AGRICULTURAL RISKS OF RAINFALL IN MAPALANA AREA

Abstract

The irregularity of rainfall in Mapalana area generates risks on agriculturecreating dry spells as well as heavy rainfall periods through out the year. Inorder to understand the behavior of rainfall in Mapalana area daily rainfalldata over 55 years (1950 - 2004) were assessed, considering the time units;days, weeks and months.

Mean daily, weekly and monthly rainfall by arithmetic mean method anddependable rainfall at 75 % probability level were calculated. AgriculturalRainfall Index values were calculated using rainfall values at 75 %probability level and pan evaporation data at Mapalana area.

Mean annual rainfall and annual dependable rainfall in the Mapalana area are2248.6 mm and 2026.5 mm respectively. The daily arithmetic mean rainfallat Mapalana area is stated as 6.2 mm (Range 0.7 - 24.3) whereas dailydependable rainfall at 75 % probability level is recorded as zero. The dailydependable rainfall at 50 % probability level is also zero for 212 days in theyear. The results reveals that even though the area receives 6.2 mm dailymean rainfall, there is not even 50 % probability of receiving rain in each dayper year.

The weekly rainfall analysis shows that weekly arithmetic mean rainfall inthe Mapalana area is 43.2 mm (Range 17.0 - 82.6). However, whenconsidering the rainfall at 75 % probability level, 12 weeks per year showedzero dependable rainfall.

The mean monthly rainfall and mean dependable rainfall at 75 % probabilitylevel are 187.4 mm (Range 88.0 - 313.1) and 104.9 mm (Range 16.2 - 205.5)respectively. According to the Agricultural Rainfall Index, seven months areunder water deficit condition.

The results revealed that yearly variation and irregularity of rainfall withinthe month is high in the Mapalana area. Although the mean rainfallcalculated for daily, weekly and monthly records high values, water deficitperiods occur in the Mapalana area through out the year except in May, June,September, October and November.