Do multi-year droughts increase floods?

Abstract

Due to climate change, drought-flood alternations have become more frequent and more severe in many countries in the world. However, little research has been done on the relationship between flood and drought. Thus, this research aims to investigate whether a relationship exists between multi-year droughts and the floods that follow in the post-drought period. The hypothesis is that vegetation decreases as a result of long-term drought, which decreases evaporation and increases the proportion of precipitation that becomes runoff, and thus, increases flood. To test this hypothesis, the United States is chosen as a case study area. Hydrological data and remote sensing data are collected for 671 basins in the case study area, from around 1980 till 2014. A list of criteria is established to detect the occurrence of long-term droughts, verify the water balance, and ensure data availability. 83 basins fulfill all criteria and these basins are investigated further. These 83 basins are plotted inside the Budyko space and Fu's equation is fitted to find the free parameter that indicates the runoff ratio. It is found that in general, no increase in runoff ratio is detected after drought. However, looking at the basins individually, changes in the runoff ratio do exist. Further investigations into the cause of these changes are carried out by looking at the effects of drought on the partitioning of hydrological fluxes and separating the basins' movements into climate effects and residual effects. Vegetation changes are also investigated to see whether the runoff changes are related to vegetation changes. The results show that although strong correlations exist between the runoff ratio, evaporative index, and residual effects, no relationship is found between the runoff ratio and the vegetation-related variables. It is concluded that using the method in this research, no significant relationship is found between the multi-year droughts and floods that occur after drought.