What about crop damages due to flood events? Simply difficult.
Flood events can result from river flow, originating from distant catchment basins, from flash flooding caused by heavy rainfall including from typhoon, and from coastal sea surge in the agricultural and aquaculture production areas in the river deltas. Traditionally, the monitoring and mapping of these events has been carried out by point measurements collected by river gauges combined with terrestrial mapping. In the past two decades, flood modeling and Synthetic Aperture Radar (SAR) have become widely accepted practices in flood forecasting and mapping. In summary, while flood modeling provides a sophisticated technique enabling, assuming the availability of very accurate input parameters, the simulation and the forecasting of flood extent, duration and depth, flood extent and duration are directly quantifiable from SAR time-series, assuming the availability of images acquired at appropriate time and scale. From a technical point of view, an operational and reliable flood risk management service must consider both techniques, because they are complementary. In particular, SAR provides to the flood modeling relevant input parameters (such as land cover map, vegetation status, Digital Elevation Model, flood extent, etc.). In turn, flood modeling timely complements flood extent information, whenever SAR data are not available. Finally, the use of independent techniques and data allows to cross-validate the resulting products.
The figure shows a typical set of output generated using high resolution SAR data. Note that the two SAR images (on the top) have been acquired immediately after the event (flooding is visible as black areas) and around one month later. The status of the land coverage six month prior (bottom left) and after (bottom right) the flood event is illustrated.
Top figures - SAR images: immediately after the flood event (left) and around one month later (right). Bottom left figure - Green tones: vegetation and rangeland; Brown and gray tones: barren land; Blue: water. Bottom right figure - Black: Water covered areas on both dates; Blue: flooded areas just after the flood event. The different blue tones correspond to the different flood severity. Yellow and Green: Cultivated areas. The two colours correspond to the different growth. Pink: Uncultivated land.
The World Bank, Assessment of Innovative Approaches for Flood Risk Management and Financing in Agriculture, Agriculture and Rural Development Discussion Paper 46, 2010.