9 the inverse distance squared weighting technique.<sup>19</sup> In particular, rainfall was considered excessive in a particular year if it exceeded 300 millimeters, 450 millimeters, or 600 millimeters in any rolling five‐day period throughout that year. By using 5‐day periods as opposed to monthly data, a more refined picture of localized flooding could be constructed.

The annual occurrence of excess rain across the grid during 2001‐2006 is presented in Figure 4. Figure 5 presents the proportion of years the 0.1 degree grid cells experienced at least one episode with more than 300 mm rain in a consecutive five‐day period. The Northcentral and Southcentral Coastal regions appear especially prone to excess rain. Localized flooding occurred also in the Red River Delta Region in 2003. More generally, localized flooding appears especially concentrated along the coast. Excess rainfall is however rather rare in the northern uplands and the south (Southeast and Mekong River Delta regions), suggesting that excess rain is closely related to tropical cyclones, and less to riverine floods. Nonetheless, there are also small inland patches with excess rain, spread across the country.

# _Riverine floods_

Given the historical importance of riverine floods in Vietnam and the limited correlation between the geographical distribution of localized floods and the location of the rivers, the localized flood maps were complemented with riverine (and coastal) flood maps constructed from the Dartmouth Flood Dataset (Dartmouth Flood Observatory, 2008). The latter is an historical worldwide flood database spanning 1985‐2007 and showing locations of floods and their major causes (monsoonal rain, heavy rain, tropical cyclones). The information is derived from a wide variety of sources including news and governmental reports based on locally gauged and remote sensing observations.<sup>20</sup> Unfortunately, the maps are not very disaggregated (ten by ten degrees in latitude and longitude). In principle, the flooding caused by any of the types of rainfall might have been picked up already by the localized flooding dataset. What would not be picked up is when rainfall in one place is transported by rivers or terrain to another place or when high tides push ocean water inland.

Given the focus here on riverine and coastal floods and to better pinpoint the locations that actually experienced riverine or coastal floods, the DFO flood maps were overlaid with the location of the rivers taken from the Hydroshed project (Lehner et al., 2008). Only the major rivers were mapped as these will have sufficiently large catchment areas exposing the downstream areas they traverse to (non‐localized) flooding. Riverine and coastal flood bands were then derived by considering DFO flooded areas within 2, 5 and 8 meters elevation difference from the closest point on the major river or the coast using a Digital Elevation Model from GLOBE (GloBe Task team, 1999).

The recorded riverine and coastal floods using bands of 2 m elevation difference for each year from 2001 to 2006 are in Figure 6. First, there is little overlap over the years between the localized and riverine flood maps, confirming the complementary nature of both flood indicators.

> 19 Because the construction of interpolated daily rainfall maps was very data and time intensive, interpolation was limited to the IDW technique only.

> 20 Many floods have now also been imaged by satellite or airborne sensors, and many of them have been translated by DFO into maps of inundation events.  Yet, as these satellite based maps did not cover all floods, partly because at times the images were obscured by cloud cover and partly because not all floods have been imaged, it was decided not to use this data source to ensure consistency.