Analysis of Drought Severity and Duration Using Copulas in Anuradhapura, Sri Lanka

Anuradhapura district is one of the largest agricultural crop production areas in Sri Lanka. But it is often affected by droughts and droughts caused severe damage for agricultural industry. Thus it is very important to identify the drought characteristics (drought duration and drought severity) and their joint probability distribution to minimize the adverse effects of droughts. Drought characteristics were defined using 3 - month standard precipitation index (SPI). It is calculated using monthly rainfall da ta from 1951 to 2007 in Anuradhapura. Occurrences of 46 drought events were indentified using the calculated SPI. Since dependency nature of the drought variables, copula based joint distribution was used to calculate the joint distribution. The joint dis tribution could be obtained by combining the marginal distributions using copula. Five copulas were examined and compared to find the best fitted copula to represent the joint distribution . The best marginal distributions were identified as the gamma distr ibutions for drought durations and drought severity using AIC, BIC and Kolmogorov - Smirnov test. Frank copula was