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1.
Summary Evidence for changes in the annual and growing season rainfall series for the period 1919 to 1985 in Nigeria are examined on a regional basis, using power-spectral and lowpass filter techniques, and the Mann-Kendall rank statistic. Four regions, the Coastal Zone, the Guinea-Savanna Zone, the Midland area and the Sahel, are used in the investigation of rainfall variation from south to north across the country.Quasi-periodic oscillations in the annual and growing season rainfall series are found to be concentrated in four spectral bands: 2.0–2.4, 2.7–2.9, 3.2–3.6 and 5.6–6.3 years. The spatial coherence of the fluctuations in annual and growing season rainfall is found to be limited to Nigeria south of 11 degrees north latitude. Evidence also emerges of a progressive decline in annual and growing season rainfall for northern Nigeria, north of nine degrees north latitude, for the period 1939–1985.With 5 Figures 相似文献
2.
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
3.
Rainfall characteristics during the annual rainy season are explored for the Mzingwane catchment of south-western Zimbabwe, for both historic period (1886–1906) and more recent times (1950–2015), based on available daily and monthly precipitation series. Annual and seasonal rainfall trends are determined using the modified Mann-Kendall test, magnitude of trends test and Sen’s slope estimator. Rainfall variability is quantified using the coefficient of variation (CV), precipitation concentration index (PCI) and standard precipitation index (SPI). Results suggest that contemporary mean annual rainfall may not have changed from that measured during the historic period of 1886–1906. However, the number of rainy days (≥ 1 mm) has decreased by 34%, thus suggesting much more concentrated and increased rainfall intensity. A notable shift in both the onset and cessation dates of the rainy season is recorded, particularly during the twenty-first century, which has resulted in a significantly reduced (p < 0.05) length of the rainy season. The combination of a reduced number of rainy days (≥ 1 mm) and a shortened rainy season suggests that long intra-season dry spells have become more common through time and have considerable negative consequences for agriculture and wetland ecosystem in the region. In addition, high spatio-temporal rainfall variability and seasonal PCI values indicate strong seasonality in the rainy season. Based on the SPI results, the El Niño Southern Oscillation (ENSO) strongly influences rainfall variability. The results further suggest high uncertainty in rain season characteristics, which requires effective planning for water needs. 相似文献
4.
Changes of Accumulated Temperature, Growing Season and Precipitation in the North China Plain from 1961 to 2009 
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下载免费PDF全文 Using the high-quality observed meteorological data, changes of the thermal conditions and precipitation over the North China Plain from 1961 to 2009 were examined. Trends of accumulated temperature and negative temperature, growing season duration, as well as seasonal and annual rainfalls at 48 stations were analyzed. The results show that the accumulated temperature increased significantly by 348.5℃ day due to global warming during 1961-2009 while the absolute accumulated negative temperature decreased apparently by 175.3℃ day. The start of growing season displayed a significant negative trend of -14.3 days during 1961- 2009, but the end of growing season delayed insignificantly by 6.7 days. As a result, the length of growing season increased by 21.0 days. The annual and autumn rainfalls decreased slightly while summer rainfall and summer rainy days decreased significantly. In contrast, spring rainfall increased slightly without significant trends. All the results indicate that the thermal conditions were improved to benefit the crop growth over the North China Plain during 1961-2009, and the decreasing annual and summer rainfalls had no direct negative impact on the crop growth. But the decreasing summer rainfall was likely to influence the water resources in North China, especially the underground water, reservoir water, as well as river runoff, which would have influenced the irrigation of agriculture. 相似文献
5.
近50年广东省分级降水的时空分布特征及其变化趋势的研究 总被引：6，自引：2，他引：6
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
6.
D. O. Adefolalu 《Theoretical and Applied Climatology》1986,37(4):205-219
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.
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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.
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7.
The study has analyzed the variability and trends in monthly, seasonal and annual rainfall and rainy days of four locations over different agro-ecological zones of Bihar, namely Samastipur (zone-I), Madhepura (zone-II), Sabour (zone-IIIA) and Patna (zone-IIIB). The Mann–Kendall nonparametric test was employed for detection of statistical significance and slopes of the trend lines were determined using the method of least square linear fitting. The variability and trends of onset of effective monsoon and length of monsoon period were also analyzed using the same method. The mean annual rainfall varies from 1137 mm at Patna to 1219 mm at Sabour. July is the rainiest month in all the zones followed by August. Maximum increase in annual rainfall was found at Sabour (40.1% of mean/30 years at 95% confidence level) and minimum for Patna (10.1% of mean/30 years). Significant increasing trend of rainfall during July, August and September at rates of 41.9, 83.2, and 112.7% of the mean/30 years, respectively has been noticed at Madhepura. Analysis of rainy days indicates that rainy days increased during winter and annually for all the sites. The mean effective onset of monsoon varies from 18th June at Sabour to 28th June at Patna. The trends in the date of effective onset of monsoon indicate that the date tends to be early in all the sites except Madhepura. But a significant delayed trend in the onset at a rate of 2.8% of the mean/30 years has been observed for Madhepura. The early trend of the effective onset of monsoon and increasing trends of length of monsoon season have been observed for Samastipur, Sabour and Patna. 相似文献
8.
