Long-term trends in rainfall erosivity–analysis of high resolution precipitation time series (1937–2007) from Western Germany
Highlights
► Analysis of 71-year data-set of 5-min precipitation from 10 stations in Central Europe. ► Slight increase in summer erosivity (1937–2007) with steeper trends within the last 35 years. ► Increase in frequency and magnitude of erosive events. ► Erosivity increase in spring and autumn especially amplify potential erosion.
Introduction
It is expected that increasing global warming will have multiple effects on the hydrological cycle (Sivakumar, 2011). In most regions an increase and in some a decrease of precipitation is projected (IPCC, 2007). However, apart from regional differences in rainfall depths, it is generally assumed that global warming will lead to a more dynamic atmosphere which potentially leads to more frequent high intensity rainfall events (Groisman et al., 2004, Nearing et al., 2004). Such potential increase in high intensity rainfall events may lead to a number of (unwanted) side effects, e.g. an increase in local flash floods or muddy floods (Boardman et al., 2003, Verstraeten and Poesen, 2000), and an increase in on-site (Lal, 2001) as well as off-site erosion damages (Bilotta et al., 2007, Haygarth et al., 2006).
To evaluate potential effects of changes in rainfall on erosion, traditionally the annual rainfall erosivity (or R factor) of the Universal Soil Loss Equation (USLE; Wischmeier and Smith, 1960), which combines rainfall intensity and depth, is used. The rainfall erosivity can be understood as a variable representing rainfall energy (derived from rainfall intensity) and surface runoff potential (derived from event-based rainfall depth). Since its introduction in the 1970s in the US, the rainfall erosivity was empirically adapted to other regions (e.g. Larionov, 1993, Schwertmann et al., 1990). Moreover, annual rainfall erosivity calculated as the sum of event-based erosivity, is a valuable proxy variable to evaluate the combined change of high rainfall intensities and depths. Hence, its change is also a generally valuable variable for other hydrological purposes.
Climate data derived from a combination of Global Circulation Models and different statistical or dynamical downscaling methods hardly provide the necessary detailed storm information (time step ≤ 10 min) needed to calculate rainfall erosivity. Therefore, existing evaluations of changes in rainfall erosivity are relatively rare and are based on (i) some long-term, high resolution rainfall data (e.g. Meusburger et al., 2012, Verstraeten et al., 2006) and/or (ii) empirical relations between rainfall erosivity and monthly to yearly rainfall depth (e.g. Diodato and Bellochi, 2009). The latter, however, can only lead to reasonable estimates of changes in erosivity if the assumption of a stable relation between rainfall depth and erosivity holds true under changing boundary conditions.
The main objectives of this study are to use one of the very rare long-term (1937–2007) high resolution (≤5-min) data sets of ten stations in the central Ruhr area in Western Germany for a detailed analysis of: (i) long-term trends in summer rainfall erosivity (here: April–November), (ii) potential changes in frequency and/or magnitude of individual erosive rainfall events, and (iii) shifts in seasonality of rainfall erosivity.
Section snippets
Test area and data
The study area is located in the central Ruhr region in Western Germany ranging from the Lower Rhine Basin in its eastern part to the Westphalian Plain in its western part. In its South, it is bordered by the hills of the Rhenish Massif. The area is relatively flat with altitudes increasing from approximately 30 m a.s.l. in the West to 150 m a.s.l. in the East. In this densely populated and traditionally highly industrialized area, the local water authorities (Emschergenossenschaft and
Results
Following the data processing (Fig. 2) and the homogeneity tests, nine of the existing ten stations were chosen for trend analysis. SNHT indicates that summer rainfall at station Bochum DMT (Fig. 1, Table 1) is not homogeneous with a shift in the data set detected for the year 1975. This shift does not correspond with information regarding changes in measuring equipment and therefore might result from unknown changes in station set-up. Although no inhomogeneity was determined for summer
Discussion
In general, our results indicate a slight but significant increase in summer rainfall erosivity for the overall observation period (1937–2007) with a most pronounced increase during the last 35 years of observation (Fig. 4, Fig. 5). Due to the comprehensive data pre-processing and the multitude of statistical tests, we can be quite confident in these results. These trends are of most importance for erosion studies but they are also relevant for other hydrological studies dealing with extreme
Conclusion
A 71-year data set of high resolution (<5-min) rainfall data from nine stations in the central Ruhr area in Western Germany was used to analyze (i) trends in summer (April–November) erosivity, (ii) changes in the magnitude and frequency of individual erosive events, and (iii) changes in the seasonality of long-term mean daily erosivity.
For the period from 1937 to 2007, we observed a slight but significant increase in summer erosivity of 4.4% (in relation to the mean value) per decade. This
Acknowledgements
This research was carried out within the framework of the project “Effects of global climate change on the spatio-temporal variability in rainfall erosivity in North-Rhine-Westphalia, Germany” funded by the “Innovationsfonds – Anpassung an den Klimawandel in NRW”. Special thanks go to Dr. Andrea Hädicke for her support of the above mentioned project. The contributions to improve the English of the manuscript by Dr. B. Maxfield and the inputs by three anonymous reviewers should be also
References (39)
- et al.
Muddy floods on the South Downs, southern England: problems and responses
Environ. Sci. Policy
(2003) - et al.
Spatio-temporal patterns in land use and management affecting surface runoff response of agricultural catchments—a review
Earth-Sci. Rev.
(2011) - et al.
Increasing occurrence of high-intensity rainstorm events relevant for the generation of soil erosion in a temperate lowland region in central Europe
J. Hydrol.
(2011) A homogeneity test applied to precipitation data
J. Climatol.
(1986)- Anonymous, 2010. Extremwertstatistische Untersuchung von Starkniederschlägen in NRW (ExUS)–Veränderung in Dauer,...
- ATV-DVWK, 1985. Niederschlag - Starkregenauswertung nach Wiederkehrzeit und Dauer, Niederschlagsmessungen, Auswertung....
- Bartels, H., Malitz, G., Asmus, S., Albrecht, F.M., Dietzer, B., Günther, T., Ertel, H., 1997. Starkniederschlaghöhen...
- et al.
Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands
Erosion. Hydrol. Proc.
(2007) - et al.
Streamflow trends in Switzerland
J. Hydrol.
(2005) - et al.
Introduction to Time Series and Forecasting
(1996)