جستجوی مقالات مرتبط با کلیدواژه « Change point » در نشریات گروه « آب و خاک »

Estimating the amount and intensity of precipitation and its spatial distribution in various return periods is necessary for flood estimation hydrological models. This information is obtained based on traditional methods through intensity-duration-frequency curves with many assumptions, such as the choice of suitable distribution for each period, and the requirement of many parameters in different return periods. If a study area has incomplete data or a lack of data, traditional methods are limited. For this reason, the fractal method is used to transform the precipitation hyetograph in different durations and transfer the precipitation data from one place to another. The fractal method is a self-similar method; It means that every part of it is similar to the whole, like a pine tree, where every branch is like a whole tree. This method has high reliability, convenient access, and less number of parameters, which can be used to create daily precipitation data over long and short periods. It is noteworthy, that in the past valuable research has been done in the field of using the fractal theory to extract IDF curves. Nevertheless, in this research, in addition to extracting the characteristics of precipitation based on the above method in all the stations inside and outside the selected watershed, the spatial distribution of the rainfall intensity mapped based on the Co-Kriging method, and has been compared with the Ghahraman method.

The study area is the Tireh watershed in the Borujerd-Dorud region, which is located between east longitudes from ̍28˚48 to ̍17˚49 and north latitudes from 51˚33 to ̍35˚33. This watershed, with an area of 2127.28 km2, is in the northernmost part of the large Karun River watershed and in the south of Oshtorinan town. The average rainfall in the mountains and plain areas has been estimated as 611.4 and 410.6 mm, respectively. The average annual temperature of the plain with an average elevation of 1493.3 m is 13.4 ̊C and in the highlands with an average elevation of 2025 m, it is 8.5 ̊C. In addition, the amount of evaporation in the highlands and plains is 1852.2 and 2148.8 mm per year respectively. In this research, the maximum intensity and amount of precipitation were estimated based on the fractal theory using the daily precipitation data for 12 stations with a statistical period of 31 years recorded from 1990 to 2021. The research method was conducted based on studies of Azhdary Moghaddam and Heravi, (2018) in the following steps. A) data extraction of the maximum amount of precipitation in different durations of 1, 2 and … days B) determining the maximum intensity of annual precipitation by dividing the maximum precipitation values by their durations C) calculating the weighted moment of the data ( ) in different orders (r) and durations (d) and then drawing linear graphs on a logarithmic scale, D) and then, using the related relationship, the maximum rainfall was calculated in the specified duration and return period. Since hourly precipitation data are not available in most of the stations, therefore, at this step, the IDF curves were extracted using the fractal theory and were compared with the experimental method of Ghahraman (which is based on the maximum daily precipitation) by the Pearson correlation coefficient. The Co-kriging method was used to create spatial distribution maps of precipitation intensity and amount based on fractal theory. The geostatistical Co-kriging interpolation method is similar to kriging and auxiliary variables can be used for better spatial analysis. Optimal spatial distribution maps of rainfall intensity and amount are provided by the existing point data extracted from the fractal method, and introducing different auxiliary layers such as maximum daily rainfall in the 24-hour continuity period.

Fractal analysis of precipitation data showed that there is a linear relationship between scale power and moment order in all stations. Therefore, the maximum precipitation data in the study area have a mono-fractal nature, which means that by using the fractal theory, the precipitation data can be converted from one duration to another. The results of the density of precipitation zoning based on fractal theory using the Co-kriging method showed that the accuracy of interpolation increases with the increase of the return period. Indeed the calculated values have a suitable fitting with the observed values and are close to the fitted line. Contrary to this, the results of precipitation zoning based on the Ghahraman method using the Co-kriging method showed the most scattered points around the fitting line; which actually shows the low accuracy of this method in estimating and zoning the area precipitation. The results of the 24-hour rainfall interpolation error using the fractal method showed an increase in the RMSE value with the increase of the return period based on only the auxiliary variable of the rainfall intensity data produced by the fractal method. The RMSE was calculated based on adding auxiliary data such as the amount and annual average of precipitation and the value of the maximum one-day precipitation intensity of the original data to precipitation intensity prepared by the fractal theory. According to this, the RMSE in the return periods of 2, 5, 25, 50, 100, 200, and 300 years equals 0.09, 0.27, 0.74, 0.18, 0.25, 0.059, and 0.13, respectively, have a decreasing trend compared to the use of only auxiliary variable of precipitation magnitude. This composition has reduced the error criterion values to less than a fifth compared to the initial state (only by precipitation intensity covariate) in the return period of over 50 years.

