首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 93 毫秒
1.
Summary Mean monthly rainfall data of 28 meteorological stations in Nigeria for the period 1911–1980 are analysed to examine trends in precipitation patterns in the country. Specifically, four 40-year periods (1911–50, 1921–60, 1931–70 and 1941–80) have been studied in detail.Results show three prominent features. First, both the amount and area of the secondary rainfall maximum at 9°–10°N latitude in Nigeria has depreciated with time. Second, the belt of relative minimum rainfall, with its east-west axis almost coincident with the channels of the rivers Niger and Benue, appears to be expanding with time. Last, while places north of 8°N latitude (the mean axis of the belt of relative minimum rainfall) receive 90–100% of the annual total rainfall from April to October, fluctuations of the wet or rainy season contribution to total annual rainfall further south is about 84–88%.These aberrations, which imply a decrease in the dry season contribution to the annual rainfall, suggest a drier environment in the long-term, especially if drought spells of the type 1969–73 and 1979–84 become a regular feature in West Africa. The planning implications are discussed in relation to water-use problems.
Zusammenfassung Mittlere Monatssummen des Niederschlags von 28 meteorologischen Stationen in Nigeria von 1911–1980 wurden analysiert, um Trends in den Niederschlagsverteilungen des Landes zu untersuchen. Insbesondere wurden die 40-Jahresperioden 1911–50, 1921–60, 1931–70 und 1941–80 bearbeitet.Die Ergebnisse zeigen drei bedeutende Merkmale. Erstens, Menge und Niederschlagsgebiet des sekundären Maximums in 9–10° nördlicher Breite wurden kleiner. Zweitens, der Gürtel eines relativen Niederschlagsminimums, dessen Ost-West-Achse fast mit den Flußläufen von Niger und Benue zusammenfällt, scheint sich auszudehnen. Und schließlich, während Orte nördlich von 8° nördl. Breite (der Hauptachse des Niederschlagsminimums) 90–100% der Jahressumme von April bis Oktober empfangen, beträgt die Schwankung des Anteils von Regen- und Trockenzeit weiter im Süden 84–88%. Diese Abweichungen, die eine Abnahme des Anteils der Trockenzeit zum Gesamtniederschlag bedeuten, lassen in Zukunft eine trockene Umwelt erwarten, besonders wenn die Trockenperioden von 1969–73 und 1979–84 sich wiederholen sollten. Es werden die Folgen für die Planung des Wasserverbrauches diskutiert.

With 8 Figures  相似文献   

2.
Annual series of light rainfall, moderate rainfall and heavy rainfall are computed for 4 zones arranged from south to north in Nigeria: Coastal, Guinea-Savanna, Midland and Sahelian zones. Daily rainfall data for the period 1919–85 are utilized. Each series is examined for evidence of change in structure in terms of pattern of decrease and increase in dry and wet years, the overall trend, and the occurrence of runs of dry and wet years. The northern Nigeria (Midland and Sahel) heavy rainfall series and the Sahel moderate rainfall series are found to depict evidence of climatic change as defined by Landsberg (1975) that climatic conditions must change to a new equilibrium position with the values of climatic elements changed significantly. On the other hand Landsberg's definition of climatic fluctuations as involving temporary deflection which can revert to earlier conditions is found to fit the 4 regional light rainfall series and the Midland area moderate rainfall series. The southern Nigeria moderate and heavy rainfall series are found to depict only evidence of high frequency oscillations about a stable long-term mean. The recent drought in Nigeria north of about 9° N is shown to be associated with a large decline in moderate and heavy rainfalls over this part of the country.  相似文献   

3.
Climate change and implications for agriculture in Niger   总被引：1，自引：0，他引：1  
Five-year moving averages of annual rainfall for 21 locations in Niger showed a decline in the annual rainfall after 1960. Correlation coefficients of the moving averages of monthly rainfall with annual rainfall showed significant correlations between the decline in the annual rainfall with decreased rainfall in August. Analysis of daily rainfall data for rainy season parameters of interest to agriculture suggested that from 1965 there was a significant decrease in the amount of rainfall and in the number of rainy days in the months of July and August, resulting in a decreased volume of rainfall for each rainstorm. In comparison to the period 1945–64, major shifts have occurred in the average dates of onset and ending of rains during 1965–88. The length of the growing season was reduced by 5–20 days across different locations in Niger. The standard deviation for the onset and ending of the rains as well as the length of the growing season has increased, implying that cropping has become more risky. Water balance calculations also demonstrated that the probability of rainfall exceeding potential evapotranspiration decreased during the growing season. The implications of these changes for agriculture in Niger are discussed using field data.  相似文献   

4.
Summary This study used monthly rainfall totals for the period 1961 to 1988 and pentad OLR values for the period 1974 to 1991 to study the structure and transition of active convention across the Congo Basin (10°S–5°N, 15°E–35°E) from the southern to the northern hemisphere summer. This involved the examination of map patterns and cross-sections of monthly rainfall and pentad OLR data.The results from the study indicated that there were two seasons observed over the Congo Basin; one is the wet season lasting from September to April and the other a dry season covering the rest of the year. The onset of the wet season takes place rapidly with active convection spreading very quickly to the south near latitude 20°S. This is due to the formation of the meridional (north–south) branch of the ITCZ over this region.This study has confirmed that the annual rainfall over the Congo Basin is reliable with the coefficient of variation of less than 30%. The wet seasons (e.g., SON and DJF) also show reliable rainfall occurrence but the dry season (e.g., JJA) has low reliability.The anomalously wet seasons are characterised by a relatively slow transition rate (1° latitude per pentad) of zones of active convection resulting in a late withdrawal of the rainy season while the dry seasons show a rapid transition rate with an early withdrawal of zones of active convection.High-rainfall months (>200mm) are concentrated within the Southern Hemisphere summer months. These high-rainfall months progress from the equator to the southern latitude following generally the movement of the overhead sun.The results further revealed that the years 1987/1984 had the lowest/highest mean OLR values over the Congo Basin within the period 1979 to 1991. The rates of transition of the zones of low OLR values were 0.9/5.0 degrees of latitude during 1987/1984, respectively.Received June 18, 2002; revised September 30, 2002; accepted November 21, 2002 Published online: June 12, 2003  相似文献   

5.
Summary  Reports of changes in the seasonal and annual rainfall in Nigeria suggests that a more detailed analyses of the geographic extent of these changes and of their impact on agriculture could be of value. Variation in the growing season (April to September) rainfall from stations across Nigeria was analysed over the 30-yr period, 1960–90. Regression analyses were used to examine long-term trends. Principal component and cluster analyses were used to group stations with similar trends in standardised seasonal rainfall. Mean accumulated standardised seasonal rainfall were used to examine short- and medium-term trends for each of the groups identified. Significant (P ≤ 0.05) decreases in rainy season rainfall were found at 8 stations mostly in the Guinea and arid/semi-arid savannas of northern Nigeria, whereas no station showed significant increases. Examination of the monthly (April through September) rainfall showed that only three – Kano, Sokoto and Potiskum in the arid/semi-arid savanna – of the twenty-three stations used in the analysis had declining rainfall trends for each of the months April to September and subsequently declining seasonal rainfall trends. However, 12 to 15 stations had consistently declining rainfall trends in atleast some but not all the growing season months. However, a similar pattern was not the case in terms of increasing rainfall trends, where only one to three stations had consistently increasing rainfall trends in some but not all of the months from April to September. Stations that showed increasing rainfall trends were in the southern parts of Nigeria. Six groups with similar patterns in standardised seasonal rainfall were identified by Principal Component and Cluster analyses. For most of the groups, the period from 1967 to 1973 was that of consistently below average seasonal rainfall. However, the timing and extent of the decline varied with location. Common to stations in four of the six groups was a negative trend in seasonal rainfall for the period considered. The geographic variation in seasonal rainfall trends has tremendous agricultural significance since there are indications that the reliability of the season is decreasing from the humid forest zone with positive seasonal trends to the arid/semi-arid savanna with significant negative seasonal trends. Received June 24, 1998 Revised December 18, 1998  相似文献   

