Mapping climate behaviour, ten days at a time
This year, the Global Information and Early Warning System (GIEWS, a project of the FAO) has brought into public domain a new rainfall and vegetation assessment indicator. The indicator takes the form of maps which describe conditions over blocks of ten days each, with each such block termed a dekad (from the Greek for ‘ten’). Thus we have visual views of divisions of thirds of a month which from a crop cultivation point of view, now lies between the weekly and fortnightly assessments regularly provided by agri-meteorological services.
In 2015, what was quickly called “out of season” rainfall was experienced in most of India during March and April. These conditions carried over into May and that is why the typical contrast between a hot and rainless May and a wet June is not seen.
The panel of maps shows the incidence of normal, below normal and above normal rain during six dekads of May and June. Greens signal above normal, yellows are normal and reds are below normal. The first dekad of May looks like what the second week of June normally does, but for the large above normal zone in the north-central Deccan. The second dekad of May has in this set had the largest number of above normal points, with more rain than usual over the southern peninsula, and over Chhattisgarh, Odisha, West Bengal. Rajasthan and Punjab.
The third dekad of May shows most of India as far below normal. This changes in the first dekad of June, with rain over the eastern coast registering much above normal for the period – Tamil Nadu, Rayalaseema, Andhra Pradesh and Odisha. During the second dekad of June, the divide north and south of the Vindhyas is visible, when northern India and the Gangetic belt continued to experience very hot days whereas over Telengana, Karnataka, Vidarbha and Madhya Maharashtra there was above normal rainfall. During the third dekad of June the picture was almost reversed as the southern states fell below their running rainfall averages.
This panel describes not rainfall but the anomalies (above and below) recorded in received rainfall. At the level of a meteorological sub-division or a river basin, the anomaly maps are a quick and reliable guide for judging the impacts of climate variability on crop phases (preparation, sowing, harvest) and on water stocks.