 4 Time (doys)

I

FIGURE 1 : Schematic diagrams of water iise in land preparation /

(2) If water is supplied with a constant discharge through the period of land preparation, the discharge is certainly smaller than that of the formula.

Equations (2) and (4) are similar to Equation (1).

' BLAEfAGWNT a . PROGRAM FOR 1RRIGATION OF TRANSPLANTED RICE 99

. Fmm 2 : Schematric diagram for proposed water use in land preparation ' 4

MODIFICATION OF CONVENTIONAL FORMULA

Under a condition that the relationship between time, n and the rate of land development, p ( w p ! is curve, starting from .

- P ( l d p) - ~(lnnd)initial~he" n=O, and ending at P(Iand p) = P(!ond p) 9

- *([andp) when = (/findp) with an enclosed area of A(* as shown in Figure 2, the discharges required for puddling the field and for the trans-

' planted rice at the time, n, will be respectively :

and the total of Q8 and Qr will be :

In order to supply a constant discharge through the whole period of land preparatiqp ijnder ' the. mpst e ~ o n ~ r n i c ~ l csndition for the detqrmiparioq

100 IClD TECHNICAL MEMOIRS NO, I

of canal or pump capacity, let &=constant, and differentiate Equation (6). Therefore,

~ Q ( I ~ P I ~ ,- p) .Ic ' w(~q. tt=. ~ i ) dr~ W ( ~ q . pud) . P(landp)

The above differential Equation (7) is a condition that (Q(,&) + Q ( , , ,i) is always equal to a constant discharge. . Its solution will be :

=- W k q . tr. ti) J dn+C : *w. P U ~ )

or

where e= the base of natural logarithm=2.7 18282.' .

C The constant . e can be evaluated from the condition that

- Pjkindp) - P(landp)lnitial at *=O, giving

Because

- e - { W ( ~ q . rr. ri)lW(lZq. pzrd))n - '('0"'~) in jftal - w(~q. t r . r i ) / W(~q. pud) 1 o

MANAGEMENT PROGRAM FOR IRRIGATION OF TRANSPLANTED RICE

. 4 9 )

If the value of P(larrdP)initiai is substituted into Equation (8), one obtains

The discharge, Q, is constant for the entire period of land preparation; therefore, it is convenient to evaluate & at n = t(landP) Equations (6) and (10) will be respectively :

A(ir) W( ;r. r i ) e- ( W ( ~ q . w. i r ) / w ( ~ q . pud) l t(londg)

Q = {W(RQ. ,r . r i )/ WCRQ. pud) 1 t ( h d p )

+ 1 -e-

If the conveyance loss of the canal is considered, the discharge, Q, will be :

w * ( i (Rp. fr, r i ) 1 Q - -1w I t 1 -(L) ...( 1 2)

1-e- (Rq.rr.ri)IW(Rq.pud) (londp)

FXNAL FORMULAS

If water conveyance efficiency, q is applied to Equation (12), one (cv)

obtains the finally modified formula,

A(ir) W(R~. tr* ri) f -' in m8Js ,

= 8'49,~,){l-e-tW(,Rq. tr. r i ) ) l W ( ~ q . p u d l ) (l+)

where

A(ir) = Area in hectares

y,,) = in decimals instead of per cent

In rotational irrigation, WcRq tr b r i ) will be y(k)/Y(ir) in which y[i,) is the depth of water for each application of rotational irrigation and Y(ir) is the rotation interval in days.

If the known factors are :

A(jr) = 100 ha = 1,000,000 m4

W ( R ~ . tr. r i ) = 10.5 rnm/day==0.0105 m/day

W'W. PW = 180 mrn -- 0.18 m

(landp) = 20 days

(I , , ) = 0.2 and q ( ~ = 0.8

From Equation (13), the required discharge will be :

From Equation (I), however, the required discharge will be

The ratio of the above two discharges is about 1.3 ; i.e., Equation (1) requires about 30 per cent of canalfpump capacity more that Equation (14) in this example.

MANAC3Ebfl3Nr PRO(tRAM POR IRRIGATION OP TRANSPLANTED RICE

1ClD TECHNICAL MEMOIRS NO. 1 r

MANAGEMENT PROGRAM FOR IRRIGATION OF TRANSPLANTED RICE f 05

From Equations (9) and (1 I), the rates of land preparation at the beginning and at the end of the period, respectively are calculated as follows :

NOMOGRAPHS FOR APPLICATION

For practical application, canal or pump capacities may be directly obtained from Figure 3, which was developed from Equation (14). Tlie per cent, A(,un)% of area to be supplied with puddling water on the 10th day from the beginning of land preparation i s shown in Figure 4 by using the following equation derived from Equation (10) :

In actual field practice, during the period of land preparation and transplanting, water flow can hardly be controlled to a discharge that varies with a straight line as assumed% the conventional formula which gives a large discharge at the end of the period. It is, therefore, be1ieve:d that the conventional equation is not practical. While the newly derived formula gives a constant discharge smaller than that of the conventional one, the smaller capacity results in lower construction cost. Furthermore, the new formula will give a sufficient amount of water as far as the total volume of water given in the period is concerned. It i s recommended that Equation (14) be used instead of the conventional one in determination of canal or pump capacity for the irrigation of transplanted rice.

- REFERENCES

(1) CHOW, LEE, 'cDevelopment of rotational irrigation in Taiwan", Journal of the Irrigation and Drainage, American Society of Civil Engineers, Vol. 86, NO. IR3, Proceedings Paper 2588. September, 1960.

106 ICID TECHNICAL ~ M O I R U NO. I

(a) FUKUDA, HITOSHI, and HIKARU TSUTSUI, "Rice irrigation in Japan," Food and Agriculture Organization of the United Nations. 1968. Rome. P 60.

(a) PtOVhcial Department of Agriculture and Forestry (PDAF), Taiwan, ''Agricu'I- tural Manual-Rice and sweet potatoes," Part I, Vol. 6. 1964. PDAF. Taichung, Taiwan. p 342. Chinese.

(4) Provincial Conservancy Bureau (PWCB), Taiwan. " I m p t ion and Drainage Engineering Handbuok," No. 66, 1967. PWCB, Taipei, Taiwan. p 463, Chinese.