6.
Summary The relationship between the all-India summer monsoon rainfall and surface/upper air (850, 700, 500 and 200 mb levels) temperatures over the Indian region and its spatial and temporal characteristics have been examined to obtain a useful predictor for the monsoon rainfall. The data series of all-India and subdivisional summer monsoon rainfall and various seasonal air temperatures at 73 surface observatories and 9 radiosonde stations (1951–1980) have been used in the analysis. The Correlation Coefficients (CCs) between all-India monsoon rainfall and seasonal surface air temperatures with different lags relative to the monsoon season indicate a systematic relationship.The CCs between the monsoon rainfall and surface-air temperature of the preceding MAM (pre-monsoon spring) season are positive over many parts of India and highly significant over central and northwestern regions. The average surface air temperature of six stations i.e., Jodhpur, Ahmedabad, Bombay, Indore, Sagar and Akola in this region (Western Central India, WCI) showed a highly significant CC of 0.60 during the period 1951–1980. This relationship is also found to be consistently significant for the period from 1950 to present, though decreasing in magnitude after 1975. WCI MAM surface air temperature has shown significant CCs with the monsoon rainfall over eleven sub-divisions mainly in northwestern India, i.e., north of 15 °N and west of 80 °E.Upper air temperatures of the MAM season at almost all the stations and all levels considered show positive CCs with the subsequent monsoon rainfall. These correlations are significant at some central and north Indian stations for the lower and middle tropospheric temperatures.The simple regression equation developed for the period 1951–1980 isy = – 183.20 + 8.83x, wherey is the all-India monsoon rainfall in cm andx is the WCI average surface air temperature of MAM season in °C. This equation is significant at 0.1% level. The suitability of this parameter for inclusion in a predictive regression model along with five other global and regional parameters has been discussed. Multiple regression analysis for the long-range prediction of monsoon rainfall, using several combinations of these parameters indicates that the improvement of predictive skill considerably depends upon the selection of the predictors.With 9 Figures  相似文献   

7.
In this paper, the characteristics of the long-term precipitation series at Athens (1858–1985) have been statistically analyzed. This study covers both the history and the analysis of the data. The ten-year mean amounts, the monthly and annual amounts averaged over the intervals 1858–1890, 1891–1985, 1951–1980, 1858–1985, the mean number of hours of precipitation and the precipitation intensity are given. The analysis of long-term time series of climatic data (in particular precipitation) is a useful tool for the study of past climate. Different statistical techniques are used in order to depict monthly, seasonal and annual variations, as well as trends, periodicities and recurrence intervals of the amount, intensity and number of precipitation days. The analysis reveals many interesting characteristics. These characteristics of the precipitation regime are extended to a time scale from seasonal variation to a semi-secular trend. The study of such long-term series may be helpful not only in practical applications of rainfall, but also for explaining the possible physical or anthropogenic mechanisms of climatic fluctuations and tendencies. The series of precipitation at Athens is one of the longest in south-eastern Europe.  相似文献   

8.
Summary An inviscid form of the hydrodynamical equations is solved with enhanced horizontal shear, which is a synoptic feature consistent with stronger African Easterly Jet (AEJ) in Sahelian dry years, for unstable waves generated along the boundary between the two tropospheric air masses in tropical north Africa (i.e. the moist south-westerlies and the dry north-easterlies). Using a two-layer model of the atmosphere in order to correctly simulate the tropospheric synoptic situation in the sub-region, results show that the mode of the waves which is known to be fundamental to the development of West African squall lines is more unstable in dry years. This instability is found to be most-pronounced when the surface of discontinuity between the south-westerlies and the north-easterlies is at 700 mb level. Further, it is shown that in Sahelian dry years, the zone of these unstable waves shifts slightly southwards. This shift causes a deficit in rainfall in West African isohyet bands north of latitude 12°. The persistence of this deficit is linked with the continuous warming, in July, August and September of the 18-year period 1969–1986, of the three oceans (Indian, Pacific and South Atlantic) whose sea-surface temperature (SST) anomalies influence rainfall in tropical north Africa. It is shown that anytime these oceans warm up anomalously, the strength of the AEJ is enhanced leading to the climate-change process of: SST anomaly, increased AEJ strength, southward shift of the zone of squall-inducing waves and consequent reduction in total annual rainfall north of latitude 12° in tropical north Africa.With 5 Figures  相似文献   

