In general, except in semi-arid areas where rice cultivation is marginal, average rainfall in rice-growing areas in Pakistan can meet at least part of the water requirement for a rice crop.
Thus, in strict terms, there is no fully irrigated rice cultivation in Pakistan. Rice is a semi- aquatic plant and does not need standing water for a successful crop. However, uncertainty of water supply, either through irrigation or lack of rain, and to reduce weed infestation, rice is always cultivated as a crop with standing water. Response to its plant to water stress varies with its growth stage and other agronomic practices.
Direct sown rice crop is less prone to drought than a transplanted crop. Highest water use during preparation of land with minimum timing and maximum use of rain water at the correct time of the season is recommended.
Water is the single most important component for sustainable rice production, especially in the traditional rice-growing areas. Reduced investments in irrigation infrastructure, increased competition for water and large water withdrawals from underground water lower the sustainability of rice production.
However, despite the constraints of water scarcity, rice production must rise dramatically to meet the food needs. Producing more rice with less water is therefore a formidable challenge for the food, economic, social and water security.
Water deficit: 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. The effect of water deficits on growth and yield of rice depends on the stage of crop growth at which the water deficits occur.
Immediately after transplanting, adequate land submergence ( five centimeters) is necessary to prevent damage to establishing seedlings from high winds and for root development. Following the early rooting stage, a shallow depth of land submergence (two centimeters) facilitates tiller production and firm root anchorage in the soil. Excessive water depth at this stage hampers rooting and decreases tiller production.
By and large, water deficit during this stage may reduce plant height, tiller number and leaf area but the yield is least affected if adequate water is provided to permit recovery of the crop before panicle primordial initiation.
Three days of moisture stress at 11 days and at three days before heading results in maximum yield loss due to high percentage of sterility. Reduction in grain yield due to water deficit during this stage is more related to the degree and duration of water deficits than to the stage of crop growth.
This stage (milk to grain maturity) is least sensitive to soil moisture stress. After yellowish ripening stage, there is no necessity for standing water. Water may be drained from the field about 7-10 days before harvest to facilitate harvesting.
Mechanisms: The effect of water stress may vary with the variety, degree and duration of water stress and the growth stage of 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 varies 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.
Therefore, maintaining saturated water regime through the crop duration is the 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 five per cent of the water absorbed through roots from soil.
Tolerance mechanisms: 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 tolerance, avoidance, escape or recovery. The “true” drought avoiding plants could possess 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.
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.
Water requirement for land preparation could be minimal with dry land preparation, which needs little or no supplementary moisture. However, majority of rice is cultivated by transplanting method. 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.
Seepage and percolation rates are highly variable depending on the soil type (porosity), topography and depth to the water table. Therefore to maintain standing water or to keep the soil saturated, water should be supplied to meet the seepage, percolation 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.
Improved varieties: High yielding varieties have more than doubled rice water productivity (against traditional) over the last decades. Hybrid rice has successfully been introduced in transplanted systems of many countries (not Pakistan). However, the direct seeding method which is gaining increased acceptance is limiting the adoption rate of the hybrid rice technology since the process requires the use of much more of the costly seeds of hybrid rice per hectare than does the alternative method of transplanting rice.
Direct seeding of hybrid rice is not economical with current hybrid seed production technologies. The new plant type (NPT) has been developed by IRRI scientists with the goal of raising the yield potential of conventional rice varieties to about 12-15 t ha-1. NPTs are targeted for direct seeding conditions in an irrigated ecology. Biotechnology could amend many abiotic and biotic constraints to sustainable rice production including drought stress and tolerance to adverse soils and cold temperature.
Agronomic management: Improving pest control and nutrient management and other technologies that enhance yields increase output per unit of water. It should be noted that IPM techniques were developed in the context of large schemes where water supply was considered a constraint. Efforts are currently under way to integrate on-farm water management, IPM, nutrient management with the improvement of crop management.
Planting date: Planting the rice at the onset of monsoon rains may reduce the need for supplemental irrigations. So nursery raising, direct seeding or transplanting should be planned keeping in view the weather forecasts.
Practices like land preparation, puddling, management of cracked soils may reduce the water losses to much extent. Water losses due to cracked soils can be reduced by measures that minimize crack development during the soil drying period through straw mulching and dry shallow surface tillage on crack formation during the fallow period, or by impeding the flow of water through these cracks. Primary tillage in dry conditions may also reduce the water requirement during land preparation and puddling.
Planting practices: Wet seeding of rice uses about 20-25 per cent less water than in traditional transplanted rice methods and drastically reduces labor for establishing the crop from 30-person days per ha for transplanting to 1-2 person days. Improved water management practices during crop establishment (the first 2 weeks from planting) are crucial to enhance the weed-suppressing advantages that can be achieved by early flooding of wet seeded rice.
Flooding techniques: With the development of new varieties and the improvement of agro-technical methods and practices, yield obtained under aerobic conditions reach the level of production as under flood irrigation.
The acceptance of all these above strategies depend on economic factors. Furthermore, they depend on improved water control management of water at the system level, as well as adequate irrigation (in particular a reticulated irrigation distribution system) and drainage facilities. Their availability in China has allowed farmers to adopt water savings techniques described above. However, typically, at that level, conveyance, field canal and distribution efficiencies are particularly sensitive to the quality of management, communication and technical control.
When water supply within the system is unreliable, farmers try to store more water than is needed. In many large irrigation systems, few control structures at any level and poor drainage structures and poor drainage networks contribute to a waste of water.