Seasonal Rainfall Trends Research Articles

Identification and trend analysis of homogeneous rainfall zones over the East Asia monsoon region

Rainfall is a complex phenomenon with high spatiotemporal variability. Identification of homogeneous rainfall zones to better analyse the rainfall intensity and extent is of vital significance for water resources management and mitigation of potential hazards from extreme events, i.e. droughts and floods. Appropriate zoning of homogeneous rainfall regions may give better understanding of rainfall patterns by resolving small scale variations. Although homogeneous rainfall zones have been established at country scale based on climatological mean behaviour, there has been little attempt to identify zones over broader scale with consistently homogeneous rainfall variability. This study employed K-means and Hierarchical clustering methods to establish homogeneous rainfall zones in the East Asia monsoon region (20 ∘ N-50 ∘ N, 103 ∘ E-149 ∘ E) using 30 years (1978-2007) monthly rainfall data at 0.5 ∘ grid resolution. Various cluster validation indices were used to assess the optimal number of homogeneous rainfall zones. The comparison of K-means and Hierarchical clustering showed that although both methods were able to define the homogeneous rainfall zones well with spatial contiguity, the K-means clustering outperformed the Hierarchical clustering in identifying more distinct zones with diverse rainfall characteristics. Mann-Kendall and linear regression tests were used for seasonal and annual rainfall trend analysis in the homogeneous rainfall zones. The study revealed that the region experiences distinct rainfall regimes over different zones. Furthermore, significant increasing and decreasing trends were observed over different zones with strong seasonal variation that indicate the aggravated stress of climate induced disasters, i.e. droughts and floods over the East Asia monsoon region.

Analysis of monthly and seasonal rainfall trends using the Holt's test

Climate change is a global issue that has impact on every living being in the world. One of the most noticeable consequences of these global phenomena is the inevitable water cycle modification, with precipitation being a major component in these processes. Consequently, research into the occurrence and distribution of precipitation has increased over the past few decades. As Malaysia is located in the tropical area where there is no distinct four seasons, analysing rainfall has therefore become one of the common assessment tools for climate change. In this study, rainfall trends of Langat River Basin were examined on a monthly and seasonal basis (monsoon and non-monsoon) for the period of 1970–2012. Only rainfall time series with duration more than 25 years and missing data less than 10% have been considered for this study. The Holt's test has been employed to model the rainfall trends for the 10 selected time series; while Kendall's Tau test and Spearman's Rho test were used to test, compare and support for the significance of the trends. For monthly rainfall trends analysis, it was found that March, July and November are among the months those have most of the stations with increasing rainfall trends; while May and September are the months with the highest number of stations showing decreasing rainfall trends. Specifically, station 2815001 shows the highest number of months with changing rainfall trends throughout the year; while station 44255 has the least number of months with changing rainfall trends. Based on the seasonal rainfall trend analysis, there are seven stations during the Northeast Monsoon that revealed upward trends and the result is found to be consistent with the monthly rainfall trend analysis.

Relating changes in synoptic circulation to the surface rainfall response using self-organising maps

The climate of a particular region is governed by factors that may be remote, such as the El Nino Southern Oscillation or local, such as topography. However, the daily weather characteristics of a region are controlled by the synoptic-scale atmospheric state. Therefore changes in the type, frequency, duration or intensity of particular synoptic states over a region would result in changes to the local weather and long-term climatology of the region. The relationship between synoptic-scale circulation and the rainfall response is examined for a 31-year period at two stations in different rainfall regimes in South Africa. Dominant rain-bearing synoptic circulations are identified for austral winter and summer as mid-latitude cyclones and convective systems respectively whereas no circulations are dominantly associated with spring and autumn rainfall. Over the 31-year period a statistically significant increase in the frequency of characteristic summer circulation modes is observed during summer, winter and spring. During autumn a statistically significant shift towards characteristically winter circulation modes is evident. Seasonal rainfall trends computed at each station corroborate those of the circulation data. Extreme rainfall is associated with particular circulation modes and trends in both circulation and station data show an earlier occurrence of extreme rainfall during the rainy season.

