ﺗﻐﯿﯿﺮات زﻣﺎﻧﯽ ﺗﻮﻟﯿﺪ ﺟﺮﯾﺎن و ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري در ﺧﺎك ﻣﺎرﻧﯽ ﺗﺤﺖ ﺷﺪت ﻫﺎي ﻣﺨﺘﻠﻒ ﺑﺎران

Temporal variation of runoff production and rill erosion in a marl soil under different rainfall intensities

ﺳﺎﺑﻘﻪ و ﻫﺪف :ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري، ﯾﮑﯽ از ﻋﻮاﻣﻞ اﺻﻠﯽ ﻫﺪررﻓﺖ ﺧﺎك در ﺳﺎزﻧﺪﻫﺎي ﻣﺎرﻧﯽ اﺳﺖ .اﯾﻦ ﺳﺎزﻧﺪﻫﺎ ﺑﺴﯿﺎر ﺣﺴﺎس ﺑﻪ ﻓﺮآﯾﻨﺪﻫﺎي ﻓﺮﺳﺎﯾﺶ آﺑﯽ ﺑﻮده و ﮔﺴﺘﺮه ﻧﺴﺒﺘﺎً ﺑﺰرﮔﯽ را در ﺑﺮﺧﯽ ﺣﻮزهﻫﺎي آﺑﺨﯿﺰ ﻣﻨﺎﻃﻖ ﺧﺸﮏ و ﻧﯿﻤﻪ ﺧﺸﮏ درﺑﺮﻣﯽ ﮔﯿﺮد .اﻃﻼﻋﺎت در ﻣﻮرد ﺗﻐﯿﯿﺮات زﻣﺎﻧﯽ ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﻃﯽ ﯾﮏ رخ داد ﺑﺎرﻧﺪﮔﯽ و ﺗﺄﺛﯿﺮ ﺧﺼﻮﺻﯿﺎت ﺟﺮﯾﺎن ﺑﺮ آن ﻣﯽ ﺗﻮاﻧﺪ در ﺷﻨﺎﺧﺖ ﻓﺮآﯾﻨﺪ ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري در داﻣﻨﻪﻫﺎ ﮐﻤﮏ ﻧﻤﺎﯾﺪ .ﺷﺪت ﺗﻐﯿﯿﺮات زﻣﺎﻧﯽ ﺧﺼﻮﺻﯿﺎت ﺟﺮﯾﺎن و در ﻧﺘﯿﺠﻪ ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﻣﻤﮑﻦ اﺳﺖ ