Drought studies in Kala Oya basin, Sri Lanka

Document Type : Original Research Paper


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 1192 mm and the mean annual potential evapotranspiration is about 1514 mm. Major part of the basin is within the dry zone and only 3% 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 15 rainfall stations for the period of 1960 to 2018. 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., 59 years) records for 15 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, 2nd 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 136 days. The average SPI results confirmed that the drought condition in the basin was "nearly normal" (-0.99<SPI<0.99). The probability of obtaining "nearly normal” results for the fifteen rain gauge stations was 53%. Similarly, almost all rain gauging stations show a declining trend, which is an indication that drought may occur more frequently in the future. Severe drought occurred in the basin during the period of 2003-2004. The basin has a good overall probability of receiving a moderate quantity of rainfall, but its erratic distribution in time and space had played major role in advancement of drought hardship in Kala Oya basin. The basin had a sufficient number of wet days (on average about 136 days). The study provides information on the potential for future droughts due to climate change.
Conclusions: Studying the long-term drought will provide guidance for future water resources planning and management. Precipitation data analysis using SPI and the rainfall departure provides the adequate information on past droughts. More than 50% of the Kala Oya basin is used for paddy cultivation. Therefore, it is especially important to understand the drought in a basin-like Kala Oya and manage water sources to supply water for cultivation and other activities. This study provides a guide for water resource managers to plan cultivation quantitatively and in a timely manner. An analysis of 59 years of long-term data shows that the basin receives a moderate amount of average rainfall (near normal rainfall). As there is a greater tendency for drought, efforts should be made to promote the crop diversification rather than conventional paddy cultivation.


Main Subjects

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