Improving groundwater representation and the parametrization of glacial melting and evapotranspiration in application of the WaSiM hydrological model within Iceland

Abstract

The WaSiM model is used for hydrological modelling at the Icelandic Meteorological Office. The model application has been improved from former use by: i) improving the representation of groundwater by activating the model’s groundwater module; ii) improving the representation of seasonal changes in the Hamon evapotranspiration scheme; and iii) by applying glacier melt parameters calibrated by mass balance measurements instead of river discharge data. Trial studies were done for two watersheds: Austari-Jökulsá, vhm 144, and Sandá í Þistilfirði, vhm 26. Comparable or better results are obtained with more physically realistic modelling. Coefficients such as recession constants for direct runoff and interflow are now within expected range. Groundwater flow is important in Iceland because of young porous postglacial lava fields and high hydraulic conductivity through tectonical faults and fissure swarms and the activation of the groundwater module is therefore the largest improvement. Two different methods were used to obtain data required for the groundwater module. Simple estimation based on geological maps was used for Sandá í Þistilfirði, vhm 26, while values for Austari-Jökulsá, vhm 144, were based on a thorough hydrogeological study. Meteorological data from PSU/NCAR MM5 numerical weather model were used as input for the hydrological modelling but comparison to interpolated data from meteorological observation stations indicates that the MM5 temperature data might by biased towards to high temperatures during winter time. Temperatures data used for Sandá í Þistilfirði, vhm 26, were therefore shifted before being used as input for the hydrological modelling.