Meteorological characteristics, postfire effects and soil erosion risk in the lake Marathon basin (Attica, Greece), using the PESERA model.

Transcription

1 C O M E C A P e-book of proceedings ISBN: Vol 1 Page 38 Meteorological characteristics, postfire effects and soil erosion risk in the lake Marathon basin (Attica, Greece), using the PESERA model. Alexiou D.S., Papanikolaou I., Kairis O., Tsiros I. Soil erosion risk assessment in the Marathon artificial lake basin in Attica, Greece was estimated through extended geomorphological analysis and a comparative study on soil erosion before and after the severe wildfire of 21 st August 2009 (data for the years 2008 and 2011 respectively). The assessment of the spatial distribution of soil erosion was achieved through the well-known and semi-detailed European PESERA soil erosion model. The Marathon basin has been significantly affected by fire events resulting on topsoil losses. In respect to this, the differentiated meteorological characteristics seem to partially control the erosion rates as shown by the higher erosion rate of the unburned area in 2008, in comparison to the year For example in 2008 the extreme upper and lower values of erosion rates extend to a wider area compared to the 2011 values. There are 8.6km 2 of additional area in 2008 characterized of negligible or no erosion and 11.9km 2 of additional area characterized of very high erosion risk compared to the 2011 values. Overall, moderate erosion rates are severely stretched in 2008 in comparison to the Meteorological parameters and the post fire effects can explain these effects. These areas may define environmentally sensitive areas to desertification. Alexiou D.S. 1*, Papanikolaou I. 1, Kairis O. 2, Tsiros I. 3 1 Laboratory of Mineralogy and Geology, Agricultural University of Athens, Iera odos 75, , Athens, Greece 2 Laboratory of Soil Science and Agricultural Chemistry, Agricultural University of Athens, Iera odos 75, , Athens, Greece 3 Laboratory of General and Agricultural Meteorology, Agricultural University of Athens, Iera odos 75, , Athens, Greece *corresponding author alexiousimoni@gmail.com

2 C O M E C A P e-book of proceedings ISBN: Vol 1 Page 39 1 Introduction The Marathon basin is one of the most important basins for the Attica region due to the Marathon artificial lake and the extended drainage which is being regarded as the main water resource for the wider area. This lake has recreated the surrounding area, forming a valuable environmental zone. A dam was constructed in 1929 to water Athens, which resulted in significant environmental alterations. The generally degraded environment and the differentiated land-uses in regard with the increase of the extreme fire events make the soil erosion an important issue. The studied area is mainly covered by cultivated fields which are very important for the far-reaching economy. The reduction on soil productivity caused by the enhanced soil erosion makes the prevention s measures a necessity. In this study, a correlation has been established between the soil erosion, the increase of fire events, the geomorphology and the climatic characteristics of the Marathon basin. The combination of the PESERA erosion model with the GIS system was applied before and after the fire of 21 st of August 2009 by inserting the available geomorphological, geological, soil, satellite, forest and climatic data followed by methodical field work. 2 Studied area The Marathon basin is characterized by a complex relief and drainage network as a result of active tectonics processes, meteorological characteristics, erosion and human interventions. The hydrographic network is complicated because of the existing active faults and the artificial junction between the Haradros and Varnavas rivers after the construction of the dam. It has to be referred that the Afidnes fault is the main active tectonic structure which is responsible for the present day setting of Haradros basin. Due to Afidnes fault there is being noticed headward erosion within the footwall catchments producing perpendicular flow directions in the hanging wall. In addition, due to the fact that the Haradros s basin is placed on the hanging wall and as a result it is being subsided, several catchments are clearly deflected into a fault parallel flow direction (Papanikolaou and Papanikolaou 2007, Ganas et al. 2005). The main area is covered mostly by post-alpine sediments of Late Miocene, and minor outcrops of Pleistocene and Holocene alluvial (Mettos 1992). Nevertheless, the Marathon basin comprises also both metamorphic and non-metamorphic rocks. The Almyropotamos unit constitutes the metamorphic basement, while the Afidnes and Katsimidi units comprise the non-metamorphic rocks. A narrow zone with low-grade metamorphic rocks (Marathon s schist and marbles) was observed along the tectonic contact of the metamorphic and the nonmetamorphic units with no correlation to each other (Papanikolaou et al. 2004). 2.1 Methodology The delineating of the burnt area was a primary target so, the NDVI factor was chosen (Rouse et al. 1973). The algorithm was executed through the ERDAS Imagine 2010 software by processing the satellite images (Landsat TM5) of May and September For the soil erosion estimation the PESERA model was selected since it is a spatial distributed model developed for quantifying soil erosion in environmentally sensitive areas based on a European scale (Kirkby et al. 2003). This method combines primary data such as the DEM, the soil depth, the soil grain size analysis, the regional climatic data and the slope map which through a close-box process, results in the final erosion risk map. For the year after the fire the climatic data were not the only fields that it had to be changed. The coverage percentage field for every month and the use type for the demarcated burnt area field was also affected by the degradation of the vegetation coverage. In the studied area other erosion

