Trends in rainfall at 39 locations of the Nile River Basin (NRB) in Africa were analyzed. Compari... more Trends in rainfall at 39 locations of the Nile River Basin (NRB) in Africa were analyzed. Comparison was made between rainfall trend results from the long-term data and those of short-term series selected over different time periods. The bias on trend results from series of short-term records was quantified. Homogeneity test was conducted to assess the coherence of the trend directions on a regional basis. Based on an assumed population (for simplicity) of rainfall data time periods in the range 75–100 years, bias in the short-term trend analysis was noted to reduce by about 10% for every 10% increase in record length. Under some conditions if respected, it was possible to derive trends at stations with short rainfall records based on those at nearby stations with longer term records but in the same region. Using the same data record length and uniform time period at all the selected stations, an improved regional coherence of rainfall trend results was obtained. In the equatorial region, trend in annual rainfall was found mainly positive and significant at level α = 5% in 4 of the 7 stations. Collectively for Sudan, Ethiopia and Egypt, trends in the annual rainfall were mostly negative and significant at α = 5% in 69% of the 32 stations. Heterogeneity in the trend directions for the entire NRB was confirmed at α = 1% in 13% of the 39 stations. These findings are vital for water and agricultural management practices.
The optimization of the operation of existing water systems such as dams is very important for wa... more The optimization of the operation of existing water systems such as dams is very important for water resource planning and management especially in arid and semi-arid lands. Due to budget and operational water resource limitations and environmental problems, the operation optimization is gradually replaced by new systems. The operation optimization of water systems is a complex, nonlinear, multi-constraint, and multidimensional problem that needs robust techniques. In this article, the practical swarm optimization (PSO) was adopted for solving the operation problem of multipurpose Mahabad reservoir dam in the northwest of Iran. The desired result or target function is to minimize the difference between downstream monthly demand and release. The method was applied with considering the reduction probabilities of inflow for the four scenarios of normal and drought conditions. The results showed that in most of the scenarios for normal and drought conditions, released water obtained by the PSO model was equal to downstream demand and also, the reservoir volume was reducing for the probabilities of inflow. The PSO model revealed a good performance to minimize the reservoir water loss, and this operation policy can be an appropriate policy in the drought condition for the reservoir.
This study compares the daily crop evapotranspiration estimated by artificial neural network (ANN... more This study compares the daily crop evapotranspiration estimated by artificial neural network (ANN), neural network-genetic algorithm (NNG), and nonlinear regression (NLR) methods. Using a 6-year (2000)(2001)(2003)(2004)(2005) daily meteorological data, recorded at Tabriz synoptic station, and Penman-Monteith FAO 56 standard approach (PMF 56), the daily crop evapotranspiration (ET C ) was determined during the growing season (April-September). Air temperature, wind speed at 2 meters height, net solar radiation, air pressure, relative humidity and crop coefficient for every day of the growing season were selected as the input of artificial neural network models. In this study, the genetic algorithm was applied for optimization of parameters which used in ANN approach. It was found that the optimization of input parameters did not improve the performance of ANN method. The results indicate that NLR, ANN and NNG methods were able to predict potato crop evapotranspiration (ET C ) at the desirable level of accuracy. However, the NLR method with highest coefficient of determination (R 2 > 0.96, P value<0.05) and minimum errors provided the superior performance among the other methods.
Projection of the future impacts of climate change on hydrological extremes is essential for adap... more Projection of the future impacts of climate change on hydrological extremes is essential for adaptation and mitigation strategies for these hazards. This work investigates historical and projected trends in extreme precipitation events in Iran using the current and future (2071-2100) climate simulations using the Couple Model Intercomparison Project 5 Phase (CMIP5) models under the RCP4.5 and RCP8.5 scenarios. The Delta change method is used for downscaling of precipitation extremes. The cross-validation of the climate models is performed and the results are discussed based on mean climate scenario.
