E. Seyhan

Estimating soil moisture from satellite microwave observations: Past and ongoing projects, and relevance to GCIP

On the basis of a series of studies conducted in Botswana and preliminary results from an ongoing study in Spain, developments in microwave remote sensing by satellite, which can be used to monitor near-real-time surface moisture and also study long-term soil moisture climatology, are described. A progression of methodologies beginning with single-polarization studies and leading to both dual polarization and multiple frequency techniques are described. Continuing analysis of a 9 year data set of satellite-derived surface moisture in Spain is ongoing. Preliminary results from this study appear to provide some evidence of long-term desertification in certain parts of this region. The methodologies developed during these investigations can be applied easily to other regions such as the GCIP area and could provide useful databases for simulation and validation studies. Additionally, they have strong potential for global applications such as climate change studies.

Groundwater discharge from areas with a variable specific drainage resistance

Abstract Based on schematic representations of hydrogeological conditions, related to the structure of the formations and topographical features, analytical solutions were derived for the behaviour of groundwater discharge in terms of a time-variable drainage resistance. This led to a general equation of the drainage resistance as a function of groundwater discharge. This physically based equation was implemented in a simple non-distributed conceptual runoff model in order to analyse continuous time series of runoff with intermittent rainfall. Application of the model to different physiographic environments, varying from high Alpine mountainous catchments to the low lying areas of the Netherlands, showed that ignoring the time-variable nature of drainage resistance may lead to significant errors in the analysis and prediction of both baseflow and direct runoff.

Multi-temporal Analysis of ERS-1 and EMISAR C-band VV Backscattering Properties of Forest and Lake Surfaces in the NOPEX Region.

Multi-temporal ERS-1 SAR data, consisting of seven acquisitions in 1994 and 1995, were analyzed to determine the potential of C-band VV polarization backscattering data to discriminate different surface types within the patchy boreal landscape of the NOPEX area. Based on an aerial photo analysis, four classes of forest density were distinguished. For independent comparison, also one fully polarimetric EMISAR C-band image, including the VV polarization signatures was analyzed. It was found that the differences between the classes were quite consistent throughout the seasons and therefore probably significant, at least in a relative sense. The same images were also studied to determine the sensitivity of backscattering properties of lake surfaces to regional winds. For this purpose the backscattering values of five lakes within the study area were analyzed. It was found that the five lake surfaces behaved very similar, except for one observation date. This behavior, therefore, seems to be dominated by regional scale wind fields, although local scale wind fields also may have an effect. Since regional and local scale wind fields are difficult to observe in a synoptic way, the use of radar might have potential for monitoring such wind fields in the Swedish boreal region, which is characterized by the occurrence of many lakes.

Statistical analysis of weather-type runoff phenomena in an Alpine environment

Abstract In order to get a better insight into the role of hydrologic regime components in an Alpine environment, runoff characteristics were analysed in relation to atmospheric circulation types over a 23-yr. period. For this purpose, atmospheric circulation types were classified into hydrologically relevant classes of weather types using Schuepps classification which was developed for the European Alpine region. It is found that the magnitude of runoff during each particular class of weather types is largely governed by autocorrelation (related to the characteristic times of the systems active during preceding classes and thus related to the preceding weather types) and by antecedent conditions, rather than by the weather types themselves. The influence of a weather class displays a distinct dependency of the time elapsed since the start of the particular class. This influence does not become manifest until after a 2–3-day period on the average.

Determination of resistance parameters of pluvio-nivo-glacial alpine systems by mathematical modeling of runoff

Abstract A nonlinear optimization technique has been applied to analyse complex hydrographs from an alpine catchment with pluvial, nival and glacial regime components, in order to determine bulk resistance parameters of the contributing systems. Both the pluvial system and the nivo-glacial system are represented by a linear system function. Application of the optimization model to monthly time series of daily runoff, revealed the time variability of the systems operation which reflects the seasonal variability of hydro (geo) logical and snow-hydrological conditions relevant for the creation of runoff. The model has been applied to a northern Italian Alpine catchment with a runoff record of 22 yr.

Desertification/Aridification Monitoring of the Iberian Peninsula By Passive Microwave Remote Sensing

Nimbus/SMMR passive microwave signatures were used to determine the spatial distribution and temporal variations in surface soil moisture over the Iberian Peninsula for the whole lifetime of SMMR (1978-1987). The analysis of surface moisture showed a significant and gradual decrease in surface moisture over the ten-year period in one large region in northeastern Spain. The developed methodology, therefore, seems to be a promising tool for large-scale degradation/aridification monitoring in desertification threatened regions.

Satellite microwave estimates of soil moisture and applications for desertification studies

Based on a series of studies conducted in Botswana and preliminary results from an ongoing study in Spain, developments in microwave remote sensing by satellite which can be used to monitor near real-time surface moisture and also study long term soil moisture climatology are described. A progression of methodologies beginning with single polarization studies and leading to both dual polarization and multiple frequency techniques are described. Continuing analysis of a nine year data set of satellite-derived surface moisture in Spain is ongoing. Preliminary results from this study appear to provide some evidence of long term decertification in certain parts of this region. The methodologies developed during these investigations can be applied to other regions, and have the potential for providing modelers with extended data sets of independently derived surface moisture for simulation and validation studies, and climate change studies at the global scale.