پژوهشنامه مدیریت حوزه آبخیز
پیاپی 2 (پاییز و زمستان 1389)

Drought as a prevalent phenomenon has seriously affect water and agricultural resources of Iran, especially in recent decade. Evapotranspiration (ET) is an important climatic factor of drought occurrence that would change as a result of global climate change. In this regard, uncertainties that almost arise in ET simulation approaches would be important and however, would affect ET projection results in upcoming decades. This research investigates uncertainties of ET simulation results under different Green House Gas (GHG) emission scenarios and different ET estimation models in Yazd meteorological station. For this purpose, reference ET was first determined using historical data of studied station through Hargerives-Samani and Thornthwaite approaches. Then, simulated daily Tmin and Tmax data of HadCM3 model that were resulted from GCM-runs based on the IPCC-SRES scenarios of A2 and B2 were used for projection of potential ET in 2010 to 2039 period, as well. Results of the study show increase of Tmin and Tmax in 2010 to 2039 compared with the historical period (1961-1990). Also the results indicate that potential ET of Thornthwaite approach would have an increase of about 0.4 mm per day in the future three decades, while the projected ET of Hargerives-Samani approach would not be increased, significantly.

Applying appropriate flood control measures leads to decrease the flood damages. Identifying of flood origin areas is one of these measures. This study investigates to identify flood origin areas at Lavidj river basin located in Northern Iran using the rainfall-runoff HECHMS model. The final goal is prioritization of flooding for the river basin using the results of the study. By computing discharge from each hydrologic unit and river routing till the basin outlet and at the final step applying successive hydrologic unit elimination technique at each model run, contribution of each unit to the basin flood is determined. This leads to prioritize hydrologic units to apply proper flood control measures. Furthermore, to remove the effect of the area for each hydrologic unit, flooding prioritization has been done using area parameter. The results showed that hydrologic unit A has the most contribution in the basin discharge and it has the highest flooding potential in the Lavidj river basin.

Marl unit include of about 4438 km sq. (some %20) of total surface area of Zanjan province. A study on the relationship among the runoff threshold, volume and run-off coefficient at Marl unit shows a quadratic regression equation to express the variations. Statistical data analysis shows the significant effects of the Marl units and slope on the runoff threshold (p<0.05). While, there is no significant effect of the climate on it. Marl unit can be classified in to three groups, based on the runoff threshold, so that Marls of Pliocene (PL) and Qom (OM) are in a separate class and upper-red (Mu), lower red and Eosen (EM) Marl units are in a unique group. These 3 groups differ significantly of view point of the runoff threshold resulted by the physico-chemical differences among them. So that the more silt content, the more rapid runoff threshold, because of lack or scarcity of water absorption capacity in these units. Investigation suggests that combined effects of slop and climate are significant on the runoff threshold.

The available statistics in all world indicate increasing the frequency and occurrence intensity of flood events. So identifying the effective regions on outlet flood of watershed, on that area, is important and basic actions in flood control and decreasing damages arising from that. Present study was conducted prioritization of main subwatersheds in Roodak basin in Tehran province using HEC-HMS simulation model. After dividing watershed to numbers of subwatersheds, providing and completing needed information for model in Geographical Information System, selecting curve number method in loss section, SCS hydrological simulation method in transportation section and Muskingum method for flood routing, was determined roles of all subwatersheds in outlet flood hydrograph by Succession Single Subwatersheds Elimination method and according to it, prioritization was done. This research studied relationship between flood and subwatersheds area too. The results of research show that, Kelookan subwatershed, has more effect than on outlet flood from basin. The results also show that, subwatersheds participation in outlet flood doesn t have direct relationship with their area.

