Extremes in hydroclimatic systems and their consequences: Evaluation of precipitation reanalysis products for rainfall-runoff modelling in Slovenia

Design discharges are a key element for flood protection measures. Their determination is a continuing challenge, especially for sparely or even ungauged catchments. While hydrological modelling and subsequently derived flood frequency analysis is a possible solution for this issue, its reliability significantly depends on the quality of the input precipitation time series. Unfortunately, observed precipitation time series are often too short, their temporal resolution is not sufficient or the network density is too low.

From the latest precipitation reanalysis products (PRP) three are most promising due to their spatial and temporal resolution for rainfall-runoff (r-r) modelling of small to mesoscale catchments: ERA5-Land (raster with ~9 km width), REA6 (6 km) and CFSv2 (22 km). These three PRP are able to cope with the dynamics of the r-r process due to their hourly resolution. The PRP are evaluated for Slovenia (Europe) with both, precipitation and runoff characteristics. For areal precipitation, continuous and event-based characteristics are evaluated as well as precipitation extreme values. Simple correction methods for identified biases are suggested and applied. It can be seen that although the PRP clearly differ from each other, there is no clear ‘favourite’ to use as input for the r-r modelling.

For r-r modelling, continuous simulations are carried out with GR4H for 20 catchments in Slovenia (55 km²-480 km²). Models are re-calibrated for each PRP input based on KGE. Simulation results of calibration and validation period are evaluated by runoff extreme values, KGE, flow duration curve and intra-annual cycle. Interestingly, first results show that the deviations of some rainfall characteristics do not necessarily transfer to deviations in runoff characteristics, which can be explained by the high nonlinearity of the r-r process. PRP lead to better, at least similar results for runoff characteristics for catchments without rain gauges in their centre.