Long term spatial and temporal rainfall trends and homogeneity analysis in Wainganga basin, Central India

Gridded rainfall data of 0.5×0.5° resolution (CRU TS 3.21) was analysed to study long term spatial and temporal trends on annual and seasonal scales in Wainganga river basin located in Central India during 1901–2012. After testing the presence of autocorrelation, Mann–Kendall (Modified Mann–Kendall) test was applied to non-auto correlated (auto correlated) series to detect the trends in rainfall data. Theil and Sen׳s slope estimator test was used for finding the magnitude of change over a time period. For detecting the most probable change year, Pettitt–Mann–Whitney test was applied. The Rainfall series was then divided into two partial duration series for finding changes in trends before and after the change year. Arc GIS was used to explore spatial patterns of the trends over the entire basin. Though most of the grid points shows a decreasing trend in annual rainfall, only seven grids has a significant decreasing trend during 1901–2012. On the basis of seasonal trend analysis, non-significant increasing trend is observed only in post monsoon season while seven grid points show significant decreasing trend in monsoon rainfall and non-significant in pre-monsoon and winter rainfall over the last 112 years. During the study period, overall a 8.45% decrease in annual rainfall is estimated. The most probable year of change was found to be 1948 in annual and monsoonal rainfall. There is an increasing rainfall trend in the basin during the period 1901–1948, which is reversed during the period 1949–2012 resulting in decreasing rainfall trend in the basin. Homogeneous trends in annual and seasonal rainfall over a grid points is exhibited in the basin by van Belle and Hughes׳ homogeneity trend test.

Observed and modelled trends in rainfall and temperature for South Africa: 1960–2010

Observed trends in seasonal and annual total rainfall, number of rain days and daily maximum and minimum temperature were calculated for a number of stations in South Africa for the period 1960–2010. Statistically significant decreases in rainfall and the number of rain days are shown over the central and northeastern parts of the country in the autumn months and significant increases in the number of rain days around the southern Drakensberg are evident in spring and summer. Maximum temperatures have increased significantly throughout the country for all seasons and increases in minimum temperatures are shown for most of the country. A notable exception is the central interior, where minimum temperatures have decreased significantly. Regionally aggregated trends for six water management zones covering the entire country are not evident for total rainfall, but there are some significant trends for the number of rain days. Temperature in these zones has increased significantly for most seasons, with the exception of the central interior. Comparison of the observed trends with statistically downscaled global climate model simulations reveals that the models do not represent the observed rainfall changes nor the cooling trend of minimum temperature in the central interior. Although this result does not rule out the possibility of attributing observed local changes in rainfall to anthropogenically forced global change, it does have major implications for attribution studies. It also raises the question of whether an alternative statistical downscaling method or dynamical downscaling through the use of a regional climate model might better represent regional and local climatic processes and their links to global change.

Trends In Seasonal Precipitation And Temperature – A Review In Doti And Surkhet Districts Of Nepal

Rainfall and temperature are the crucial factors for affecting the yield of cereals. The annual mean temperature in Surkhet and Doti district is 21.47°C and 22.84°C respectively. The annual total rainfall in Doti and Surkhet district is 1145 mm and 1609 mm respectively. The average seasonal rainfall in (July-November) in Doti and Surkhet is 670.7 mm and 1109 mm. The average seasonal rainfall in (November-March) in Doti and Surkhet is 168 mm and 135 mm. The average seasonal rainfall in (March-June) in Doti and Surkhet is 352 mm and 389 mm. The average seasonal mean temperature in (July-November) in Doti and Surkhet is 25.76°C and 23.7°C. The average seasonal mean temperature in (November-March) in Doti and Surkhet is 16.67°C and 15.48°C. The average seasonal mean temperature in (March-June) in Doti and Surkhet is 25.5°C and 24.69°C. The analysis of meteorological data indicated that the temperature of both districts is on a rising trend. It has been observed no any trend in total seasonal rainfall during wheat and barley growing season (November-March) in Surkhet. Also, the seasonal total rainfall during maize growing period (March-June) has not shown any trend in Doti. Besides, decreasing trend of annual rainfall has been observed in Doti whereas increasing trend of annual rainfall is observed in Surkhet. The temperature in 1985 in Doti and Surkhet has increased and the rainfall in the same year and same districts has decreased that has negatively affected the yield of all the major cereals except wheat. International Journal of Environment, Volume-2, Issue-1, Sep-Nov 2013, Pages 269-279 DOI: http://dx.doi.org/10.3126/ije.v2i1.9227

