Investigation of relationships among the environmental factors and water erosion changes using EPM model and GIS

Transcription

1 International Research Journal of Applied and Basic Sciences. Vol., 3 (5), , 2012 Available online at www. irjabs.com ISSN X 2012 Investigation of relationships among the environmental factors and water erosion changes using EPM model and GIS Maryam Barmaki 1*, Ebrahim Pazira 2, Najaf Hedayat 3 1 Ph.D. Student in soil science, Science and Research Branch, Islamic Azad University, Tehran- Iran 2 Member of scientific world, Science and Research Branch, Islamic Azad University, Tehran-Iran 3 Member of scientific world, Islamic Azad University, Dezful-Iran *Corresponding Author barmaki2355@yahoo.com Abstract Soil erosion and as a result of that, sedimentation process particularly where the topography involves rugged mountainous regions in the watersheds like those in Iran. The case in point is the Khiav Chay Watershed, in the Ardabil Province of Iran. The methodology involves assessing the environmental factors and water erosion relationship, focusing on topography, soil & ground, vegetation and human factors. NDVI, land use and drainage density were studied, related maps were then produced and compared in two time series using aerial photos (1968) and satellite images (2007) to determine the influence of environmental factors on soil erosion. EPM model was also used to estimate the specific erosion. Keywords: Soil erosion, environmental factor, EPM, GIS Introduction Soil erosion by hydraulic flow is of the challenging hydraulic and environmental engineering phenomenon which not only erodes an astronomical volume of highly fertile soils of sloppy lands on the skirts of mountain ranges but is also emerging as one of global destructive land degradation problems (Eswaran et al., 2001). This is becoming particularly critical in mountainous Iran, where the mean soil erosion rate is estimated at 20 ton per acre per year, yielding annual sediment amounting to about 2 billion cubic meters (UNDP, 1999). This happens as Iran is situation in a place where more than 70% of its total area is exposed to soil erosion. Water erosion is controlled by climatic characteristics, topography, soil properties, vegetation, and land management. Detachment of soil material is caused by raindrop impact and drug force of flowing water. Detached particles are transported by overland flow (sheet- or interrill erosion) and concentrated flow (rill erosion) and deposited when flow velocity decreases (Lal, 2001). Gullies by nature can develop as enlarged rills, but their genesis is generally more complex, involving sub-surface flows and sidewall processes (Bocco, 1991). Satellite images and the parameters derived from their composite bands such as Normalized Difference Vegetation Index (NDVI) (Momeny and Saradjian, 2007 and Jong et al, 1999) and Geographic Information System (GIS) are universally applied in various research studies involving soil erosion (King et al Kheir et al, 2007 and Miller et al, 2007). Various types of water-related soil erosion typical of the area under investigation are observed. The aim of this study is to find the relationships among environmental factors influencing soil erosion in Khiav Chay watershed and estimate the water erosion in two time series, the results of which are compared to assess the erosion risk.

2 Materials and Methods The study area is located in the southeastern regions of Sabalan Mountain ranges in the Ardabil Province situated between longitudes 47-38' to 47-50' and latitudes 38-12' to 38-32'. It covers approximately 800 km 2 with 2325 m.s.l of mean elevation. The mean annual precipitation is mm, increasing with elevation. The outlet of watershed is connected to Garasu River which collects all the runoff flows in the water basin. Research for this study was as follows: 1- Investigation of variable environmental factors causing water erosion in a period between 1968 and 2007 Vegetation cover, land use and drainage density are three important variables affecting the soil erosion which studied in this research. 1-1 Vegetation cover Vegetation cover is one of the very changeable factors during the time. During 39 years ( ), vegetation cover has undergone fundamental change spatially and has shown to have decreased significantly. Various methods, namely the Aerial photography, satellite images and NDVI have been applied to assess the vegetation cover condition. Vegetation maps were prepared in two time series, in the first time (1968) using aerial photos and in the second time (2007) the satellite image, GIS software and remote sensing capabilities to reach out. Prepared NDVI for the region was calculated and classified according to equation [1] which was performed by the application of GIS and ILWIS software, bands 3 and 4 of Landsat images and "NDVI" algorithm. ETM 4 ETM 3 [1] NDVI = ETM 4 + ETM 3 Where, ETM3 and ETM4 are visible (red) and near-infrared region in Landsat images respevtively. This relatively simple algorithm produces output values in the range of -1.0 to 1.0. Increasing positive NDVI values, shown in increasing shades of green on the images, indicate increasing amounts of green vegetation. NDVI values near zero and decreasing negative values indicate non-vegetated features such as barren surfaces (rock and soil) and water, snow, ice, and clouds. Figure 1. NDVI map (1968) Figure 2. NDVI map (2007) 2-1 Land use Land use describes various ways in which land resources are controlled and managed. The land use changes in two time sections of 1968 and 2007 were studied. Land use maps in 1968 were prepared from the aerial photos at a scale of 1:20,000 and in 2007 were generated by the use of satellite images, Landsat

