R-factor and Erosivity Density in Greece

Rainfall erosivity considers the effects of rainfall amount and intensity on soil detachment. Rainfall erosivity is most commonly expressed as the R-factor in the Universal Soil Loss Equation (USLE) and its revised version, RUSLE. Several studies focus on spatial analysis of rainfall erosivity ignoring the intra-annual variability of this factor. This study assesses rainfall erosivity in Greece on a monthly basis in the form of the RUSLE R-factor, based on 30-minutes data from 80 precipitation stations covering an average period of almost 30 years. The spatial interpolation was done through a Generalized Additive Model (GAM). The observed intra-annual variability of rainfall erosivity proved to be high. The warm season is 3 times less erosive than the cold one. November, December and October are the most erosive months contrary to July, August and May which are the least erosive. The proportion between rainfall erosivity and precipitation, expressed as erosivity density, varies throughout the year. Erosivity density is low in the first 5 months (January – May) and is relatively high in the remaining 7 months (June – December) of the year.  The R-factor maps reveal also a high spatial variability with elevated values in the western Greece and Peloponnesus and very low values in Western Macedonia, Thessaly, Attica and Cyclades. The East-West gradient of rainfall erosivity differs per month with a smoother distribution in summer and a more pronounced gradient during the winter months. The aggregated data for the 12 months result in an average R-factor of 807 MJ mm ha-1 h-1 yr-1 with a range from 84 to 2825 MJ mm ha-1 h-1 yr-1 . The combination of monthly R-factor maps with vegetation coverage and tillage maps contributes to better monitor soil erosion risk at national level and monthly basis.

Monthly maps of rainfall erosivity in Greece

The 12 maps of monthly rainfall erosivity in Greece show a gradient of high erosivity in Western Greece, Ionian Islands, Peloponnesus and western Crete to lower erosivity in Northern Greece, Thessaly, Attica and the Cyclades islands. The spatial resolution of the maps is at 100m as all selected covariates (WorldClim, DEM) were available at this resolution.  The highest mean R-factor is noticed in November (144.6 MJ mm ha-1 h-1 month-1), December (136.2 MJ mm ha-1 h-1 month-1) and October (111.8 MJ mm ha-1 h-1 month-1) while the lowest values are found in August (32.3 MJ mm ha-1 h-1 month-1), July (33.3 MJ mm ha-1 h-1 month-1), May (36.3 MJ mm ha-1 h-1 month-1) and June (37.1 MJ mm ha-1 h-1 month-1). The rainfall erosivity is almost 3 times higher during the cold and wet season (October – March) compared to the warm and dry season (April – September). However, there are regions such as Western Macedonia and Thessaly where mean summer rainfall erosivity is almost equal to mean winter rainfall erosivity. On the contrary, western part of Greece, Ionian Islands, Western Crete and Dodecanese show a pronounced seasonal variability with mean winter R-factor 4-5 higher than the summer one.

The aggregated annual R-factor map of Greece has an average value of 807.4 MJ mm ha-1 h-1 yr-1 and a standard deviation of 527.7 MJ mm ha-1 h-1 yr-1. The range of R-factor in Greece is 84.2 – 2,825 MJ mm ha-1 h-1 yr-1 with few high values over 2,000 MJ mm ha-1 h-1 yr-1 (in western Greece).

Monthly erosivity density

Each monthly erosivity dataset is divided by the corresponding monthly precipitation dataset resulting in monthly erosivity density datasets. Erosivity density values higher than 1 mean that a certain precipitation amount may cause relatively higher rainfall erosivity. The erosivity density values are directly proportional to the average monthly 30-min rainfall intensity (Foster, 2008) and thus indicate the seasonal variation in rainfall intensity at a location. . The seasonal variability of erosivity density is also very high as the summer has the highest mean erosivity density with 1.89 MJ ha−1 h−1 followed by autumn with 1.36 MJ ha−1 h−1, winter with 0.85 MJ ha−1 h−1 and spring with 0.78 MJ ha−1 h−1

 

 

 

 

 

 

 

 

 

 

 

Data

The Rainfall Erosivity and the other climatic data is in Raster format. The public user can download the following datasets a) Rainfall erosivity in Europe (R-factor) b) Erosivity Density c) The standard error of the estimates d) The R-factor in Switzerland (as calculated in 2012) e) Monthly R-factor maps of Greece and seasonal Erosivity Density. To get access to the data and the code, please compile the request form; instructions will then follow how to download the datasets.

References

A complete description of the methodology and the application in Europe is described in the paper:
Panagos, P., Ballabio, C., Borrelli, P., Meusburger, K.  2016. Spatio-temporal analysis of rainfall erosivity and erosivity density in GreeceCatena, 137, 2603, pp. 161-172

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