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Comparing the NDVI of the north and south hillsides of the Natural Reserve of Mounts Belezma by decision tree DEKHINAT S., HADJADJ M.F.& CHAOUI A. 1

University of Batna, Algeria, [email protected]

Abstract. Many works on the Maghreb (North Africa) made reference to the exposure of hillsides, especially in the forest domain, to explain the distribution of the species, and we speak mostly about the hillside North as being the best exposed to the pluviometric influences coming from the méditerranée; and thus the best watered with regard to the South hillside. With decision tree of the software ENVI 4.5, the tool of analysis of satellite images, we were able to make this comparison to the national park of Belezma ( Algeria). Indeed, the coupling of the DEM (digital elevation model) and multispectral images Landsat (B4, B3, B2, B1) of the mission 2003 showed that there is a considerable difference at the level of the occupation of the grounds of the chlorophyllous vegetation between the Northwest and Southeast hillsides. Key words: Forest, Exposure, Distribution, Coupling, DEM, Landsat. 1. Intoduction The NDVI (Normalized difference vegetation index) is usually used in the studies which are interested in the liveliness of the plant place setting. His application in several studies on the forest areas by means of multiple satellite platforms (NOAA/AVHRR (Advanced Very High Resolution Radiometer), MODIS (Moderate Resolution Imaging Spectroradiometer), Landsat TM (Thematic Mapper), ETM+ (Enhanced Thematic Mapper Plus), and Spot/Vegetation) showed the efficiency of this algorithm in the measures and the prediction of the evolution spatio-temporal of plants. The DEM stands out as major tools for the modelling of the space thanks to the power of calculation of the software as ArcGis [1]. The data of levelness and relief can be exploited by mathematical models to generate maps on the orientation, the visibility, the slope and the aspects of the relief. So, many authors used the digital model of ground for their diverse works to be known about spatializing, modelling of reliefs and structural analysis [2, 3, 4]. In this study, we are going to test this technology, by coupling the DEM and the satellite imaging Landsat TM, to calculate, on one hand, the total surface of the chlorophyllous vegetation situated unless 20 % of slope of the national park of Belezma, and to compare, on the other hand, the occupation of the grounds of the North and South hillsides, to verify the vegetable situation of every hillside. 2. Description of study area The park is located in the mounts of Belezma (Algeria)( Fig.1) with a surface of 274297474 m2. It is situated between the UTM coordinates: 3930 and 3940 km North, and 760 and 790 km east. The climate is semi-arid. 3. Methods The satellite images are widely used in the domains of the forestry, the hydrology, the occupation of grounds, etc., for the modelling and the mapping of the natural surfaces. The product Landsat of 30 of resolution lends itself to the analysis by pixel of the vegetation which has a chlorophyllous activity thanks to algorithms Band Math and Decision Tree of ENVI 4.5. The NDVI is calculated by using the reflectance of the red band (0,58 - 0,68 µm) and of the infrared band(0,73-1,10 µm) respectively B4 and B3:

Fig. 1: The study area

NDVI= B4 - B3/B4 + B3 [5]. To calculate the slope and the exposure, we used the digital elevation model ( shuttle radar topographic mission , USGS) of 30 m of resolution. To calculate the NDVI we used the multispectral images Landsat ( B1, B2, B3, B4, B5, B7). The images and the DEM are georeferenced in UTM, ellipsoïde WGS 84 to work in ideal conditions of overlaying. The dimension of pixels is not binding even if it is different from a file in the other one. 4. Results The model (decision tree) can be executed in the following way (Fig.2):

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Fig.1: Decision tree • Class 1: NDVI above 0.3, slope greater than or equal to 20 degrees • Class 2: NDVI above 0.3, slope below 20 degrees, non-North-facing aspect • Class 3: NDVI above 0.3, slope below 20 degrees, North-facing aspect • Class 4: NDVI less than or equal to 0.3, band 4 greater than or equal to 20 • Class 5: NDVI less than or equal to 0.3, band 4 below 20 • Class 6: band 4 equal to 0 5. Discussion Figure 3 express the total NDVI of the natural reserve of Belezma. It is about a temporal situation of 2003, date which corresponds at the time of the recording Landsat. The darkest zones are relative to a NDVI upper to 0,3 and are situated in a slope lower than 20 % (accessible to the leisure activities). The exposure was chosen according to the orientation of the wrinkling that is Southwest Northeast and the influence of rains. The NDVI is vectorised then transferred in ArcGis 9.3. The sum of surfaces is executed with map calculator. The obtained results are:
Total surface of the park: 274297474 m2
Surface of the NDVI: 83916598 m2 (31 % of the total surface).
The North hillside: 59580784 m2 ( 70 % of the surface).
The hillside Southwest: 24335813 m2 (30 % of the surface). The NDVI is well enough represented in the park of Belezma (31 % of the surface of the park). This demonstrates that the park contains a long-lived plant place setting in spite of a dry climate especially in summer. This vegetation seems correlated to a relatively good pluviometry (+ of 300 mm on average a year); moreover this last one plays a large part in the chlorophyllous activity [6].

Fig.2 : global distribution of NDVI

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The comparison between hillsides (Fig.4 and 5) confirms the thesis according to which the vegetation develops better on facades exposed to the influences of the North (70 % of the NDVI). The opposite part is less provided on the cover vegetable (30 % of the NDVI) because of the influences of meteorological agents which come from the South (warm winds, the deposits of sands). Moreover, the major part of the forest species begins at 1500 m in South side; contrary to the North part where it begins at 1000 m.

Fig.4 : NDVI of North area

Fig. 5 : NDVI of South area

6. Conclusions The coupling DEM with images Landsat showed that the park of Belezma contains rather important vegetable surfaces in spite of the aridity of the region. The comparison of both North and South hillsides confirmed that the face turned to the méditerrannée is more favored than the one who undergoes the meteorological actions of the South. References [1] Kamp U., Benjamin J. Growley , Ghazanfar A. Khattak , Lewis A. “GIS-based landslide susceptibility mapping for the 2005 Kashmir earthquake region”. Geomorphology, 2005, 101: 631–642. [2][Murphy W. “Remote Sensing of active faults: case studies from Southern Italy”. Z. Geomorphol. N. F., Suppl. Bd., 1993, 94: 1 –23. [3] Wdowinski S., Zilberman, E. “Systematic analyses of the large-scale topography and structure across the Dead Sea Rift”. Tectonics, 1997, 16: 409– 424. [4] Zhou G. , T. Esaki, Y. Mitani, M. Xie, J. Morib.”Spatial probabilistic modeling of slope failure using an integrated GIS Monte Carlo simulation approach”. Engineering Geology,2003, 68: 373–386. [5] Julien Y., José A. Sobrino, Wout Verhoef. “Changes in land surface temperatures and NDVI values over Europe between 1982 and 1999”. Remote Sensing of Environment, 2006, 103: 43–55. [6] Boniface O., Rolf A de By and Andrew K Skidmore. “Interannual variability of NDVI and bird in Kenya species diversity”. JAG , 2000, 2: 31- 53.

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