IDF Curves at Ungauged Sites under Anticipated Changing Climate

The impact of a changing climate is already being felt on several hydrological systems both on a regional and sub-regional scale of the globe. Southeast Asia is one of the regions strongly affected by climate change. With climate change, one of the anticipated impacts is an increase in the intensity and frequency of extreme rainfall which further increase the region's flood catastrophes, human casualties and economic loss.. Optimal mitigation measures can be taken only when stormwater systems are designed using rainfall Intensity-Duration-Frequency (IDF) curves derived from a long and good quality rainfall data. Developing IDF curves for the future climate can be even more challenging especially for ungauged sites. The current practice to derive current climate's IDF curves for ungauge sites is, for example, to 'borrow' or 'interpolate' data from regions of climatologically similar characteristics. Liew et al. (2013) presented a novel approach to derive IDF curves for present climate in which rainfall data were extracted from a high spatial resolution Regional Climate Model (RCM; 30 x 30 km over the study domain) driven by ERA-40 dataset. This approach was demonstrated on an ungauged site (Java, Indonesia) and the results are quite promising. In this paper, the authors extend the application of the approach to other ungauged sites particularly in Peninsular Malaysia. The study is performed by first identifying the nearest meteorological stations where IDF curves exist. Biases resulting from these meteorological sites are captured and serve as very useful information in the derivation of present day IDF curves for ungauged sites. The present day climate's derived IDF curves at the ungauged sites fall within the suggested bias correction range. This range allows designers to decide on a value within the lower and upper bounds, normally subjected to engineering, economic, social and environmental concerns. It also provides policy makers better information on the adequacy of storm drainage design, for the current climate at the ungauged sites, and the adequacy of the existing storm drainage to cope the impacts of climate change.