9.
Variability and associated mechanisms of summer rainfall over east China are identified and described using both observations and a general circulation model (GCM) simulation. The observations include two data sets: the 90-station, 1470–1988 annual drought/flood index and the 60-station, 1889–1988 monthly mean precipitation measurements. The GCM data set is a 100-year equilibrium simulation of the present climate. Spectra of the drought/flood index indicate decadal cycles which decrease from north (47 y) to south (21 y). Correlation coefficients show decadal variability in the relationship between index values along the Yangtse River valley and those over northeast and southeast China. Analysis of the measured data confirms this result; for example, the correlation was small during 1889–1918, but significantly negative during 1930–1959. When compared with precipitation measurements, the GCM better simulates monthly means and variances along the Yangtse River valley. Three distinct 30-year periods of interannual variability in summer rainfall are found over this area. During each period, rainfall is negatively correlated with spring surface temperature over a remote region and is identified with variations in a specific component of the east Asian monsoon circulation: (1) when Eurasian temperatures decrease, the thermal contrast across the Mei-Yu front increases and frontal rainfall intensities; (2) lower temperatures over the Sea of Japan/northwest Pacific Ocean are identified with enhanced easterly flow, moisture transport and rainfall; (3) when tropical east Pacific Ocean temperatures decrease, rainfall associated with the low latitude monsoon trough increases. Given that the GCM generates decadal changes in the relationship between the physical mechanisms, the east Asian monsoon and planetary general circulations and east China rainfall, future studies should focus on the predictability of these changes with the use of improved and much longer GCM simulations.  相似文献   

10.
Summary A simple water balance method is used to compute the dates of the onset and termination and length of the growing season from long-term rainfall series in northern Nigeria. For most of the stations, the time series of onset and termination dates and growing season length are homogeneous and random, and can be taken as normally distributed. There is a progressive decrease in the length of the growing season from a mean of about 200 days in the south to less than 155 days in the extreme northern part. While there is no statistically significant trend in the onset dates, there is some evidence for statistically significant decreasing trend in the termination dates and the length of the growing season over the region. The results indicate that recent trends in the length of the growing season are more sensitive.to large interannual fluctuations in the start of the rains than to variations in the cessation dates.With 7 Figures  相似文献   

11.
Summary Annual and interannual variations of rainfall over Brazil are discussed. First, rainy and dry seasons for several stations of Brazil are determined using the data of 21 years (1958–1978). The progressive movement of the Intertropical Convergence Zone seems to be associated with the progresive variation of rainfall seasons in the equatorial eastern Brazil. The annual migration of deep tropical convection from Central and Southern Portion of the Amazon basin in austral summer to the northwestern sector of South America in austral winter seems to be responsible for the annual cycle of rainfall in the Amazon basin. The conncection between the interannual variation of rainfall over Brazil and the Southern Oscillation is also discussed. The correlation coefficient between the Southern Oscillation index and the rainfall is generally small over most of Brazil except over Rio Grande do Sul. The correlation between the spring rainfall of Rio Grande do Sul and the Southern Oscillation index of the same or of the previous season is significantly high and shows prospects for seasonal rainfall prediction.With 5 Figures  相似文献   

