Seasonal forecasting of local-scale soil moisture droughts with Global BROOK90
Abstract. Prolonged deficit of soil moisture can result in significant ecosystem and economical losses. General slowdown of vegetation growth and development, withering of foliage cover, reduction of carbon, nutrient and water cycling, increase of fire and insect outbreaks are just a few examples of soil moisture drought impacts. Thus, an early and timely warning via monitoring and forecast could help to prepare for the drought and manage its consequences.
In the study, a new version of Global BROOK90, an automated framework to simulate water balance at any location is presented. The new framework integrates seasonal meteorological forecasts from European Centre for Medium-Range Weather Forecasts (ECMWF). Here we studied how well the framework can predict the soil moisture drought on a local scale. Twelve small European catchments (from 7 to 115 km2) characterised by various geographical conditions were chosen to reconstruct the 2018–2019 period, when a large-scale prolonged drought was observed in Europe. Setting the ERA5-forced soil moisture simulations as a reference, we analysed how the lead time of the ECMWF hindcasts influences the quality of the soil moisture predictions under drought and non-drought conditions.
It was found that the hindcasted soil moisture fits well with the reference model runs only within the first (in some cases until second and third) month of lead time. Afterwards significant deviations up to 50 % of soil water volume were found. Furthermore, within the drought period the ECMWF hindcast forcing resulted in overestimation of the soil moisture for most of the catchment, indicating an earlier end of a drought period. Finally, it was shown that application of the probabilistic forecast using the ensembles’ quantiles to account for the uncertainty of the meteorological input is reasonable only for short-to-medium range lead times (up to three months).
