Surface Runoff Discrepancy in Urban‐PLUMBER Land Surface Models

Enhanced surface runoff in urban environments reduces water availability and limits the evaporative cooling potential. We evaluate surface runoff in 18 urban land surface models (ULSM) in Urban-PLUMBER for 6,570 rainfall events across 21 urban sites. Surface runoff occurs when rainfall exceeds the infiltration, saturation, or interception capacity. Ten models omit at least one of these processes, while seven fail to increase runoff with increasing imperviousness. Surprisingly, some models lack any runoff during intense (>50mmh^-1) or prolonged (>20 mm) rainfall. Urban land surface models (ULSMs) turn 0%–86% rainfall into runoff. Most models produce runoff in agreement with an empirical comparison offered by the CN method, especially for high imperviousness. However, ULSM runoff exceeds CN runoff estimates for low impervious fraction, particularly by models with incomplete process description and for low-intensity rainfall. The large discrepancy between ULSMs calls for advancing the urban hydrology representation in ULSMs, which is essential for correct simulation of evaporative cooling in cities.

Plain Language Summary: Precipitation that is lost as surface runoff is not stored to evaporate and thus to cool the environment at a later time. Urban areas are characterized by many impervious surfaces like roads and buildings, which promote runoff and reduces water availability for evaporation. This decreased water availability can exacerbate heat in cities. This study examines how ULSM predict runoff, which is crucial for simulating energy exchange between the surface and the atmosphere. We find that many models perform well for high impervious fraction and heavy rainfall events, but substantial discrepancy in modeled runoff appears for low‐intensity events and at green sites. Improving these models will enhance estimation of water availability and evaporation, and enabling more effective urban cooling and water management strategies.