1. Dry spells – The interval between the end of a seven day wet spell, beginning with the onset of effective monsoon and another rainy day with 5 e mm of rain (where “e” is the average daily evaporation) or the commencement of another seven day rainy spell with four of these as rainy days (satisfying the third criterion) and with a total rain of 5 e mm or more during this spell is called the first dry spell. If the duration of this dry spell exceeded certain value, depending on the crop-soil complex of the region, this dry spell was called a critical dry spell. Criteria for forecasting rainfall characteristics (like onset of effective monsoon) are as follows:
• The first day rain in the 7-day spell signifying the onset of effective monsoon, should not be less than “e” mm.
• The total rain during the 7-day spell should not be less than 5 e + 10 mm.
• At least four of these seven days should have rainfall, with not less than 2.5 mm of rain on each day.
2. Wet spell – A wet spell is defined as a rainy day with “X” mm of rainfall or a 7 day spell where the total amount of rainfall equals “x” mm or more and the condition that three out of these seven days must be rainy with rainfall more than 2.5 mm on each day. In this, “x” is the amount of rainfall, which brings the top 50 cm soil layer to field capacity. The water holding capacity varies with the type of soil. For example, the value of “x” is equal to 83 mm for light soils, 125 mm for medium soils and 166 mm for heavy soils of Punjab.
3. Critical Dry Spell (CDS) – CDS is defined as the duration between the end of a wet spell and the start of another wet spell during which a 50% depletion of available moisture occurs in the top 50 cm soil layer. It is calculated by the following formula. AMD CDS ET = Where CDS in day; AMD = 50% of the available soil moisture in the top 50 cm soil layer, expressed in terms of depth (mm); ET = Average maximum daily ET of a crop (mm/day).
4. Drought – Drought has varied meanings for different people. In general, drought may be defined as a complex phenomenon, which results from the prolonged absence of precipitation in conjunction with high rate of evaporation. This causes abnormal loss of water form water bodies, lowering of the water table and dehydration of the root zone of the soil, thus upsetting water supply to plants. The term drought can be defined by several ways.
• The condition under which crops fail to mature because of insufficient supply of water.
• The situation in which the amount of water required for transpiration and evaporation by crop plants in a defined area exceeds the amount of available moisture in the soil.
• A situation of no precipitation in a rainy season for more than 15 days continuously. Such length of non-rainy days can also be called as dry spells.
5. Aberrations in rainfall – Aberration means the deviation from the normal behaviour of the rainfall. As the principal source of water for dry land crops is rain, a major portion of which is received during the monsoon period. Bursts of rain alternated with “Breaks” are not uncommon. There are at least four important aberrations in the rainfall behaviour.
• The commencement of rains may be quite early or considerably delayed.
• There may be prolonged breaks during the cropping season (Intermittent drought).
• The rains may terminate considerably early (early cessation of rain) or continue for longer periods.
• There may be spatial and/or temporal aberrations.
(a) Early or delayed onset of monsoon – To quantity the aberrations in the onset of monsoon, 50 years of data to be analyzed for the date of onset of monsoon has to be studied for different regions of the country. The aberrations require changes in crops and varieties with the normal onset of NEM (Sep.–Oct.). Crops like sorghum, bajra, pulses and oil seeds can be grown in Kovilpatti tract of Tamil Nadu with the onset of monsoon. If monsoon is delayed up to late October, crops like bajra, pulses, sunflower etc., can be raised. If it is very much delayed up to first week of November, crops like sunflower can be sown.
(b) Breaks in the monsoon rains-Intermittent drought – The breaks can be of different durations. Breaks of shorter duration (5–7 days) may not be a serous concern, but breaks of 2–3 weeks or even more, lead to plant-water stress causing reduction in production. This intermittent drought can be of different magnitude and severity, and affect different crops in varying degrees. The yields of sensitive crops are seriously affected but not drought resistant crops. Another aspect of the breaks or intermittent drought is the stage of the crop at which the drought occurs. The effect on crop will be different at different stages. Another important factor is the effect of breaks or intermittent drought depends on the physical properties of the soil particularly its water holding capacity. Deep black soils have capacity to store as much as 300 mm of available soil moisture in one meter depth, whereas light soils can store only as little as 100 mm or so. Hence drought is more pronounced in the soils having less storage capacity.
(c) Early withdrawal of monsoon – The normal withdrawal of SWM in Rayalaseema region will be between 25th September and 15th October. But, in 4% of the years out of 55 years, monsoon withdraws during first fortnight of September and in 10% of the years, it withdraws during the month of December. Since, crops and varieties in any given region are selected based on the normal length of growing season. Persistence of rains much beyond normal dates creates an extraordinary situation. Under Kovilpatti (Tamil Nadu) conditions, short duration bajra and sunflower will be suitable under early withdrawal of monsoon. Cultural practices to mitigate the effect of moisture stress due to intermittent drought and early withdrawal of monsoon are:
(i) shallow intercultural operations to eradicate weeds,
(ii) maintain soil mulch to conserve soil moisture,
(iii) application of surface mulch,
(iv) thinning of crops by removing alternated rows as in sorghum and bajra,
(v) recycling of stored run off water,
(vi) ratooning in crops like sorghum and bajra, and
(vii) 2–3% urea spray after a rain for indeterminate crops like castor and red gram.
(d) Uneven distribution of monsoon rains, in space and time – This situation is encountered almost every year in one or another part of the country during monsoon period leading to periodical drought and flood situations. High variability of rainfall is the single factor which influences the high fluctuations in the crop yields in the different parts of the country.
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