Tải bản đầy đủ - 0 (trang)
Chapter 2. Rainwater Utilization Efficiency in Rain-Fed Lowland Rice

Chapter 2. Rainwater Utilization Efficiency in Rain-Fed Lowland Rice

Tải bản đầy đủ - 0trang

86



PRADEEP K. SHARMA AND SURJIT K. DE DATTA



conserved rainwater for part of its water requirements. Upland rice also uses

water from local rains, but unlike rain-fed lowland rice, it grows in unbunded

fields where water does not stagnate and it has no access to irrigation. Another difference between upland and rain-fed lowland rice is the method of planting. Whereas upland rice is always dry seeded, rain-fed lowland rice may be

either dry seeded in a dry-tilled field or transplanted in puddled fields (De Datta,

1981).

Rain-fed lowland rice covers about 38 million ha, which is 28% of the rice

area of the world (Garrity er af., 1986), contributing an estimated 19% of the

global rice supply. Of world’s rain-fed lowland rice area, 95% is in Asia. A

majority of the rice-growing areas in South and Southeast Asia are rain-fed lowlands (Fig. 1). India has the largest rain-fed lowland rice area, with about 16.5

million ha, which is about 40% of the total rice area in India (Pande and Reddy,

1984; De Datta, 1986). Eastern India alone (states of Assam, West Bengal, Bi-



Figure 1 Predominantly rain-fed lowland rice areas in south and southeast Asia.



EFFICIENT UTILIZATION OF RAINWATER BY RICE



87



Table I

Rain-Fed Lowland Rice Areas in South and Southeast

Asian Countriesa

Area (lo00 ha)



Country



Total rice area



Rain-fed

lowland

rice area



Rain-fed

lowland



143

5679

805

5716

2932

12,779

807

277

1415

2086

222

153

1378



76

65



(%)



~~



Bhutan

Thailand

Nepal

Bangladesh

Myanmar

Eastern India b

Cambodia

Laos

Philippines

Vietnam

Sri Lanka

Malaysia

Indonesia



I89

8677

1262

10,012

5317

26.763

203 1

695

3515

5573

760

735

8204



64

57

55



48

40

40

40

37

29

21

I1



“Adapted from Huke (1982).

bEastern India includes the states of Assam, West Bengal,

Bihar, Orissa, Madhya Pradesh, and Uttar Pradesh.



har, Madhya Pradesh, and Uttar Pradesh) leads all other countries by a wide

margin in the area of rain-fed lowland rice, with about 15 million ha (Table I).

In Thailand, an area of -6.2 million ha (62% of total rice area) is planted in

rain-fed rice, and is the second largest in Asia. In all rain-fed areas, the actual

rice yields are generally much lower than the potential yields because of several

soil, environmental, and socioeconomic constraints. One key factor in increasing

the production of rain-fed lowland rice is to increase the rainwater utilization

efficiency in these areas.



11. CONSTRAINTS

Rain-fed lowland rice environments are highly diverse and unpredictable, with

insufficient or excess water as the major limiting factors. Most of the monsoonal

Asian countries receive 50-90% of the annual rainfall during May-September



88



PRADEEP K. SHARMA AND SURJIT K. DE DATTA



(Fagi et al., 1986), and in most areas these rains are concentrated within a short

span of 4-6 weeks per monsoonal season (Abeywardene, 1987). High-intensity

rains, exceeding the infiltration rates of soil, are not only lost as runoff but may

also cause serious soil erosion.

Spatial and temporal fluctuations are the characteristics of monsoonal rains.

One such example, showing variations in the time of onset and withdrawal

of monsoon and its distribution during the cropping season for the Tarai region of Nepal and northern India, is shown in Fig. 2. Saenjan et al. (1990)

analyzed rainfall data collected at 40 locations in northeast Thailand for 30 consecutive years. The coefficient of variation for the total seasonal rainfall, which

is a measure of the reliability of rainfall at a given location, varied between 40

and 72%.