Long-term trends and variability of rainfall extremes in the Philippines

Owing to the increasing concerns about climate change due to the recent extreme rainfall events in the Philippines, long-term trends and variability in rainfall extremes in the country are investigated using 60-year (1951–2010) daily rainfall data from 35 meteorological stations. Rainfall extremes are described using seven extreme precipitation indices (EPI) that characterize daily rainfall in terms of intensity, accumulation, and duration on a seasonal perspective. The nonparametric Mann–Kendall test is employed in combination with the moving blocks bootstrapping technique to detect significant trends in EPI. The results suggest a tendency toward a drying condition for the dry season, January–March (JFM), as indicated by statistically significant decreasing trends in seasonal wet days total rainfall (PCPTOT) associated with increasing trends in maximum length of dry spell (LDS). In contrast, statistically significant increasing trends in maximum 5-day rainfall (RX5day) and decreasing trends in LDS denote a wetting condition during the July–September (JAS) season, particularly at stations located in the northwest and central Philippines. The trends obtained are further assessed by examining the longer time series of EPI at four meteorological stations (Aparri, Dagupan, Iloilo, and Masbate) that have rainfall data from 1911 to 2010. The longer historical data revealed that the trends obtained in the shorter period (1951–2010) could either be consistent with the continuous long-term trends, as observed in RX5day during JAS at Aparri and Masbate, or represent interdecadal variability as was observed at Dagupan and Iloilo. The long-term (1911–2010) southwestward extension of the western North Pacific subtropical high associated with a weakening of the 850-hPa westerly wind over the South China Sea partly provides a possible cause of the trends in EPI during JAS, whereas the weakening of the East Asian winter monsoon contributed somewhat to the trends obtained during JFM. Furthermore, interannual variations in EPI are found to be influenced greatly by the El Niño–Southern Oscillation (ENSO). Composite analyses suggest that El Niño (La Niña) events are associated with statistically significant drier (wetter) conditions over the Philippines, especially during the seasons close to ENSO mature stage.

Temporal and spatial variability of annual and seasonal rainfall over Ethiopia

Characterization of the seasonal and inter-annual spatial and temporal variability of rainfall in a changing climate is vital to assess climate-induced changes and suggest adequate future water resources management strategies. Trends in annual, seasonal and maximum 30-day extreme rainfall over Ethiopia are investigated using 0.5° latitude × 0.5° longitude gridded monthly precipitation data. The spatial coherence of annual rainfall among contiguous rainfall grid points is also assessed for possible spatial similarity across the country. The correlation between temporally coinciding North Atlantic Multidecadal Oscillation (AMO) index and annual rainfall variability is examined to understand the underlying coherence. In total 381 precipitation grid points covering the whole of Ethiopia with five decades (1951–2000) of precipitation data are analysed using the Mann-Kendall test and Moran spatial autocorrelation method. Summer (July–September) seasonal and annual rainfall data exhibit significant decreasing trends in northern, northwestern and western parts of the country, whereas a few grid points in eastern areas show increasing annual rainfall trends. Most other parts of the country exhibit statistically insignificant trends. Regions with high annual and seasonal rainfall distribution exhibit high temporal and spatial correlation indices. Finally, the country is sub-divided into four zones based on annual rainfall similarity. The association of the AMO index with annual rainfall is modestly good for northern and northeastern parts of the country; however, it is weak over the southern region. Editor Z.W. Kundzewicz; Associate editor S. Uhlenbrook Citation Wagesho, N., Goel, N.K., and Jain, M.K. 2013. Temporal and spatial variability of annual and seasonal rainfall over Ethiopia. Hydrological Sciences Journal, 58 (2), 354–373.

