Increasing river flood preparedness by real-time warning based on wetness state conditions

• Soil moisture monitoring for two purposes.
• General overview of soil moisture state by hydrological modelling and indirect observation techniques.
• Combining soil moisture and rainfall to calculate discharge exceedance probability, by logistic regression.
• Exceedance probability is mapped and can be used for flood warning purposes.

SummaryHigh wetness state levels can be considered as a primary indicator for potential river flooding. Therefore it is advisable to visualise real-time soil moisture information in flood forecasting or warning systems. Monitoring of soil moisture, however, is not an easy task due to its variable nature in time, space and depth. This paper presents and compares methods to assess the severity of the soil moisture state of hydrological catchments considered in a typical operational flood forecasting system. The severity of the relative soil moisture state is obtained and mapped by comparing the actual simulation result with the historical simulation results of a lumped conceptual hydrological model, directly by making use of the soil moisture component of the model or indirectly considering the baseflow component. Another approach uses rainfall, evapotranspiration and river flow observations. By applying a baseflow filter to the river flow observations and an advanced method for empirical catchment water balance computation, two indirect soil moisture indicators were defined, namely the filtered baseflow and the water balance based relative soil moisture content. It is shown that each of the methods allows to obtain useful estimates of the soil moisture state of a catchment in real time. The severity level of the soil moisture state is computed after comparison with long term statistics derived from a long term simulation. The severity level moreover is used to calculate the probability of exceedance of a predefined riverflow threshold, e.g. flood threshold, at the outlet or a given location in the catchment. This is done by means of a logit relation of the river flow probability of exceedance with the soil moisture indicator. The different soil moisture indicators are compared in their predicting capabilities by calculating and comparing the Brier score. Interestingly, the application of the logit relation or the use of a simple water balance computation for the catchment, based on real-time rainfall, evapotranspiration and river flow observations, leads to more reliable probability of exceedance estimates than the common direct use of total runoff results from a state-of-the art rainfall–runoff model. Mapping the probability of exceedance for the different hydrological catchments together with the width of the confidence interval on this probability is proposed as a useful tool to increase the preparedness for potential floods.