In desert dune environments, vegetation may be an indicator of dune stability and rates of sediment transport. As topography and underlying controls of vegetation vary over large spatial areas variations in dune form make scaling up of field research difficult. To mitigate this, and to identify spatial variations in vegetation distributions in a Saudi Arabian sand sea, spectral information from high resolution satellite images was classified to map polygons of shrub vegetation over 360 km 2 of well-defined linear dunes, broken linear dunes and dome dune forms. When compared to topographic characteristics of the landscape extracted from a digital elevation model, vegetation densities were often highest on 10 to 20-degree slopes elevated above interdune salt deposits on dune flanks. Spatially this was confined to small areas, and density was not always related to dune form, more to the presence of groundwater which could also encourage vegetation on the tops of some dunes. Field observations identified shrubs of mainly Calligonum genus whose size is related to the amounts of salts, moving sediment and access to the water table that varies within and between dune forms. Shrub vegetation density is likened to surface roughness to better understand sediment movement in this environment.
In central Saudi Arabia the redness of sands observed on satellite imagery, often related to iron oxide amount, was used to better interpret geomorphic processes operating in the area. To compare variations between dune and interdune morphology, linear and dome dunes, and an ephemeral river partly buried under a sand sea, Nafud Al‐Thuwayrat, a multi‐scale approach was used to identify the spatial variation in causes of iron oxide amounts and colour. Macro‐scale mapping of Image Spectral Redness identified that the extent and intensity of colour grading across the orientation of well‐defined linear and dome dune types, interdunes and the Wadi Al‐Rimah, indicated sediment mixing of different provenance. Meso‐scale chemical, physical and colour analysis of surface samples indicated that iron oxide coated grains were present within all the samples and was not the sole control on Image Spectral Redness. The reddest sediments were the best sorted medium‐to fine‐quartz sands with iron oxide bearing chemistry and the paler sediments contained coarser sands with additional feldspars, calcite and gypsum. Micro‐scale analysis of grain characteristics found dune–interdune contrasts and that the reddest sands with iron coatings were of aeolian origin and the paler sands contained larger fluvial material mixed with reworked aeolian deposits. Dust in the area consists of high amounts of iron oxide and appears to be a major contributor to redness in the sand dunes. These controls on colour show the sand seas are paler westwards because a major river and local drainage systems deliver fluvial sediments from the carbonate and sandstone bedrock into the quartz‐rich aeolian material from the linear dunes. Evaporite deposits dominate in the paler interdunes, and south of the Wadi the dome dunes are the reddest as they are not migrating but building up. This multi‐scale approach has provided a modern analogy of processes for palaeoenvironmental studies. © 2018 John Wiley & Sons, Ltd.
Rainfall trend analysis, on different spatial and temporal scales, has been used during the past century as the indicate of global climate change by the scientific community. According to some recent studies, the Nature Reserve (ITBA) has been suffering a rainfall decrease, especially in the second half of the studied period (1993-2018). The aim of the present study is to analyze the rainfall trends using the time series of 52 years in five rain stations (Linah, Rafha, Ha’il, Dawmat Al Jandal and Hafr Al Batin) surrounding the Nature Reserve of (ITBA) that extended on area of about 91500 km2 in the Northeast of Saudi Arabia. The trends analysis of annual rainfall, rainy days, shows that the fluctuations or variations in climatic parameters is a recurring phenomena in the studied stations. Inter-annual variability of the rainfall and the actual mean of daily rainfall are characterized by the high coefficients of variation, varying from 0.73 (Linah) to 0.83 (Ha’il) and from 0.53 (Linah) to 1.09 (Rafha), respectively. The variance test of Fmax-ratio method indicates the homogeneous annual rainfall distribution at Linah, Rafha and Ha’il, the homogenous rainy days distribution at only Ha’il and the homogeneous distribution of the actual mean of the daily rainfall at only Linah. The trends of annual rainfall, rainy days and actual mean of the daily rainfall obtained by using T-student test are significant in Ha’il and Dawmat Al Jandal, in all stations and in only Hafr Al Batin, respectively.
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