Shifts in streamflow and flood timings over the Upper Ping River Basin, Thailand

This study investigated spatiotemporal changes in streamflow and flood timing within the Upper Ping River Basin of Thailand, a major area providing a source of inflow to the largest reservoir, the Bhumibol Dam. To investigate this, a robust circular statistics framework was applied to examine three timing metrics: mean streamflow, streamflow center and peak flood timing. Daily streamflow data from 17 gauging stations spanning the period 1995-2024 were used for the analysis. The results reveal that the spatial gradient of these timings similarly occurs earlier in the upstream mountainous area and progressively later in the downstream floodplain areas. To confirm the seasonality strength of streamflow and flood timing, the resultant length and Rayleigh uniformity tests were used. The results indicated that flood and streamflow timings were significantly concentrated around their respective mean dates. Furthermore, their temporal distribution was found to be unimodal, indicating strong seasonality. Analysis of temporal trends revealed a significant delay in all metrics with 0.71, 0.84 and 0.86 days per decade for mean streamflow, streamflow center and peak flood timings respectively. Moreover, the pronounced delay in peak flood timing, coupled with its positive correlation to streamflow center timing, suggests that delaying flood timing may be a significant driver of the change in basin hydrological timing.