Variations in the onset and termination of monsoons have technical as well as

production consequences. Late onset delays land preparation and crop establishment of rain-fed lowland rice, and early termination causes drought stress at the

reproductive stage. The uncertainty in the frequency and amount of rainfall also

Rainfall (mm)



C V % 125 93

70 74 66 200

149 106 73 123 99 198



Figure 2 (A) Annual rainfall distribution illustrating the extent of variability (dashed line) associated with monsoon onset and withdrawal periods, and (B) rainfall distribution and variability

common to the Terai region of Nepal and northern India (Chang et al., 1979).



EFFICIENT UTILIZATION OF RAINWATER BY RICE



89



Table I1

Approximate Probability of Deficient Rainfall during the Monsoon in Various Meteorological

Subdivisions of India“

Recurrence of highly

deficient rainfall



Meteorological subdivisions

Assam

West Bengal, Madhya Pradesh, Konkan, coastal Andhra Pradesh, central Maharashtra, Kerala, Bihar, Orissa

Southern interior Mysore, eastern Uttar Pradesh, Vidarbha

Gujrat, eastern Rajasthan, western Uttar Pradesh, Tamil Nadu,

Kashmir, Rayalaseema, Telengana

Western Raiasthan



Very rare, once in 15 years

Once in 5 years

Once in 4 years

Once in 3 years

Once in 2-5 years



“From Kateswaram ( 1 974).

bRainfall deficiency is equal to or greater than 25% of normal rainfall.



makes predictions about rainfall difficult. One rice crop may suffer from both

drought and excess water during the same growing season. Table I1 shows the

probability of drought occurrence in India. Based on the recurrence interval of

highly deficient rainfall, every part of India, except Assam and the adjacent

states, faces the probability of a drought once in 5 years. Some provinces, such

as Gujrat and Rajasthan, and regions such as Rayalaseema and Telengana, are

subject to drought once in about 3 years.

It has generally been accepted that rain-fed rice grows best in areas receiving

more than 200 mm of rainfall per month for a minimum period of 3 months,

without dry periods exceeding 7- 10 days (Krishnamoorty, 1979; Hundal and

Tomar, 1985; Abeywardene, 1987). The 200-mm rainfall per month is based on

6-7 mm of daily evaporation. On this basis, IRRI (1974) developed a breakdown of the climatic zones of the rice-growing regions of Southeast Asia

(Table 111). If a monthly rainfall of 200 mm is considered as the lower limit

for lowland rice, then rain-fed areas belonging to climatic zones 11-3, 111-1,

111-2 and IV require serious efforts toward increasing the effectiveness of the

normal rainfall, particularly if year-to-year fluctuations in the monthly rainfall

are sharp.

An international committee divided the rain-fed lowland ecosystem into five

subecosystems based on two major hydrological stresses, i.e., inadequate or excess moisture (IRRI, 1984):

1. Rain-fed shallow, favorable environments, areas in which drought or submergence is not a serious constraint and crop management practices are essentially similar to those in fully irrigated systems.



90



PRADEEP K. SHARMA AND SURJIT K. DE DATTA

Table I11

Climatic Zones in Southeast Asian Rice-Growing Regions“



Climatic

zone

I

11-1

11-2



11-3



111-1



111-2



IV



Description



Utilization



More than 9 consecutive wet months

with 200 mm of rainfallimonth

From 5 to 9 consecutive wet months

with 100-200 mm of rainfalhnonth

From 5 to 9 consecutive wet months

with 100-200 mm of rainfalhonth

during the remaining part of the year,

and with another rainfall peak

From 5 to 9 consecutive wet months

with at least 2 months of rainfall


From 2 to 5 consecutive wet months

with at least 100 mm of rainfall/

month during the remainder of year

From 2 to 5 consecutive wet months

and a pronounced dry season with at

least 2 months of <200 mm of rainfalllmonth

Less than 2 consecutive wet months



Year-round cropping with two crops of

puddled rice

Year-round cropping with one crop of

puddled rice

Suitable for multiple cropping; farmers

are likely to grow two crops of

puddled rice

Possible to grow two crops in a year



Limited possibility of growing two

crops in a year

Limited possibility of growing two

crops in a year



Not suitable for any type of agriculture



~



“IRRI (1974).



