How precipitation lapse rates shape runoff simulations and flood frequency estimates in mountainous regions

Abstract. Precipitation lapse rates (PLRs) play a key role in hydrological simulations of mountainous catchments. However, they are often poorly represented in the precipitation estimates and are typically simplified as constant and positive values in the hydrological models. In this study, we combine a stochastic weather generator with a hydrological model to investigate how PLRs affect runoff simulations for several mountainous catchments in Switzerland. In the weather generator, the PLR adjusts precipitation from station elevation to mean catchment elevation, while in the hydrological model it redistributes precipitation among elevation zones. By systematically varying the PLRs in both the weather generator and the hydrological model between 0 % and 10 %, we found effects on mean seasonal and annual runoff, as well as on extreme floods, depending on catchment characteristics and precipitation network properties. Specifically, higher-elevation catchments were less sensitive compared to lower-elevation catchments. Increasing PLRs tended to increase summer floods, while decreasing PLRs tended to increase winter floods. In addition, the seasonality of frequent floods was more sensitive to changes in PLRs than that of rare floods. Moreover, flood seasonality was primarily controlled by the PLR in the hydrological model, while flood magnitude was mainly driven by the PLR in the weather generator through its effect on precipitation amounts. These findings highlight the need for a more comprehensive investigation of the assumption of a constant lapse rate in hydrological models, particularly in mountainous regions where precipitation gradients are strong and observations are limited.