One of the major dilemmas in rainfall-regime research is regarding the representativity of rainfall amounts. The annual total (TOTAL) represents the summation of the contributions of all rainfall events. However, this parameter is obtained mainly by contributions of some major events, whereas, many others may contribute minute quantities of a negligible impact. A selected daily rainfall threshold (DRT) is used to filter these small events and to retain only the effective ones (for a wide range of activities), and enable focusing on them in order to better detect any important temporal changes in the rainfall regime. Two different procedures are applied to the data in order to filter out slight and/or sporadic rains: (a) definition of a DRT for each station which filters out the slight rains all year around regardless of their timing, and (b) definitions of the rainy season beginning date (RSBD) and the rainy season ending date (RSED), which filter out sporadic rain events at the beginning or the end of the rainy season, regardless of their amount. The current study presents the appropriate DRT and effective rainy season length (RSL) in 41 Mediterranean stations, and analyzes their influence on some other parameters. The main conclusions are: (1) the filtering process reduced considerably the number of rain-spells (NRS) and therefore the average intensities increased; (2) the filtering process reduced the average time that elapsed between the beginning of two consecutive rain-spells, and (3) the RSL within the study area has a clear spatial distribution, longer in the northern parts and shorter in the southern parts. 相似文献
9.
Using the high-quality observed meteorological data,changes of the thermal conditions and precipitation over the North China Plain from 1961 to 2009 were examined.Trends of accumulated temperature and negative temperature,growing season duration,as well as seasonal and annual rainfalls at 48 stations were analyzed.The results show that the accumulated temperature increased significantly by 348.5℃ day due to global warming during 1961 2009 while the absolute accumulated negative temperature decreased apparently by 175.3℃ day.The start of growing season displayed a significant negative trend of-14.3 days during 1961-2009,but the end of growing season delayed insignificantly by 6.7 days.As a result,the length of growing sea.son increased by 21.0 days.The annual and autumn rainfalls decreased slightly while summer rainfall and summer rainy days decreased significantly.In contrast,spring rainfall increased slightly without significant trends.All the results indicate that the thermal conditions were improved to benefit the crop growth over the North China Plain during 1961-2009,and the decreasing annual and summer rainfalls had no direct negative impact on the crop growth.But the decreasing summer rainfall was likely to influence the water resources in North China,especially the underground water,reservoir water,as well as river runoff,which would have influenced the irrigation of agriculture. 相似文献
10.
Seyni Salack Alessandra Giannini Moussa Diakhaté Amadou. T. Gaye Bertrand Muller 《Climate Dynamics》2014,42(1-2):189-201
Intra-seasonal drought episodes (extreme dry spells) are strongly linked to crop yield loss in the West African Sahel, especially when they occur at crop critical stages such as juvenile or flowering stage. This paper seeks to expose potentially predictable features in the sub-seasonal to inter-annual occurrence of “extreme dry spells” (extDS) through their links to sea surface temperature anomalies (SSTAs). We consider two kinds of extreme dry spells: more than 2 weeks of consecutive dry days following a rain event (often found at the beginning of the rainy season, after the first rain events) and more than a week (observed towards the end of the rainy season, before the last rain events). We extract dry spells from daily rainfall data at 43 stations (31 stations in Senegal over 1950–2010 and 12 stations in Niger over 1960–2000) to identify the intra-seasonal distribution of extDS and their significant correlation with local rainfall deficits. Seasonality of distribution and high spatial coherence are found in the timing and the frequency of occurrence of extDS in different rainfall regions over Niger and Senegal. The correlation between the regional occurrence index (ROI), necessary to capture the spatial extent of extDS, and observed global sea surface temperature anomalies (SSTAs) sheds light on the influence of the external factors on the decadal, interannual and sub-seasonal variability of extDS over the West African Sahel. When the global tropics and the Atlantic are warmer than normal, more coherent and delayed June–July extDS are observed after onset of rainy season, as well as early cessation type in August–September. When the Indo-Pacific is cooler and the equatorial south Atlantic is warmer than normal little to no extDS are found in the onset sub-period of the monsoon season. Mostly late types of extDS occur in October as a result of late cessation. These results show potential predictability of extreme dry spells after onset and before cessation of monsoonal rain based on global patterns of sea surface temperature anomalies. 相似文献
11.
