SPATIAL VARIABILITY OF NUMBER RAINFALL DAY IN EAST JAVA: APPLICATION OF EXPLORATORY SPATIAL DATA ANALYSIS (ESDA)

Abstract

This article expose the spatial variability of number of rainfall day in East Java region. Daily rainfall data was collected from 946 pluviometres spread around the regions. Number of rainfall-day per year is averaged from the availbale periods of record for each locations. Spatial statistics were analysed by means of ESDA (Exploratory Spatial Data Analysis) techniques available on Geostatistical Analyst extention of ArcGIS (9.x.). Statistical tools exploited to analise the data are: (1) Histogram, (2) Voronoi Map, (3) QQ-Plot dan (4) Trend Analysis. The result show that histogram and QQ-Plot of rainfall day close to normal distribution. Statistical value obtained from the analysis are: minimum = 4 day/year, average = 81 day/year, maximum = 184 day/year, and median = 80 day/year. Other statistical value summarised are: standard deviation = 23,74; skewness = 0,28; and curtosis = 3,6. Furthermore, 3D-visualisation of spatial distribution of rainfall-day using trend analysis tool shows no significant trend on both direction (XZ and YZ). The research demonstrate the capability and benefit of those statistical tool to describe spatial variability of rainfall-day. Finally, thematic map visualized the spatial variability of rainfall-day on this regions is produced using Inverse Distance Weighting (IDW) interpolation methods.