Rice (Orysa sativa L) is cultivated either as
a rainfed or as a supplementary or fully irrigated crop. The system of
rice cultivation mainly depends on the available rainfall and it's distribution.
In general, except in semi arid areas where rice cultivation is marginal,
average rainfall in rice growing areas of Sri Lanka can meet at least part
of the water requirement for a rice crop during it cropping season. Thus
in strict terms there is no fully irrigated rice cultivation in Sri Lanka.
Rice is a semi aquatic plant and does not need standing water for a successful
rice crop. However, uncertainty of water supply, either through irrigation
or rain, and to reduce weed infestation rice is always cultivated as a
crop with standing water. Response of the rice plant to water stress vary
with its growth stage and other agronomic practices. Direct sown rice crop
is less prone to drought than a transplanted crop. Highest water use is
during the preparation of land, thus land preparation with minimum timing
and maximum use of rain water at the correct time of the season is recommended.
Effect of water deficit
Stress has been define as "any environmental
factor capable of inducing a potentially injurious strain in plants". Water
is a major constituent of tissue, a reagent in chemical reaction, a solvent
for and mode of translocation for metabolites and minerals within plant
and is essential for cell enlargement through increasing turgor pressure.
With the occurrence of water deficits many of the physiological processes
associated with growth are affected and under severe deficits, death of
plants may result. The effect of water stress may vary with the variety,
degree and duration of water stress and the growth stage of the rice crop.
Water requirement is low at the seedling stage. Unless there is severe
water stress the effect during this stage could be recovered. Water stress
during vegetative stage reduces plant height, tiller number and leaf area.
However, the effect during this stage vary with the severity of stress
and age of the crop. Long duration varieties cause less yield damage than
short duration varieties as long vegetative period could help the plant
to recover when water stress is relieved. Leaf expansion during vegetative
stage in very sensitive to water stress. Cell enlargement requires turgor
to extend the cell wall and a gradient in water potential to bring water
into the enlarging cell. Thus water stress decreases leaf area which reduces
the intercepted solar radiation. Rice leaves in general have a very high
transpiration rate thus under high radiation levels rice plant may suffer
due to mid day wilting. Rice plant can transpire its potential rate even
when soil moisture was around field capacity. Therefore maintaining saturated
water regime through the crop duration is best for saving water and increasing
grain yield. However, if the weed pressure is high maintaining standing
water until the closure of the canopy and then maintaining saturated soil
conditions could increase water saving without reduction in yield. Soil
cracking should be prevented to reduce percolation during subsequent irrigation.
In general rice plant uses less than 5% of the water absorbed through roots
from the soil. The rest is lost through transpiration which helps to maintain
leaf energy balance of the crop. Decrease leaf water potential closes stomata
and decrease transpiration which in turn increase leaf temperature. Stomatal
closure could be due to the accumulation of Abscisic acid which is a drought
tolerant mechanism. Even though closure of stomates improve water use efficiency
under water stress conditions this decreases carbon assimilation due to
reduction in physical transfer of CO2 molecule and increase leaf temperature
reduces the biochemical processes. Decrease solubility of CO2 may also
reduce CO2 assimilation. Translocation of assimilates may also decrease
under desiccating conditions due to reduction is source and sink capacity
and decrease water potential. There is a marked genotypic variation in
rooting pattern in rice in response to water stress. Drought resistant
varieties possess deep and thick roots in contrast to this and shallow
roots. Thick roots in rice are positively correlated with xylem vessel
area which are vital to the conductance of water from soil to the upper
parts of the plant to meet the evaporative demand. It has also observed
that water stress reduces the uptake of nutrients which could be due to
the fact that most of the elements are absorbed via roots through passive
diffusion. Direct seeded rices are more tolerant to water stress than transplanted
rice which could be due to its superior root system. Increase N nutrition
increases the susceptibility of the rice plant to water stress. Rice is
most susceptible to water stress during reproductive stage. Water stress
at or before PI reduces panicle number most, but all stresses regardless
of crop stage or duration significantly reduce panicle number. Water stress
after PI reduces the potential spikelet number. Water stress at heading
reduces rate of exertion of the panicle. Anthesis and ripening stages are
the most sensitive stage for water stress. Water stress during anthesis
increased unfilled spiklets. Spikelet sterility decreases with decrease
leaf water potential during meiotic stage of pollen development. Mild stress
affect sink more than source, whereas severe stress affects both. Stress
during grain filling decrease translocation of a assimilates to the grain
which decreases grain weight and increase empty grains. Increase canopy
temperature above 280C due to water stress linearly increase relative spikelet
sterility. The ability of a plant to grow satisfactorily when exposed to
periods of water stress is called drought resistance. Mechanism of drought
resistance in rice could be either tolerance, avoidance, escape or recovery.
