The increasing frequency of extreme hydrological events, particularly floods, highlights the need for improved understanding and modeling of surface runoff generation processes. Saturation excess and infiltration excess are two primary surface runoff generation processes. However, most extensively used models either focus on saturation excess runoff only or treat saturation and infiltration excess runoff separately. These limitations are especially problematic in arid regions where infiltration excess primarily dominates runoff generation. Our study aims to address this gap by augmenting the SCS-PDM model, a multi-spatiotemporal scale saturation excess-based water balance model that has been effective only in humid regions. We extend the SCS-PDM model by representing saturation and infiltration excess runoff in a single framework. The enhanced version is denoted as SCS-PDM+. The performance of the SCS-PDM+ model was evaluated using over 181 catchments across the continental United States. When compared to the original saturation excess-only model, SCS-PDM+ demonstrates superior performance, better capturing daily streamflow and annual mean flood peaks, particularly over arid catchments. Importantly, our modifications to the SCS-PDM model enable us to quantify the contribution of infiltration excess to total runoff generation at both regional and global scales. Such capability could pave the way for improved assessments of climate impacts on flooding in a changing environment. For example, in the future the SCS-PDM+ will be integrated into the global hydrological model Xanthos for improved flood simulation capacity.