Analysis of Soil Moisture Data Retrieved from C-band Scatterometers in Southeast Asia

Kurzfassung

The C-band scatterometer data have been demonstrated in many studies [1-9] to be valuable for monitoring of surface soil moisture using the so-called TU Wien change detection method [10-11]. High temporal sampling in all weather conditions, multi-viewing capability and availability of long-term measurements make the European C-band scatterometers excellent observation tools for soil moisture change detection. The observations of the ERS-1/2 scatterometers together with the new series of advanced scatterometers (ASCAT) onboard Metop satellites ensure long-term global observation (from 1991 until at least 2020). Soil moisture is recognized as an important component of the water cycle in hydrological and natural environmental processes. Information on surface and profile soil moisture is demanding for a wide range of applications concerning water supply, agriculture, weather forecasting, climate modeling, and etc. This study presents an in-depth evaluation of the soil moisture products retrieved from C-band scatterometer data (from 1991-2000 and 2007-2010) in regional scale and demonstrates application examples in lower Mekong Basin in Southeast Asia. The Mekong River is the longest river in Southeast Asia and is one of the ten longest rivers in the world. It rises in the Tibetan highlands and flows through six states and drains an area of 795,000 km². It crosses the southeast Chinese province of Yun-Nan, forming the border between Myanmar and Laos and in the lower reaches of a large part of the border between Laos and Thailand. It flows through Cambodia and into South Vietnam branched into several mouth-arms that make up the vast Mekong delta where it empties into the South China Sea. The climate of the Mekong region is influenced by the Southwest and Northeast monsoons. The tropical monsoonal regime in lower Mekong area generates a distinctly biseasonal pattern of wet and dry periods of more or less equal length. This results in an annual flood pulse and therefore a distinct seasonality in the annual hydrological cycle between a flood season and a low-flow season. The strong seasonal variations in rainfall leads to extreme conditions for the people of the lower Mekong Region: Large-scale and long-lasting floods alternating with periods of drought and water shortage. Floods and droughts can occur anywhere in the basin imposing large economic and social costs on the people [12]. Therefore monitoring of soil moisture conditions in Mekong basin is valuable for many hydrological and agricultural applications. The study includes a catchment-base noise analysis of soil moisture data in lower Mekong basin to identify the areas where soil moisture retrieval is robust and applicable. In general, the seasonal variability of soil moisture in Mekong basin is well captured by the retrieval method especially in agricultural areas. Comparison of the soil moisture data with topography and land cover classifications showed that the soil moisture noise increases in highlands with complex topography and in areas covered by very dense vegetation. It is found that the quality of soil moisture is strongly degraded during oversaturated soil situations particularly by large flooded events in delta region in Vietnam and Tonle Sap basin in Cambodia during the peak of the wet season. Furthermore, a catchment-base statistical analysis of the soil moisture data has been carried out to evaluate the relation between the Basin Water Index (BWI), an indicator of the basin soil moisture condition, with in-situ hydrometeorological measurements in different months of year. The results of analysis also showed significant correlations between Enso events and monthly BWI anomaly measurements.