The "True" drought avoiding plants could posses mechanisms to maintain
favourable water status, either by conserving water or by their ability
to supply water to above ground organs even during drought stress. Root
depth is a plant trait most strongly related to drought avoidance in upland
rice culture which is an avoidance mechanism. Rice plant that can escape
or evade drought through the adjustment of the life cycle is also an important
trait for Drought resistance. Leaf rolling or reduced leaf area, stomatal
closure and delayed flowering under water stress conditions compared to
well watered condition could be escape mechanisms. Tolerance implies the
plant tissues to withstand water stress. The degree of tolerance in rice
vary among varieties and among growth stages within a variety. Osmorogulation
in certain varieties of rice in a tolerance mechanism. Recovery of a rice
plant after reliving drought stress vary with the variety, the severity
of stress and growth stage.
Excess water effects
To be developed
Water requirement of a rice crop in Sri Lanka
Water requirement for a successful rice crop
varies with the method of land preparation, method of crop establishment
and duration of the rice crop. It also varies with the soil, environmental
conditions and the management of the subsequent rice crop.
Method of water loss
Water is lost through evaporation (E)
from free water surface, transpiration (T) from the crop, seepage and percolation
of the soil, bunt leakages and runoff from the field. Seepage and percolation
vary with the edaphic environment which could be partially controlled through
proper management. However, evapotranspiration is determine mainly by the
vapor pressure deficit and the canopy size which is beyond the control
of a farmer. Bund leakages and runoff from the field is totally under the
farmer's control. Therefore the main determinants of water requirement
(WR) are evapotranspiration, seepage and percolation (S & P) rates,
which could be summarized as follows.
WR = E + T + (S + P)
Water requirement for
Land Preparation
Water requirement for land preparation could
be minimal with dry land preparation which is popularly known as "Kekulan"
or "Manawary" system of cultivation, which needs little or no supplementary
moisture. However, majority of rice is cultivated as lowland crop. The
duration of land preparation mainly determine the amount of water required
which is dependent on the type of land class and the weed infestation.
Water requirement for lowland land preparation is determine by the amount
required for soil soaking, losses during operations and maintaining standing
water in the field. Water requirement for soaking the land depends on the
initial soil moisture content and surface conditions of the land and soil
type. The requirement may vary from 30 mm. to 125 mm. of water as there
may be losses through cracks and other ways. After first ploughing field
is inundated with water to keep the soil and weeds under water which facilitate
decomposition. During the period when standing water is maintained on the
surface, water is lost through evaporation, seepage and percolation. Average
rate of evaporation in a sunny day in the Dry Zone during "Maha" is about
3.5 mm and during "Yala" is about 6 mm. Seepage and percolation rates are
highly variable depending on the soil type (porosity), topography and depth
to the water table. Reddish Brown Earth (RBE) soil has an average S &
P rate of 7-10 mm/day and Low Humic Gley (LHG) soils it is around 3-4 mm/day.
Therefore to maintain standing water or to keep the soil saturated, water
should be supplied to meet the S, P and evaporation requirements. Thus
the water requirement increases with the increase in duration of land preparation.
A minimum period of two weeks is required for conventional method of land
preparation. In general water requirement for land preparation in the dry
zone of Sri Lanka vary from 150 mm on LHG to 300 mm on RBE
Water requirement during
crop growth
Water is lost from a rice field mainly through
evapotranspiration, seepage , percolation, surface runoff & bund leakages
which could vary depending on crop, environment and the field management
factors. Evapotranspiration from a rice crop canopy is a function of the
size of the crop (leaf area), water availability and the environmental
conditions. Evapotranspiration increases with increase leaf area. Variation
in rice crop ET during its growth is shown in fig. 1. Evapotranspiration
is low at early stages of crop growth and achieve maximum towards heading.
Hence the frequency of irrigation should increase accordingly towards flowering
to meet the increasing demand for water. Experiment conducted at Agriculture
Research Station, Mahailluppallama showed that the total ET in the dry
zone during in Yala season in higher than during Maha season (Table 1).
Table 1.Total Evapotranspiration (mm) form a
4 1/2 and 3 ? month rice crop during Yala and Maha seasons at Mahailluppallama
| Method of estimation | Evapotranspiration per season, mm | |||
| 4 1/2 month | 3 1/2 month | |||
| Yala | Maha | Yala | Maha | |
| Experimentally determined ET | 830 | 455 | - | - |
| *Calculated ET | 770 | 520 | 465 | 665 |
* Calculated using modified Penman method using long term average climatic values.
