A. D. S. Iresh - Senior Irrigation Engineer, Hydrology and Disaster Management Division, Irrigation Department, Sri Lanka.

Introduction: Kala Oya basin is the third largest basin in Sri Lanka. The basin receives an average annual rainfall of about ۱۱۹۲ mm and the mean annual potential evapotranspiration is about ۱۵۱۴ mm. Major part of the basin is within the dry zone and only ۳% of the basin area is in the intermediate zone, where the river emanates in central mountains. Materials and Methods: Drought is a common feature of the climate in the Kala Oya basin. Drought in this region had occurred mainly due to erratic behaviour of monsoon, especially due to long breaks in monsoon, high-intensity shorter duration rain etc. In this study, seasonal and annual rainfall departures have been calculated using ۱۵ rainfall stations for the period of ۱۹۶۰ to ۲۰۱۸. Seasonal and annual rainfall departure analysis indicates a seasonal and annual rainfall deficiency during a drought year. Results: The analysis showed that the departure of annual rainfall follows the trend of seasonal rainfall indicating that the drought events in basin are largely governed by the monsoon seasonal rainfall. The average frequency of basin drought does not follow a clear frequency pattern. Then probability distribution analysis has been carried out considering long term (i.e., ۵۹ years) records for ۱۵ stations. The standardised precipitation index (SPI) represents a statistical z-score or the number of standard deviations (Following Gamma probability distribution transformation to normal distribution). SPI has been applied in basin to quantify annual and seasonal precipitation deficit anomalies on multiple time scale. The estimated values of SPI demarcate precipitation events over a specified period into surplus (heavy precipitation). The analysis revealed that the drought condition in the area is dominantly driven by the total rainfall during the period from September to December. Monthly departure values indicated that May, Jun, July, August, and September months are the dry months of basin. Whereas, ۲nd inter monsoon and northeast monsoon heavy rainfall was received in October, November, December, January and February. The average wet days in the basin were about ۱۳۶ days. The average SPI results confirmed that the drought condition in the basin was "nearly normal" (-۰.۹۹

Kala Oya, Drought, Rainfall departures, Standard Precipitation Index, Sri Lanka