12.
Information from a variety of sources, including an airborne field expedition in November 1985, is used to produce estimates of the annual emissions of some hydrocarbons from bushfires, and isoprene from trees, in tropical Australia. For the continent north of 23° S the annual bushfires (biomass burning) input was estimated, in units of Tg carbon, to be 2 TgC (uncertainty range 0.8–5 TgC), emitted predominantly during the May to October dryseason. Isoprene emissions during this period were estimated also to be 2 TgC (uncertainty range 0.5–8 TgC), but were estimated to be an order of magnitude higher during the November to April wet season, at a level of 23 TgC (uncertainty range 6–100 TgC).The large annual emission of isoprene over the tropical part of the Australian continent yields ppbv levels of isoprene measured at the surface in summertime. Isoprene reactivity with hydroxyl radical is such that at these concentrations isoprene must be a dominant factor in controlling the concentration of OH radical in the convective boundary layer. Simple arguments based on the convective velocity scale suggest that the shape of the isoprene vertical profile in November 1985 would be consistent with available data on the OH-isoprene reaction rate if OH concentration in the boundary layer averaged about 2.5×106 cm-3 over the middle part of the day.Temporarily at the International Meteorological Institute, Stockholm University, S-106 91, Stockholm, Sweden.  相似文献   

13.
Accurately predicting precipitation trends is vital in the economic development of a country. Ground observed data from the Nigeria Meteorological Agency (NIMET) was analyzed to study the long-term spatio-temporal trends of rainfall on annual and seasonal scales for 23 stations in Nigeria during a 40-year period spanning from 1974 to 2013. After testing the presence of autocorrelation, Mann–Kendall (modified Mann–Kendall) test was applied to non-autocorrelated (autocorrelated) series to detect the trends in rainfall data. Theil and Sen’s slope estimator test was used to find the magnitude of change over a time period. Pettitt’s test, Standard Normal Homogeneity Test, and Buishand’s test were further used to test the homogeneity of the rainfall series. The results show an increasing trend in annual rainfall; however, only nine stations have a significant increase during the period of study. On the seasonal time scale, a significant increasing trend was observed in the pre- and post-monsoon seasons, while only nine stations show a significant increasing trend in monsoon rainfall and a significant decreasing trend in the winter rainfall over the last 40 years. During the study period, 15.4 and 13.90 % increase were estimated for annual and monsoonal rainfall, respectively. Furthermore, seven stations exhibit changes in mean rainfall while majority of the stations considered (Eighteen stations) exhibit homogeneous trends in annual and seasonal rainfall over the country. The performance of the different tests used in this study was consistent at the verified significance level.  相似文献   

14.
Summary An objective approach similar to the forward selection of independent variables in the multiple linear regression has been attempted to optimize the network of raingauges for the summer monsoon rainfall (June–September total) series (1871–1984) of India as well as its 29 selected meteorological subdivisions prepared involving the data of 306 raingauges. For the all-India monsoon rainfall series twenty seven gauges entered the selection whose mean showed the correlation coefficient (CC) of 0.9869. Keeping in view the difficulties of getting data from all the 306 gauges, 35 India Meteorological Department (IMD) gauges with mean showing CC of 0.9898 have been identified for updating this series. The constructed all-India monsoon rainfall series for the period 1871–1992 using 35 selected observations is presented. It was interesting to note that the set of 35 gauges selected for the monsoon total has shown equally promising results for the all-India monsoon monthly (June–September) as well as the annual rainfall series.For the 29 subdivisional monsoon rainfall series, however, in total 188 IMD-gauges (62% of the total of 306 gauges) entered the selection. For 17 subdivisions the CC exceeded 0.98, for 3 subdivisions it varied between 0.97 and 0.98, for 5 subdivisions between 0.96 and 0.97 and for the remaining 4 subdivisions between 0.90 and 0.94. They showed equally encouraging results for the monsoon monthly and annual rainfall series for the different subdivisions.Limitations and implications of the optimization technique are also briefly discussed.With 9 Figures  相似文献   