Seepage and precolation
losses
Rates of seepage and percolation, when compared
with ET which is relatively stable in a given period within a given agro-ecological
region with uniform climate, vary very much from place to place. Seepage
and percolation rates are mainly govern by the profile characteristics
and topography and are much grater in sandy than clay soils. It increases
with increase depth of standing water. The rate of S & P are about
6 mm/day in well drained and 3 mm/day in poorly drained soils. In general
RBE soils have greater S & P compared to LHG. Further dry land preparation
increases S & P rates due to increase porosity, hence puddling decrease
S & P by clogging the pores and forming a hardpan below the plough
layer. Poorly constructed bunds and crab holes increase seepage.
Total water requirement
for lowland rice
Total water requirement for lowland rice increases
with the age of rice crop and could be summarized as follows. Water requirement
(WR) per season = Sum of daily ET + Sum of daily S & P As suggested
earlier S & P rates are highly variable between locations thus WR varies
accordingly. Experiments conducted under controlled situations at Agriculture
Research Station, Mahailluppallama suggest the following total WR for the
Maha season (Table 2).
Table 2. Total water requirement for a rice
crop at ARS, Mahailluppallama during Maha season
|
|
|
|
| 3 month | 4 month | |
| RBE moderately drained | 1057 | 1232 |
| LHG | 948 | 1128 |
Irrigation requirement
and frequency
Water loss through ET, S & P should be
supplemented by either natural means such as rain, and seepage from adjoining
plots or through irrigation. If an average of 5 mm of water is lost per
day by ET, and about 3 - 6 mm/day by seepage and percolation from poorly
drained and well drained soils respectively, a total of 8 to 11 mm of water
is lost per day from a low land rice field. If irrigation water could be
supplied to a depth of about 7.5 cm per issue, irrigation frequency should
be maintained at 7 to 10 days interval. When initial water height in the
field is lower, frequent irrigation is needed. However, in this system
of irrigation field will be kept without standing water towards later days
after irrigation. If soil moisture level drops below field capacity, subsequent
formation of soil cracks increase
Irrigation systems in
Sri Lanka
Water for rice culture in Sri Lanka is received
through rainfall or through irrigation. In areas where rain fall distribution
during the season is satisfactory to meet the crop water requirement of
rice culture, crop is raised completely as rainfed crop. In this case crop
depends on direct rainfall to the field and seepage and run off from surrounding
areas. There is no properly constructed system of channels for directing
of distribution of water. Dykes are constructed to retain water in the
field and they are maintained well to prevent water leakages. In areas
where rainfall is not assured to supply water requirement of the crop,
supplementary irrigation is provided through distributory channel systems
from tanks and anicuts. These irrigation networks essentially designed
for rice culture are divided into two main categories based on command
area namely (1) Minor irrigation system (2) Major irrigation system by
the Irrigation Department. The minor irrigation systems are the systems
where command and area is less than 80 ha. Both tank and anicut systems
are included. the major irrigation systems are with command areas greater
than 80 ha. They also include both tanks (reservoir) and anicut systems.
Minor irrigation systems These systems come within the justification of
Agrarian services department. Since the command area is comparatively smaller
and distributory channelled lengths are shorter, better regulation can
be expected. The water availability in these systems depend on the catchment
area rainfall tank capacity and the size of command area. Major irrigation
systems These systems came within the authority of either the Irrigation
Department of Mahaweli Authority of Sri Lanka. The tanks and streams which
are used for anicut systems depend on their own catchments for water in
many systems. However, some tanks are benefitted by water diverted to them
from other catchment through transbasin channels. The water supply under
these reservoirs are more assured than the tanks which depend on their
own catchments. The distributory channelled system in these systems are
better equipped with control structures than in the minor irrigation schemes.
Hence somewhat controlled water management practices have been introduced
into these systems. Water is issued mostly on a pre scheduled rotation
in major tank systems.
Problems related to water
management
Salinity Development in Paddy fields Wrong
water management practices cause salinity built up in paddy fields, Observations
show that lack of surface drainage is the main cause of salinity development
in Sri Lankan paddy fields. Seepage and runoff water which collects in
depressions in inland scape evaporate, leaving salts dissolved in them
causing salinity built up. Collection of water in these depressions or
low lying areas can be due to purposeful blocking of drainage ways or by
mere negligence by the farmers. Improvement of drainage will correct the
problem. Iron toxicity Iron toxicity is a problem largely found in the
rice soils of intermediate zone and up and low country Wet Zone soils.
The problem is commonly observed in flat valleys and its occurrence is
mainly confined to those positions in the flat valley where interflow streamlines
from adjacent landscape emerge within the valley. Interception of those
interflow is a water management practice that can alleviate the problem.
