Identifying Patterns in Extreme Precipitation Risk and the Related Impacts

Extreme precipitation can harm human life and assets through flooding, hail, landslides, or debris flows. Flood risk assessments typically concentrate on river or mountain torrent channels, using water depth, flow velocity, and/or sediment deposition to quantify the risk. In addition, extreme events with high recurrence intervals are often the main focus. However, damages from short-term and localized convective showers often occur away from watercourses. Also, damages from more frequent small scale extremes, although usually less disastrous, can accumulate to considerable financial burdens.

Extreme convective precipitation is expected to intensify in a warmer climate, and vulnerability patterns might change in tandem with changes in the character of precipitation and flood types. This has consequences for adaptation planners who want to establish effective protection measures and reduce the cost from natural hazards.

Here we merge hydrological and exposure data to identify patterns of risk under varying synoptic conditions. Exposure is calculated from a database of 76k damage claims reported to the national disaster fund in 480 municipalities in south eastern Austria from 1990-2015. Hydrological data comprise sub-daily precipitation (59 gauges) and streamflow (62 gauges) observations. We use synoptic circulation types to identify typical precipitation patterns. They indicate the character of precipitation even if a gauge is not in close proximity, facilitating potential future research with regional climate model data.

Results show that more claims are reported under synoptic conditions favoring convective precipitation (on average 1.5-3 times more than on other days). We find that Alpine districts are particularly vulnerable to high-intensity convective storms that trigger torrential events and debris flows. Also in agricultural districts most damage claims are reported under convective weather situations. However, long low-intensity precipitation events causing flooding and landslides are also responsible for almost as many damage claims in agricultural districts.

Unlike in stratiform events, streamflow levels do not reflect the risk from isolated convective precipitation events, even if they cause major damages (>50 claims per municipality). The synoptic conditions under which these small scale extremes occur make up only 10% of the days. Yet a disproportional share of approx. 15% of the overall cost can be attributed to these events, underlining the need to include small scale events in risk assessments.

The results indicate distinct patterns of vulnerability within the study area. Thus, measures targeted to increasing resilience should take into account different regional vulnerabilities to specific precipitation conditions and accompanying hydrological hazards.

In addition to possible observational error, uncertainty is present in damage reporting errors, claims from private insurers and adaptation effects after damaging events. As for the latter, preliminary results indicate that investments regularly occur after big events, which may skew subsequent damage claims. Their effectiveness, though, needs to be analyzed in future research.