3 C O M E C A P e-book of proceedings ISBN: Vol 1 Page 40 models have also been applied. Xanthakis (2011) applied the RUSLE equation, the RMMF model and the Gavrilovic method in the Marathon basin. The required data were also collated from field surveys and remote sensing techniques. Overall, the upcoming conclusions were extracted from the comparison of both erosion maps and evaluated through systematic field work. 3 Results The fire of 21 st of August 2009 lasted 4 days destroying acres. It was characterized as an unprecedented fire event for the Attica region. The human interventions in the drainage by constructing the dam imprinted on the reduction of the principal stream s class, the number of streams by class, the change in streams s length by class and the change in hydrographic density. The remarkable alteration in land uses in the last decades leads to environmental demoting and increase of runoff. According to Xanthakis and Xanthopoulos (2007) there was high increase of agricultural land and a significant decrease of wetlands and forests from 1880 to This fact combined to the extraordinary raise in fire events (Migiros et al. 2010) has resulted in reduction on vegetation s productivity. More specifically, an abandonment of arable land due to declining productivity is a land use change that may result from soil erosion (Kosmas et al. 2000, Bakker et al. 2005). The PESERA model was applied two years after the wildfire, so the burnt area was not regarded as bare land; on the contrary the vegetation coverage had been differentiated enough from In 2011, it was noticed that there was a significant ecosystem s recovery which according to the satellite images of that year fluctuated between 20-25%. For example, the previous burnt forestry areas had been replaced by scrubs. There is an important diversification between the soil erosion of the year 2008 compared to the year 2011(Fig. 1). In 2008, the total basin shows higher percentages of potential eroded zones compared to 2011, due to different rainfall heights. Fig. 1. Soil erosion risk map for the years 2008 and 2011 using the PESERA model. Based on meteorological data, in 2008 the rainfall was higher for about 120mm (Fig. 2). Additionally, in 2011, more rainfall was distributed on March-June (Fig. 3). During this period, there was enough vegetation to decrease the energy of falling water, the runoff and consequently, the erosion. On the contrary, during the year 2008, the main rainfall was located on the autumnal-hibernal period (October-December) in which, most of the region was bared of vegetation and the topsoil coverage had been exposed. It is clear that the burnt area corresponds to an area of high erosion risk due to lack of vegetation.

4 C O M E C A P e-book of proceedings ISBN: Vol 1 Page 41 Fig. 2. Rainfall distribution for the year 2008 Fig.3. Rainfall distribution for the year Discussion- Conclusions The geomorphology of the basin is being affected by the active tectonic faults and also, by the human interventions regarding, principally, to the construction of the Marathon dam. Such dam is an important engineering project, because it has been fully integrated into the environment, forming the microclimate of the entire Attica, demarcating an area of environmental protection and defining a crucial environmental zone between the National Parnitha s forest, the Penteli s massif and the Southern Euboea s Gulf. The recent fire events in NE Attica have increased the soil erosion processes. The regional climatic characteristics enhance the risk and some precaution measures need to be taken. The annual runoff is being correlated with the annual erosion risk. In respect to this, the PESERA method is a spatial analysis model which is defined by the primary data that are being entered to the algorithm. The soil erosion risk maps that have been exported by the PESERA model are being regarded as accurate because of the detailed processing of the embedded data. The slope factor is being taken into such a serious consideration, so to characterize some forest fields as high risk areas due to slope value greater that 25 o. It has to be noticed that when a correlation of the erosion risk per class (both extracted by the PESERA model) was made for both years, it was clarified that the year 2011 was characterized by less percentage of high and extremely high erosion risk zones, even though it would be expected that after the fire the greater part of the basin would be of extreme erosion risk. This could be explained by the decrease in the rainfall of the year 2011 in comparison to the rainfall rates of Tables 1 and 2. Soil erosion risk classes expressed as percentage of the total area for 2008 and Erosion (2008) Area (Km 2 ) Area (%) No Erosion 35,6 30,48 Very Low 3,5 3,03 Low 1,3 1,09 Moderate 3,7 3,17 High 12,6 10,8 Very high 60,2 51,43 Erosion (2011) Area (Km 2 ) Area (%) No Erosion 27 23,9 Very Low 16,8 14,3 Low 9,5 8,1 Moderate 8,5 7,3 High 5,8 5,1 Very high ,3 These changes on the erosion classes of 2011 correlated with the ones of the year 2008 are being imprinted in the map below (Fig. 4). A large part of the map is being characterized as an area with no alteration on soil erosion risk s classes, even within the burnt area of Generally, the part of the map that remained unchanged is mainly related to the forestry areas. The most vulnerable sites correspond to agricultural land because of the agricultural practices which reduce the vegetation coverage and leave the ground almost bare.