Multiple time scale characteristics of rainfall and its impact on soil salinization in the typical easily salinized area in Huang-Huai-Hai Plain, China

On the basis of monthly rainfall data over 50 years, we analyzed temporal properties of annual and seasonal rainfall variability by using and comparing two kinds of continuous wavelet transforms. Furthermore, we deduced its impacts on soil salinization in the area of Huang-Huai-Hai Plain, China. Results showed that the annual rainfall collected by the studied area showed a decreasing trend during period 1960–2009 at a climatic tendency rate of 14.43 mm/10 years. The Morlet wavelet is better at capturing local feature of rainfall oscillatory behaviors at fine time scales, and the Mexican Hat wavelet is better at revealing global feature of rainfall period. Combining Wavelet transform with wavelet power spectrum and its confidence test, we found the 8-year dominant period of annual rainfall and the 5-, 6-, 10- and 12-year dominant periods of seasonal rainfall, respectively. The annual and seasonal rainfall trends in the near future and their impacts on soil salinization are deduced based on the rainfall dominant periods and the prominent correlation between rainfall and soil salinity changes. The annual rainfall will still be in a relatively low period in the subsequent year after 2009 and then revert to a relatively high period during 2011–2014. The subsequent summer variability of rainfall may be helpful for reducing the soil salinization, while the opposite trends of spring and autumn rainfall variability may aggravate the soil salinization. This suggests sustainable irrigation and drainage measures to prevent soil salinization in this area.

Interdecadal Variability of Summer Rainfall in Taiwan Associated with Tropical Cyclones and Monsoon

Abstract This study investigates interdecadal variability of summer (June–August) rainfall in Taiwan for the 1950–2008 period. Summer rainfall in Taiwan is partitioned into two components: tropical cyclone (TC) rainfall caused by TC passage and seasonal monsoon rainfall associated with monsoon southwesterly flows. The joint interdecadal mode of TC rainfall and seasonal monsoon rainfall is extracted by empirical orthogonal function (EOF) analysis. The first interdecadal mode features an increasing trend plus a near-20-yr oscillation. The spatial patterns of this mode are uniform in sign over the entirety of Taiwan with positive anomalies for TC rainfall and negative anomalies for seasonal monsoon rainfall. These results reveal that TC rainfall and seasonal monsoon rainfall tend to vary inversely in interdecadal variability, with a positive trend in TC rainfall and a negative trend in seasonal monsoon rainfall. Large-scale regulating processes associated with this interdecadal rainfall mode are interpreted from the correlation patterns. Significant warm sea surface temperature (SST) anomalies exist in the tropical central and eastern Pacific and the Indian Ocean. At the low levels, an anomalous large-scale divergent center occurs in the Australian regions, which in turn evokes an anomalous cyclonic circulation in the subtropical North Pacific. Taiwan is on its western edge and affected by anomalous northeasterly flows, in company with weakening in the prevailing southwesterly flows and moisture transport from the South China Sea into Taiwan. As such, negative seasonal monsoon rainfall anomalies occur in Taiwan with a decreasing trend. The subtropical anomalous cyclonic circulation also weakens vertical wind shear over the major TC genesis region, that is, the Philippine Sea. Warm SST anomalies in this region and accompanying anomalous ascending motion provide additional favorable conditions for TC genesis. More TCs are thus formed in the Philippine Sea. The appearance of an anomalous cyclonic circulation in the subtropical North Pacific reflects a weakening of the Pacific subtopical high, which tends to retreat eastward and provides southeasterly or southerly flows on its western boundary to guide TCs formed in the Philippine Sea northwestward toward Taiwan. TC frequency and TC rainfall thus increase in Taiwan with an increasing trend.

