Analysis of Z-R Relationship for Rainfall Estimation by Takhli Radar

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Ratchawatch Hanchoowong


The local water management has suffered from the shortage of accurate areal rainfall data collection in use of automatic rain gauge with partial pixel of 200 cm2 caused by the lack of the device installation in the area and the extreme complication of natural phenomenon involving precipitation. In response to that, the adoption of remote sensing technology in weather radar has been introduced to rainfall measurement for the past 50 years. When interacting with raindrop particles, the weather radar can detect radar reflectivity which varies depending on rainfall drop size distribution. This reflectivity subsequently becomes applicable as a support method to the conventional rainfall estimation after translated into rainfall intensity (R (mm/cm)) by using relationship Z-R (Z=aRb). This study had begun and concluded the data collecting process during August 25, 2018 to August 31, 2020 and accumulated 230 rainfall event data, including hourly rainfall data from 174 automatic rain gauges and radar reflectivity from Takhli radar operating under its measuring radius of 240 km, in analysis of altered Z-R relationship correspondingly to periodical rainfall drop size distributions to compare with the result by the use of Climatological Z-R relationship,  Z=200R1.6 and Z=300R1.4. The result shows the improvements in accuracy of radar measurements of rainfall by incorporating the altered Z-R relationship correspondingly to periodical rainfall drop size distribution, raising up to [9.05%,10.68%,12.42%], [18.96%,22.50,26.39%], [18.59%,21.60%,25.78%] and [512.14%,580.58,621.36%] when considering from RMSE (Root Mean Squared Error), MSE (Mean Squared Error), MAE (Mean Absolute Error) and BIAS respectively, in comparison to that of Climatological Z-R relationship,  Z=200R1.6 and Z=300R1.4.

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