Environmental Monitoring and Changes in the Watershed Area Analysis(2/3)

ABSTRACT

As one of the top three countries that suffers from threats of debris flow, watershed conservation and disaster mitigation has been the ultimate goal that the government tries to achieve. Concerning aspects of watershed management, erosion control, slopeland conservation and sustainable development, Soil and Water Conservation Bureau has employed various engineering practices such as check dam, submerged dam, river regulation, revetment, levee and water drainage work in torrent erosion, gully erosion and landslide. However, it is crucial to track and assess the efficiency of every management practice while considering the landscape variation and disaster susceptibility analysis of each management watershed.

To perceive results of engineering works and disaster susceptibility history of critical management watersheds, this project aims to employ Formosat-2 satellite imagery database (processed through procedures of coregistration, orthorectification, geometric registration, radiance normalization, spectral summation intensity modulation, edge sharpening and adaptive enhancement) in landscape variation analysis. Then overlaying data of engineering sites on Formosat-2 imagery to distinguish management watersheds, sub-watersheds and slopelands and outline the areas that might be influenced by disasters according to land use map and data of protected objects. Furthermore, this study applies three quantitative methods. Fist, use NDVI, Greenness index and gradient to delineate vegetation areas. Second, employ pre and after disaster Formosat-2 imagery with DEM to delineate landslide area. Third, apply river course map to delineate variation of river course. Moreover, a more detailed analysis will be processed on the outcomes of each engineering work and disaster susceptibility as the reference of future management works.

This project establishes a database and proceeds a thorough analysis of 87 sub-watersheds (48 of them have been done last year, and 39 of them are updated this year). Landscape variations are interpreated for those 39 updated sub-watersheds, with intention to analyse the current status of sub-watersheds and critical management watersheds. Furthermore, 6 out of 39 sub-watersheds are selected for UAV photo shooting and onsite investigation. The most updated and high-spatial-resolution aerial photos of the important engneering works and critical management watersheds are successfully acquired. The critical management watershed of Daxixi, I-Lan County, is also selected to establish a high-spatial-resolution DEM from the UAV aerial photos. The same new technique would be applied to establish an after-event DEM, if there were any disaster in the future. The changes in the critical management watershed can be quantitatively evaluated by comparing the before- and after-event DEMs, which would significantly benefit the detailed analysis of the results of engineering works.

Trough integrating basic data and efficiency assessment of 39 sub-watersheds as KMZ (Keyhole Markup Language) files, they are published on the GEE platform of SWCB which is available to other divisions as the reference of relative policy making. To promote and educate the public how does a management practice influence their lives, the results of 6 sub-watersheds are edited as short films and posters.

Keywords:Formosat-2, Critical Management Watershed, Management Efficiency, Variation Analysis.