Inundation Mapping using time series RADASART-SAR images In the Lower Making River Basin
Akihiko Imazato*, Atsushi Rikimaru**, Yasumi Yamaguchi, Takatumi Amano *** and Yoshiaki Otsubo*
*The Japanese institute of Irrigation and Drainage
** Hoseri Univ.,***Japan Weather Association
*Toranomon NN building, 1-21-17, Toranomon,
Minato-ku, Tokyo,105-0001, JAPAN
Tel : +81-3-3502-1387 , Fax : +81-3-3502-1329
E-mail : akihiko-imazato@jiid.or.jip
Introduction
It is important for agricultural development to grasp the geographical distribution of inundation that occurs during the rain season every year. Such date is essential for agricultural and rural improving in many countries. The data is used for inundation mapping, which cab be any map that considers agricultural damage caused by inundation. To assess the distribution of inundated fields that may cover a huge area and that may change rapidly; observation from the air is more effective than observation ion the round. Aerial photographs and satellite images captured by optical sensor have been used so far to create inundation maps . however, such optically-captured images do not always provide enough data for analysis s the images are often obscured by the very clouds that cause inundation in the rain season. This study reports on the development of a method for creating inundation maps by means of Synthetic Aperture Radar (SAR). In this method, time series observation using SAR is carried out to provide image of data analysis. These images immediately show inundating conditions in the annual rain season. This study, involving the Lower Mekong River Basin, is part of a program to build a monitoring system for agricultural and rural supporting.
Inundation Mapping using RADASAT-SAR images.
Synthetic Aperture Radar (SAR) can easily detect water surfaces . the microwaves emitted by SAR undergo back-scattering and are reflected like form a mirror when impinging on a water surface =. Microwaves penetrate clouds and reach the round without depending on sunlight. SAR is ideal for observing inundating phenomena that occur frequently during the rain season when there are many clouds. The RADASAT satellite used on this study allows various types of observation by changing the modes of the relevant elements including at beam angle. The user cab specify and observation for any given place under any desired conditions and establish a sequential observation system at intervals as short as two to five days. This interval range is one of the most remarkable features of RADARSAT, compared with J-ERS ad ERS that were typical satellites used before RADARSAT having obsevation intervals of as long as one month or more. These characteristics of the sensor and operating conditions of the RADARSET satellite make RADARSET very useful for acquiring SAR images quick observation of inundating conditions. In this study , SAR study, SAR image taken over several times are subjected to various corrections and overlapped on chronological order, thus creating a inundation map that shows how the inundation changes over time. We also conducted site investigation to verify the validity of the inundation map against actual conditions.
Time series observation and inundating condition.
For agricultural and rural development, it is vitally important assess actual inundating conditions and to define the maximum range on inundated areas. However, defining the maximum range of peak inundated area is not sufficient for determining agricultural damage; it is also necessary to know bow long the corps have remained submerged . that is, including observation should be carried out at three different times as a minimum: 1) the peak tie of inundation, 2) when the inundated area has shrunk to the minimum area and 3) the dry season. Observations in the dry season are used to define the water area heat normally exists throughout the year.
For the investigated area, the correlation between the submerged period due to inundation and agricultural damage is to clear. However, based on ground investigations in advance, we compared the actual agricultural damages and the results of SAR observation. In this comparison, we assumed that the time taken to assess a creation level of agricultural damage and changes of distribution of inundated area is about one week to ten days. After observations at the peak of inundation , we continued to make observations at intervals of the is time span until the inundation shrank to the minimum range . table 1 summarizes the relationship between the SAR observations and agricultural damage.
| | Peak of inundation | After 10 days | After 40 days | Dry season |
| Non-inundated area, undamaged | Non-submerged | Non-submerged | Non-submerged | Non-submerged |
| Short-term inundated area, slightly damaged | submerged | Non-submerged | Non-submerged | Non-submerged |
| Medium-term inundated area, moderately damaged | submerged | submerged | Non-submerged | Non-submerged |
| Long-term inundated area, seriously damaged | submerged | submerged | submerged | Non-submerged |
| Regularly submerged area | submerged | submerged | submerged | Submerged |
Table 1 Relationship between SAR observations at intervals and agricultural damage