This can be achieved by digging drains at the boundary between paddy land
and adjacent highland.
Water management in relation
to other practices
Water management in relation to weed control
It is not an exaggeration that total success of rice weed control is a
function of better water management. Abundance, composition and temporal
distribution of weeds in rice fields are controlled by the depth and duration
of water availability. Most of the weed seeds are highly sensitive to soil
moisture and standing water. Usually, optimum soil moisture regime for
weed seed germination is below the saturated conditions. Increasing soil
moisture above saturated levels progressively reduces the seed germination
and maintenance of standing water for one to two inches can arrest more
than 90% of the potential weed emergence. On the other hand, periodic wetting
and drying of rice soil provides and ideal soil moisture condition for
a prolific weed growth. Therefore, maintaining standing water right from
the inception of crop establishment is and effective method to reduce weed
growth in rice. In transplanted rice where seedlings are fairly tall, an
effective level of standing water can be maintained right from the planting.
In fact, post planting weed competition could be completely eliminated
in transplanted rice through management of water. In broadcast rice, however,
standing water can only be maintained in rice when the seedlings are at
least 7/8 days old. Water management in relation to plant disease control
Moisture on foliage or standing water in the field is very important condition
for fungal and bacterial disease occurrence and development. Fungal spore
germination requires a moisture film on the plant surface. High relative
humidity is essential in maintaining this leaf wetness that often occurred
through condensation. Since a normal paddy cultivation provides above conditions
it is very difficult to use water management methods for disease control.
However, prevention of rice field submerges by stormy rain water could
prevent out-break of bacterial blight, bacterial leaf streak and sheath
blight epidemics. On the other hand upland dry soil condition completed
with cool weather condition favour occurrence and development of blast
disease.
Mitigation options
Field water requirement for a rice crop depends
mainly on the growth duration of the crop and its growing environment.
It is calculated that about 30-40% of the total water supplied to an irrigated
crop is often supplied before the establishment of the rice crop and the
amount is dependent on the soil drainage class, weed density and time taken
for land preparation. Time taken for land preparation could be minimised
to about 2 weeks using total killing herbicides (e.g. Paraquat) which also
would help to reduce one tillage operation and conserve irrigation water.
Dry sowing could be an alternative for the well drain or sandy soils where
water use is very high. Mulching straw after seeding could conserve moisture
which facilitate early and uniform germination and suppress weeds to a
certain extent. However, poor plastering of bunds and "not puddling" the
field would increase subsequent water use due to rapid percolation and
lateral seepage. The potential of existing rainfall for growing rice in
under utilized. Timely cultivation with maximum utilization of rain water
has a tremendous potential for increased rice production. It will also
maximize irrigation water use efficiency. Initial land preparation with
the onset of rains when soil is moist could not only conserve irrigation
water but also help to plough deep into the soil and facilitate growing
a longer duration rice crop without exposing to terminal drought. Selection
of an age class to suit the available water would increase the field irrigation
water use efficiency. In general lowering the age decreases the water requirement
for paddy but at the expense of yield. Cost of land preparation and other
agronomic practices would be the same or higher except a small decrease
in use of fertilizers and pesticides with short age varieties. However,
short growing season demand better weed control and optimum timing which
could increase cost of production. Very short duration (75 days) varieties
(Bg 750) could be used in drought prone areas to avoid terminal drought
but potential yield of such varieties are rather low (about 70 bu/ac).
These varieties could be used as an escape mechanism. Similarly Kakulan
type varieties with good weed competitive ability (e.g. 62-355) could also
be used in areas with short growing season. Scientist have so far unsuccessful
in developing varieties for drought avoidance or tolerance due to its complexity
and difficulty in combining those desirable traits. New techniques in breeding
could be a solution to these problems. One reason farmers keep rice fields
continuously flooded is to keep down weeds, which complete less well with
rice under such conditions and also as an insurance against moisture stress.
Minimising percolation and seepage losses by proper land preparation and
plastering of bunds could keep standing water in the field for a long time
which help in both conserving irrigation water and keep weed pressure low.
Rice does not require standing water to maximize yields. Maintaining saturated
condition could save up to 40% of water in clay loam soils (IRRI, 1995)
without yield reduction, however weed control should be made through chemical,
mechanical and manual means. Failure to maintain saturated condition (drying)
could increase soil cracking which could increase percolation through soil
cracks. Weed control by chemicals would eventually be an alternative with
scarce water and labour, however risk of development of weeds resistant
to herbicides, human health and environment hazzads and cost could increase
with the increase usage of herbicides. New frontier research is ongoing
in many parts of the world to Kill weeds by infecting their own natural
pathogens. Suppress growth using allelopathic activity against weeds. New
plant types to smother weeds.