## 1 Introduction  
Rice is one of the most important nutrition components in the world and a supply of stable food in many countries, especially in Southeast Asia. Up-to-date information on areas under rice cultivation and changes in the cultivated area is essential for national political decision-makers and for international trade organizations for planning and monitoring purposes. Due to the human population growth the global food security has to be ensured. Irrigated rice fields are also among the major sources of methane emissions. On the other hand, awareness of changes in rice growing conditions is beneficial for estimating the long term impacts of climate change on the rice production. According to the International Rice Research Institute, Vietnam is the leading producer of rice in the world; the Mekong Delta region being the most important rice production area [1]. In some of the coastal parts of the Mekong Delta region, United Nations International Fund for Agriculture (IFAD) has given recommendations to rice farmers to change from rice to other crops due to the salinity intrusion in the area. Therefore, information on areas under aquaculture and other crops is needed as well. Land-use changes can be further used to evaluate the effect of salinity intrusion to land use in the study area and to define the rice growing areas that need to transit to other land uses as a result of increasing salinity currently and through forecasting based on the sea level rise and reduced river flow. Although, some central descriptive data on cultivated areas are recorded by the national authorities, there is still need for more accurate and timely information. Moreover, it would be advantageous to extend the time-line of changes in rice land-use to the past decades, starting even from the 1970s. The use of Earth Observation (EO) satellites and their historical data archives is a reliable and cost effective way to obtain this information. The main challenge in estimating the area under rice cultivation using EO data is to obtain well-timed satellite data, which enables interpretation of rice areas from other crops and land-use classes. Therefore, a multi-temporal data set is typically needed, _i.e._, a sufficient number of images during the rice growing season (there may be one to even three growing seasons in a year). Due to the extensive cloud cover during the rice growing season, the use of cloud-penetrating imaging system, Synthetic Aperture Radar (SAR) on-board of satellite, appears to be an ideal solution for monitoring of agricultural areas [2]. Using _a priori_ knowledge of common cultivation practices in the target area (crop calendar), it is possible to order well-timed SAR images for detecting rice areas. It is also advantageous to exploit optical EO data to identify areas that are permanently non-irrigated, since there are different interactions with the target for active microwave SAR and daylight-based optical sensors. In addition, optical satellite missions have been operational longer than SAR satellites and have collected long time series of image data. Examples of integrated use of SAR and optical data for land cover and crop type mapping are presented in [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. A review of remote sensing of rice crop areas has been published [17]. In Mekong Delta, several land cover studies based on optical EO data have been carried out (e.g., [18, 19, 20, 21]). The German-Vietnamese WISDOM (Water-related Information System for the sustainable Development of the Mekong Delta in Vietnam) project is currently designing and implementing an information system for the Mekong Delta, containing information from the fields of hydrology, sociology, information technology and earth observation [22]. A considerable amount of scientific research has been carried on the matter of rice crop and land cover monitoring using SAR data, e.g., [2, 23, 24, 25, 26, 27, 28, 29, 30].