Principles Excess / limited / no water leads to reduction in yield. Every crop plant tries to survive under extreme drought conditions but never survives for atleast 1 – 2 days under excess moisture situations. Rice a semi – aquatic plant requires near submergence Submergence helps in - suppressing weed growth more availability of certain nutrients Daily consumptive use of rice is 6 – 10mm Total water requirement of rice is 1240mm 5000 litres of water required to produced 1kg of rice Rice accounted for 33% of gross irrigated area Rice consuming 66% of total available irrigation water. Principles of water management Highly saline and brakish water not good for irrigation. Application of FYM or incorporation of green manures reduce adverse effects of excess salts. Application of FYM or compost or green manures increase water holding capacity of light textured soils and thus saving of water. Field management for higher water use efficiency. Through puddling impermeable layer reduces deep percolation losses. Perfect levelling maintains uniform depth of water throughout field. Maintenance of water depths in field as recommended for high water use efficiency and yield.

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Utilization of water under different growth stages Stage of growth Avg. water requirement (mm) % of total water requirement Nursery 40 3.2 Main field preparation 200 16.1 Planting to P.I 458 37.0 P.I to flowering 417 33.7 Flowering to maturity 125 10.0 Total 1240 100.0 P.I = Panicle Initiation

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Water requirement stage wise Water management in nurseries Puddle the nursery bed thoroughly Level the land perfectly. Formulate channels to irrigate or drain the field. Drain water completely and broadcast seed uniformly Allow water for 12 hours and again drain (if we broadcast dry seed allow water in the nursery for 24 hours). Allow nursery beds drying upto four days.  Give irrigation and maintain 2cm of water till pulling of seedling Avoid excess irrigation poor growth of seedlings. Avoid stress deep root growth pulling of seedlings is difficult Stage of crop Depth of water (cm) Advantages in maintaining the recommended depth of water At transplanting Shallow (2– 3cm) Shallow transplanting is possible Helps in good tillering After transplanting (24 – 48 hours) Deep (5 – 7cm) Better establishment of seedlings Quick development of new roots Reduction in transpiration Duringtillering (60–65 days) Shallow (2 –3cm) Excess water reduces tillering Mid – season drainage (24 – 48 hours) 20 – 25 DAT (heavy soils) 20 – 25 DAT very heavy 40 – 45 DAT soils - Stimulate rooting Check unproductive shoots Non – accumulation of toxic substances Drains excess N Sterility % reduced Supply of oxygen to elongate internodes Prevent excess mineralisation Prevent lodging Avoids Zn deficiency Reproductive stage Panicle emergence Booting Heading Flowering Deep (5 –7cm) Moisture stress – Impaired panicle growth Increased sterility Excess water – delay in heading stem weakens a lodging Ripening stage (21 days after full flowering) Milk stage Dough stage Drain the field gradually Early with drawal increase of immature grains broken grains Late with drawal lodging Toxic substances : Poor root development, root rot, poor tillering, anaerobic respiration, low nutrient uptake Note Drain the field to a very thin film of water a day before top dressing with N reflood after 48 hours to reduce N – loss.

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Critical stages of water requirement Active tillering phase -- 30% yield reduction Reproductive phase (Panicle Initiation)(Flowering) -- 50-60% reduction Critical stage refers to a stage when water scarcity or deficit of water causes comparatively greater reduction in yields which cannot be made by favourable water supply at earlier or later stages. Hence, water deficit during these stages should be avoided.

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Losses of water in rice fields Evaporation (E) 180 –300mm unavoidable Transpiration (T) 200 – 500mm unavoidable Seepage and percolation 200 – 500mm reduced to some extent by thorough puddling and perfect levelling Average Evapotranspiration (ET) is 7mm/day (kharif) and 10mm/day (rabi) Application of phosphatic fertilizers increases the transpiration. Application of FYM or compost or green manures reduces evaporation, percolation and seepage. Evaporation losses can be minimized by 50% when the soil is kept at saturation. Needless to say such a shallow water application practice requires perfectly levelled land. Addition of clay or tank silt (to light textured soils only) @ 150m3/ha reduced percolation loss by 20 –25%. Addition of bentonite @ 7000kg/ha. Higher the transpiration higher the yield. A normal crop of 4.5t/ha with an irrigation period of 100 days consumes 6mm/day by transpiration. The figure declines to 1.4mm/day when the yield decreases to 1t/ha and increases 10.5mm with a crop yield of 7.5t/ha. Seepage varies according to topography, depth of ground water table and farming practice. Seepage facilitates removal of toxic salts and supply of oxygen to roots. Disadvantages with seepage are (1) wastage of water (2) nutrient leaching.

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Water quality and critical values Major considerations of water quality are, Salinity Water infiltration rates Specific ion toxicity Types of problems and critical values Potential problem Units No probelm Slight to moderate problem Severe problem Ph No units 6.5-8.5 <6.5: > 8.5 <6.5: > 8.5 Salanity ECw Ds/m=m mol/cm <2.0 2.0-2.6 >2.6 Salanity ECe Ds/m <3.0 3.0-3.8 >3.8 TDS* Mg/l <450 450-2000 >2000 Infiltration Mm/day       Specific ion toxicity No units <3 3-9 >9 Chloride me/l <4 4-10 >10 Boran me/l <0.7 0.7-3.0 >3 Bicarbonate me/l <4 >4 >>4 * TDS = Total dissolved salts For each 1ppm element in the water, the input is 1000mm equivalent to 10kg/ha If pH is out of range (6.5 – 8.4) but with low salinity (<0.2Ds/m) then there is likely no problem as the water has very low buffering. However, additional checks should be pursued for possible nutrient imbalance Abnormal pH, damages the equipment as the water is corrosive High bicarbonate levels in irrigation water can cause Zn deficiency Large masses of silt must be avoided for heavy soils High sodium water causes deflocculation of soil particles leads to increase stickyness and compactness and decrease permeability Specific conductivity of ideal water : (K x 106) < 7500 Boron < 1ppm S.A.R. Index < 10 Ecw = Irrigation water salanity Ece = Soil salanity a measure on a saturation extract SAR = Na/(Square root (Ca + Mg/2)); Na, Ca and Mg in me/l

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