Assessment of agricultural drought using a simple water balance model in the Free State Province of South Africa

Water Requirement Satisfaction Index (WRSI) at three different probability levels (20%, 50%, and 80%) was used to quantify drought affecting rain-fed maize production in the Free State Province of South Africa based on climate data from 227 weather stations. Results showed high spatial variability in the suitability of different areas: the southern and southwestern localities are unsuitable due to high drought incidences; the northern, central, and western regions are marginally suitable; the eastern, northerneastern areas and a few patches in the northwest are highly suitable with relatively low drought severity. Proper choice of maize varieties to suit conditions at different localities is crucial. The Mann–Kendall test and coefficient of variation were further used to determine trends and temporal variability, respectively, in the WRSI, seasonal rainfall, and seasonal maize water requirements. Results of this analysis revealed no significant positive trends in the WRSI, no significant negative trends in seasonal rainfall, and no significant positive trends in maize water requirements, contradicting previous findings of increased drought severity. Seasonal rainfall and the WRSI showed high interseasonal variability, while seasonal maize water requirements showed low variability. In view of these observations, it is apparent that realignment of management practices is an overdue prerequisite for sustainable maize production.

Trends in rainfall amount and number of rainy days in river basins of India (1951–2004)

This study aims to determine trends in annual and seasonal rainfall and rainy days over different river basins across India. The data used consists of daily gridded rainfall at 1° × 1° resolution for the period 1951–2004. Sen's non-parametric estimator of slope was used to estimate the magnitude of trend whose statistical significance was assessed by the Mann–Kendall test. Among 22 basins studied, 15 showed a decreasing trend in annual rainfall; only one basin showed a significant decreasing trend at 95% confidence level. Of the 6 basins showing an increasing trend, 1 basin showed a significant positive trend. The monsoon rainfall increased over 6 basins, decreased over 16 basins and a decreasing trend for 2 basins was found statistical significant. With the exception of Ganga, Brahmaputra and EFR4, all river basins experienced the same direction of trend in monsoon and annual rainfall. Four river basins experienced increasing (non-significant) trend in annual rainy days; three basins did not show any change in annual rainy days whereas 15 basins have shown a decreasing trend in annual rainy days. The decreasing trend in three basins was statistically significant. Most of the basins have shown the same direction of trend in rainfall and rainy days at the annual and seasonal scale.

Factors affecting the trends in evaporation during different crop growing seasons over India

Evaporation, rainfall and meteorological data for the period 1971-2004 for 58 well distributed stations over India were selected for the study. Trends of evaporation and rainfall in five regions, viz., Northwest, North, Northeast, Central and Peninsular regions of India during different crop growing seasons, viz., kharif, rabi and summer and the meteorological factors contributing towards the trend were analyzed. Annual evaporation shows decreasing trend in all the regions of the country. Trends in seasonal evaporation during kharif, rabi and summer seasons also showed decreasing trends in Northwest, North, Central and Peninsular regions of the country while few locations in Northeast India, viz., Guwahati, Dibrugarh and Tocklai showed significant increasing trend in evaporation during kharif and rabi seasons. No significant trend in annual and seasonal rainfall was observed in Indian region except a few stations in peninsular India where increasing trend was observed.
 Normalized anomalies of maximum temperature, relative humidity and vapour pressure showed increasing trend in Northwest and Northern regions during all the three crop growing seasons while decreasing trend or no trend in wind velocity was observed in all the regions except in central region where increasing trend was observed during summer season.
 As evaporation relates to the meteorological elements, viz., temperature, sunshine duration, wind velocity and relative humidity, the likely causative meteorological factors for such changes are studied. Increasing trends in maximum temperature was observed in central and peninsular inland regions of the country during rabi and summer seasons while slight decreasing trends were observed in the Northeast region during kharif season. High positive correlation found between maximum temperature and wind velocity indicates that the trend in evaporation is mostly influenced by these two factors. Increase in humidity and decrease in bright sunshine hours were both important and found correlated with the decrease in evaporation.

