Simplified approach to considering watershed heterogeneity in direct runoff estimation using SCS-CN model

This study investigates the effect of impervious surfaces on the application of the SCS–CN direct runoff estimation method and introduces a simplified approach to account for watershed heterogeneity in SCS–CN model parameter estimation. Unlike existing approaches that rely on more complex parameterizations, this method incorporates the spatial distribution of impervious surfaces in a simple and physically meaningful way when estimating Curve Number (CN) values from rainfall-runoff measurements at the basin outlet. The method was evaluated across multiple experimental watersheds and synthetic datasets and demonstrated performance comparable to more complex approaches, achieving R2 values between 0.868 and 0.992, compared with 0.877 to 0.993 for the more complex Two–CN method. Although slightly less accurate, the proposed method requires fewer parameters and offers a more practical solution for water management applications, particularly where simplicity and computational efficiency are essential. Its performance was also comparable to methods based on spatially distributed CN estimates, confirming its robustness without additional methodological complexity. The results highlight the critical role of impervious surface distribution in improving CN estimation, especially in highly permeable watersheds.