The climatic characteristics of the precipitation in Guangdong province over the past 50 years were analyzed based on the daily rainfall datasets of 86 stations from 1961 to 2010. The rainfall was divided into five categories according to its intensity, and their spatiotemporal characteristics and variation trends were investigated. The annual rainfall amount was within 1,500 to 2,000 mm over most parts of Guangdong, but substantial differences of rainfall amount and rainy days were found among different parts of the province. There were many rainy days in the dry seasons (October to March), but the daily rainfall amounts are small. The rainy seasons (April to September) have not only many rainy days but also heavy daily rainfall amounts. The spatial distributions of light rainy days (1 mm 100 mm) are generally concentrated in three regions, Qingyuan, Yangjiang, and Haifeng/Lufeng. The average rainfall amount for rainy days increases form the north to the south of Guangdong, while decreasing as the rainfall intensity increases. The contributions from light, moderate and heavy rain to the total rainfall decreases form the north to the south. The annual rainy days show a decreasing trend in the past 50 years. The light rainy days decreased significantly while the heavy, rainstorm and downpour rainy days increased slightly. The annual total rainfall amount increased over the past 50 years, which was contributed by heavy, rainstorm and downpour rains, while the contribution from light and moderate rains decreased. 相似文献
12.
Sharon E. Nicholson 《Climatic change》2001,50(3):317-353
This article describes a historical archive of proxy and actual precipitation data that extends the African climate record back to the early nineteenth century. The `proxy' archive includes verbal, documentary references which contain information related to rainfall conditions, such as references to famine, drought, agriculture or the nature of the rainy season. The precipitation archive includes all observations made in Africa during the nineteenth century. It consists of records for 60 stations in Algeria, 87 stations in South Africa and 304 stations scattered over the rest of Africa. Information is particularly plentiful for the 1880s and 1890s. The two parts have been be combined into a semi-quantitative regional data set indicating annual rainfall conditions in terms of anomaly classes (e.g., normal, dry, wet). This data set extends from the early nineteenth century to 1900 and distinguishes seven anomaly classes, using numbers ranging from –3 to +3 to represent very wet, wet, good rains, normal, dry, drought, and severe drought. The regionalization is based on 90 geographical regions shown via studies of the modern precipitation record to be climatically homogeneous with respect to the interannual variability of rainfall. The regional aggregation allows the voluminous fragmentary information available in historical sources to be used systematically to produce multi-year time series that can be directly integrated into the modern record for each region. The resultant time series can also be subjected to statistical analysis, in order to investigate nineteenth century climate over Africa. Spatial detail is added to the data set by utilizing a unique methodology based on climatic teleconnections established from studies of rainfall variability over Africa. The historical information and station records have been combined into a file containing a regional anomaly value for up to 90 geographic regions and the years 1801–1900. Gaps necessarily remain in the matrix, but as early as the 1820s over 40 regions are represented. By the 1880s generally around 70 regions or more are represented. 相似文献
13.
Temporal rainfall variability in the Lake Victoria Basin in East Africa during the twentieth century
Michael Kizza Allan Rodhe Chong-Yu Xu Henry K. Ntale Sven Halldin 《Theoretical and Applied Climatology》2009,98(1-2):119-135
Water resources systems are designed and operated on assumption of stationary hydrology. Existence of trends and other changes in the data invalidates this assumption, and detection of the changes in hydrological time series should help us revise the approaches used in assessing, designing and operating our systems. In addition, trend and step change studies help us understand the impact of man’s activities (e.g. urbanisation, deforestation, dam construction, agricultural activities, etc.) on the hydrological cycle. Trends and step changes in the seasonal and annual total rainfall for 20 stations in the Lake Victoria basin were analysed. The seasonal rainfall for any station in a given year was defined in two ways: (1) fixed time period where the rainy seasons were taken as occurring from March–May (long rains) and from October–December (short rains); and (2) variable periods where the rainy seasons were taken as the three consecutive months with maximum total rainfall covering the period of January–June (long rains) and July–December (short rains), to take into account the fact that the onset of rainy seasons within the basin varies from year to year and from one station to the next. For each station, sub datasets were derived covering different periods (all available data at the station, 1941–1980, 1961–1990, 1971–end of each station’s time series). The trends were analysed using the Mann-Kendall method, while the step changes were analysed using the Worsley Likelihood method. The results show that positive trends predominate, with most stations showing trend being located in the northern part of the basin, though this pattern is not conclusive. In all, 17% of the cases have trends, of which 67% are positive. The 1960s represent a significant upward jump in the basin rainfall. Seasonal rainfall analysis shows that the short rains tend to have more trends than the long rains. The impact of the varying month of onset of the rainy season is that the results from analyzing the fixed-period and variable-period time series are rarely the same, meaning the two series have different characteristics. It may be argued that the variable-period time series are more reliable as a basis for analysing trends and step changes, since these time series reflect more closely the actual variability in rainy seasons from one year to the next. The fixed-period analysis would, on the other hand, find more practical use in planning. 相似文献
14.
