Event and model dependent rainfall adjustments to improve discharge predictions

Most conceptual rainfall–runoff models use as input spatially averaged rainfall fields which are typically associated with significant errors that affect the model outcome. In this study, it is hypothesized that a simple spatially and temporally averaged event–dependent rainfall multiplier can account for errors in the rainfall input. The potentials and limitations of this lumped multiplier approach are explored by evaluating the effects of multipliers on the accuracy and precision of the predictive distributions. Parameter sets found to be behavioural across a range of different flood events were assumed to be a good representation of the catchment dynamics and were used to identify rainfall multipliers for each of the individual events. An effect of the parameter sets on identified multipliers was found, however it was small compared to the differences between events. Accounting for event–dependent multipliers improved the reliability of the predictions. At the cost of a small decrease in precision, the distribution of identified multipliers for past events can be used to account for possible rainfall errors when predicting future events. By using behavioural parameter sets to identify rainfall multipliers, the method offers a simple and computationally efficient way to address rainfall errors in hydrological modelling

Imbalanced land-surface water budgets in a numerical weather prediction system

There has been a significant increase in the skill and resolution of numerical weather prediction models (NWPs) in recent decades, extending the time scales of useful weather predictions. The land-surface models (LSMs) of NWPs are often employed in hydrological applications, which raises the question of how hydrologically representative LSMs really are. In this paper, precipitation (P), evaporation (E) and runoff (R) from the European Centre for Medium-Range Weather Forecasts (ECMWF) global models were evaluated against observational products. The forecasts differ substantially from observed data for key hydrological variables. In addition, imbalanced surface water budgets, mostly caused by data assimilation, were found on both global (P-E) and basin scales (P-E-R), with the latter being more important. Modeled surface fluxes should be used with care in hydrological applications and further improvement in LSMs in terms of process descriptions, resolution and estimation of uncertainties is needed to accurately describe the land-surface water budgets

Area averaging of land surface-atmosphere fluxes in NOPEX: Challenges, results and perspectives

The NOPEX experimental campaigns dealt with the land-surface-atmosphere exchange of momentum, heat, water and CO2 on local and regional scales. In this paper emphasis is put on the NOPEX experiences with respect to the spatial integration of fluxes of momentum, heat, humidity and CO2 over the mosaic of forest, agricultural land, lakes and mires that make up the southern part of the NOPEX area. It is found that the forest dominates both the regional momentum and heat fluxes but in very different ways. Furthermore, results from a NOPEX experiment in the northern zone of the boreal forest in Finnish Lapland highlights the very unique processes associated with the energy exchange within a forest in wintertime. The interaction of the forest canopy with sunshine provides a considerable energy source, particularly at low solar angles. A considerable improvement in model simulations of fluxes and surface temperature was achieved when this effect and the heat storage of the canopy were taken into account