15.
During the last 30 years, the climate of the West African Sahel has undergone various changes, especially in terms of rainfall. This has large consequences for the poor-resource farmers depending mainly on rainfed agriculture. This paper investigates the impacts of current climate variability and future climate change on groundnut and cowpea production in Niger for three major agricultural regions, including the groundnut basin.Niger was one of the largest West African groundnut producing and exporting countries. Groundnut production – as a cash crop – dropped fromabout 312,000 tons in the mid 1960s (about 68% exported) to as low as 13,000tons in 1988 and increased again to 110,000 tons in 2000. Cowpea, a food crop, showsa different tendency, going from 4,000 tons in the mid fifties to a maximum of 775,000 tons in 1997, and its cultivated area is still increasing. It is also a cash crop in local economies (especially for women).To highlight the impact of climate change on groundnut and cowpea production (significantly determined by rainfall in July, August and September), the following components of the rainfall regime were calculated for the period 1951–1998: mean annual and monthly rainfall, beginning, end and lengthof the rainy season, number of rainy days per month, amount of rainfall per rainy day and the maximum length of dry spell per month. Three sub-periods whose duration varied per region were defined: for Dosso 1951–1968,1969–1984 and 1985–1998; for Maradi 1951–1970, 1971–1987 and1988–1998; and for Zinder 1951–1966, 1967–1984 and 1985–1998. A change in rainfallregime components was observed between the three sub-periods, which were characterized in chronological order by wet, dry and intermediate conditions. To assess the impact of climate variability and change on groundnut and cowpea production, a statistical modeling approach has been followed, based on thirteen predictors as described and discussed in the preceding paper. Climate change is mimicked in terms of reduced total amount of rainfall for the three main rainfall months and an increased temperature, while maintaining other significant predictors at a constant level. In 2025,production of groundnut is estimated to be between 11 and 25% lower, while cowpeayield will fall maximally 30%. Various strategies to compensate thispotential loss are presented for the two crops.  相似文献   

16.
利用广东省86个常规气象观测站1961—2010年的逐日降水资料,分析近50年广东省降水气候特征,探讨不同等级降水空间分布及随时间变化特征。结果表明:广东省降水丰沛,年均降水量多为1 500~2 000 mm;降水气候特征的区域差异较大,不同区域降水量与降水日数分布差异显著;各月的降水日数差异没有降水量月分布的差异明显,非汛期的日降水量较小,而汛期降水日数多且日降水量大;小雨日和中雨日的区域差异小,大雨日、暴雨日、大暴雨日的大值中心主要集中在广东省的三大暴雨中心地区 (清远中心、阳江中心、海陆丰中心),雨日量级分布大致由北向南逐渐增强,且随着降水等级的增加降雨日数迅速减少;小雨、中雨和大雨的降水贡献率均由粤北地区向沿海地区递减,暴雨和大暴雨的贡献率由粤北向沿海递增;小雨日数显著减少、大雨以上日数略有增多,总降水日数也呈减少趋势;小雨和中雨的贡献率呈减少趋势,大雨以上贡献率增多,使年均降水量呈增多趋势。   相似文献   

17.
Since the 18th century systematic measurements of rainfall have been collected in Italy. The daily rainfall series observed in Milan (1835–2001), Genoa (1833–2000), Bologna (1813–2001) and Palermo (1797–1999) are examples of available long rainfall records. These data series can help analyzing the evolution of precipitation. The present paper deals with long term evolution of: (i) annual rainfall amount; (ii) annual number of rainy events; (iii) intensity of rainfall, (iv) inter-annual rainfall partitioning, i.e. the duration of wet and dry periods, and (v) maximum annual values of daily rainfall amount, duration of wet and dry periods. The evolution is studied analyzing the first two order statistics and the 30-year return period quantiles via moving window analysis. Confidence intervals are introduced to check the statistical significance of the estimated statistics and quantiles. The results are compared with those provided by the traditional Mann-Kendall test. The analysis shows how the annual precipitation exhibits a negative trend in the first half of 20th century, with a subsequent positive trend in northern Italy (Genoa, Milan and Bologna). Conversely, the dataset for Palermo (southern Italy) displays only a negative trend. Because the number of precipitation episodes is found to decrease in the investigated period, the average rain rate is significantly increasing especially in northern Italy. This is also associated with shorter duration of rain episodes with an evident effect on rainfall extremes. Dry periods tend to be longer with increasing variability. The Mann-Kendall test and its progressive form have shown to be well suited for monotonic trend, but the confidence interval analysis, introduced here, is more appropriate if oscillations are significant.  相似文献   