5 C O M E C A P e-book of proceedings ISBN: Vol 1 Page 42 Fig. 4. This map indicates the changes on soil erosion risk s classes for the year By examining only the burnt area a significant alteration in erosion classes is being observed. In tables 3 and 4, all classes have increased in 2011 apart from the high erosion risk class, which has decreased. The affected areas with higher erosion rates correspond to cultivated areas, grasslands and scrubs, while some forestry areas have not been affected by the change due to the adequate soil depth. Tables 3 and 4. Soil erosion risk classes expressed as percentage of the burnt-only area for 2008 and Erosion (2008) Area (Km 2 ) Area (%) No Erosion 0,9 4,5 Very Low 0,1 0,7 Low 0,7 3,8 Moderate 1,2 6,1 High 5,7 29,8 Very high 10,6 55,1 Erosion (2011) Area (Km 2 ) Area (%) No Erosion 1,1 5,6 Very Low 0,3 1,7 Low 1,3 6,8 Moderate 2 10,4 High 1,1 5,9 Very high 13,4 69,6 During field work the severe soil removal in the basin has been noted in a small artificial dam in the Spartorrema river, NE of the Marathon s lake, which was constructed by gabions soon after the fire event to prevent the soil transfer downstream. A significant increase on the sediments was observed after the intense flood season on December 2012 and January It is estimated that the sediments height in this small dam was raised with a mean range of 50cm over the last 3,5years. Both the PESERA (this paper) and the RUSLE models (see Xanthakis 2011) recognized and highlighted the spatial distribution of soil erosion risk. Controlling erosion rates is of major importance in order to prevent the diminishing soil productivity and to proceed to a rational management of the Marathon s artificial lake. a) b) Fig. 5. The first photo displays the first flooding event after the construction of the dam, while the second photo represents the status of the dam on January 2013; the apparent raise on sediments height is very important for the validation of the soil erosion risk s models. In conclusion, all these methods require validation to standardize the differences between abundant combinations of different soil types, meteorological and geomorphological characteristics.

6 C O M E C A P e-book of proceedings ISBN: Vol 1 Page 43 References Bakker MM, Goversb G, Kosmas C, Vanackera V, Kristof van Oostb, Rounsevella M (2005) Soil erosion as a driver of land-use change. Agriculture, Ecosystems and Environment 105, (DOI: /j.agee ). Cosmas K, Gerontidis St, Marathianoy M (2000). The effect of land use change on soils and vegetation over various lithological formations on Lesvos (Greece). Catena, 40, (DOI: /S (99) ) Ganas A, Pavlides S, Karastathis V (2005). DEM-based morphometry of range front escarpments in Attica, central Greece and its relation to fault slip-rates, Geomorph., 65, Kirkby MJ, Jones RJA, Irvine B, Gobin A, Govers G, Cerdan O, Van Rompaey AJJ, Le Bissonnais Y, Daroussin J, King D, Montanarella L, Grimm M, Vieillefont V, Puigdefabregas J, Boer M, Kosmas C, Yassoglou N, Tsara M, Mantel S, Van Lynden GJ, Huting J (2004) Pan-European soil erosion risk assessment: the PESERA map, version 1 October Explanation of Special Publication Ispra 2004 no.73 (S.P.I.04.73). European Soil Bureau Research Report no. 16, Luxembourg, EUR 21176, p. 18. Mettos AI, (1992). Geological and paleogeographical study of the continental Neogene and Quaternary deposits ofne Attica and SE Beotia, Unpublished Ph.D. thesis, Department of Geology, University of Athens. (in Greek) Migiros G et al, (2010). A complete management of the crisis after the devastating fire of 2009 in the Agios Stefanos Municipality: Report and assessment of the current situation, planning for restoration and protection. Mun, of Agios Stefanos, Attiki. Papanikolaou D, Papanikolaou I, (2007). Geological, geomorphological and tectonic structure of NE Attica and seismic hazard implications for the northern edge of the Athens plain. Bulletin of the Geological Society of Greece 40, Papanikolaou DI, Lozios SG, Soukis K, Skourtsos E (2004). The geological structure of the Allochtonous "Athens Schists", Bulletin of the Geological Society of Greece 36, Rouse, J.W., Haas, R.H., Schell, J.A. and Deering, D.W. (1973). Monitoring vegetation systems in the Great Plains with ERTS. In 3rd ERTS Symposium, NASA SP-351 I, Xanthakis M (2011). A study on soil erosion in mountainous basins with current technological tools. Ph.D., Harokopio University of Athens, p.228 Xanthakis Μ, Xanthopoulos G (2007). Determination of land use changes in the Marathon basin during a century by using the G.S.I.. 13 o Pan-Hellenic Forestry Conference, vol. ΙΙ,