Species like nitrate (NO 3 − ), hydrogen peroxide (H 2 O 2 ), and formaldehyde (HCHO) are ubiquitous trace compounds in snow. Photochemical reactions of these compounds in the snow can have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. Therefore, we performed laboratory experiments to investigate such reactions in artificially produced snow samples. Artificial snow samples allow to execute experiments under defined and reproducible conditions and to investigate single reactions. All reactions were carried out under comparable experimental conditions and indicated that the photolysis of H 2 O 2 and NO 3 − occurred equally fast, while the photolysis of HCHO was considerably slower. Moreover, the photolysis of HCHO was only observed if initial concentrations were much higher than found in natural snow samples. These results indicate that the H 2 O 2 and NO 3 − reactions are possibly equally important in natural snow covers regarding the formation of OH radicals, while the photolysis of HCHO is probably negligible. Nitrite (NO 2 − ) was observed as one of the products of the NO 3 − photolysis; however, it was itself photolyzed at a higher rate than NO 3 − . After a certain photolysis period (≥8 h) the NO 3 − and NO 2 − concentrations in the snow remained constant at a level of 10% of the initial nitrogen content. This is probably due to a recycling of the anions from nitrogen oxides in the gas phase of the reaction cells indicating that the chemical reactions occur in or near the surface layer of the snow crystals.
Abstract:Rainfall variability has strong impact on food security, livelihood and socio-economic activities as farming in West Africa is mainly rain-fed. The annual, seasonal and decadal rainfall variability over Ghana has been studied and their periodicities analysed using wavelet analysis. A rainfall time series from 1901-2010 from the Global Precipitation Climatology Center (GPCC) was used in this analysis. It was observed that high mean annual rainfall totals ranging from 900-1900 mm are recorded over the entire country. In addition, very high totals between 1500-1900 mm are recorded at the South-Western part of the country whereas low totals (900-1200 mm) are recorded in the Savannah and East coast of the country. In general, a decreasing trend was observed for the annual rainfall over all the agro-ecological zones except for the coastal zone, where a slight increasing trend of 0.1600 mm per year was seen. The seasonal trend analysis revealed a significant decreasing trend at 0.01 significance level in all the agro-ecological zones except for the Savannah during the DJF season indicating an intensification of the Harmattan. The Coastal zone recorded the lowest mean rainfall values for all seasons with the highest of about 150 mm in MAM. The Forest zone on the other hand recorded very high rainfall values for all seasons with the maximum of about 200 mm in JJA. The Transition zone, however, recorded almost quite stable rainfall amount for all seasons except for DJF. On the decadal time scale, below normal rainfall values were observed between the 1901-1920 and 1980-2010 periods for almost all the agro-ecological zones except for the Savannah which showed above normal rainfall values within the 1901-1940 period. Indicating that, the decreasing trend observed in recent years is not solely due to antropogenic factors but have a strong contribution from a natural climate variability. The wavelet analysis also revealed a strong annual periodicity over all the agro-ecological zones except for the Coastal and Forest zones where the annual periodicity was accompanied by 4-8 months signal. The results of both the 5 year moving average and the decadal anomaly confirm a significant decrease in rainfall amount. This will have negative consequences on agricultural practices, water resource management and food security.
This study used the double cumulative curve and visual image interpretation methods for the selection of spatiotemporal Landsat data to evaluate the land degradation by anthropogenic activities in the Pra River Basin (PRB) of Ghana. Unsupervised and supervised classification procedures were used to map the land use and land cover (LULC) distribution from 1986 to 2016. Assessment of LULC showed that the PRB has been subjected to six different rates of land degradation in the years 1986, 2000, 2004, 2008, 2013, and 2016. This is due to increase in settlement, cropland, and mining activities to about 130%, 198%, and 304%, respectively. The Markov chain and cellular automation integrated model was successful in predicting LULC distribution in 2016 and the outcomes were comparable to the actual LULC for 2016. The projected LULC for 2025 showed that land degradation is significant in the western and the eastern parts where cropland and forest are, respectively, converted to mining activity. The northern, southern, and middle parts of PRB are expected to experience high settlement expansion, sedimentation in the rivers, and cropland expansion, respectively. The results will aid natural resources management, planning, and sustainable development at PRB. In addition, the research method serves as guideline for other related studies in an attempt to investigate, quantify, and project LULC change in forest ecological areas.
The Lower Pra River Basin (LPRB), located in the forest zone of southern Ghana has experienced changes due to variability in precipitation and diverse anthropogenic activities. Therefore, to maintain the functions of the ecosystem for water resources management, planning and sustainable development, it is important to differentiate the impacts of precipitation variability and anthropogenic activities on stream flow changes. We investigated the variability in runoff and quantified the contributions of precipitation and anthropogenic activities on runoff at the LPRB. Analysis of the precipitation–runoff for the period 1970–2010 revealed breakpoints in 1986, 2000, 2004 and 2010 in the LPRB. The periods influenced by anthropogenic activities were categorized into three periods 1987–2000, 2001–2004 and 2005–2010, revealing a decrease in runoff during 1987–2000 and an increase in runoff during 2001–2004 and 2005–2010. Assessment of monthly, seasonal and annual runoff depicted a significant increasing trend in the runoff time series during the dry season. Generally, runoff increased at a rate of 9.98 × 107m3yr−1, with precipitation variability and human activities contributing 17.4% and 82.3% respectively. The dominant small scale alluvial gold mining activity significantly contributes to the net runoff variability in LPRB.
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