ارزیابی درازمدت اثرات هیدرولوژیک تغییر کاربری اراضی بر میزان رواناب سالانه در حوضه آبریز زیارت

مهمترین مداخله انسانی در چرخه هیدرولوژیک حوضه ها، تغییرات کاربری اراضی است. عدم رعایت قابلیت توسعه زمین در انتخاب کاربری ها و حتی مدیریت غیر اصولی کاربری ها، باعث افزایش حجم رواناب و فراوانی وقوع سیلاب می شود. هدف از پژوهش حاضر، بررسی آثار تغییر کاربری اراضی حوضه آبریز زیارت در دوره زمانی 21 ساله و در فاصله زمانی 13941373 بر عمق و حجم رواناب است که به این منظور از مدل lthia بهره گیری شده است. مدلlthia ، مدلی برای ارزیابی آثار بلند مدت حاصل از تغییر کاربری زمین بر منابع آبی است که با استفاده از داده های اقلیمی بلند مدت، نقشه کاربری اراضی و نقشه خاک قابل اجرا هست. در این راستا از تصاویر ماهواره ای لندست، سنجنده های tm (1994) و oli (2015)؛ و تکنیک های دورسنجی نظیر طبقه بندی نظارتی و تفاضل تصاویر برای شناسایی و پایش تغییرات استفاده شده است. نتایج طبقه بندی حاکی از پنج کلاس کاربری شامل جنگل، مناطق مسکونی، زمین کشاورزی، مرتع و مناطق آبی می باشد. نتایج بررسی تغییرات کاربری اراضی بیانگر سیر صعودی سطح مناطق مسکونی (96.3%) و اراضی کشاورزی (37.8%) و سیر نزولی مساحت اراضی مرتعی (6.35%)، جنگلی (1.2%) و مناطق آبی (4.2%) می باشد. نتایج مدل سازی نشان می دهد که در طی 21 سال، علی رغم روند کاهشی میزان بارندگی در منطقه و ثابت بودن ویژگی های هیدرولوژیکی در طول زمان، عمق رواناب تولیدی در حوضه به طور متوسط 14.2 میلی متر بر اثر تغییرات در کاربری زمین افزایش داشته است که سهم مناطق مسکونی و زمین های کشاورزی در افزایش حجم رواناب به ترتیب 37.26 و 28.91 درصد بوده است. این مدل با ارائه نقشه توزیع مکانی رواناب، امکان شناسایی مناطق حادثه خیز، پهنه بندی سیل و همچنین مدیریت سیل را فراهم می سازد.

Investigating the Long-Term Hydrologic Impacts of Land Use Change on Annual Runoff in the Ziyarat Basin

Introduction Land use change has a significant impact on water resources. At a watershed scale, land use change can increase runoff, flooding, and nonpoint source pollution and can degrade downstream water bodies. Thus it is important to assess the potential hydrologic impacts of landuse change prior to watershed development. To evaluate the hydrologic impacts of LULC change, many hydrological models have been developed. Some of these models mentioned above require many types of data inputs and parameter estimation that are usually not readily available for landuse planners. Therefore, it is necessary to develop a much easier to use model to evaluate hydrological effects of the landuse changes. The LongTerm Hydrologic Impact Assessment (LTHIA) model is developed by Purdue University as a tool to assess how landuse change affects annual average runoff in a watershed. The model uses only readily available data, such as daily rainfall, land uses, and hydrologic soil group. The objectives of this paper are: 1) to derive LULC classification maps of the study area; 2) to quantitatively assess the effects of LULC change on direct runoff using the LTHIA model. Materials MethodsStudy areaThe Ziarat River Basin is located between 54 °23' to 54 °31' E longitude and 36 °36' to 36 °43' N latitude. It has an area of 9923 hectares.The basin has experienced dramatic urbanization in the past decade, resulting extensive land use changes. Therefore, it is important to assess hydrologic response to land use changes for future land use planning and management purposes.Model descriptionLTHIA has been developed as a straightforward analysis tool that provides estimates of changes in runoff, recharge and nonpoint source pollution resulting from past or proposed land use changes. It gives longterm average annual runoff for a land use configuration, based on actual longterm climate data for that area. LTHIA results can be used to generate community awareness of potential long term problems and to support physical planning aimed at minimizing disturbance of critical areas. Recent concern over urban sprawl has focused on several land use change issues, including the failure to account for hydrologic aspects of land use change that can result in flooding, stream degradation, and erosion. LTHIA is based on computations of daily runoff obtained from long term climate records, soil data, CN value and land use of the area.Discussion of ResultsDatabase preparationIn order to investigate changes in land use, land use maps were generated from Landsat images in 1994 (TM) and 2015 (OLI). The images were geometrically corrected and Orthorectified using Ground Control Points and Digital Elevation model. In next stage, Training samples were collected for image classification, using field works, digital topographic maps and interpretation of false color composite. Finally, Image classification was done using supervised classification maximum likelihood. Based on this method, five land use category such as agriculture land, forest, city, grassland and water bodies were defined. Also, the digital soil data were reclassified into hydrologic soil groups A (Sand, loamy sand, or sandy loam); B (Silt loam or loam); C (Sandy clay loam) and D (Clay loam, silt clay loam, sandy clay, silt clay, or clay) according to rules of hydrologic groups classification for the LTHIA application. Once the land use and soil grids are prepared, the Curve Number can be calculated by using the Calculate the CN option from the LTHIA menu. The comparison between the landuse map and the CN layer was determined that most of the areas with curved number of 80 located in agricultural and urban land uses. After, the generation of CN layer, the LTHIA model was used to estimate annual depth and volume runoff using daily rainfall data with different LULC maps of 1994 and 2015.ConclusionLand use change is the most important human intervention in hydrological cycle of each watershed area. This paper investigates the hydrological effects of land use change in Ziarat basin in north of Iran, based on the LongTerm Hydrologic Impact Assessment (LTHIA) model. This is a hydrological model that requires the meteorological data, land use pattern and soil hydrology map. In this way, the Landsat satellite images of TM (1994) and OLI (2015) sensors; and techniques of remote sensing, such as supervised classification and Image difference were applied to identify and monitor the land use changes. The Classification results show five land use types, including residential areas, agricultural lands, rangelands, forests and water areas. The result of land use changes indicate an increase in the area of residential (96.3%), and agricultural (37.8%) lands, and a decrease in the area of rangelands (6.35%), forests (1.2%) and water surfaces (4.2%). Besides, the land use change during the last 21 years, has led to a 14.2 mm increase in the average amount of runoff depth, of which the portion of residential areas and agriculture lands were 37.26 and 28.91 percent, respectively. The results indicate that the LTHIA model have a good ability to assess the effects of land use and land cover change in the depth and volume of runoff. This model could provide the possibility of identifying accident prone areas, flood zones and flood management by spatial distribution map of runoff.Keywords: Land use change, Runoff depth, Hydrological models, LTHIA.