Global warming during 20 th century led to change in precipitation pattern in many parts of the w... more Global warming during 20 th century led to change in precipitation pattern in many parts of the world, but the change is not globally uniform. Since changes in precipitation patterns may lead to floods or droughts, the study of the trends and fluctuations in precipitation has been received a great deal of attentions. The aim of this research was to investigate the trends of the annual, seasonal and monthly precipitation time series at seven raingauge stations in Hamedan province for a 40-year period. Three non-parametric tests namely Kendall, Spearman and Mann-Kendall at the 95% confidence level were used for the trend analysis and the Theil-Sen's estimator was applied for determining the magnitudes of the trends. The obtained results showed an insignificant trend in the annual and seasonal precipitation series at the stations. Moreover, only two significant trends were observed at the February precipitation of Aghajanbolaghi station and the March precipitation series of Khomigan station.
The purpose of this study was to evaluate the trend of river discharge at the monthly and annual ... more The purpose of this study was to evaluate the trend of river discharge at the monthly and annual scale in the West Azarbaijan province using non-parametric methods. Two statistical tests including Mann-Kendall and Spearman were used for detection of the trend in river discharge data at the 95% confidence level. For this purpose, discharge data from 14 hydrometric stations which have data during 1980-2009 were used and the tests were applied on data from these stations. The results of this study indicate the increasing trend in the annual discharge at all the stations except Marze Sero during the past three decades. The application of the above mentioned procedures on monthly discharges has shown a decreasing trend for most months and there was any significant trend for months of February and March. Maximum number of stations with significant trends was found for the June series. Although there was any significant increasing trend detected by both statistical tests, but in the monthly scale, a significant decreasing trend was confirmed in the July, June, May and October series by both employed tests.
Drought is one of the most serious problems arising for human societies and ecosystems from clima... more Drought is one of the most serious problems arising for human societies and ecosystems from climate variability. This phenomenon causes billions of dollars in global damages annually and affects more people than any other form of natural disasters. In this study, the trends of rainfall and meteorological drought time series were investigated at ten stations located in the east of Iran for the period of 1966–2005. Amounts of drought severity were computed using standard precipitation index (SPI) for a 12-month time scale. The trend analyses of the data were also performed using the Mann–Kendall test and the Sen’s slope estimator. The results of this study showed that the rainfall and drought data had high variations to average values in the study period, and these variations increased with increasing aridity towards the east of the study area. The results of the trend tests revealed statistically significant decreasing trends in the rainfall time series of Sar-Pol-Zohab, Bijar, Sanan...
Trends in rainfall at 39 locations of the Nile River Basin (NRB) in Africa were analyzed. Compari... more Trends in rainfall at 39 locations of the Nile River Basin (NRB) in Africa were analyzed. Comparison was made between rainfall trend results from the long-term data and those of short-term series selected over different time periods. The bias on trend results from series of short-term records was quantified. Homogeneity test was conducted to assess the coherence of the trend directions on a regional basis. Based on an assumed population (for simplicity) of rainfall data time periods in the range 75–100 years, bias in the short-term trend analysis was noted to reduce by about 10% for every 10% increase in record length. Under some conditions if respected, it was possible to derive trends at stations with short rainfall records based on those at nearby stations with longer term records but in the same region. Using the same data record length and uniform time period at all the selected stations, an improved regional coherence of rainfall trend results was obtained. In the equatorial region, trend in annual rainfall was found mainly positive and significant at level α = 5% in 4 of the 7 stations. Collectively for Sudan, Ethiopia and Egypt, trends in the annual rainfall were mostly negative and significant at α = 5% in 69% of the 32 stations. Heterogeneity in the trend directions for the entire NRB was confirmed at α = 1% in 13% of the 39 stations. These findings are vital for water and agricultural management practices.
The optimization of the operation of existing water systems such as dams is very important for wa... more The optimization of the operation of existing water systems such as dams is very important for water resource planning and management especially in arid and semi-arid lands. Due to budget and operational water resource limitations and environmental problems, the operation optimization is gradually replaced by new systems. The operation optimization of water systems is a complex, nonlinear, multi-constraint, and multidimensional problem that needs robust techniques. In this article, the practical swarm optimization (PSO) was adopted for solving the operation problem of multipurpose Mahabad reservoir dam in the northwest of Iran. The desired result or target function is to minimize the difference between downstream monthly demand and release. The method was applied with considering the reduction probabilities of inflow for the four scenarios of normal and drought conditions. The results showed that in most of the scenarios for normal and drought conditions, released water obtained by the PSO model was equal to downstream demand and also, the reservoir volume was reducing for the probabilities of inflow. The PSO model revealed a good performance to minimize the reservoir water loss, and this operation policy can be an appropriate policy in the drought condition for the reservoir.
