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13 - Flood in the Lower Mekong basin2. Causes and contributing factors to floodingThe hydro-meteorological causes of floods in the Mekong Basin are prolonged, heavy rains onsaturated soils. This leads to an increase in surface run-off that can only slowly be discharged throughthe river system into the South China Sea. Other natural and man-made factors can also affectflooding in the basin.2.1 Climate change: Climate change and global warming are likely to be contributing factors toincreased frequency and intensity of severe flooding, particularly in low-lying coastal and estuaryareas. Warmer water temperatures in the South China Sea may increase the number and intensity oftyphoons, in turn causing sea surges that inundate low-lying areas. Some estimates indicate that15,000-20,000 km 2 of land in the Mekong Delta could be threatened. 7 However if there is lowerrainfall in the basin as a result of climate change, which is possible under some modeling scenarios,there may be decreased flooding in some parts of the basin.2.2 Deforestation: Deforestation and landclearance in upland areas of the basinresults in increased volume and speed ofsurface run-off. This, in turn, contributesto increases in peak discharges and peakwater levels. The impact of these factorson flooding is more pronounced for smalland medium sized floods along tributariesof the Mekong. 82.3 Land degradation: Land degradationoccurs in areas of intensive shiftingcultivation, shortened fallow periods andwhere agricultural systems are not adaptedto topographic and soil conditions. Thisleads to increased erosion and sedimentdeposit in the Mekong and tributaries thatcan reduce drainage capacity, thuscontributing at a regional scale to increasedflood hazards. 92.4 Changes in flood storage capacity:The construction of flood embankments andother man-made structures designed toprotect areas along the Mekong results in aloss of natural floodplain storage capacity.Flood waters continue downstream withoutthe mitigating effects of that storage. Ingeneral, the impact of a single embankmentmay not be that great. However, thecumulative effects of many embankmentscan result in the loss of a significant volumeof floodplain storage that causes increaseddischarge and higher flood levels bothupstream and downstream. 10Deforestation and land clearance in upland areas canincrease the volume and speed of surface run-off andlead to small- and medium-scale flooding in tributariesReclamation and infilling of low-lying wetlands foragriculture and other purposes contributes toflooding in the LMB273