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Delft University of Technology (TU Delft) 2020

The aftermath of multiyear droughts

van Terwisga, Stijn

Titre : The aftermath of multiyear droughts

Auteur : van Terwisga, Stijn

Université de soutenance : Delft University of Technology (TU Delft)

Grade : Master of Science in Civil Engineering 2020

Résumé
In this report the hydrological response to multiyear droughts has been researched. Long-term droughts have resulted in an increase in tree die-off in affected areas. As the hydrological response is related to the vegetation in a catchment, it is of interest to find out whether catchment response regarding rainfall-runoff is different after a long-term drought relative to the situation before the drought. It is important to know this, because a good prediction of water availability is required for making decisions regarding water resources. The research is conducted in Australia, where a long-term drought took place from 1997 up to 2008. The two research questions that have been investigated during this research are : 1) Is there a change in runoff ratio (the fraction of precipitation that becomes runoff) in catchments after a long-term drought relative to the situation before the drought ? 2) Is there a change in the root-zone storage capacity (the amount of storage available in the root-zone of the soil) determined with the mass curve technique following a multiyear drought ? In this research, the runoff ratio observed after the drought has been compared to the runoff ratio before the drought. This is done to determine whether a change has taken place in the rainfall-runoff relationship of the catchment. In the catchments where the rainfall-runoff relationship shows a change from the situation before the drought, a decrease in runoff ratio is mostly observed. In 54 out of the 196 analyzed catchments, the runoff ratio is lower than the 10 percentile of the runoff ratio before the drought. In 27 catchments, the runoff ratio after the drought is above the 90 percentile of the runoff ratio before drought. In most of the studied catchments no clear difference in the rainfall-runoff relationship can be found due to a long-term drought. Factors that were found to play a role in the differences between the responses to a drought were the drought severity, which is a combination of the drought length and the intensity of the drought, and the seasonality of the precipitation. Catchments that had an increase in runoff ratio were more likely to have a longer drought period and a summer-based precipitation pattern. For the second research question, the root-zone storage capacities have been determined using an earlier derived mass curve technique method, combined with a method that has been developed for recovering ecosystems. In order to compare root-zone storage capacities before and after a drought, a variable is introduced that describes the differences between the values for the root-zone storage capacities before and after the drought. Catchments that showed an increase in the runoff ratio were also more likely to have a negative change in the root-zone storage capacity. This indicates that the root-zone storage capacity decreases in catchments that show an increase in the runoff ratio after drought.

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