Economic impact upon agricultural production from extreme flood events in Quang Nam, central Vietnam

Quang Nam province, central Vietnam, is situated within the tropical monsoon and typhoon zone of south-east Asia and is susceptible to extreme floods. Historical water level data from 1976 to 2009 for the Vu Gia-Thu Bon river system have been used to simulate flood frequency, concluding that the floods experienced in 2004, 2009 and 2007 were congruent with 1:10-, 1:20- and 1:100-year floods, respectively, all occurring within the last decade (Institute of Geography 2012; Institute of Water Resources Planning in Review and update the flood prevention plan for central provinces: Vu Gia-Thu Bon river. Water Resources Planning Institute, Hanoi, 2011). Since the most productive agricultural land is concentrated along the low-lying sections of river systems, losses to agriculture in extreme flooding can be significant. Using ex-post data, this study estimates the direct damage to agricultural production caused by three flood classes, 1:10-, 1:20- and 1:100-year floods in Quang Nam. Utilising geo-spatial inundation maps together with the timing of the floods with respect to crop rotation, calculation is made of flood-depth susceptibility rates for the four main crop types. These susceptibility rates are then applied to calculate the damage value and also the percentage loss in value for the four crop types under the three flood classes. Benefit-cost ratios were calculated under ‘with’ and ‘without’ extreme flood events. In addition, both scenario and sensitivity analyses were conducted. The estimated value of direct losses to the four main crops for a 1:10-, 1:20- and 1:100-year flood is approximately VND22 billion, VND115 billion and VND147 billion, respectively. These represent a percentage loss in value in the inundated areas for 1:10-, 1:20- and 1:100-year floods, of 12, 56 and 62 %, respectively. Benefit-cost ratios, already very low for subsistence farmers, are further eroded in years of extreme floods, with many farmers experiencing a net loss. This study will help to inform flood management decision-makers in central Vietnam.