18.
Abstract

The relationship between sea surface temperature (SST) and rainfall index anomalies over sub‐Saharan Africa for the 15‐year period, 1970–84, has been examined. The objectively analysed monthly mean SST data were used for the global oceans between 40°S and 60°N. The rainfall data consist of annual mean rainfall indices for the Sahel and Soudan belts over north Africa.

An Empirical Orthogonal Function analysis of the SST data has been carried out for the Atlantic, Indian and global ocean regions. The results show that the most dominant eigenmode, EOF1, is characterized by warming over the central eastern Pacific, cooling over the eastern mid‐latitude Pacific and warming over the entire Atlantic and Indian ocean basins. The second EOF for the Atlantic Ocean SST analysis shows a dipole (north‐south see‐saw) pattern. The third EOF for the Atlantic SST analysis has the same sign over the entire Atlantic basin. Global SST EOF2 and EOF3 correspondió Atlantic SST EOF3 and EOF2, respectively.

The correlation between the sub‐Saharan annual rainfall index, which mainly represents the summer season rainfall from June to September, and SST EOFs shows that EOF1 has statistically significant monthly correlations for the Sahel and Soudan regions and that the warm El Niño‐like phases of SST EOF1 correspond to drought conditions. This result suggests that the large‐scale SST anomalies may be responsible for a significant component of the observed vacillation of sub‐Saharan rainfall. Some preliminary GLA GCM simulation results that support the above findings are also presented.  相似文献   

19.
Over 20 million forest farmers practice slash and burn agriculture in the forests of the Congo Basin (Bahuchet and de Maret, 1995. State of Indigenous Populations Living in Rainforest Areas, European Commission DG XI Environment, Brussels). They rely on the long dry season (December–February north of the equator) to ensure that their new fields, cleared from regrowth forest, burn sufficiently well to deposit nutrients into the soil and to minimize the labor required to prepare the field for planting. Data from the Ituri forest in northeastern Democratic Republic of Congo show that the strength of the annual dry season (a) has a direct positive impact on the size of fields cleared each year by slash and burn farmers, and consequently on food production and the severity of the subsequent year's pre-harvest hunger period; and (b) is inversely related to total annual rainfall. These results suggest that the 1 mm/d increase in rainfall predicted for much of the Congo Basin by the 2050s may cause a basin wide increase in the frequency of heavy rains during the dry season, causing a reduction in the size of slash and burn farmers’ fields, and potentially a substantial increase in the food insecurity of poor rural families across the region.  相似文献   

20.
Regional variations in fluctuations of seasonal rainfall over Nigeria   总被引：1，自引：0，他引：1  
Summary Previous work on rainfall variations over Nigeria has concentrated on country-wide averages, which approach has tended to mask the regional contrasts in the country's rainfall. In this paper, variations of rainy season rainfall over the Southern, Middle Belt, and Northern regions of Nigeria as well as the country as a whole are examined over a 72-year secular period (1916–1987). The extent and nature of nonrandom changes, such as fluctuations, trend and persistence, are investigated.The trend analysis showed a tendency towards decreasing seasonal rainfall totals in all the regions, though only those for the Northern region and the country as a whole were significant. No significant persistence was however evident in any of the regions.Power spectrum analysis revealed the occurrence of significant oscillations with time periods of 2.53 to 2.67 and 3.69 to 4.36 years only in the Middle Belt, and 8.00 to 9.60 years in all the regions. The climatological identity of the Middle Belt and the implications of its rainfall oscillations for the large scale agricultural projects planned for the region are pointed out.With 3 Figures  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号