Contents & Foreword, Characterizing And ... - IRRI books

The radiation values range from less than 12.5 to more than 16.5 MJ m –2 d –1 inJanuary and they start increasing thereafter until May. In May, the radiation valuesreach their peak at more than 25.0 MJ m –2 d –1 in eastern Uttar Pradesh. In the northeasternstates, however, the peak values are reached in April and decrease thereafter,as premonsoon and then monsoonal rains start. The solar radiation values in easternIndia are lowest in July and August and range from less than 8.5 to slightly more than12.5 MJ m –2 d –1 (Fig. 7A,B). For better crop growth and development, a minimumvalue of 12.0 MJ m –2 d –1 is needed.Figure 2 also shows the temporal dynamics of solar radiation at three representativestations. At Cuttack, where the monsoon starts earlier (mid-June), the solarradiation is less than 12.0 MJ m –2 d –1 from July to September, whereas, in Patna,where the monsoon starts late, in the first week of July, solar radiation values are lessthan 12.0 MJ m –2 d –1 only in August and September. In the rest of the months, thevalues are more than 12.0 MJ m –2 d –1 . In the case of Silchar, however, the solarradiation values are lower than 12 MJ m –2 d –1 from June to September. This is a clearindication that, in higher rainfall areas, solar radiation constrains the rice crop’s growthand development.Summary and conclusionsTo develop strategies for improving rice productivity, a thorough inventory of thethree components of climate—moisture, thermal, and radiation regimes—is needed.In eastern India, rice is grown mostly under rainfed conditions in three subecosystems:rainfed lowland, upland, and flood-prone. In this region, rainfall or moisture regimeis an important climatic component because most of the area is under upland andrainfed lowland drought-prone ecosystems. Rainfall amount and rainy days are simplemeasures for characterizing rainfed rice environments. Using rainfall amounts, easternIndia is divided into eight zones, of which three are under flood/submergenceproneconditions and three are under drought-prone conditions. Only two zones comeunder favorable conditions. In both the flood-prone and drought-prone areas, betterwater management practices need to be developed.Rainy days, though readily available, are not a good measure for assessing thegrowing season as the growing season is always longer than rainy days due to intermittentdry spells. Therefore, moisture availability periods are a relatively better measurefor assessing the crop-growing season and cropping pattern. The humid period(P > PE) is the best period for the rice crop’s vegetative and reproductive phases;therefore, varietal duration needs to be assessed based on the humid period. The ricebasedsecond crop in the winter season can be grown in areas with a longer durationof humid and moist II periods and in soils with a higher moisture retention capacity.In spite of such detailed analysis, in eastern India average rainfall matches withonly 30% or 40% probability, implying that any strategy based on average rainfallcan be successful only once in three years. Hence, a concept of desirable rainfallperiod has been developed by defining a desirable rainfall period as the period whendaily rainfall is 7 mm or more. The rainfall fluctuations above this threshold value do228 Sastri and Singh