Delineating the boundaries of numerous watersheds is essential for hydrological analysis in the big data era. Various techniques, such as ArcGIS, have been developed to delineate watershed boundaries. However, existing techniques suffer from two laborious manual processes: (a) Determine flow accumulation thresholds through trial and error to derive digital stream networks; (b) Manually relocate watershed outlets to the correct stream networks. Here, we proposed an automatic outlet relocation (AOR) algorithm for rapid watershed delineation without manual processing. The AOR algorithm relocates outlets to correct stream networks by analyzing flow accumulation gradients along the flow directions. An experiment using the AOR algorithm in 1,398 watersheds across the contiguous United States showed that 94.1% of the watersheds were correctly delineated, compared to 81.3% accuracy of the ArcGIS relocation algorithm (Snap Pour Point). In addition, the AOR algorithm took 58 min to delineate the 1,398 watersheds on a 90 m‐resolution flow direction, while the ArcGIS algorithm required 586 min to perform the same task. Thus, the AOR algorithm offers an opportunity to automatically delineate a large number of watersheds for hydrological simulations in the big data era. Software with a graphical user interface for the AOR algorithm is freely available on GitHub (https://github.com/xiejx5/watershed_delineation).

Terrestrial actual evapotranspiration (ET), referring to the water transferred from the land surface to the atmosphere, provides a critical nexus between water, carbon, and energy changes Liu et al., 2022;Zhang et al., 2016). Accurate estimation of ET is crucial for a better understanding of the changes in regional climate characteristics and plays an important role in agricultural drought forecasting, water management, and atmospheric-terrestrial carbon exchanges (

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