Trends in floods in small catchments – instantaneous vs. daily peaks Donna Wilson, Hege Hisdal, Deborah Lawrence Hydrological Modelling Section, Norwegian Water Resources and Energy Directorate

Instantaneous flood peaks Introduction Climate change is expected to cause increased precipitation in Northern Europe, resulting in increased flood magnitudes in many areas. This study compares trends in both the magnitude and frequency of high flow events in small catchments, using both instantaneous and daily data Regional, longer-duration precipitation Local, short-duration precipitation + + Daily average flood peaks Instantaneous flood peaks

Purpose Have floods increased and/or become more frequent in Norway? Improving flood estimates for climate change adaptation.

Aim Investigate spatial and temporal changes in daily and instantaneous values of: (a) the magnitude of annual maxima (AM) flood peaks (b) the frequency of peak-over-threshold (POT) flood peaks (c) ratio between daily and instantaneous values of the AM and POT series’.

Data Pristine small catchments (<60km2) for the period 1980-2011 31 annual maxima stations 24 POT stations Periods of missing data affect a maximum of eight separate years (i.e. 8/32 years). Missing data rarely corresponds with the timing of the largest flood. An inter-event time of 2 days used to identify separate flood events (based on Engeland et al., 2004) AM & POT stations AM stations only

Method Preparation of the AM series Maximum value for each year. Preparation of the POT series Threshold selected to give an average of two events per year over the whole period of record. Examination of autocorrelation in dataseries No stations were found to have significant levels of positive autocorrelation (at the 95% level). Trend analysis using the Mann- Kendall test A non-parametric test for the detection of trend in a time series. It is simple, robust and can cope with missing values.

Results: Trends in annual maxima flood Magnitude of AM flood based on daily data Magnitude of AM flood based on instantaneousdata Positive trends : 23% Negative trends: 13% Positive trends : 19% Negative trends: 10%

Results: Trends in frequency of POT floods Frequency of POT based on daily data Frequency of POT based on instantaneousdata Magnitude of AMF based on daily data Strong positive trend : 21% Strong positive trend : 8%

Results: Trends in ratio of daily and instantaneous peaks Ratio between frequency of daily and instantaneous POT flood events Ratio between magnitude of daily and instantaneous AM flood events

Conclusions Trends in the frequency of high flow events are more systematic than the trend in the magnitude of annual maxima flood events. Similar spatial patterns are obtained when using daily and instantaneous flood peak data. The number of stations showing a trend in both AM and POT series is less pronounced when considering daily as compared to instantaneous data.

Future work Larger dataset of daily average floods Only rain-dominated floods Magnitude of POT flood events