Rainfall distribution in West Timor: Temporal and spatial characteristics, regional frequency analysis and drought identification

This study of rainfall distribution in West Timor is concerned with the analysis of temporal and spatial rainfall characteristics, regional frequency analysis of annual daily maximum rainfall and pentad (5-days) drought analysis. Several statistical properties such as persistency, trend and seasonality of rainfall series, and also physical factors such as topography and prevailing winds are used to describe the general characteristics of rainfall. The index-rainfall and L-moment approaches are used to provide a regional probablistic model of annual daily maximum rainfall and, for comparison, the at-site frequency of selected gauges is analyzed. Probability analysis is used to describe pentad drought properties such as onset and end of the rainy season, severity, and vulnerability of drought. The results of this study show that the rainfall characteristics in West Timor are variable in time and space. The prevailing wind system and topography are the dominant physical factors affecting the spatial and temporal distribution of rainfall in this region. West Timor can be classified into one homogenous rainfall region based on annual daily maximum rainfall analysis, but into two homogenous rainfall regions based on monthly rainfall analysis. The Gumbel distribution is an acceptable model for at-site and regional frequency analysis. The severity and vulnerability of drought is significant even in the wet season. At site 6 for example, only two months in the wet season are wet enough for rice, and only four months for dry land crops at a reliability of 75%. The results of this study should provide better guidance to engineers involved in irrigation and agricultural planning in West Timor. In addition, the results obtained provide an update of an earlier study by Crippen Consultants (1980).