Abstract:
Weather significantly influences agronomic production in Sri Lanka. This
study develops a spatially based rainfall model for Matara District, analyzing
average annual and seasonal rainfall (Southwest monsoon (SWM), First-inter
monsoon (FIM), Northeast monsoon (NEM), and Second-inter monsoon
(SIM)) using data from 2011 to 2020 (09 rainfall stations). Objectives
included creating rainfall maps via interpolation models in GIS, generating
Isohyetal maps, and identifying the best interpolation model for rainfall
distribution using zonal statistics and spatial analysis. Methods involved
raster interpolation techniques Inverse Distance Weighting (IDW), Kriging,
and Spline to map average annual and seasonal rainfall. Isohyetal maps
portrayed rainfall boundaries, while zonal statistical analysis determined
minimum, maximum, and mean rainfall by Divisional Secretariat Division
(DSDs) in Matara. Statistical tests (sample test, paired t-test, and One-way
ANOVA) assessed the significance of differences between interpolation
models within DSDs and seasons at 1% and 5% significance levels. Findings
showed significant annual average rainfall variation between southern and
northern DSDs. Kotapola DSD recorded the highest mean annual and
seasonal rainfall across all interpolation methods: IDW (294.99 mm),
Kriging (294.18 mm), and Spline (382.45 mm). In contrast, Dewinuwara
DSD had the lowest mean rainfall using the IDW method (153.47 mm),
while Thihagoda DSD recorded the lowest values using the Spline (154.69
mm) and Kriging (154.42 mm) methods. One-way ANOVA indicated no
significant difference at the 5% level between interpolation types for each
rainfall pattern. Mean comparisons revealed the Spline method had the
lowest Standard Mean Error (±6.67) for annual average rainfall, while IDW
had the lowest errors for seasonal rainfall during monsoon and intermonsoon
periods. The study concludes that Spline is suitable for mapping
average annual rainfall, whereas IDW is best for seasonal rainfall patterns in
the district, supported by Isohyetal lines.