An Overview of the Second Generation Adjusted Daily Precipitation Dataset for Trend Analysis in Canada

A second generation adjusted precipitation daily dataset has been prepared for trend analysis in Canada. Daily rainfall and snowfall amounts have been adjusted for 464 stations for known measurement issues such as wind undercatch, evaporation and wetting losses for each type of rain-gauge, snow water equivalent from ruler measurements, trace observations and accumulated amounts from several days. Observations from nearby stations were sometimes combined to create time series that are longer; hence, making them more useful for trend studies. In this new version, daily adjustments are an improvement over the previous version because they are derived from an extended dataset and enhanced metadata knowledge. Datasets were updated to cover recent years, including 2009. The impact of the adjustments on rainfall and snowfall total amounts and trends was examined in detail. As a result of adjustments, total rainfall amounts have increased by 5 to 10% in southern Canada and by more than 20% in the Canadian Arctic, compared to the original observations, while the effect of the adjustments on snowfall were larger and more variable throughout the country. The slope of the rain trend lines decreased as a result of the larger correction applied to the older rain-gauges while the slope of the snow trend lines increased, mainly along the west coast and in the Arctic. Finally, annual and seasonal rainfall and snowfall trends based on the adjusted series were computed for 1950–2009 and 1900–2009. Overall, rainfall has increased across the country while a mix of non-significant increasing and decreasing trends was found during the summer in the Canadian Prairies. Snowfall has increased mainly in the north while a significant decrease was observed in the southwestern part of the country for 1950–2009.

Rainfall variability and the trends of wet and dry periods in Bangladesh

AbstractSpatial patterns of annual and seasonal rainfall trends of Bangladesh over the time period 1958–2007 has been assessed using rainfall data recorded at 17 stations distributed over the country. Mann–Kendall trend test and the Sen's slope method are used to detect the significance and the magnitude of rainfall change, respectively. Historical dry and wet months are identified by using standardised precipitation index method and their trends are analysed to assess the possible change in wet and dry events in Bangladesh. The result shows a significant increase in the average annual and pre‐monsoon rainfall of Bangladesh. The number of wet months is found to increase and the dry months to decrease in most parts of the country. Seasonal analysis of wet and dry months shows a significant decrease of dry months in monsoon and pre‐monsoon. Copyright © 2009 Royal Meteorological Society

Trends in extreme rainfall events of Bangladesh

A study of the variability of the extreme rainfall events in Bangladesh during the time period 1958–2007 has been carried out in this paper. Quality-controlled homogeneous daily precipitation records of nine stations distributed over Bangladesh are used for the study. A total of 15 annual and seasonal indices of rainfall are examined. Variability of annual and seasonal rainfall trends is also assessed. The Mann–Kendall statistic and Sen's Slope model are used to reveal the trends and estimate the magnitude of change, respectively. A significant increase of annual and pre-monsoon rainfall in Bangladesh is observed. In general, an increasing trend in heavy precipitation days and decreasing trends in consecutive dry days are observed. Significant change in most of the extreme rainfall indices are observed in Northwest Bangladesh.

Evidence for long‐term regional changes in precipitation on the East Coast Mountains in Mauritius

AbstractGlobal climate change and its impacts are being increasingly studied and precipitation trends are one of the measures of quantifying climate change especially in the tropics. This study uses daily rainfall data to determine if there are changes in the long‐term trends in rainfall variability in the East Coast Mountains of Mauritius during the last few decades, and to investigate the factors influencing the trends in the inter‐annual to inter‐decadal rainfall variability. Statistical modelling has been used to investigate the trends in total seasonal rainfall, the number of rain days and the mean amount of rain per rainy days and the local, regional and large‐scale factors that affect them on inter‐annual to inter‐decadal time scales. The strongest inter‐decadal trend was found in the number of rain days for both rainfall seasons, and the other variables were found to have weak or insignificant trends. Both local factors, such as the surrounding sea surface temperatures and large‐scale phenomena such as Indian Monsoon and the El Niño Southern Oscillation were found to influence rainfall patterns. Copyright © 2009 Royal Meteorological Society

Trends in seasonal and annual rainfall and rainy days in Kashmir Valley in the last century

