On the Spatial Characteristics and Scaling Behaviour of the LISFLOOD and the ERS/SCAT Derived Soil Moisture Fields
Laguardia, G.1; Niemeyer, S.2
1European Commission, DG Joint Research Centre; 2European Commission - DG Joint Research Centre, Institute for Environment and Sustainability
In a large scale modelling framework it is common to encounter a series of problems related to the evaluation of the reliability of the produced spatial information due to the uncertainties inherited by the static spatial data (e.g. the soil maps) or by the spatial mapping of the point input data (e.g. the meteorological inputs).
The LISFLOOD model is a distributed hydrological model performing continuous hydrological simulations within the European Flood Alert System (EFAS) on a 5 km spatial resolution. It comprises modules for the modelling of vegetation, soil, groundwater, snow cover, runoff generation, and stream routing in major European rivers. The soil compartment of LISFLOOD consists of a two-layer soil model. Infiltration of effective precipitation, soil evaporation and plant water uptake take place from the upper soil layer, while the lower soil layer represents essentially a storage term that produces a slow runoff component and recharges the groundwater compartment. Accordingly the soil moisture content of the upper soil layer mirrors well the balance of water between precipitation supply and climate and vegetation demand. The observed meteorological input information is derived from measured and spatially interpolated meteorological point data provided by the MARS-STAT activity of IPSC JRC (so called JRC-MARS).
We obtained the ERS Scatterometer soil moisture data of the Global Soil Moisture Archive. Such data have been derived by the scatterometer on board the ERS-1 and ERS-2 satellites, operated by the European Space Agency (ESA), whose data have been processed globally by means of a soil moisture retrieval technique based on a change detection approach. The information on the soil surface layer water content has been processed in a semi-empirical modelling approach to estimate the status of the profile soil moisture content, deriving the Soil Water Index (SWI). SWI can be transformed into any other soil moisture measurement once determined the soil water retention features.
In the framework of the comparison between the LISFLOOD modelled and the ERS Scatterometer derived soil moisture fields, we assess the spatial features of the two products by means of variogram analysis and scaling behaviour of such products.
The analysis of the variograms shows that both the datasets present similar behaviour at the larger distances. The ERS scatterometer variogram shows a quite linear behaviour, giving the impression that it has not reached yet the sill; the LISFLOOD one has a rapid increase, then achieving a rather asymptotic behaviour. At the short lags the larger variability of the higher resolution LISFLOOD data can be detected.
When analyzing the scaling behaviour, it is possible to observe a power law decay of the variance with respect to the averaging area. The exponents for the two datasets, despite being quite similar, show differences in their temporal variability.