Introduction   Cotton has a taproot that grows quickly reaching 20-25 cm depth before the seedling emerges from the ground. The root depth may reach 90 to 150 cm when the plant is 20 to 25 cm tall. The main root reaches about 200 cm. Most of the lateral roots are concentrated in the upper soil layer to a depth of 50 cm and may extend laterally to 100 cm. Cotton is sensitive to soil moisture conditions. Moisture stress restrains vegetative growth and encourages early flowering. Principles of Water Management A satisfactory yield of fibre depends on maintenance of a proper balance between leaf production and boll (fruit) production. A planned moisture regime that will restrain vegetative growth without adversely affecting yield is essential. Desirable that the plant completes most of its vegetative growth before the flowers appear and this is the best achieved by moisture regime that promotes regular and rapid but not excessive development of the young plant. During early part of the season, less water is used by the plant and more water is lost by evaporation than transpiration. The use of water increases from about 3 mm reaching a peak of 10 mm a day when flowering is at its height. The peak is reached when the plant is loaded with bolls and water consumption then begins to decline. Cotton seeds will not germinate until they absorb half their weight of water which is considerable. The soil should be wetted to a depth of 120 cm prior to planting. Pre-planting irrigation ensures good germination and rapid plant development. Yields are in proportion to the wetted depth of soil at planting in several cases. Ample moisture during flowering and boll formation is essential. Moisture stress during these periods results in flower drop, boll shedding, poor development of bolls, low ginning percentage and ultimately low yield of fibre. Irrigation is continued until the first boll of last flushes opens. Otherwise the quality of lint is poor in black clayey soils. In light soils, one or two irrigation's may be necessary after the first boll of last flush opens.

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Methods of Irrigations Furrow method of irrigation is the best and most commonly adopted (20-30% saving in water). No significant reduction in yield by irrigating alternate furrow. Paired planting irrigate in the same furrow. For every two planted rows leave one fallow furrow. Mulching the inter spaces reduces 3 irrigation's. Drip irrigation - 60% water saving - Weed management is less - Initial cost high. Irrigation water use efficiency Pre-sowing irrigation may be unnecessary, if early Kharif rains are properly used for cultivation. Laying out the cotton field into ridges and furrows along the slope of 0.4 to 0.5% can result into substantial water conservation. If the slope is of higher order, the ridges may have to be planned across the slope. Contour furrow cultivation is very useful in conserving rain water and can save atleast 1-2 irrigation's

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Critical Stages of Water Requirement The stages of flowering and boll formation are critical in their demand for assured water supply. Exact duration of these stages will depend on varieties and the climate conditions of the place. Lack of adequate moisture at these stages resulted in premature flower drop, boll shedding, poor development of bolls, low ginning percentage and ultimately low yield of cotton. Delay in the first irrigation resulted in good growth and yield of cotton. The germination and seedling stages were found to be very susceptible to excess water. Planting on ridges or raised beds reduced the damage due to excess water. Total water requirement is 900-1200 mm. Upto flowering (20% of total water requirement) Flowering (40% of total water requirement) Flowering - Boll bursting (30% of total water requirement) Boll bursting - maturity (10% of total water requirement)

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Scheduling of irrigation Careful observation shows a change in the appearance of the cotton plant before signs of wilting occur. The foliage has a slightly bluish tinge and in drier spots of the field the plant will appear somewhat darker. These drier parts in the field should be watched closely, for they are the first to change in colour and serve as a guide for scheduling irrigation. Water has to be applied when leaves wilt on the upper third of the plant. Irrigation to cotton may be scheduled by soil moisture depletion approach as well as climatological approach. The optimum irrigation schedule is at 50% depletion of available soil moisture. Under water shortage conditions, irrigation can be scheduled even at 75% depletion of available soil moisture. Irrigation based on climatological approach varied with regions and it is 0.6 to 0.9 IW/CPE ratio. 6- 8 irrigation's in clay or clay loam soils. 13-14 irrigation's for red sandy soils.

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Response of cotton to irrigation Planned moisture regimes will helps in avoiding excess moisture and moisture stress-both are detrimental to cotton. Excess moisture causes more vegetative development. During vegetative growth period if the crop is subjected to favourable moisture regime or initially moisture stress and subsequently receive ample moisture, the plant produce excessive vegetation which in turn increase shading and restrict air movement. As a result the opening of bolls is delayed and time needed for maturity increased. Crop subjected to moisture stress at bud stage, bud shedding is increased considerably. A time lag of at least 4 weeks may take to put up fresh cycle of buds by the time lot of vegetative growth is put up by the plant in the absence of competition from the flowering parts of the plant. Lint yields were found to be closely associated with boll-set. Moisture stress at peak boll-set stage depressed yields considerably. Frequent irrigation had a definite delaying effect on crop maturity, regardless of the soil fertility level. When cotton is grown entirely on stored soil-moisture, maturity may be hastened by several weeks as compared to irrigated cotton. The entire crop ripens uniformly. Favourable moisture regime lengthens the flowering period, so that the total number of flowers produced is increased. Moisture stress at boll developing stage- the leaves are capable of withdrawing water from the bolls - continued moisture stress for more than three days there is marked reduction in boll size and in boll shedding. Alternate furrow irrigation not only results in saving of water, but also produce more number of bolls as the plants are more compact. Maturity is advanced by 8-10 days and higher yields obtained.

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Problems due to excess moisture Excess moisture at later stages delayed opening of bolls. Bolls are susceptible to boll rot pathogens. Lodging of plants. Excess moisture generally has adverse effect on fibre quality, mainly because of increase in proportion of fibres that fail to mature properly.

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