Abstract
High-resolution rainfall data are essential for basin-scale hydrological analysis, but long-term rainfall products are often available only at coarse spatial resolutions. This study proposes a patch-based upsampling method using Bézier surfaces to enhance the spatial resolution of gridded rainfall data over the Chao Phraya River Basin, Thailand. Daily rainfall observations from 79 Thai Meteorological Department stations during 1981 – 2021 were quality controlled and interpolated using Inverse Distance Weighting (IDW) to generate 15-arc-minute rainfall grids. Each coarse grid was subdivided into rectangular patches based on local rainfall peak locations. A tensor-product Bézier surface was fitted to each patch and sampled to produce a 5-arc-minute rainfall field. The proposed method was evaluated using spatial comparisons and monthly basin-average rainfall over the Chao Phraya River Basin. The upsampled fields preserved the broad spatial rainfall pattern of the coarse input while providing smoother rainfall gradients at the target resolution. Monthly basin-average rainfall from the Bézier -upsampled fields showed near-perfect agreement with the directly interpolated 5-arc-minute IDW reference field, with R^2=0.999 for 1981 – 2021. Strong agreement was also obtained with the T12 dataset (R^2=0.981) and K10 dataset (R^2=0.844), whereas agreement with the Thiessen polygon dataset was moderate (R^2=0.663). The results