The analysis of statistical moments showed that the precipitation maximum intensity data has a mono-fractal nature. In other words, the changes in the power of the scale are completely linear with respect to moment order, and it can be used to produce the data in different durations. The statistical analysis of the results of estimating the intensity of precipitation in the different return periods and durations with this method compared to the Ghahraman method showed that in most stations there is a significant relationship with a correlation coefficient of over 99 % at a confidence level of 99 %. Generally, the results of the spatial distribution error of precipitation intensity using the Co-kriging method based on fractal in the return periods of 2, 25, 100, and 200 years showed acceptably reduced interpolation error by adding different auxiliary data.

Reference evapotranspiration as an important indicator of demand for air evaporation is an important factor for climatic and hydrological studies. Due to the occurrence of climate change and the occurrence of large fluctuations in precipitation and the occurrence of mild to severe droughts, it is important to study the evapotranspiration changes. In this study, to investigate the temporal-spatial changes of trend and change point in the reference evapotranspiration time series for different seasons in Mazandaran province, 40 years data (1981-2020) from satellite networks (with a resolution of about 5 km) were used. Non-parametric Mann-Kendall, Sen's slope, Pettitt's test (Non-Parametric Rank Test) and quantile regression were used to investigate the changes in the trend and to present the range of fracture point changes in the evapotranspiration. The correlation coefficient between network evaporation-transpiration data and ground station data was estimated to be more than 0.9 in most of the stations and the average value of BIAS was 0.24. The results of Pettitt's test showed the time of sudden changes in reference evapotranspiration in spring, autumn and winter seasons in 2013, 2007-2010 and 1999, respectively, in most cases. Very high values of evapotranspiration increased in spring in the eastern half, in summer in the northern half and in winter in the southern and western strips (with a seasonal slope percentage of 45, 75 and 120 percent, respectively), but they decreased in autumn in the north of the province (with a slope of -45 percent). In general, significantly increasing slope rates for high values of evapotranspiration were higher than average. A significant increase in high amounts of evapotranspiration, especially in the dry season, will reduce water resources and disrupt the agricultural sector. Therefore, scientific and practical methods for managing reference evapotranspiration in the province should be considered.

Climate changes and human activities are the two main factors altering the hydrological cycle of watersheds causing changes in the spatial and temporal distribution of water availability. The river flow, as the most critical component of the hydrological cycle, is the most vulnerable component that is affected by these changes resulting in worrying consequences on water demand by different sectors. Since there is growing controversy on the contribution of two main factors (climate change and human interventions) affecting river flow alteration, assessment and quantification of their contribution is of utmost importance to the water resources managers. Quantification of climate change and direct human intervention contributions to streamflow alteration is a prerequisite for developing adaptation strategies and policies for regional water resources planning and management. To this end, various approaches and methods have been developed and proposed during the last decades in order to separate the contribution of natural and human-induced factors on river flow regime conditions. The current research investigates the contribution of climate change and direct human interventions on the discharge decline of the Hablehrood watershed.

