Statistical analysis and combination of active and passive microwave remote sensing methods for soil moisture retrieval

摘要

Knowledge about soil moisture and its spatio-temporal dynamics is essential for the improvement ofclimate and hydrological modeling, including drought and flood monitoring and forecasting, as wellas weather forecasting models. In recent years, several soil moisture products from active andpassive microwave remote sensing have become available with high temporal resolution and globalcoverage. However, for the improvement of a soil moisture product and for its proper use in modelsor other applications, validation and evaluation of its spatial and temporal patterns are of greatimportance.In chapter 2 the Level 2 Soil Moisture and Ocean Salinity (SMOS) soil moisture product and theAdvanced Scatterometer (ASCAT) surface soil moisture product are validated in the Rur and Erftcatchments in western Germany for the years 2010 to 2012 against a soil moisture reference createdby a hydrological model, which was calibrated by in situ observations. Correlation with the modeledsoil moisture reference results in an overall correlation coefficient of 0.28 for the SMOS product and0.50 for ASCAT. While the correlation of both products with the reference is highly dependent ontopography and vegetation, SMOS is also strongly influenced by radiofrequency interferences in thestudy area. Both products exhibit dry biases as compared to the reference. The bias of the SMOSproduct is constant in time, while the ASCAT bias is more variable. For the investigation of spatiotemporalsoil moisture patterns in the study area, a new validation method based on the temporalstability analysis is developed. Through investigation of mean relative differences of soil moisture forevery pixel the temporal persistence of spatial patterns is analyzed. Results indicate a lower temporalpersistence for both SMOS and ASCAT soil moisture products as compared to modeled soil moisture.ASCAT soil moisture, converted to absolute values, shows highest consistence of ranks and thereforemost similar spatio-temporal patterns with the soil moisture reference, while the correlation of ranksof mean relative differences is low for SMOS and relative ASCAT soil moisture products.Chapter 3 investigates the spatial and temporal behavior of the SMOS and ASCAT soil moistureproducts and additionally of the ERA Interim product from a weather forecast model reanalysis onglobal scale. Results show similar temporal patterns of the soil moisture products, but high impact ofsensor and retrieval types and therefore higher deviations in absolute soil moisture values. Resultsare more variable for the spatial patterns of the soil moisture products: While the global patterns aresimilar, a ranking of mean relative differences reveals that ASCAT and ERA Interim products showmost similar spatial soil moisture patterns, while ERA and SMOS products show least similarities.Patterns are generally more similar between the products in regions with low vegetation. [...]