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4.1 Introduction
4.2 Growing Paddy Rice
4.3 Rice Growth Stages
4.4 Irrigation water need of paddy rice
4.5 Irrigation scheduling of paddy rice

4.1 Introduction

Paddy rice is usually grown in level basins (Figure 17) which are flooded with water throughout most of the growing season.

Figure 17. Paddy rice, at various growth stages, grown in level basins

The main reason for flooding the rice fields is that most rice varieties maintain better growth and produce higher yields when grown in flooded soils, than when grown in dry soils. The water layer also helps to suppress the weeds.

4.2 Growing Paddy Rice

To grow a paddy rice crop, the following activities are usually carried out:

i. Preparation of the rice nursery

The nursery is usually 5 - 10% of the size of the total area to be planted; for example, if the total field size is 1200 m2, then the nursery should be between (0.05 x 1200 =) 60m2 and (0.10 x 1200 =) 120 m2.

Preparation of the nursery starts one month before sowing the nursery. The soil of the nursery should be loose, without weeds, moist and fertile. When sowing the nursery, it is very important to select good rice seeds.

ii. Preparation of the rice fields

Preparation of the rice fields starts about one, or sometimes two, months before the rice is transplanted. The fields are usually first flooded. A few days after flooding, the field is ploughed. Ploughing is the initial breaking and turning over of the soil. Flooding makes ploughing easier. Ploughing is done by hand (with a hoe), by animal traction (with oxen or buffaloes, see Figure 18) or mechanically.

Figure 18. Ploughing the rice field

It is also possible to plough the dry soil; this is, however, much heavier and is in practice only done if tractors are available. Ploughing the dry soil does save some water.

After ploughing, the soil is puddled. During puddling, the big soil clods are broken. Puddling reduces the permeability of the soil and therefore also reduces the percolation losses.

After puddling, the soil is levelled; that is, the soil is made flat (Figure 19). To facilitate the levelling, the soil is flooded with a shallow water layer. This way it is possible to see where the high spots are. Levelling can be done with a shovel, a rake, a levelling board, etc.

Figure 19. Levelling the rice field

iii. Transplanting of the seedlings

The seedlings should be transplanted approximately one month after sowing the nursery. The seedlings will then have four to five leaves. Only strong seedlings are transplanted (Figure 20). The seedlings must be transplanted into the very wet rice field. The seedlings are planted in straight rows with proper spacing between them.

Figure 20. Transplanting the seedlings

iv. Water control

This is discussed separately in section 4.4 and 4.5.

v. Weeding

Weeds prevent rice from growing and tillering well. Weeding usually starts two weeks after transplanting and continues as necessary throughout the growing season.

vi. Fertilization

Fertilizers are usually applied just before transplanting, one month after transplanting and one month before flowering.

vii. Pest control

Rats, birds and insects often do much damage to the rice crop. Ask the extension service how best to control them.

viii. Harvest

The rice is cut when the heads are yellow. Cutting is usually done with a sickle.

4.3 Rice Growth Stages

Usually a distinction is made between the four growth stages of rice (Figure 21).

Figure 21 Rice growth stages


from sowing to transplanting; duration approximately one month.

Vegetative stage:

from transplant to panicle initiation; duration varies from 1½ to 3 months. Vegetative stage includes the tillering. Tillering means that several stems develop on one plant (Figure 22).

If the rice is sown directly (broadcast), the two stages combined are called the vegetative stage.

Mid season or reproductive stage:

from panicle initiation to flowering; duration approximately one month. This stage includes stem elongation, panicle extension and flowering. Late tillers may die.

Late season or ripening stage:

from flowering to full maturity; duration approximately one month. This stage includes grain growth.

Figure 22. The vegetative stage includes tillering

4.4 Irrigation water need of paddy rice

The determination of the irrigation water need of rice has been explained in Volume 3. The steps involved are briefly repeated below; the datasheet to determine the Irrigation water need of paddy rice is given in Annex III.

Step 1: Determine the reference crop evapotranspiration: ETo
Step 2: Determine the crop factors: Kc
Step 3: Calculate the crop water need: ET crop = ETo x Kc
Step 4: Determine the amount of water needed for land preparation: SAT

In the month before sowing or transplanting, water is needed to saturate the root zone. The amount of water needed depends on the soil type and rooting depth. For the purpose of this manual it is however assumed that the amount of water needed to saturate the root zone is 200 mm. Thus:

SAT = 200 mm

Step 5: Determine the amount of percolation and seepage losses: PERC

The percolation and seepage losses depend on the type of soil. They will be low in very heavy, well-puddled clay soils and high in the case of more sandy soils. They can best be determined locally. If no local data can be obtained, the following values may be used:

for heavy clay

: PERC = 2 mm/day = 60 mm/month

for more sandy soils

: PERC = 8 mm/day = 240 mm/month

on average

: PERC = 5 mm/day = 150 mm/month

Step 6: Determine the amount of water needed to establish a water layer: WL

A water layer is established after transplanting. The amount of water needed for maintaining the water layer has already been taken into account with the determination of the percolation and seepage losses. The amount of water needed to establish the water layer, however, still has to be considered. Various approaches are being used with respect to the depth of the water layer. Sometimes a water layer of 100 mm is established after transplant and maintained throughout the growing season. In other cases the water layer is reduced to 20 to 50 mm during the latter part of the vegetative stage and brought back to 100 mm during the mid-season stage (see Figure 23).

Figure 23. The depth of the water layer may vary during the growing season

Also a common practice is to drain all the water from the field before applying fertilizers and to re-establish the water layer after the fertilizer application. This, of course, has a significant effect on the total irrigation water need of the paddy rice.

Step 7: Determine the effective rainfall: Pe

The effective rainfall is calculated using the following formulae:

Pe = 0.8 P - 25


P > 75


Pe = 0.6 P - 10


P < 75


Step 8: Calculate the irrigation water need: IN

The irrigation water need is calculated using the following formula:

IN = ET crop + SAT + PERC + WL - Pe

4.5 Irrigation scheduling of paddy rice

It has to be decided if the irrigation water is to be supplied to the field continuously or if it is to be given in rotation (see also Volume 5, section 2.4 Irrigating basins).

To determine the size of the flow to be used with continuous irrigation, the irrigation water need is multiplied by the area to be irrigated, which gives a volume of irrigation water needed per unit of time. This is the net flow of irrigation water which has to be supplied to the field continuously. This quantity of course varies over the growing season as the irrigation water need varies.

If, for example, the Irrigation water need during the month of June is 15 mm/day and the area is 1.6 ha, then the continuous net flow would be:

Another way of calculating the continuous flow is using the "rule of thumb" indicated in section 3.4 and multiplying the flow per hectare with the area to be irrigated.

For an area of 1.6 ha, the continuous flow would be: 1.6 x 1.74 = 2.8 l/sec.

If water is supplied to the same field on a rotational basis the net irrigation flow has to be increased. If, for example, irrigation water is supplied during one day out of every 7 days and during 12 hours out of 24 hours then the flow should be: 24 hours/12 hours x 7 x 2.8 l/sec = 39.2 l/sec. If water would be supplied every 5 days during 8 hours, then the flow should be: 24 hours/8 hours x 5 x 2.8 = 42 l/sec.

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