In this research, the upstream area of the Hablehrood watershed draining to the Bonekouh hydrometry station, located within the Tehran Province jurisdiction, has been studied. The hydrological condition of the Hablehrood watershed, as a main drainage channel of the watershed, has drastically altered during recent years and its discharge has significantly decreased. In this study, first, the long-term statistics of Simindasht and Dalichay hydrometric stations were collected and subjected to pre-processing. Then, using the Pettit and Lanzante statistical tests, the significant change point in their annual discharge time series was identified. Then, the empirical methods of the slope change ratio of accumulative quantity (SCRAQ) and double mass curve (DMC) were applied to separate the effect of climate change and human interventions on the discharge decline of the Hablehrood watershed and its main tributaries. Since rainfall, temperature, and potential evapotranspiration are considered the main climatic elements in both the used methods, the relationship between discharge, and precipitation, between discharge and precipitation as well as temperature, between discharge and precipitation as well as potential evapotranspiration were analyzed in order to compute the contribution of climatic drivers first. According to the empirical methods, after calculating the contribution of climatic variables to the changes in water flow, the remaining changes in water flow are attributed to human interventions. In this study, the observed data from the hydrometry stations of Simindasht and Dalichay during 1980 – 2017 were used. For SCRAQ, the slope change of the ratio of cumulative discharge and cumulative values of rainfall and temperature as well as the slope change of the ratio of cumulative discharge and cumulative values of rainfall and evapotranspiration were computed to separate the effects of climate change and human interventions.

The results of both tests showed that the hydrological regime of the basin changed in the mid-1990s in both hydrometric stations and the average annual discharge drastically decreased in the period after the change point. Results of different calculation methods of SCRAQ (the slope change of the ratio of cumulative discharge and cumulative values of rainfall and temperature (first method) and the slope change of the ratio of cumulative discharge and cumulative values of rainfall and evapotranspiration (second method)) showed that human intervention is the main cause of discharge decline within the Hablehrood watershed. According to the first calculation method, the contribution of climate change on discharge decline at the Simindasht and Dalichay stations were, respectively, 15.53 % and -37.08 %, and for the second calculation method, 0.55 % and -39.72 %, respectively. The positive values for the contribution of climate change to the Dalichay station indicate that climate change has resulted in an increase in its discharge. Results of different calculation methods of DMC between cumulative values of discharge and climate variables showed that climate change has increasing effects on the discharge of both Simindasht and Dalichay sub-watersheds.

Human intervention is the main cause of discharge decline in the Hablehrood watershed and the contribution of climate change is very small and incremental in some cases, particularly for the Dalichay sub-watershed. Therefore, management policies and priorities should be focused on managing human interventions, promoting public awareness, optimal using of water resources, and preventing over-exploitation of water resources across the watershed. The results of this study can provide a scientific guidefor the development, utilization, and management of regional water resources and ecological environment protection.

In this research, gradual and rapid changes of hydro-climatological variables, trends were analyzed for the Lighvanchai basin. Also, the natural (stationary) and impacted periods were identified. For this purpose, the traditional Mann-Kendall (MK1), modified (MK3) and Pettitt methods were used and for determination of the trend line slope the Senʼs estimator was used. Results showed that streamflow values of Lighvanchai River (in Hervy station) had a downward trend in the all time scales. In the spring, all precipitation related variables showed a decreasing trend. In the case of temperature variables, there was an increasing trend at the most of the studied time scales, which were significant from January to  May. Furthermore, the differences between Tmax and Tmin showed an increasing trend. On the other hand, air relative humidity time series showed a decreasing trend (specially in winter). It could be concluded that the combination of these changes led to decreasing trend in basin runoff. The result of Pettitt test showed that  the streamflow of  Lighvanchai (in Hervy) had a sudden change point, so that the average discharge in the recent period had been decreased about 35% compared to the first period. Most of studied climatic variables such as air temperature and RH, had a sudden change point that the date changes of these variables and the basin discharges (at Hervy station) were occurred in the mid-1990s. Regarding the discharges of Lighvan station (upstream of basin) the null hypothesis of abrupt change point were rejected. Therefore, along with the effects of climatic factors the effectiveness of the human factors on reducing the runoff in the basin outlet at Hervy station was confirmed.