The effect of Urban Development on Watershed Hydrological Properties (case study: Tajrish Watershed)

The effect of Urban Development on Watershed Hydrological Properties (case study: Tajrish Watershed) Throughout the human history, societies and rivers have been closely linked, so that the human civilization began from the riverside (Stevaux et al. 2009 (. The quantitative and qualitative characteristic of river is vulnerable to land-use changes (Kang et al. 2010). Natural and urban watersheds are influenced by the rapid land use change due to urban development (Furusho et al. 2013). Hence the importance of land use as an environmental variable have made its changes as a major issue in environmental changes and sustainable development) Verburg et al. 2009). The development of urbanization and industrialization of cities and communities have undesirable effects on the hydrological response of watersheds. It increases the magnitude of runoff and contamination, reduces the base flow and the groundwater recharge. Hence, urban authorities are urged to pay more attention to the environmental damaging effects of the urbanization process and the increase of construction. In this regard, attention should be paid to the effect of type of land cover and land use on urban runoff and hydrological changes in surface flow. Tehran as the largest metropolis in Iran has ascending trend of land cover and land use changes due to the growing population. In this research, the effect of urban development on the hydrological characteristics of the Tajrish sub-watershed (in Darband watershed) located north of Tehran has been investigated. Results of this study indicated that the river Darband is exposed to hydrological hazard due to human need for space and land use and land cover changes. The studied area is affected by decreasing pervious area, increase of runoff coefficient, and change in water quality parameters. Darband River watershed consists of two streams of Darband and Golabdareh which are considered as the major rivers of the Tehran-Karaj Basin. This river originates from the mountains of the Tochal located in northern Tehran. The catchment area of Darband River in the studied area is 39.88 square kilometers. In this study, aerial photos of years 1345, 1358 and also   and IkONOS images in year 2011 were used to detect the changes in land cover and land use in the Tajrish watershed. Pas-Ghale sub-watershed in upstream of Tajrish was selected as benchmark since its land use doesn't affected by human interventions. SCS-CN method developed by the United States Department of Agriculture (USDA) was used to estimate the quantitative changes in surface storage and runoff volume. Man-Kendall test was used for temporal trend detection of discharge and chemical parameters of surface water and also. The change of water type was identified annually using the Piper diagram in the aqQA software. Frequency analysis was carried out for peak discharge data using the weibull’s empirical method. During three considered periods, the curve numbers (CN) and runoff coefficient (C) in Tajrish watershed significantly increased. Significant trend was also observed for the chemical parameters of surface flow in Tajrish. While the surface storage and initial abstraction ratio (λ) indicate decreasing trend.  Relationships of CN and λ with rainfall depth (P) were also computed for both studied watersheds. According to the Piper diagram, the distribution of ions in the cation diagrams at both Maghsudbeik and Pasghaleh stations is generally more directed toward sodium. In the triangles of anions, both of the stations studied tend to show more calcium biocarbon content. Presence of sodium ion in the surface water is due to igneous formations in the watershed. The surface water in Pasghale station, indicate a neutral type of water. Whereas, saline water type is detected in the Maghsudbeik station. The increase of urban utilization over the past three decades could be the main cause of changes in the hydro chemical characteristics and water type along the Darband River. Investigation of land use changes in the Darband watershed indicate that the impervious surface has increased during years of 1996 to 2011. Results also indicate that the CN and λ values in Pas-ghale watershed are more correlated to Pin compared with ones observed in Tajrish. The results also reveal that hydrological modeling in watersheds undergoes land use changes and urbanization will result in imprecision results.  Many chemical parameters of the water quality of Darband River have been increasing at the Maghsudbeik station such as Chlorine, sulfate, sodium, electric conductivity and TDS and in the coming years, it can be considered inappropriate in terms of agriculture in the water class.   Keywords: Darband River, land cover and land use, Piper diagram, SCS-CN.