An improved technique of rainfall analysis: a case study in Mapalana catchment

Abstract

Apart from the magnitude o f rainfall, measurement o f onset, intensity and duration o f each rainfall event is o f paramount importance in Agriculture. A study was conducted to determine the above parameters and to find out the relationship between intensity and duration o f rainfall. In this regard, an analysis o f rainfall data in relation to the number of rainfall events received, magnitude, duration and intensity o f rainfall during day time, night and twenty four-hour period for Yala and Maha seasons and for the year was carried out using daily rainfall data recorded by Tipping Bucket Recording Type Rain Gauge at Mapalana meteorological station during 2005 and 2006. The highest number of rainfall events o f 41 and the longest duration o f rainfall o f 960 min. were recorded in October for Maha season and for the year whilst for Yala season they were in May (23) and June (580 min.) respectively. The greatest amount o f rainfall o f 368.5 mm and the highest rainfall intensity o f 31 mm h r1 were noted in November and February for Maha and for the year respectively. For the Yala season, they occurred in June (207.5 mm) and April (21 mm h r 1) respectively. The amount o f rainfall received was high (261 mm) in November during day time whereas it was high (155.5 mm) in June during night. The lowest number o f rainfall events o f 5 was noted in April during Yala and during the year and in December (9) during Maha season. The amount and duration o f rainfall were less during August and April. The relationship between intensity and duration o f rains received in less than one-hour duration in 2005 was determined using rainfall intensity - duration curves. The following regression equation was formulated between intensity and duration o f rainfall. y = axh where y = intensity (mm/hr), x — Duration (min), a, b = constants The a and b constants for Mapalana were 64.58 and 0.36 respectively. The formulated equation was tested with an independent rainfall data set recorded by the same Tipping Bucket Recording Type Rain Gauge in 2006 and the r2 value was 0.91. Therefore, the formulated equation can be recommended for Mapalana to determine rainfall intensity-duration relationship. However, values o f a and b will vary from catchment to catchment depending on the characteristics o f rainfall received. The analysis will enhance the quality and accuracy o f rainfall related studies as it determines the onset, duration, amount and intensity o f each rainfall event.