This study compares the daily crop evapotranspiration estimated by artificial neural network (ANN... more This study compares the daily crop evapotranspiration estimated by artificial neural network (ANN), neural network-genetic algorithm (NNG), and nonlinear regression (NLR) methods. Using a 6-year (2000)(2001)(2003)(2004)(2005) daily meteorological data, recorded at Tabriz synoptic station, and Penman-Monteith FAO 56 standard approach (PMF 56), the daily crop evapotranspiration (ET C ) was determined during the growing season (April-September). Air temperature, wind speed at 2 meters height, net solar radiation, air pressure, relative humidity and crop coefficient for every day of the growing season were selected as the input of artificial neural network models. In this study, the genetic algorithm was applied for optimization of parameters which used in ANN approach. It was found that the optimization of input parameters did not improve the performance of ANN method. The results indicate that NLR, ANN and NNG methods were able to predict potato crop evapotranspiration (ET C ) at the desirable level of accuracy. However, the NLR method with highest coefficient of determination (R 2 > 0.96, P value<0.05) and minimum errors provided the superior performance among the other methods.
Projection of the future impacts of climate change on hydrological extremes is essential for adap... more Projection of the future impacts of climate change on hydrological extremes is essential for adaptation and mitigation strategies for these hazards. This work investigates historical and projected trends in extreme precipitation events in Iran using the current and future (2071-2100) climate simulations using the Couple Model Intercomparison Project 5 Phase (CMIP5) models under the RCP4.5 and RCP8.5 scenarios. The Delta change method is used for downscaling of precipitation extremes. The cross-validation of the climate models is performed and the results are discussed based on mean climate scenario.
Global warming during 20 th century led to change in precipitation pattern in many parts of the w... more Global warming during 20 th century led to change in precipitation pattern in many parts of the world, but the change is not globally uniform. Since changes in precipitation patterns may lead to floods or droughts, the study of the trends and fluctuations in precipitation has been received a great deal of attentions. The aim of this research was to investigate the trends of the annual, seasonal and monthly precipitation time series at seven raingauge stations in Hamedan province for a 40-year period. Three non-parametric tests namely Kendall, Spearman and Mann-Kendall at the 95% confidence level were used for the trend analysis and the Theil-Sen's estimator was applied for determining the magnitudes of the trends. The obtained results showed an insignificant trend in the annual and seasonal precipitation series at the stations. Moreover, only two significant trends were observed at the February precipitation of Aghajanbolaghi station and the March precipitation series of Khomigan station.
The purpose of this study was to evaluate the trend of river discharge at the monthly and annual ... more The purpose of this study was to evaluate the trend of river discharge at the monthly and annual scale in the West Azarbaijan province using non-parametric methods. Two statistical tests including Mann-Kendall and Spearman were used for detection of the trend in river discharge data at the 95% confidence level. For this purpose, discharge data from 14 hydrometric stations which have data during 1980-2009 were used and the tests were applied on data from these stations. The results of this study indicate the increasing trend in the annual discharge at all the stations except Marze Sero during the past three decades. The application of the above mentioned procedures on monthly discharges has shown a decreasing trend for most months and there was any significant trend for months of February and March. Maximum number of stations with significant trends was found for the June series. Although there was any significant increasing trend detected by both statistical tests, but in the monthly scale, a significant decreasing trend was confirmed in the July, June, May and October series by both employed tests.
Drought is one of the most serious problems arising for human societies and ecosystems from clima... more Drought is one of the most serious problems arising for human societies and ecosystems from climate variability. This phenomenon causes billions of dollars in global damages annually and affects more people than any other form of natural disasters. In this study, the trends of rainfall and meteorological drought time series were investigated at ten stations located in the east of Iran for the period of 1966–2005. Amounts of drought severity were computed using standard precipitation index (SPI) for a 12-month time scale. The trend analyses of the data were also performed using the Mann–Kendall test and the Sen’s slope estimator. The results of this study showed that the rainfall and drought data had high variations to average values in the study period, and these variations increased with increasing aridity towards the east of the study area. The results of the trend tests revealed statistically significant decreasing trends in the rainfall time series of Sar-Pol-Zohab, Bijar, Sanan...
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Papers by Hossein Tabari