Water Management

   Crop water requirement is the water required by the plants for its survival, growth, development and to produce economic parts.  This requirement is applied either naturally by precipitation or artificially by irrigation.
    
  Average Water requirement – 1100 mm
  Average Water requirement (SRI) – 700 mm

     The daily consumptive use of rice varies from 6-10 mm and total water is ranges from 1100 to 1250 mm depending upon the agro climatic situation, duration of variety and characteristics of the soils.

• Irrigation only to moist the soil in the early period of 10 days
• Restoring irrigation to a maximum depth of 2.5cm after development of hairline cracks in the soil until panicle initiation
• Increasing irrigation depth to 5.0 cm after PI one day after disappearance of ponded water.

• Puddling and leveling minimizes the water requirement
• Plough with tractor drawn cage wheel to reduce percolation losses and to save water requirement up to 20%.
• Maintain 2.5 cm water over the puddle and allow the green manure to decompose for a minimum of 7 days in the case of less fibrous plants like sun hemp and 15 days for more fibrous green manure plants like Kolinchi (Tephrosia purpurea).
• At the time of transplanting, a shallow depth of 2 cm of water is adequate since high depth of water will lead to deep planting resulting in reduction of tillering.
• Maintain 2 cm of water up to seven days of transplanting.
• After the establishment stage, cyclic submergence of water is the best practice for rice crop. This cyclic 5 cm submergence has to be continued throughout the crop period.
• Moisture stress due to inadequate water at rooting and tillering stage causes poor root growth leading to reduction in tillering, poor stand and low yield.

• During first one week just wet the soil by thin film of water.
• Depth of irrigation may be increased to 2.5cm progressively along the crop age.
• Provide adequate drainage facilities to drain excess water or strictly follow irrigation schedule of one day after disappearance of ponded water.  Last irrigation may be 15 days ahead of harvest.

• As that of irrigated rice, canal water is used for irrigation
• For drill sown rice care should be taken to drain out excess rain water during first 10-15 DAS and the water level in the field should not be more than 2.5cm height during tillering stage.
• In drill sown rice carry out hodta operation (Planking) in standing water at 40 DAS, and impound sufficient rain water through the crop growth period.

• The crop is irrigated from 30-35 days onwards, utilizing water impounded in the tanks.
• Irrigation may be to a depth of 2.5 -5.0cm only. Follow the schedule of one day after disappearance of ponded water in order to save water and to bring additional area under this type of rice cultivation.

• The field plot can be 25 to 50 cents depending on the source of irrigation.
• Field to field irrigation should be avoided.
• Small bund may be formed parallel to the main bund of the field at a distance of 30 to 45cm within the field to avoid leakages of water through main bund crevices.
• To minimize percolation loss, the depth of stagnated water should be 5cm or less.
• In water logged condition, form open drains, about 60cm in depth and 45cm width across the field.
• Care should be taken not to allow development of cracks.
• In double cropped wetland of command area, raise groundnut / pulse in the place of Kuruvai rice if water is a constraint.

• Active tillering
• Panicle initiation
• Booting
• Heading and
• Flowering.
  

    During theses stages, the irrigation interval should not exceed the stipulated time so as to cause the depletion of moisture below the saturation level.

• Continuous land submergence for rice is, usually, practiced due to the associated major advantages of increase in availability of nutrients and less weed management problems.
• Shallow submergence of water up to 5 cm depth throughout the crop period is optimum for high yield.
  
  

• Continuous land submergence requires huge quantity of water for rice production, so to minimize the irrigation requirement, the approach of intermittent submergence has been practiced.
• Under high humidity and low evaporative demand, the practice of intermittent submergence i.e submergence during the critical stages of crop  and maintenance of saturation or drying up to hair cracking stage during the rest of the crop stages.
• Intermittent period varied from one to nine days depending on rainfall pattern, depth of water table and soil texture.
• This practice saves about 30% water.

• Standing water in lowland rice minimize the irrigation needs leading to high water use efficiency compared with continuous flowing irrigation water from field to field. 

• Flowing water from field to field increases grain yield of rice by preventing accumulation of harmful salts in the soil. 
• However, nitrogen losses will be higher with continuous flowing irrigation water.  Flowing irrigation water is ideal for problem soils.

