Stratigraphic analysis of alluvial/colluvial sequences and 14C dating have been used as proxies for Holocene climate changes in the highlands of Tigray (northern Ethiopia). The studied records show alternations of buried soils and peaty–clayey sediments, pointing to wet, stabilization phases, and organic-free colluvium layers resulting from the abrupt occurrence of dry-climate episodes. The 14C dates, mostly unpublished, cluster in the 11,090–9915, 9465–9135, 8450–7330, 6720–3635, 2710–2345, and 1265–790 cal yr B.P. time spans. Evidence of subsequent pedogenesis is lacking in the area, apart from a buried humified horizon dated at 300 ± 60 14C yr B.P. (460–295 cal yr B.P.). Both the timing and the pattern of Tigray paleoclimatic events fit the corresponding framework, based on lake level changes, previously implemented for the Main Rift Valley. These findings give further support for arguing that the forcing mechanisms of the wet/dry fluctuations during the Holocene were effective over a large scale. 相似文献
ABSTRACT Understanding recharge processes is fundamental to improve sustainable groundwater resource management. Shallow groundwater (SGW) is being developed for multiple purposes in Ethiopia without consideration of monitoring. We established a citizen science-based hydro-meteorological monitoring network, with a focus on SGW recharge estimation, in Eshito micro-watershed, Ethiopia. Citizen scientists collected rainfall, groundwater-level and stream water-level data. We characterized the shallow aquifer using pumping tests. The data were used to estimate SGW recharge using three methods: chloride mass balance, water-level fluctuation (WLF) and baseflow separation. Approximately 20% and 35% of annual rainfall amount contributes to recharge based on the chloride mass balance and WLF results, respectively. Baseflow separation showed recharge values for the watershed vary from 38% to 28% of annual rainfall at the upstream and downstream gauging stations, respectively. This study shows that the recharge in previously unmonitored micro-watersheds can be studied if citizens are involved in data generation. 相似文献
Dire Dawa city is identified as one of the most flood-affected cities in Ethiopia. Classifying village-level flood vulnerability using flood indicators is a new approach to Dire Dawa city. Analysis of different flood vulnerability factors underpins sustainable flood risk management and the application of Flood Vulnerability Index (FVI) approach is the hub of this study. Relevant data were collected from 110 households sampled from purposely selected 10 villages found in Dire Dawa city. The flood vulnerability index was used to compare, classify and rank villages in terms of their flood vulnerability levels. For this purpose, 24 sets of indicators which are strongly affecting the levels of flood vulnerability were assessed from social, economic and physical perspectives. The FVI of each village was computed with unequal method of weighting indicators. The findings of the study revealed that Dire Dawa city villages were experiencing varying levels of flood vulnerability. Accordingly, villages 05, 06, 07 and 09 were identified with high flood vulnerability level while villages 03, 04 and 08 and villages 01, 02 and extension village were identified with medium and low level of vulnerability, respectively. Interestingly, the findings of the study confirmed that social factors contributed much for flood vulnerability in Dire Dawa city. Hence, future urban flood risk planning and management endeavors in the city of Dire Dawa must be underpinned by proper utilization of the flood vulnerability map developed addressing social vulnerability component through both structural and non-structural urban flood risk management measures.
Errors inevitably exist in numerical weather prediction (NWP) due to imperfect numeric and physical parameterizations. To eliminate these errors, by considering NWP as an inverse problem, an unknown term in the prediction equations can be estimated inversely by using the past data, which are presumed to represent the imperfection of the NWP model (model error, denoted as ME). In this first paper of a two-part series, an iteration method for obtaining the MEs in past intervals is presented, and the results from testing its convergence in idealized experiments are reported. Moreover, two batches of iteration tests were applied in the global forecast system of the Global and Regional Assimilation and Prediction System (GRAPES-GFS) for July-August 2009 and January-February 2010. The datasets associated with the initial conditions and sea surface temperature (SST) were both based on NCEP (National Centers for Environmental Prediction) FNL (final) data. The results showed that 6th h forecast errors were reduced to 10% of their original value after a 20-step iteration. Then, off-line forecast error corrections were estimated linearly based on the 2-month mean MEs and compared with forecast errors. The estimated error corrections agreed well with the forecast errors, but the linear growth rate of the estimation was steeper than the forecast error. The advantage of this iteration method is that the MEs can provide the foundation for online correction. A larger proportion of the forecast errors can be expected to be canceled out by properly introducing the model error correction into GRAPES-GFS. 相似文献
The Gilt Edge Superfund Site is a former heap-leach gold mine that currently is being remediated in the Black Hills of South Dakota. Mine runoff water is treated before release from the site. The field pH, before treatment, is about 3; the water contains arsenic at low levels and some trace metals at elevated levels, in addition to total dissolved solids concentrations of more than 1,900 mg/L. In the Keystone area of the Black Hills, naturally occurring arsenic has been detected at elevated concentrations in groundwater samples from wells. The City of Keystone’s Roy Street Well, which is not used currently, showed arsenic concentrations of 36 parts per billion and total dissolved solids of 320 mg/L. With field samples of water from the Gilt Edge site, a limestone-based method was successful in reducing trace metals concentrations to about 0.001 mg/L or less; at the Keystone site, the limestone method reduced arsenic levels to about 0.006 mg/L. The results are significant because previous research with the limestone-based method mainly had involved samples prepared with distilled water in the laboratory, in which interference of other ions such as sulfate did not occur. The research indicates the potential for broader applications of the limestone-based removal method, including scale-up work at field sites for water treatment. 相似文献
Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelm?ya(Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent(~12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance(more than five times the thickness of the sill)from the intrusions. We present evidence that the sedimentary sequence can be divided into two units:(1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60-70 ℃,and(2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures(around 140 ℃). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ~40 ℃. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization(illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T 120-140 ℃ at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals. 相似文献