2 . Rain-jed shallow, drought-prone environments experience frequent and

severe water deficits at any growth stage. The rainy period may be short

(90- 110 days) or may continue for longer periods, but with highly uncertain

distribution.

3. Rainfed shallow, submergence-prone areas frequently experience shortterm flooding that may damage or destroy the crop when specifically adapted

varieties and crop management practices are not used.

4. Rain-fed shallow, drought- and submergence-prone ricelands may experience both water deficits and short-term flooding on a frequent basis.

5 . Rain-jed medium-deep, waterlogged ricelands accumulate water at depths

of 25-50 cm for a substantial portion of the growing season.

Based on the source of the water supply, rice-growing areas exist in three

topographic sequences (O’Toole and Chang, 1978; Greenland and Bhuiyan,

1982): pluvial, phreatic, and fluxial. Rainfall is the only source of water in the

pluvial toposequence; rainfall, seepage water, and the water table are sources in

the phreatic toposequence; and rainfall, seepage water, the water table, and runoff are sources in the fluxial toposequence. Pluvial and phreatic toposequences



EFFICIENT UTILIZATION OF RAINWATER BY RICE



91



have drought constraints, whereas floods may occur in fluxial toposequences.

Most of the rain-fed lowland rice areas are in the fluxial and phreatic toposequence categories, and only some are in the pluvial category.

Based on soil texture, growing period, and amount of rainfall, Garrity et al.

(1986) estimated that 25% of the rain-fed lowland rice areas in South and Southeast Asia are favorable, 21% are intermediate, 32% are drought prone, and 22%

are highly drought prone. Mackill (1986) estimated that 12% of the Asian lowlands are submergence prone and that 8% are drought and submergence prone.

In addition to hydrological constraints, there are limitations imposed by an

insufficient potential growing season, high temperatures (>35"C), and decreased

solar radiation. There are various soil-related constraints, such as coarse texture;

low water-holding capacity; low organic matter content; low CEC; low buffering

capacity; N, P, K, and Zn deficiencies; Fe toxicity; and salinity, alkalinity, and

organic and acid sulfate conditions (Garrity et af., 1986; Goswami and De Datta,

1986). Coupled with these are socioeconomic problems that deter high-level

rice production. Most rain-fed lowland rice farmers are poor and cannot take

risks. Because of the high risks of drought and flooding and unstable rice

yields, they usually invest very little in fertilizers, herbicides, pesticides, etc.,

for rice production in rain-fed lowland areas (Pa., 1979; Alcantara et af., 1984;

De Datta, 1986). Adoption of modern rice cultivars and associated production

technologies has been limited and nonexistent in some areas. Lack of rice cultivars responsive to improved management yet tolerant of the important biophysical stresses of the rain-fed lowland areas is another major limitation to growing

rain-fed lowland rice. De Datta (1984) stressed the need for identifying constraints in relation to specific target environments in order to make rain-fed technology viable.



1II.POTENTIALS

A large gap exists between rice yields in rain-fed and irrigated areas. For

example, 60% of the rain-fed rice contributes to only about 40% of the total rice

that is produced in India (Krishnamoorty, 1979). Also, rain-fed lowlands dominate the worldwide rice-growing regions, and thus are the areas that will allow

potential future increases in the world's rice production, if there is an effort to

focus on technologies that will stabilize yields, require a minimum of purchased

inputs, and minimize risks to the environment. One key step in this direction is

to improve rainfall effectiveness and water-use efficiency of rain-fed lowland

rice. This can be achieved by proper soil, water, and crop management techniques, and the following discussions review some of the findings, possibilities,

and challenges that are significant.



