تحلیل روند بارندگی و دبی در حوزه¬ی آبخیز کشف¬رود دکتر مهدی وفاخواه ، محمد بخشی¬تیرگانی ، مجید خزائی

Introduction
The human activates and change of the land use has made changes in the peak and base flow discharge of the rivers. Also increasing the global average temperature has made anomalies in meteorological and hydrological variables such as precipitation and evapotranspiration. For better management of water resources, the data of river flow rate changes and their creating elements of such changes are required. Therefore the review of these anomalies as the trend determination in time series of hydrological and meteorological variables in different areas and their relation with each other can have an especial importance. In recent years, a great deal of studies have been made for the possible impacts of climatic changes on the river flow, which most of them have studied the changes of long term climatic averages and hydrological characteristics. Generally these changes have been studied in two cases. Firstly, the analysis of registered and available statistics of precipitation and river flow and the other is the impacts of different scenarios of climatic changes on the river flow by hydrological models. The most common method for analysis of hydrometeological time series is the review of the existence or non- existence of trend in them by using statistical tests.  Generally the existence of trend in these series may be due to gradual natural changes and climatic changes or the impacts of human activities.  Various methods have been presented up to now for analysis of time series trend which are dividable in to two groups of parametric and non parametric. Non-parametric methods have a more extensive use in comparing with parametric methods. The present research was performed to the aim of detecting the existence and amount of discharge trend and precipitation at Kashafrood watershed in 13 meteorology  and hydrometric stations with statistical length of 1972-2006 at the north east part of Iran which is c
nsidered as the areas with low precipitation in Iran.
 
 
Material and Methods
Characteristics of the study area
Kashafrood watershed is located at geographical longitude of 58° 20′ up to 60° 8′N and geographical latitude of 35° 40′ up to 36° 3′) and from  the north is limited to Hezar Masjed heights and  from south is limited to Binalood heights . The basin area is about 16500 km² which dedicated itself a vast part of Khorasan province. Around 500 km² of the watershed area are the plain area and the remaining is the heights.. The characteristics of the study watershed are low rainfall and high evapotranspiration.  The climate of the study watershed is semidry-cold. The characteristics of the study watershed are low rainfall and high evapotranspiration.
 
Research Methodology
13 meteorological  and hydrometric stations (Table 1) have been analyzed in this research.
 
Table 1 : The Characteristics of the selected stations in Kashafrood basin





Period of series


Elevation(m)


Latitude (N)


Longitude (E)


River


Station name


Code of station




1972-2006


1200


59° 61′


36° 48′


Radekan


Imamzadeh


64-003




1972-2006


1300


59° 12′


36° 28′


Golmakan


Golmakan


64-011




1977-2006


1350


59° 11′


36° 24′


Kaho


Dolatabad


64-013




1974-2006


1340


59° 22′


36° 42′


Ardak


Bande Saroj


64-015




1977-2006


1950


59° 09′


36° 18′


Zoshk


Zoshk


64-017




1974-2006


1750


59° 13′


36° 20′


Zoshk


Shandiz


64-019




1967-1989


950


59° 40′


36° 36′


Kardeh


Anderekh


64-021




1985-2006


1200


59° 26′


36° 20′


Jaghargh


Zirbande Golestan


64-027




1967-1987


1240


59° 31′


36° 10′


Torogh


Kortian


64-029




1972-2006


840


59° 51′


36° 14′


Kashafrood


Olang Asadi


64-033




1967-2006


620


60° 51′


36° 00′


Kashafrood


Aghdarband


64-037




1987-2006


1650


58° 28′


36° 50′


Chekneh


Chekneholya


64-043




1986-2006


990


60° 14′


35° 56′


Kalateh Monar


Kalateh Monar


64-962





 
For the purpose of performing the present research, firstly the data of precipitation and seasonal and annual flow rate of each station at different years from old times to the new were sequenced and then by using rank-based nonparametric Mann–Kendall  test  , the trend existence in the flow rate and precipitation data of each stations were evaluated separately and the obtained results were drawn as a graph. Also, Sen Test was used for magnitude of trend. For better indication and general assessment of the area, the obtained results in GIS environment for precipitation and annual and seasonal flow rate were presented in the form of map. In the following, the relation of Mann–Kendall  test   and Sen Test is described.
 