Modeling is an important approach in evaluating and appraisal of catchment systems especially in better understanding of hydrologic and erosion processes. to this end, a wealth of various types of models has been developed to simulate catchment processes. the physically-based models are the most accurate and complicated ones. The LISEM is an example of physically-based models. This hydrologic and soil erosion model was designed and applied first in 1990s in the hilly loessial area in the Netherlands in order to get sustainable management strategies. The LISEM model is a process based raster model that operates on catchment scale, which is limited to single rain events. Due to deep loessial soils, population growth, overexploitation of lands and climate conditions, the undulating fertile lands in eastern part of the Golestan Province experience extensive rill, gully and channel erosions as well as destructive floods. Having in mind the abovementioned conditions and consequent financial damages and life loss in the area, investigating on sustainable methods of soil and water resources management becomes a necessity. In the present study, due to the similarity in the edaphic and climatic conditions of the study area with the hilly loessial area in the southern Netherlands for there the LISEM model has been developed, it is tried to evaluate the efficiency of the hydrologic component of the LISEM model for the Kechik catchment which is a well equipped experimental paired catchment. The hydrologic component is the prerequisite of the soil water erosion component of the model. The results of application the LISEM model in the Kechik catchment indicated that there is a good agreement between the observed and simulate hydrographs. Whereas, both correlation coefficient and Nash Satclif criteria were above 0.80. Moreover, the hydrograph characteristics such as peak discharges, total discharge and time to peak for the simulated and observed hydrographs were in a good agreement. These results indicate the efficiency of the LISEM model in simulation of rainfall-runoff processes in the Kechik watershed.

The phenomenon of sedimentation in dam reservoirs is the most important threatening factor in huge investment of capitals in hydraulic projects. Uncertainty analysis for each phenomenon shows the real view of its parameters and creates real understanding of the effective factors on the phenomenon. In this research, to estimate the volume of sediments in reservoir, using hydrometric and sediment data of Karaj River from 1961 to 2003, factors affecting sediment transportation and a model considering non-uniform sediment sizes. Then by using Monte Carlo simulation method and statistical input parameters, effective factors on sediment volume in reservoir, random input parameters and Latin square sampling method, some samples were obtained. At the end, the uncertainty of each parameter was introduced using statistical characteristics. The Results from uncertainty of annual sediment of reservoirs, expressed as coefficient of variations, was introduced as 56% Sensitivity analysis showed that annual mean flow rate and sediment rate were the most important factors in estimation of reservoir sediment volume.

The exact time of hydrological events occurrence is not predictable, but it is possible to investigate previous extreme events and estimate probability of future occurrences in different return periods. Having a long time series of hydrological variables is important parameter in frequency analysis and outlying data has an important role in quintiles prediction. In present study, we investigate the effect of outlying data in rainfall frequency analysis using L-moment method in Halilroud basin. First, all of recorded maximum annual rainfall was used in frequency analysis and then outlaying data were identified and removed form time series. In this stage, frequency analysis was done without these data. Results showed that outlying data don t have any effect on regional distribution function, but affect at-site distribution function and consequent estimated quintiles in different return periods.

Since both evaporation and transpiration are redaction factors of hydrology and have the same role in the water balance equation, they are considered and calculated together in a region. Maximum atmospheric potential evaporation, is called potential evapotranspiration, in fact it is the most water height which can return into the atmosphere through evaporation and plant transpiration- which depends on weather condition in that area. In order to measure evaporation in Iran, class A pan is used, which is an empirical method. Beside empirical methods, there are many mathematical methods through which the potential evapotranspiration can be estimated. One of these methods is Thornthwaite method. Although these methods have few parameters for estimating evapotranspiration, the data results have more errors than the pan. To investigate this fact, 5 stations were selected in south of Iran and the climate of these stations were determined using De Martonne method from 1990 to 2005 and their evapotranspiration were determined using Thornthwaite method through changing the equation exponent (Alfa) in the given time range and were compared with class A pan. The best result is obtained when using Thornthwaite method without changing the exponent. Actually if the error range of the pan be increase up to 30%, a high percentage of Thornthwaite data will be acceptable.