By increasing the intensity of Rainfall, runoff, which is considered as a very important factor in soil erosion process, occurs. Climate, geometrical characteristics of watershed, geological formations, soil, vegetation and land use are among factors affecting runoff generation. Although in plots under Rainfall simulation, the experiment time is more limited and the effect of Rainfall duration on the results is more tangible, this effect has been less considered. The present study was therefore conducted to evaluate the effect of Rainfall duration in 6 levels of 5, 10, 15, 20, 25 and 30 minutes with 3 replications under constant Rainfall intensity of 40 millimeter per hour on runoff parameters including commencement time, volume, coefficient and end time and sediment concentration. The field plots under Rainfall simulation located in an eastern highland slope of Kojour Watershed were selected to achieve the study purposes. The Results indicated that the runoff parameters were significantly increased (p≥ 0. 05) with increasing Rainfall duration while the rate of increase was not the same in different Rainfall duration levels. In other words, under the present study conditions, the turning point of hydrological response to Rainfall duration was occurred between 25-30 minutes, where the gradient of runoff parameters with Rainfall duration was considerably increased. The results also showed that the sediment concentration decreased with increasing Rainfall duration.

Introduction The consequences of climate change, changes in precipitation characters, including the amount, time and it’ s duration are expected. Considering that the rain provides the water resources on the planet, change in regime, amount and duration of Rainfall, caused a disturbance in the ecosystem of the Earth. It also affects the environmental conditions. Kurdistan province has an agricultural economy, thus variation in the Rainfall duration can affect agricultural activity and other activities. To achieve success in the environmental management planning and efficient use of water resources over an area, it is essential to have information about Rainfall variation. An important parameter of atmospher is precipitation. It has a lot of changes over the time and space. It is a basic element in the formation of the activities and prospects of the environment. Several studies have done by researchers on the properties of precipitation in the different regions of the world and also in Iran. In general, precipitation showed a negative trend in many regions (Gorgio, 2002: 675). For example, an assessment of summer Rainfall in eastern China showed a positive and a negative trend in the north (Gemmer et al, 2004: 39; Gong et al, 2004: 771). Annual Rainfall has decreased in southern parts of Italy and the decrease in winter precipitation was larger (Marco et al, 2004: 907). An increase in summer Rainfall, especially, in June and July has been reported in the Yangtze River basin (Tong et al, 2007: 1016). A decrease in winter Rainfall and an increas for other seasons has been showed in Turkey (Kahya and Partal, 2007: 43). Evaluation of maximum daily Rainfall at the global scale showed an increasing trend in Rainfall (Sethwestra et al. 2013: 3904). Negative anomalies of precipitation was reported for the most stations in southern west Ethiopia (Girma et al. 2016: 3037). Based on Iran's annual Rainfall, positive and negative trends in annual Rainfall have been showed (Asgari and Rahimzadeh, 2006: 67). A decrease in Rainfall, especially in the decade of 1995-2005 revealed in Iran using annual Rainfall (Asakereh and Razmi, 2012: 159). Assessment of changes in seasonal patterns of Rainfall in Hamedan, showed that the beginning of the rain tend to the summer and the end of winter (Movahedi et al., 2013: 23). The results of precipitation extreme indices on Iran showed a positive trend in the west and the south west and a negative trend in the north (Masoodian and Darand, 2013: 239). Methodology For this study, the daily precipitation obseravtions obtained from synoptic stations in Kurdistan province during 01. 01. 1989 to 31. 12. 2014 were anlayzed. A database with dimensions of 9526 * 8 was created. The time was set on rows (9526 days) and the Rainfall was set on columns. Homogeneous and heterogeneous monthly Rainfall data were assessed by apply cumulative deviations test and Vercelli maximum of likness. Mann-Kendall approach was implemented to extraxt the trend at the significant level of 90%, 95%, and 99 %. The significant differences in the mean of time series data before and after a mutation year by Mann-Whitney test were evaluated. The statistical calculations were done in the Matlab software. Results and Discussion The results showed that during the study period, duration of Rainfall for autumn, winter and spring, in most of the stations, has been reduced. The results indicated that the Rainfall duration for summer showes an increases in Rainfall. Which is in line with the result of many previous studies. The reduction in the Rainfall in the rainy season and an increase in Rainfall in summer were obsorved. As a result the duration Rainfall also has been changed. Annual Rainfall has decreased in southern Italy and decrease in precipitation in winter is more (Marco et al, 2004: 907). Movahedi et al. 2013, By studying the seasonal Rainfall in Hamadan, They found that the Rainfall began to ward the winter and their end to the summer have changed. Conclusion Evaluation of duration time series of Rainfall over different months of the year showed that in the rainy months of autumn, winter and spring Rainfall duration has decreased. For example, Baneh station showed a decline of 0. 3 day in December, and Marivan showed a decline of 0. 6 day in January. The average rate of decline in Rainfall duration in March for the Qorveh station was 0. 4 day per decade. In addition, a decline in spring Rainfall duration was observed as well. Bijar station showed a decline of 0. 2 day in May. However the Rainfall duration in summer showed an increase. For example, Zarinah station obtained an increase of 0. 2 day per decade in August.