92



PRADEEP K. SHARMA AND SURJIT K. DE DATTA



IV.EFFICIENT UTILIZATION OF RAINWATER

Physiologically, water-use efficiency refers to the ratio of total biomass production (or economic yield) and total crop water use. There are two aspects of

increasing the efficiency of rainwater use under rain-fed situations: (1) raising

the yield of the rain-fed crop when rainfall is normal and ( 2 ) preventing yield

loss when rainfall is inadequate and drought occurs frequently. The technology

for improving the efficiency of rainwater use in rain-fed lowlands aims at

( 1) increasing in situ rainwater interception, soil infiltrability, and profile water

storage, ( 2 ) decreasing nonproductive water losses, such as percolation and seepage, (3) collecting runoff water for providing supplementary irrigation at critical

growth stages, if needed, and (4) properly managing rice crops for efficient utilization of conserved soil moisture. These objectives can be achieved by employing various soil and water management practices.



A. SOILMANAGEMENT

PRACTICES

1. Puddling versus Dry Seeding

Either of two planting systems for growing rain-fed lowland rice may be used,

depending on the hydrological situation of the area. In one method, the soil may

be puddled and inundated and then planted with rice. This technique is used in

areas that either receive high rainfall or receive supplemental irrigation from

stored rainwater. The second system starts with dry-seeded rice and ends up with

a wetland rice culture. It is practiced in areas with relatively low rainfall. In this

system, rice is dry seeded in the early part of the monsoon; when heavy rains

arrive, the system is converted to a wet area by impounding water. Dry seeding

is practiced in a limited area in Asian countries (De Datta et af., 1979). To

decrease soil permeability, to remove weeds, and to reduce crowding of seedlings, the field is wet-plowed in both directions with a native plow, preferably

under standing water, when the seedlings are at the three- to four-leaf stage. At

the same time, the gaps are filled with the seedlings from the crowded parts of

the field. This practice is called “halod” in Himachal Pradesh, “biasi” in Madhya Pradesh, and “beusani” in Orissa (states of India), and “gogo rancah” in

Indonesia. This method takes advantage of early rainfall and allows time for a

second crop of rice to be grown. The major problems with this system are vigorous weed growth, increased risk of drought damage, poor stand establishment,

and low grain yield.

For lowland rice, puddling is the most common technique of land preparation.

Puddling destroys soil aggregates and peds, creating plastic mud, and thus elimi-



EFFICIENT UTILIZATION OF RAINWATER BY RICE



0



93



=0.982



A = 0.972

A ~0.982



=0.908



I



0



I



2



I



4



I



6



8



1



Puddling depth (cm)



0



Figure 3 Effect of depth of puddling on water flux through soils of different texture (Is, loamy

sand; 1, loam; sil, silt loam; cl, clay loam) (Sharma and Bhagat, 1993).



nates most macropores, which transmit water. The remaining macropores are

partially filled by dispersed fine particles (Sharma and De Datta, 1986; Adachi,

1990). Consequently, there is a drastic reduction in percolation losses of water

and nutrients (Sharma and De Datta, 1985, 1986; Sharma etal., 1988). Sharma

and Bhagat (1993) reported a nonlinear reduction in water flux through soils with

an increase in puddling depth (Fig. 3). This benefits the rice plant by increasing

grain yields and decreasing water requirements, thus, improving water-use efficiency. In a study at IRRI on a clay soil (Alfisol), rice grown on puddled soil

used half as much water as rice grown on nonpuddled soil (Fig. 4). Rice production on puddled soil was 2.5 times more efficient in water use than rice grown

on nonpuddled soil because of decreased water percolation losses and greater

soil moisture retention in puddled soil (De Datta and Kerim, 1974). In a clay

loam soil (Typic Argiudoll), the average daily water use for transplanted rice

during a 90-day irrigation period was 501 mm day-' with zero tillage, 263 mm



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 2. Rainwater Utilization Efficiency in Rain-Fed Lowland Rice

Tải bản đầy đủ ngay(0 tr)

×