The influence of precipitation lapse rate in flood estimates using long continuous simulations

Precipitation lapse rates (PLRs) describe how precipitation varies with elevation. PLRs strongly influence the water balance of high-elevation catchments and affect the seasonal dynamics of snow accumulation and related snowmelt runoff, as well as the longer-term mass balance of glaciers. Despite their importance, precipitation variations with elevation remain poorly understood due to the complex and highly localized nature of orographic precipitation as well as the limited availability of high-elevation precipitation observations. Consequently, streamflow simulation in mountainous environments is challenging, and many hydrological studies rely on the simplifying assumption of a constant (usually positive) PLR.

The representation of PLR in both the input data and the hydrological models plays a key role in streamflow simulations. Here, we used a combination of long synthetic time series from a stochastic weather generator (GWEX) and a hydrological catchment model (the HBV model) to study the influence of PLR on runoff simulations for several Swiss catchments.  To better understand the influence of PLR on the simulations, particularly on flood estimates, we conducted two experiments. In the first experiment, we varied the PLR parameter in HBV between 0% and 10% (0%, 2.5%, 5%, 7.5%, and 10%). This parameter redistributes the mean catchment precipitation from GWEX between elevation zones without altering the total precipitation amount. In the second experiment, we applied the same PLRs to first adjust precipitation for the difference between station and mean catchment elevation, before interpolation to mean areal precipitation using Thiessen weights. For each simulation run, GWEX inputs were adjusted using the same PLR that was applied in the hydrological model. Through these experiments, we assessed the sensitivity of flood estimates to changes in PLRs applied solely within the hydrological model and to the combined application of PLRs in both precipitation input and hydrological model.

Our findings show variable responses in the monthly water balance, flood seasonality, and changes in the flood estimates for both experiments, reflecting differences in catchment characteristics. This evaluation highlights the importance of PLRs in hydrological studies and demonstrates that the use of a fixed PLR can be misleading. Instead, PLR assumptions should be context-dependent and carefully considered in hydrological applications.