Though water resources engineering and management is vital for development activities of any country, water data collection is limited in most of the developing nations. Until recently, streamflow was gauged only in about 40 locations within the 103 river basins in Sri Lanka. The demand for data is on the rise with the increase in computational power of computers, the advances in mathematical modelling of hydrology, the demand for physics based distributed models and the demand for more water due to population rise. The biggest problem faced by civil engineers who deal with water resources is the lack of gauged data. Application of mathematical models to estimate, forecast and determine design parameters for ungauged watersheds require very clear up-to-date guidelines.