The study has analysed seasonal and annual rainfall and rainy days at five stations namely Srinagar, Kulgam, Handwara, Qazigund and Kukarnag to decipher rainfall trends over the Kashmir Valley. Rainfall data collected by the India Meteorological Department (IMD) were used. Due to the varying length of the available data, analysis was performed for two common periods: 1903–1982 (80 years) at three stations and 1962–2002 (41 years) at three stations. The 102 years of data at Srinagar were also analysed to examine the trends for last century. Time series of annual and seasonal rainfall/rainy days were examined for trends by analysis of anomalies and application of statistical tests. During the period 1903–1982, Srinagar, Kulgam and Handwara stations experienced a decreasing trend in annual rainfall; the maximum decrease was found for Kulgam (−20.16% of mean/100 years) and minimum for Srinagar (−2.45% of mean/100 years). All three stations showed a decreasing trend in monsoon and winter rainfall and an increasing trend in pre-monsoon and post-monsoon seasonal rainfall. The decreasing trend in winter rainfall was found to be statistically significant (95% confidence) at Kulgam and Handwara, whereas none of the increasing trend in the pre-monsoon and post-monsoon season was significant. Srinagar and Handwara witnessed a decreasing (non-significant) trend in annual rainy days, whereas Kulgam experienced the opposite trend. All the stations experienced a decreasing trend in monsoon and winter rainy days. Qazigund and Kukarnag experienced decreasing annual rainfall, and Srinagar showed increasing annual rainfall during the period 1962–2002. Pre-monsoon and post-monsoon rainfall decreased at all three stations. None of the increasing/decreasing trends were found to be significant. Annual, pre-monsoon, post-monsoon and winter rainfall increased (non-significant) whereas monsoon rainfall decreased (non-significant), at Srinagar during the last century.

Eighty years of meteorological observations at Wageningen, the Netherlands: precipitation and evapotranspiration

AbstractAnnual and seasonal characteristics of precipitation and evapotranspiration were analysed of an 80‐year data record (1928–2008) at Wageningen (the Netherlands). The precipitation series shows a mean annual rainfall of 765 ± 130 mm with a relatively high interannual variability [coefficient of variation (CV) = 17.0%]. Both the annual and seasonal rainfall trends show a small but statistically insignificant increase. Potential evapotranspiration was estimated by Makkink's formula, using observations of incoming solar radiation and air temperature as inputs. This provides a mean evapotranspiration of (525 ± 50) mm annum−1 with a relatively low interannual variability (CV = 9.5%). The annual and seasonal trends appear to be statistically significant, except for the summer season. In addition, since 1992, actual evapotranspiration is measured by the eddy‐covariance technique and these results were found to be highly correlated with the potential evapotranspiration (Eact ≈ 0.75Epot). A comparison is made with a class A evaporation pan operating between 1984 and 1989. Copyright © 2009 Royal Meteorological Society

Rainfall Variability and Agricultural Vulnerability in the Amhara Region, Ethiopia

Ethiopian agriculture is mostly rain fed, whereas inter-annual and seasonal rainfall variability is high and droughts are frequent in many parts of the country. Rainfall variability has historically been a major cause of food insecurity and famines in the country. Surprisingly, however, the relationships between rainfall variability and fluctuations in agricultural production at regional and sub-regional scales have not been studied in detail. The objective of this study is to analyze rainfall variability and trends, and examine vulnerability of food grain production to rainfall variability in the Amhara Region of Ethiopia. The data used for the study were historical rainfall records from 12 stations (for the 1975-2003 period) and available time series data on area coverage, production and yield of cereals during the meher season- main growing season (for the 1994-2003 period). The results reveal that there are significant intra-regional differences in rainfall amount, variability and trend; and the variability increases as rainfall amount decreases. Rainfall amount is higher and its variability lower in the western part of the region than in the eastern. Four drought years have occurred over the period 1975-2003, of which two were extreme in severity. Examination of trends in annual and seasonal rainfall generally shows absence of any systematic patterns of change across the region. Significant correlations were observed between the seasonal rainfall and crop production. Inter-annual and seasonal rainfall variability is therefore an important influence on food security of farming households; it also suggests that predicted climatic change will have significant impacts. Hence, water resources development including household level rainwater harvesting needs to be widely undertaken as it has a potential to serve as an adaptation strategy to current rainfall variability as well as to future changes in the climate.Ethiopian Journal of Development Research Vol. 29 (1) 2007: pp. 1-34