جستجوی مقالات مرتبط با کلیدواژه "نیمه خشک" در نشریات گروه "آبخیزداری، بیابان، محیط زیست، مرتع"

To better managing of rangeland the vegetation map is one of major factors, because plant communities is planning units of rangeland management and vegetation map shows the current status of plant communities. This research was conducted to produce vegetation type's map using Landsat 8 image classification in Behbahan, Khuzestan province. Rangelands of the study region is warm semi steppe and winter grazing. Geometric correction of satellite image was performed by ground control points with an error of less than one pixel. Atmospheric correction of existing data using the dark object subtraction was done. Field visits for vegetation type's border controlling and sampling training area was conducted. Eight supervised classification algorithms included Parallelepiped (PP), Minimum Distance to mean (MD), Mahalanobis distance (MAH), Maximum Likelihood (ML), Neural Net (NN) and Support Vector Machine (SVM) was performed. The results showed that ML algorithm has the highest overall accuracy (87.5 percent) and kappa (0.867) and PP algorithm has the lowest overall accuracy (67.1 percent) and kappa (0.571). It is suggested that, along with digital methods of classification of satellite images, visual interpretation should be used to clarify the boundary of the obtained vegetation types map.

Drought is one of the most costly natural hazards and is more dangerous in arid and semiarid regions which suffer from limited water resources. Climate change can also affect drought intensity; therefore, it is essential to analyze the trends of drought, particularly in these regions. This study surveyed the trends of meteorological and agricultural drought in hyper-arid, arid and semiarid regions of the world using the standardized precipitation index (SPI). Short time scales of 1-, 3- and 6-month SPI (moderate to severe and extreme drought) were derived for the period of 1970 to 2014 from the gridded precipitation data provided by CRU. The trends of the time series were computed using the Mann-Kendall nonparametric statistical test. The results showed that non-significant trends were dominant; however, some territories showed significant upward and downward SPI trends in different parts of the study regions. The significant negative SPI trends, which imply increased drought, were observed in the Middle East, central Australia and western North America. An increase in drought severity can increase consumption of water resources, particularly in developing countries.

The lack of sufficient fresh water resources, climate change and lengthy and severe droughts are of the major today’s world challenges particularly in arid and semi-arid regions. In the current conditions, especially in the case of droughts and in order to water resources planning, it is essential to analyze the changes and trends in drought particularly in hyper-arid, arid and semi-arid regions of the globe. In this study, Mann-Kendall statistics were applied to determine the significant and non-significant positive or negative trends in long term drought time series (9, 12, 18 and 24 monthly time scales) in moderate to severe and extreme classes of droughts derived based on the Standardized Precipitation Index (SPI). Results indicated the both significant positive and negative trends can be found in these regions. By increasing the time scales of droughts time series, the percentage of significant upward or downward trend grow. For example, in the case of decreasing trend of SPI (increasing in the drought), the percentage of 17.8 for 9 monthly SPI increased to 21.9, 28.48 and 31.71 for the 12, 18 and 24 monthly SPIs, respectively. The increasing in long severe and extreme classes of drought can be found in Middle East and west of America. The upward trends of drought particularly in Middle East region which is suffered by expensive tensions among the communities can intensify current critical situations. Therefore, more attention should be paid to this subject particularly in regional and international scales.

Making rain water harvesting micro catchments systems for hoarding rainwater is one of the forward ways to supply water. Performance of these systems is affected by climate specially intensity, duration and frequency of rainfall events, time distribution and their technical specifications such as figure, area and runoff coefficient. By this means rainwater plots built with dimensions of 4m×5m with surface treatments of impediment, semi impediment and natural (undisturbed) with 4 blocks that located on eastern hillside with slope of 15 to 25 percent. Daily rainfall and likes produced runoff for period of 4 years and totally 68 daily rainfall events that caused runoff (at least at 1 plot of impediment treatment from 4 repetitions) were measured. Results showed that in semiarid climate, for average frequency of daily rainfalls with depth of equal or more than 1 and 5 mm for warm months (season of plants growth and production) include July, August and September were 1 for every 1 year and 1 for every 3 years respectively. So the threshold of daily rainfall for runoff occurrence due to treatments of impediment, semi impediment and natural respectively nearly were 2.1, 3 and 5 mm and average percentage of runoff coefficients 34.93, 3.81 and 0.68 for period of May to November of every year. Conclusion is that application of impediment rainwater catchments associated with runoff reserve systems from rainy months (April and May) and reserved water distribution systems in more water needy months of plants (July, August and September) is recommended.

In view of the fact that extensive areas of ‘Iran’ are located in arid and semi arid regions, considerable damages on natural and economic resources by wind erosion and sand storms are expected annually. Lack of proper input data for physical and quantitative models and difficulties of access to these data can limit the models application. In these conditions it is necessary to devise an informative-computational model based on geographical information systems and use remote sensing to prepare as much as possible layers of parameters, accurately. Therefore, in this research a qualitative model for wind erosion assessment (IRIFR) has been embedded in geographical information system together with remote sensing to prepare layers of information and parameters. The major targets of this research are to get benefits of earlier approaches in relevant researches and compile relevant techniques and methods in GIS and RS to study wind erosion. The most important achievements of this research are; i) providing layers of information by remote sensing, ii) omitting primary common map of study units, iii) determining and defining separated maps of study units which are needed for a few parameters, iv) omitting the map of other parameters, and v) reducing uncertainties arising from experts judgment. Furthermore, the model could be repeated for any time duration. Consequently, investigations would be organized much better, more accurate and faster.