desertification and playa expansions in everglades of iran (case study: meghan lake)

introduction desertification phenomena is a critical problem in dry climates. everglades, playas and shallow lakes are the main landscapes in the closed basins of iran, which are formed under tectonic and climatic influences during the pleistocene and holocene periods. duo to climate changes, lack of precipitation, human activity and mismanagement of water resources, entering water has been reduced and caused seasonally or permanently dryness. the drought led to deposition of dissolved salt minerals and the pond has turned into the desert. meghan is drained catchment area of approximately 5,528 square kilometres, which has been faced to water reduction and desertification phenomena. this study was performed to investigate spreading of desert in the meghan everglade. meghan wetland is located in the northeast of arak in farahan plain. its area is range from about 100 to 110 square kilometres. meghan basin is made along the fault lines and its water is salty. its height is 1,660m above sea level and is composed of two parts: the mountainous area and the sediment plain. material and methods the data is included to topography map 1:25000, aster data, geology map 1:100000, 6 spectral bands and panchromatic of etm+ images and 4 spectral bands of irs-p6 images 2007. the method has the following steps: • principal components analysis based on correlation matrix was used to detect deposited faces from etm+ and irs-p6 images. • to examine sedimentology of faces, 11 samples were taken of the innermost and outermost zones, which obtained from the principal components analysis. • granulometry were performed and sorting coefficients were calculated by the standard deviation of the integrated graphics provided by folk(equation 1) equation 1 • regular rate of deposition was calculated according to equation 2. • what ever between the maximum and minimum diameters of the sediments are low, sediments are smoother and more regular. equation 2 in the above equations is the negative logarithm of the size of particle diameter scale is the base2 (equation3) equation3  • for microscopic analysis, thin section samples were prepared of deposits remaining on the 80-mesh sieve, and using colour polarising microscopy to calculate grain morphology, shape, roundness, transparency and opacity, also 120 seeds from each sample were selected to calculate friction coefficients. results the result of image analysis of etm +, were detected levels of a surface (inner surface) belong to the pc2 and the outer surface to the pc1. also, two levels of irs image were extracted same as etm +. compared the obtained levels were similar in colour and shape; so field observations was done based on the levels. microscopic examination of the sections was shown that clay minerals as the most grains in samples. also there were alkali feldspar minerals in sediment samples, which indicated they were under water during wet season. a highlight from the microscopic scale, already kaolinite was seen more than around meghan. angular grains of quartz were found in the all samples. in general, samples 1, 2, 3, 4, 7, 9, 10 and 11, with opaque particles have much more transparent debris that indicated the movement of particles in wind erosion control. on the other hand, angular particles in the samples have become increased, thus, distance transport (by wind or runoff) was not far. among the samples, the highest coefficients of friction belong to samples 6, 7 and 8, but the particles sorted in almost all cases were inappropriate. conclusion the results were concluded that due to the deposition of alkali feldspar and clay minerals, the weather was wetter. generally intact feldspar was indicated the dry weather and weathering shown wet climate. also there is kaolinite sediments in the lagoon surface and reduce the amount of feldspar minerals in the pond, was proved humid climate during the recent years. some of examples were angular and opaque particles that could be indicated wind erosion. the above evidence was shown that recent zone had runoff erosion in winter and wind erosion in summer, after the last glaciers and reduced rainfall has occurred in the lagoon meghan. it represented the dominant evaporated limestone and carbonate mineral are less solubility and was deposited earlier therefore have formed the carbonate zone. in the following period sulphite minerals was started to deposit. in general, whenever the climate was wetter with more precipitation, the center of meghan was wet and salty and in the drier period, solute salt was deposited, so lime and gypsum faces were seen around the meghan. now the biggest threat for the wetlands is decreasing of entering water to the lake in the boreal summer. and in the winter, flash floods caused widespread damage zones and spread salt in summer, decrease the amount of entering water and cause drought and therefore more dust is generated.