Mann-Kendall test
This test that was proposed by Mann (1945) and then extended by Kendall (1975) is considered as the most frequent nonparametric methods of analysis of time series trend. Use of this method is recommended for two reasons: (1) it is applicable for various types of abnormal, incomplete and seasonal data (2) it has the highest capability for data analysis. Also this test in comparing with the other trend tests is more suitable for determining the trend of hydrologic time series. The process for calculation of this test statistics would be as the following:
a)     





 
Analysis of Rainfall and Discharge Trend in Kashafrood Watershed
 
 
 





Calculating the difference between each observes with each other and using sign function and extraction of S parameter which is obtained from relation (1) :
 
 
                                                                                     (1)
In which , n is the number of observes and وx_j  and  x_k are _jth and _kth series respectively. The sign function is calculated by relation (2) :                                                
                                                            (2)
b) variance is calculated from the following relation:
          If        n>10                  (3)
                                              If        n<10                        (4)       
Where n is the number of observed data, m is the number of series with at least one repeating data, and t_i is the data with similar value. In cases where the sample size n >10, the standard normal variable Z is computed using Eq. (5):
 
                                                                                   (5)
In a bilateral test, for finding the trend of data series, the assumption of zero is accepted where the following relation is used:
                                                                                                           (6)
 
In which α is a  meaningful level which is considered for the test and Z  is a standad normal deviate at a meaningful level, which due to the two slope of the test, α/2has been used.
In this research, Mann–Kendall test has been implemented for 95% and 99% of confidence, which Z α/2 is equal to 1.96 and 2.65 respectively. The presence of a trend is accepted if Z is statistically significant if Z<-Zα/2 or Z>Zα/2. Positive values of Z indicate increasing trends, while negative values of Z indicate decreasing trends.
In addition to identifying whether a trend exists, the magnitude of a trend was also estimated by a slope estimator β, which was extended by Hirsch et al. (1982) from that proposed by Sen (1968), defined as
      where                                                 (5)
In other words, the slope estimator β is the median over all possible combinations of pairs for the whole data set. A positive value of β indicates an ‘upward trend’ (increasing values with time), while a negative value of β indicates a ‘downward trend’.
 
Discussion and Results
There is a decreasing trend of discharge and precipitation in Dolatabad station. The decreasing trend of discharge and precipitation in Dolatabad is due to the direct relationship between discharge and precipitation. This decreasing trend cannot relate to situation of the station due to Golmakan, Zoshk and Shandiz stations located in the nearest of the station having different trend in discharge and precipitation. Thus, the precipitation shows increasing trend for all seasonal and annual series expect spring season in Golmakan station. Also, the discharge shows increasing trend in autumn and summer seasons. There are different decreasing and increasing trend of precipitation and discharge in Zoshk and Shandiz stations. It may be due to other factors such as harvest and usage of rivers water and land use.
The Aghdarband station located on the main river, precipitation shows increasing trend in seasonal and annual series. The increased precipitation of this station can be signs of climate change. While discharge of this station shows decreasing trend in seasonal and annual series due to situation of the station located on main river and outlet of watershed. Also, Placing of the station on the main river and flat region and usage of water resources in agricultural and urban parts around Mashhad plain causing decreasing trend of discharge in this station. This result is consistent with Xu et al.(2010) results.   
 The Klateh Monar, Kortian, Anderekh and Chakneholeya sub-watersheds shows increasing trend in seasonal and annual discharge. The trend of precipitation is observed different in the stations. The trend of precipitation in Klateh Monar station shows a decreasing trend in seasonal and annual data except autumn season data. Also, in Kortian station is observed increasing trend in only winter season but two stations namely Anderekh and Cheknoleya station are observed increasing and decreasing trend in seasonal and annual data.
The trend of precipitation in Bande Saroj station was increasing trend in seasonal and annual data but the trend of discharge was decreasing trend except autumn season. The Imamzadeh, Olange Asadi and Zirbande Golestan stations were observed different trend in seasonal and annual precipitation and discharge data.  





 
Analysis of Rainfall and Discharge Trend in Kashafrood Watershed
 
 
 





Conclusion
 
This study was carried out for detection of rainfall and discharge trends in Kashafrood watershed, one of the low rainfall watersheds located in north-east of Iran, on 13 meteorological and hydrometry stations with data from 1972 to 2006. In this study, Mann – Kendall test, non-parametric test, was used for assessment of existence and nonexistence trend and also Sen Test was for magnitude of trend. The results of rainfall and discharge analysis showed that rainfall increased in the most stations in autumn as 9 stations have the increasing trend. While 10 out of 13 stations showed the decreasing trend in spring. In summer and winter, the numbers of stations with increasing and decreasing trends are almost equal. The annual trend analysis of rainfall and discharge showed that 5 stations had increasing trend and 8 other stations had decreasing trend in rainfall data. But no stations had increasing trend in discharge data.  So that no trend was found for data in 2 stations and the rest had increasing trend. This is probability due to increasing harvest and usage of rivers water. The usage of rivers water increased with increasing population. Thus consulting dams and increasing agricultural land under cultivation was tried for more water control and productivity by human.
 
Keywords: Trend, Discharge, Rainfall, Mann–Kendall test, Sen Test, Kashafrood Watershed.
 
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Analysis of Rainfall and Discharge Trend in Kashafrood Watershed
 
 
 





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Analysis of Rainfall and Discharge Trend in Kashafrood Watershed
 
 
 





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