The flood that didn’t come

The period from August 14/15 to 19 2015 brought some exceptionally intense rainfall for parts of Germany. For parts of Saxony, rainfall sums were comparable to the June-2013 Flood.

Let’s make use of weather radar data and draw a first rough comparison (keep in mind that radar data might be erratic to a certain extend and the analyses have been carried out for the purpose of a relative comparison only):

Comparison of catchment-specific rainfall sums for the May/June-2013 precipitation and the recent August-2015 event. Radar data provided by German Weather Service (RADOLAN-RW product). Abbreviations indicate Saxon catchments, i.g., ZM-O3 means parts of the Zwickauer Mulde above an elevation of 300 m, and so forth..

However, a closer look on the regional precipitation distribution reveals partly tremendous differences in the compared rainfields (left: 2013; right: 2014; difference plot in the center). Radar data provided by German Weather Service (RADOLAN-RW product).

Thus, the recent rainfall event was a major heavy/continual rain event. However, hydrological consequences were totally different; whereas by end on May 2013, soils were extensively saturated (or even over-saturated), in 2015 we were starting from an extensive drought situation with very low initial soil moisture.

Comparison of pre-event modeled usable field capacities showing the tremendous differences in the antecedent wetness conditions of the two considered events. Imagery provided by German Weather Service.

This is directly reflected in a comparison of the reached August-2013 flood levels with the recent hydrological situation with no alert level reached at none of the >100 Saxon flood warning gauges.

Number of gauges with reached/exceeded alert levels (AL) in Saxony during the considered events:
2013: (lowest) AL1: 17 — AL2: 32 — AL3: 31 — (highest) AL4: 36
2015: zero (total: 105)

 

2015 post-event field capacity distribution showing the wetting of soils as a consequence of the preceding rainfall event. Image provided by German Weather Service.

Last question: where did the water go? Well, event-specific runoff coefficients were very moderate for the recent heavy/continual rain event (around 20 per cent for larger catchments) and there was almost no impact on the generally falling groundwater levels (which is a consequence of the ongoing drought). Conclusion: a large amount of water went to the soils, which is reflected in the post-event increase of soil moisture/field capacity.