Dokument

Summary:
Wind erosion and shifting sand dunes as a land degradation process is a serious problem in Iran. There are significant gaps in our knowledge of Iran sand dunes in national scale for the English speaking international scientific community as well as wind erosion and sand transport in Sistan plain. Identifying active dunes and monitoring areas with migrating sand are important prerequisites for mitigating these damages. In addition, wind erosion is one of the most serious problems in the Sistan region, located in the East of Iran and near the border of Afghanistan. This thesis has two major purposes: (1) to assess sand dune activity in national scale of Iran (2) to investigate wind regime and investigated spatial and temporal patterns of wind erosion process in Sistan region. With regard to first objective, the spatial variation of the wind energy environment based on the sand drift potential (DP), with using Fryberger and Dean's (1979) method, was calculated from 204 meteorological stations in and around Iran’s deserts. Three commonly dune activity models – the Lancaster mobility index (1988), the Tsoar mobility index (2005), and the index developed by Yizhaq et al. (2009) – were used for the evaluation of Iran’s sand dune activity. The analysis of the indices showed that the dunes activity was characterized by great spatial variations across Iran’s deserts. All three models identified fully active dunes in the Sistan plain, the whole of the Lut desert, as well as in the Zirkuh Qaien and Deyhook regions, while the dunes in the northern part of Rig Boland, Booshroyeh and in the Neyshabor dunefields were categorized as stabilized dunes. For other dunes, the models show a less unified activity classification, with the Lancaster and Yizhaq models having more similar results while the Tsoar model stands more apart. Overall the three models delivered comparable results in some instances and diverging results in others. The reasons for this are the use of different parameters and their impact on the model construction. The main contradictions of the three models results are revealed when the wind blows for only short times but with a high energy, like in the north of the Dasht-e Kavir desert (Damagan, Foromad) and at some stations in the wind of the 120 days’ domain (Sedeh Birjand). Field observation demonstrated that dunes in these areas are completely active, but the Lancaster mobility index (LMI) classified them as inactive or semi-active because of a low to moderate percentage of wind events above the transport threshold. At the same time the DP in this region showed high values and thus the Tsoar mobility index (TMI) classified the dunes as active, while the Yizhaq model (YMI) classified them as active or semi-active. In fact, in spite of high wind energy, the percentage of winds above threshold (W%), that is upper 12 knot or 6.2 m s-1 in this study, was rather low, as high speed winds only occur during the warm season, while the rest of the year is characterized by calm weather. The nature of the wind power parameter varies from the LMI, that is the percentage of winds above threshold (W%) to the TMI and YMI that is DP. DP reflects the quantity (frequency) and quality (intensity) of the wind power, but W% only shows its quantity (frequency of winds above the transport threshold). It seems that if DP was used in the LMI instead of W%, it would provide more favourable results . In addition, the statistical analysis (correlation coefficient) between DP values and the percentage of wind events above the transport threshold (%W) at the meteorological stations in the study area shows a moderate correlation. Based on this argumentation, W% has been replaced by the DP (vu) as the wind energy parameter in Lancaster mobility index (1988) and a Modified Lancaster Index (MLI) was developed. Based on these model results and fields observations, a modified Lancaster mobility index has been applied to show a more realistic spatial variation of sand dunes activity in Iran’s desert areas. With regard to second objective, the wind regime, formation of aeolian dunes and the rate of sand transport in Sistan and Registan were investigated. The Sistan region in eastern Iran and the Registan region in South-western Afghanistan are strongly influenced by the Sadobist Roozeh wind (the wind of the 120 days), which is blowing along the Iran-Afghanistan border from North to South, then shifts it’s direction toward the Southeast into the Sistan region and, finally, continues eastward into south-western Afghanistan, forming the Registan sand seas. It blows during the hot season due to the pressure gradient between the Turkmenistan High and the Pakistan Low. In order to determine the wind regime and the sand transport, wind roses based on long-term datasets from 16 meteorological stations, DP, the Resultant Drift Direction (RDD), the Resultant Drift Potential (RDP) and the RDP/DP ratio have been calculated using Fryberger and Dean's (1979) method. The distribution of the Registan sand dunes was surveyed by using Landsat ETM data, Google Earth scenes and field operations (the latter only in the Iranian part). The spatial differences of the drift potential were simulated using GIS and geostatistical methods overlaying the sand dune map. The results show that DP increases from north to south along the border between Iran and Afghanistan and reaches to highest values in the Sistan region, then decreases gradually in the Registan sand seas. The highest wind energy, based on DP matches, was determined exactly where the ephemeral lakes in the northern part of the Sistan plain are located, which function as a source area of intense dust and sand storms during the dry season. The annual DP calculated in Sistan is one of the highest values (2516 vector units) in inland desert and categorized it into the windiest desert in the world. The temporal trend of the DP showed an increase between 1999 and 2007, followed by a decrease until 2015 in Sistan. The results show that the wind regime in the Sistan and Registan regions is unimodal during the wind of 120 days (the Sadobist Roozeh) period, which is also supported by the dominance of transverse, barchanoid and barchans dunes in both regions . In continue second objective, we measured the rate of sediment transport in Hamoun-e Baringak, as one of the six ephemeral lakes, producing more aeolian sediment into the atmosphere in Sistan region. In order to investigate the land surface sensitivity to aeolian transport, 74 graduated pins were embedded randomly in the ephemeral Baringak Lake bed and the aeolian transport rates were measured for three events in 2013 individually as well as for the total study period. The spatial and temporal variation of the aeolian transport was also mapped using GIS and geostatistics methods for these events and for the total duration of 103 days. The resulting variogram revealed a high spatial dependence of the different events and showed that geostatistical techniques are a valid tool for the mapping of aeolian sediment transport. The average transport rate in terms of the detected drift height on the dry lake bed was 1.93 cm or 31 kg/m2 between the 5th of August and the 17th of November in 2013.

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