3 Enhanced Thematic Mapper (ETM+) data. Five different users consisting of the ranges, agricultural, gardens, residential and rocky lands were then identified. Figure 3. Land use map (1968) Figure 4. Land use map (2007) 3-1 drainage density Drainage density is the total length of all the streams and rivers in a drainage basin divided by the total area of the drainage basin. It is a measure of how well or how poorly a watershed is drained by stream channels. It is equal to the reciprocal of the constant of channel maintenance and equal to the reciprocal of two times the length of overland flow. Drainage density maps in two time series of 1968 and 2007 were prepared by using topographic maps (1:50,000 scale, 20 m contour interval), aerial photos and satellite images in GIS environment. Figure 5. Drainage density map (1968) Figure 6. Drainage density map (2007) Assessment of soil erosion by using EPM model The erosion potential method is a model for qualifying the erosion severity and estimating the total annual sediment yield of a catchment area. This model is initially developed in Yugoslavia by Gavrilovic (1988). EPM considers six factors such as; surface geology and soils, topographic features, climate (including mean annual rainfall and mean annual temperature) and land use. From these factors; exposed rock and soil,

4 topography and climate are limited in natural class, but land use effect is depended on the human activities. The erosion potential method calculates the coefficient of erosion and sediment yield (Z) of a catchment area using the following equation: Z = y * Xa * ( + I 0.5 ) [2] Where, y, Xa, is the land use coefficient, from 1.0 to 0, is the coefficient value for the observed erosion processes, from 1.0 to 0.1, based on the severity of erosion. The factor I is the average land slope in percentage (Solaimani, 2007). For sediment production, the following equation is used: Wsp=T*H**Z 1.5 [3] Where, Wsp, is the average annual specific production of sediment per m3/km2/y, T is temperature coefficient, which is calculated as: T= (t/10+0.1) 0.5 [4] With t = the mean annual temperature in degrees Celsius, H = the mean annual precipitation rate in mm/y, and Z is the coefficient of erosion calculated from Eq. 2. Figure 7. Specific erosion map in 1968 Figure 8. Specific erosion map in Comparison of soil erosion s result in two time sections At this stage, results of soil erosion in two time sections were compared to determine the changes. Results According to analyzes of land use maps in mentioned two time sections, the percentage of range land has changed from 92.23% in 1968 to 88.25% in 2007, agricultural land from 2.7% in 1968 to 6.8% in 2007 and residential land from 0.16% to 0.33%. Based on these findings, it can be observed that the emerging population increase of the last four decades show has brought with it corresponding increased for the land allocated for residential dwelling at the detriment of the range land which has seen a significant decline. Drainage density was 2.96 km/km 2 in 1968, whereas its corresponding figure considerably increased to 4.16 km/km 2 in 2007, which suggests the prevalence of rill erosion in the area under investigation. The results concerning the risk associated with erosion (Z coefficient in EPM model) showed that 46.03% of lands fall under very severe erosion classification, 30.92% severe, 18.4% mediocre, 2.47% low and 2.16% very low erosion in In 2007 on the other hand, 45.01% of lands fall under very severe erosion classification, 31.25% severe, 18.08% mediocre, 2.33% low and 2.22% very low. Comparison of erosion intensity in mentioned times suggest a considerable increase in erosion during the study periods.

5 By placing prepared maps in the erosion equation, specific erosion map in two time section were obtained which were the basis upon which to calculate the amount of specific erosions at the two time sections(figure 7and 8). As can be seen, the amount of specific erosion of m 3 /km 2.yr in 1968 has considerably increased to m 3 /km 2.yr in Discussion Study of the major environmental factors and water erosion changes for the 39 years period of investigation vividly show significant variations in the spatially distribution patterns of erosion types. More often than not, the assessment of the factors that determine erosion is not rigorous, because of the need to study these in time series and the problems that go with it, we require due regard for their inherent variability during the time. Factors related to vegetation cover, land use and drainage density are very changeable in the study area that cause remarkable variations in the area percentage of each water erosion types. In the study area, during the time, the drainage density factor has been increased and as such, the percentages of each type of water erosions have also been increased. Sheet and rill erosions have increased due to the deterioration of the vegetation cover. However, no well-defined relations were detected in channel and riverbank erosions. Also, the land use type and its associated changes during the study times, is shown to be the underlying reason for the emergence of different water erosions types. The findings demonstrate the useful application features of the new technologies such as the GIS and RS capabilities that can be used as powerful analytical tools in cases involving time variations, in assessing various geological, environmental, hydrological components in various arena particularly water erosion and land management under uneven topography such as those in the Ardabill Province in Iran. References Bocco G, Riemann H, Quality assessment of polygon labeling. Photogram. Eng. Remote Sens., 63 (4), Eswaran HR Lal and Reich PF, Land degradation: an overview. In: Bridges, E.M. I.D. Hannam, L.R. Oldeman, F.W.T. Pening de Vries, S.J. Scherr, and S. Sompatpanit (eds.). Responses to land Degradation. Proc. 2nd. International Conference on Land Degradation and desertification, Khon Kaen, Thailand. Oxford Press, New Delhi, India. Jong SM, Paracchini ML, Bertolo F, Folving S, Megier J and Roo APJ, Regional assessment of soil erosion using the distributed model SEMMED and remotely sensed data. Catena 37: Lal R, Soil Degradation by Erosion. Land Degrad. Develop. 12: Mbagwu JS, Lal R and Scott TW, Effects of desurfacing of Alfisols and Ultisols in southern Nigeria. I. Crop performance. Soil Science Society of America Journal, 48: Momeni M and Saradjian MR, Evaluating NDVI-based emissivity of MODIS bands 31 and 32 using emissivity derived by Day/Night LST algorithm. Remote Sensing of Environment, 106: Refahi H, Water erosion and its control. Tehran University Press. Solaimani K, Principles of engineering watershed management. Mazandaran University, Note for M.Sc students, 112. UNDP, Human Development Report of the Islamic Republic of Iran. Chapter l2l.