One of the important relationships which are used in the estimation of river discharges and floods is Intensity-duration-Frequency (IDF). The accuracy of this relation is dependent on the accuracy of its parameters which need to be found based on short-duration Rainfall depths (such as 15, 30, 60 minutes, and so on) for a long term (i. e. 30 consecutive years). Unfortunately, only 24-hour Rainfall depths are available in many Rainfall stations in Iran. Various empirical relations are available to convert 24-hour Rainfall depth to sub-daily. One of these methods is IMD and its accuracy in some regions is low. In this research, the IMD method was transformed into a single-parameter equation and then, this parameter is evaluated for some Rainfall stations in Iran. To do this, maximum 24, 12, 6, and 3-hour Rainfall depths were extracted and their frequencies were calculated using Weibull and Gumbel methods. Regional coefficients in the modified IMD method were estimated using a linear regression method. Although the power of the IMD method is 0.33, results showed that this parameter for the Rainfall stations ranged from 0.28 to 0.35. To make more comparison, the IDF relation of Kordan’s watershed was calculated using the short-duration Rainfall depth which was estimated using the modified IMD, and then, this IDF was compared to observed data and Ghahraman’s relation which is commonly used in Iran. The comparison showed that the modified IMD relation could estimate the short-duration Rainfall data better than Ghahraman’s relation. After calibration of the modified IMD relation for various regions in Iran, the sub-daily Rainfall depth can be obtained with high accuracy.

The technique of relating areal Rainfall depths to area by analyzing several storms gives depth- area relationships for different specific durations. So it is necessary to formulate maximum depth- area- duration curves to determine the maximum depths of precipitation from a given storm within various durations and areas. In this research, DAD curves have been prepared for Khorasan Province. There are more than 180 rainguage stations in the Province. Daily Rainfall of these stations was collected from the beginning up to 2001 based on water years. For each year the number of days when the most stations having Rainfall with the precipitation center station is determined. The investigations showed that there is only 14 days with more than 80 percent areal extension in the study area. Thus the rainy days with more than 50 percent areal extension and with 50 percent of stations having at least 5 mm Rainfall, were selected as design storms. For drawing isohyetal map for each storm, GS+ software was used for determining the suitable interpolation methods. The mentioned maps were prepared using GIS for 41 selected maximum storms of the Province. Then the depth-area curves for each storm and final depth-area-duration curves were prepared for 3, 6, 9, 12, 18 and 24 hours durations. A mathematical model with the form of P/PM =[1-EXP (-a*Ab)] was fitted to area reduction factor curves, where P, PM and A are mean maximum areal Rainfall, maximum point Rainfall in the area in mm and watershed area in km2 respectively. The research showed that the accuracy of 18 hours DAD curve is less than the other of durations. It is recommended that the DAD and area reduction factor curves to be used only for areas between 10 to 30000 km2.

The first step form soil erosion is the soil splash that this step creates by the raindrops effect on soil surface. The raindrops can separate the soil particles and they cause the change of soil structure and increase soil erosion. Therefore, the study of effectiveness factors on splash erosion are including Rainfall erosivity and soil erodibility can be effective in control of splash erosion. Two variables of Rainfall duration and initial soil moisture can have more effect on amounts of splash erosion that have been perceived. The present study aims to the effect of Rainfall durations with rates of 5, 10, 15, 20, 25 and 30 min and initial soil moistures with rates of 20 and 40 percent studied on splash erosion in Rainfall intensity of 30 mm h-1 and scale of splash cup. The results showed that the variables of Rainfall duration and initial soil moisture had significant on the rates of total and net splash in time unit and level of 99 percent. Whereas, the interaction effect of initial soil moisture and Rainfall duration had not significant on total and net splash in time unit. The results also showed that, there was the inverse and significant correlation between Rainfall duration and total and net splash in time unit, because the total and net splash in unit time decreased with increasing Rainfall duration in both moistures of 20 and 40 percent. The results showed that the effect of initial soil moisture of 40 percent had more on total and net splash in time unit toward initial soil moisture of 20 percent.

the missing data in database must be reproduced primarily by appropriate interpolation techniques. Radial basis function (RBF) interpolators can play a significant role in data completion of precipitation mapping. Five RBF techniques were engaged to be employed in compensating the missing data in event-wised dataset of Upper Paramatta River Catchment in the western suburbs of Sydney, Australia. The related shape parameter, C, of RBFs was optimized for first event of database during a cross-validation process. The Normalized mean square error (NMSE), percent average estimation error (PAEE) and coefficient of determination (R2) were the statistics used as validation tools. Results showed that the multiquadric RBF technique with the least error, best suits compensation of the related database.

One of the most important parameters for design of hydraulic constructions is standard project storm which is derived from IDF curves for specific durations and return periods. The previous methods to estimate IDF curves are time consuming. In addition, they required many input parameters that tend to reduce their reliability. In previous methods, Rainfall data should be recorded in different durations to extract the IDF curves. In some regions, only 24-hour Rainfall data are recorded that they cannot build the IDF curves. In this study, temporal scaling properties of Rainfall were used to extract the IDF curves for short durations from daily Rainfall data. The used method has less complex stages and less number of parameters compares to previous methods. The proposed method was used in Saveh hydro-meteorological station as a case study and the results indicated its reasonable accuracy.