基于对汛期的理解和认识, 利用Samel等人设计的半客观统计分析方法、Mann-Kendall突变分析、滑动t检验等方法, 通过分析和研究1957—2006年华北台站的日降水资料, 确定了华北汛期起讫的日期。结果表明:华北汛期始于6月30日, 止于8月18日, 持续期为50d。华北汛期的起讫日期、持续天数以及空汛发生的频次, 具有鲜明的地域特征:冀北山地汛期开始最早, 结束较迟, 持续天数较长, 空汛发生频次最少; 黄土高原汛期开始较迟, 其北部汛期结束最迟, 持续期也最长, 发生空汛的频次也比较多; 黄河下游地区汛期开始比较早, 结束最早, 汛期最短, 发生无大汛的频次较大; 河北平原地区, 汛期开始最迟, 结束较迟, 汛期较长, 发生无大汛的频次最多。与华北汛期开始和结束日相对应的东亚大气环流特征是:当西太平洋西部上空500hPa存在正的位势高度距平, 华北上空存在负的位势高度距平, 同时地面为“东高西低”的异常海平面气压场配置时, 异常偏南风到达30°N, 华北汛期开始; 当华北上空500hPa为较小的位势高度正距平, 日本海为位势高度正距平, 而地面上, 我国大陆和西太平洋之间为“西高东低”的异常海平面气压场配置时, 异常偏北风控制我国东部地区, 华北汛期结束。 相似文献
15.
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
16.
基于Copula函数的中国南方干旱风险特征研究 总被引：2，自引：0，他引：2
利用1961—2012年1710个地面气象台站观测的逐日降水资料,对中国大陆雨季特征进行了分析。结果表明,雨季最早到来的地方是藏东南、滇西北的横断山脉中西部地区;接着江南地区进入雨季;之后随着夏季风的爆发,全国大部分地区进入雨季。而雨季结束最早的地区在江南地区,较迟结束的地区在华西地区、南海地区,最迟是在新疆西北部地区。就雨季持续时间而言,华北地区和西藏西部及其与新疆南部相接的部分地区雨季持续时间最短,横断山脉中西部地区、华西地区和南海地区雨季较长。通过对多年平均逐候降水进行经验正交函数(EOF)分解,能较好地将主要的降水季节演变特征分离出来,分析发现,第1、第2和第3模态分别反映了夏季、春季和秋季降水为主要特征的降水空间结构。 相似文献
17.
Summary The object of this study is the determination of the number of rainy days in the area of the Aegean sea, based on data obtained from 14 observation stations during the period 1950–1975.The results showed considerable differences from North to South. The seasonal number of rainy days as well as that of rainy spells were examined for the period 1950–1975. The spells of rainy days play an important role in the agricultural activities, especially over the southern part of the Aegean sea basin, where the annual amounts of rainfall are insufficient.From the data was concluded that during the summer, more than four consecutive rainy days were recorded at the northern most stations only and these of infrequent occurrence; whereas during the winter, it was possible to encounter up to 15 consecutive rainy days.Finally, we give a theoretical distribution for the rainy spells for each station and for each season and year, using Polya's method. We found that this distribution fitted well the 95% of the confidence level in the majority of the cases.With 2 Figures 相似文献
18.
19.
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
20.
Summary Variability of rainfall in the semi-arid regions can cause problems in evaluating experimental trials. To describe the spatial rainfall patterns over a large experimental station, rainfall was monitored during the 1986 and 1987 rainy seasons using 18 raingages over the 500 ha experimental station of ICRISAT Sahelian Center, in Niger, West Africa. Average relative variability of individual rain storms, defined as the percentage deviation from the mean, varied from 2 to 62%, while the variability over the rainy season was 17.1%. Isohyetal patterns of individual rain storms as well as seasonal totals showed distinct coherence in the spatial pattern over the station. The effects of total volume, duration, direction and intensity of storms and the time of year on the spatial correlations were analyzed. Storm value showed a large influence on the correlation decay with distance. Correlations in the W — E and SW — NE directions were higher in comparison to those in the N — S and NW — SE directions. Point rainfall measurements were better correlated with the network average rainfall than with the rainfall recorded at the meteorological station. Variograms among raingages revealed that the distance of independence was approximately 1 000 m for almost all storms. Use of a network of raingages over agricultural experiment stations reduces the average relative variability of areal rainfall estimates and provides a means to develop simple relations for estimation of point rainfall for individual applications.With 8 Figures 相似文献