جغرافیا و توسعه (Dec 2012)
Analysis of Rainfall and Discharge Trend in Kashafrood Watershed Dr. Mehdi Vafakhah Assistant Professor of Watershed Management University of Tarbiat Modares Mohammad Bakhshi Tiragani M.Sc Student of Watershed Mangement University of Tarbiat Modares Majid Khazaei M.Sc Student of Watershed Mangement University of Tarbiat Modares
Abstract
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 وxj and xk 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 n10, 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 ZZα/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. References Asakereh, Hosein (2007). Linear regression application in annual temperature trend of Tabriz. Geographical Research, 22(4). 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