Rotational Irrigation

• Required quantity of irrigation water is applied at regular intervals, such that there may not be any standing water in field between two irrigations.
• Irrigation interval is adjusted in such a way that the crop will not experience water deficits at any period. 
• It is usually followed at time of deficit water supplies. 
• Major advantage of rotational irrigation is possibly the more effective use of rainfall.
• Shallow submergence is advantageous during critical period of the crop

   The principal losses of water from lowland rice fields can be grouped into two: vapour losses and losses in liquid form.       

• Vapour losses are through transpiration from leaf surface and evaporation at water surface collectively referred to as evapotranspiration.
• The two types of liquid losses are deep percolation plus seepage and runoff of excess water over the field levees
  
  

• Evaporation is highest at early growth stages, when the leaf area index (LAI) is small and accounts for most of the evapotranspiration (ET) losses. 
• In most of the tropics, the ET requirements during wet season, on an average, is 4-5 mm day -1.   During the dry season, it is 8-10 mm day-1.
  
  

• Percolation occurs in vertical direction and seepage by horizontal movement through levees. 
• Percolation is largely affected by topography, soil characteristics and depth of water table.  In a heavy soil with water table close to the soil surface, percolation loss could be around 1.0 mm day-1 as against as high as 10.0 cm day-1 in light soils.
• Seepage, normally, flows into soil surface or streams, rivers or drains, while percolation, usually, contribute to water table.
  

Surface runoff

• It represents a major loss of water in lowland rice production, especially with flowing irrigation water from field to field.
• When the source of irrigation water is wells, there may not be any surface runoff losses. 
• Surface runoff losses are most severe in wet season especially at times of high intensity rains.
  

Tips to reduce water losses

• Seepage and percolation reduced to considerable extent by intensive land puddling and perfect leveling
• Application of FYM or compost or green manures reduce evaporation, percolation and seepage
• Lining of canals and irrigation channels can be formed to check the seepage.
• Evaporation losses can be minimized by 50% when the soil is kept at saturation under  levelled field conditions
• Addition of clay or tank silt (to light textured soils only) @ 150m3/ha reduce the percolation loss by 20 -25%.
• To reduce percolation loss:
• Growing rice in large blocks rather than in isolated small holdings.
• Subsoil compaction
• Rainwater harvesting and levees management can substantially minimize the surface runoff during wet season.

Moisture stress means the action of excess or deficit of water on plants. However, moisture stress is generally used to imply water deficits.

Water deficit occur in the plant whenever transpiration exceeds uptake. The effect of water deficits on growth and yield of rice depend on the stage of crop growth at which the water deficits occur.

• Immediately after transplanting, adequate land submergence (5 cm) 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 (2 cm) facilitates tiller production and firm root anchorage in the soil.
• Moisture stress occur at active tillering phase will reduce the yield by 30%.

• Rice crop is most sensitive to water deficit from panicle primordial development to heading.
• 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.
• Moisture stress occur at reproductive phase will reduce the yield by 
  50 - 60%.
  
  

• 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.
  
  

• Spray Cycocel @ 1000ppm (1ml of commercial product in one litre of water) under water deficit situations to mitigate ill effects of  moisture stress.
• Split application of potassium 50% at basal and 25% each at tillering and panicle initiation stage along with Azospirillum (seed inoculation, seedling dipping or soil application) alleviates harmful effects of the soil moisture stress.
• Seed hardening with 1% KCl for 16 hours (seed and KCl solution 1:1) and shade dried to bring to storable moisture. This will enable the crop to withstand early moisture stress.
• Foliar spray of kaolin 3% or KCl 1% to overcome moisture stress at different physiological stages of rice.
  
  

• Response of rice crop to excess water conditions is different for different growth stages, varieties and seasons. 
• Turbid water greatly damages the crop than clear water.
• Seedling establishment to maximum tillering stage is least sensitive. Excess water during the vegetative phase hinders tillering.
• Flowering to maturity is more sensitive to excess water than the stage from maximum tillering to flowering.  
• Fields that are kept flooded beyond the dough stage will mature in a non-homogenous way and harvest will be delayed.
  
  

• Levelling of land removes excess water by runoff.
• Drainage removes excess water from the root zone that is harmful for plant growth.
• The best time for drainage in ill drained soils is at late tillering stage.
• Controlled irrigation reduces excess use of water in water-logged area.
• Flood control measures: Construction of bunds may check water flow from the rivers to the cultivable lands.