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Water stress effects, water management systems & irrigation requirements for rice in Sri Lanka

ice (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
Soil type Age of the crop

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.
 

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