THE EFFECTS OF SEASONAL CHANGE ON THE ACCURACY OF FORECAST RAINFALL DATA IN THE UPPER PHETCHABURI RIVER BASIN
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Abstract
The objective of this study was to determine the effects of seasonal change on the accuracy of forecast rainfall data in the Upper Phetchaburi River Basin by the spatial and positional accuracy of forecast rainfall data from Numerical Weather Prediction-NWP model of Thai Meteorology Department comparing with ground rainfall data. Theses areal forecast rainfall data would be then used for the estimation of the forecast inflow of Kaeng Krachan Reservoir during the rainy season. In the study area, there are few rainfall stations because the area is the upstream part of river basin and all area are forests. For the data collection, daily rainfall data during July-October 2020, July - October 2021 and July-October 2022 were selected and two spatial estimation methods were used, Thiessen-polygon and inverse distance weighting in order to spatial compare with forecast rainfall from NWP model with selected three boundaries of grid forecast rainfall data namely d01 (18X18 km), d02 (6X6 km) and d03 (2X2 km). One days-forecasted rainfall data and compare positional accuracy with point-to-point of location of 6 ground rainfall stations were used. After that, the results of the spatial and positional rainfall estimation were examined by three accuracy indexes namely the Mean Error between the estimated value and the actual value (ME), Mean Absolute Error (MAE) and Root Mean Square Error (RMSE). The result found that forecast rainfall from Inverse Distance Weighting were a little bit better in all forecasts of Thiessen Polygon methods. Both methods gave in line of MAE and RMSE index values, and the spatial rainfall estimates of the forecasted rainfall gave mostly higher values than the estimates from ground rainfall values. Forecast rainfall data with d03 (2X2 km) grid boundary is the most accurate result because it has a fine resolution of 633 grids in the Upper Phetchaburi basin. It gives better rainfall estimation results than point rainfalls. and in October there is the greatest discrepancy, because of the changing seasons. The wind has changed from the southwest monsoon to the northeast monsoon while in August there is the lowest discrepancy because it is in the rainy season that it rains regularly.
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The published articles are copyright of the Engineering Journal of Research and Development, The Engineering Institute of Thailand Under H.M. The King's Patronage (EIT).
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