A COMPARISON OF METHODS USED IN ESTIMATING MISSING RAINFALL DATA

Abstract

Precipitation or rainfall (in tropics) is an important climatic parameter and the studies on rainfall are commonly hampered due to lack of continuous data. To fill the gaps (missing observations) in data, several interpolation techniques are currently used. However, the lack of knowledge on the suitability of these methods for Sri Lanka is a practical problem. In view of this problem, this study is aimed at comparing a few selected methods used for the estimation of missing rainfall data with a new method introduced by the authors to determine their suitability in Sri Lankan context. The methods studied were Arithmetic Mean (Local Mean) method, Normal Ratio method and Inverse Distance method. The new method introduced by the authors is named as Aerial Precipitation Ratio method. In this approach, rain gauging stations where complete monthly rainfall data sets are available were selected in such a way that the selected stations represent each of the seven major Agro-ecological zones of Sri Lanka. This selection procedure of stations makes it possible to generalize the results to the entire country. The period of data ranged from 15 years in the case of mid country intermediate zone to 28 years Up country intermediate zone and Mid country wet zone. Subsequently, monthly rainfall data of each station were estimated using the data of surrounding stations based on the above selected methods so that actual data and the estimated data can be compared. Each estimated series was compared with the actual data series using different statistical comparison techniques. These comparisons include Descriptive Statistics of Error, Root Mean Square Error, Mean Absolute Percentage of Error and Correlation Coefficient. Results of the study show that the Inverse Distance method is the most suitable method for all three Low-country zones (wet, intermediate, and dry). However, for Mid-country and Upcountry Intermediate zones, Normal Ratio method is the most suitable method. Further, Arithmetic Mean method is more appropriate for Upcountry Wet zone while Aerial Precipitation Ratio method is more suitable for Mid-country Wet zone.