Drought is universally acknowledged as a phenomenon associated with scarcity of water. Drought characterization is essential for drought management operations. Using drought indices is a pragmatic way to assimilate large amounts of data into quantitative information that can be used in applications such as drought forecasting, declaring drought levels, contingency planning and impact assessment. Using monthly mean precipitation data for a period of 1982-2012 from 12 raingauge stations in the Manjalar sub-basin of Vaigai using Standardized Precipitation Index (SPI) is produced for the drought analysis with the time scale of 3 months (SPI-3), 6 months (SPI-6) and 12 months (SPI-12) as they are applicable for agriculture and hydrological aspects, respectively. It was observed that the basin experienced frequent droughts for all months of the year. The highest percentage of occurrence of drought was observed in the month of July (15.3), May (15.4) and August (15.6) at SPI-3, SPI-6 and SPI-12 respectively. On an average we observed 32.6, 8.6, 5.6 and 2.3 percentages of drought occurred by mild, moderate, severe and extreme drought respectively with respect to SPI-12. The results showed that mild droughts occur most frequently and extreme droughts occur least frequently and the basin suffered severe drought during the year of 1985, 2004 and 2006. The central and south eastern parts of the basin had more potential sensitivity to the droughts in comparison with the other areas of the basin.
Timely and accurate medium range weather information is critical to conquer the impact of highly dynamic next few days’ weather on the farming. Advances in weather forecast models, as well as their increased resolution, have resulted in more accurate and realistic forecasts. An attempt was made during 2019 – 2021 at Agro Climate Research Centre, Tamil Nadu Agricultural University, Coimbatore to develop cluster of village level (@ 3km resolution) Medium Range Weather Forecast (MRWF) for Tamil Nadu with higher accuracy. In this study, Weather Research and Forecast Model (WRF v4.2.1) with four microphysics viz., Kessler, WSM3, WSM5, WSM6 schemes were tested for Tamil Nadu during CWP, HWP, SWM and NEM 2020. The MRWF generated from the WRF model v4.2.1 with WSM3 had better BSF, higher Forecast Accuracy Index (FAI) and Forecast Usability Percent (FUP) for Tamil Nadu followed by Kessler scheme. The WSM5 and WSM6 were poor performer during the study. In general, CWP had higher FAI followed by HWP, NEM & SWM. The FAI from WSM3 was 0.65 - 0.74 during NEM and 0.55 - 0.69 during SWM. Among the season, the MRWF generated during SWM were over forecasted the rainfall quantity, where the NEM and HWP had better rainfall forecast nearing actuals. The FUP was higher in NEM followed by CWP, SWM & HWP, which was 57 – 88 per cent during NEM and 46 – 82 per cent during SWM. A decreasing trend in the quantitative FUP was observed with increase in lead times, irrespective of the microphysics and seasons. Finally, the study concluded that the accuracy of village level medium range rainfall forecasts from WRF model v4.2.1 varied temporally by season and the WSM3 microphysics option having superiority in all seasons.
Drought is the most extensive hydro-meteorological pattern of prolonged period of water scarcity affecting natural resources and environment. However, it has significantly different characteristics from one region to another. Tamil Nadu agonizes from various disasters including droughts. Generally rainfall deviation from the long-term mean continues to be a widely adopted indicator for drought intensity assessment. The application of this indicator is strongly limited by its inherent nature of its dependence on mean. Thus a standardized procedure that overcomes these limitations becomes a basic need. Moreover stability of rainfall in recent years has been erratic and hence to understand the stability of rainfall over years Precipitation Ratio (PR) was used. Further to understand the drought climatology of southern zone of Tamil Nadu, Standardized Precipitation Index (SPI) was used. PR was studied for three decades (1981-2010) covering seven locations. In most of the locations the rainfall was stable during 1981-90 and thereafter a gradual decline in stability was noticed confirming the weather extremes in the recent decade. SPI had an annual range from +2.69 to -4.94 while Monsoon period had +2.41 to -3.43 and +2.69 to -3.14 respectively for Southwest and Northeast monsoon seasons. As per SPI index, Southern zone is prone to moderate drought followed by severe and extreme drought category. Among the periods studied (Annual, SWM and NEM) number of moderate drought occurrences had no much variation while in severe and extreme drought category, variations could be observed. In particular, during NEM period there was almost no occurrence of extreme drought.
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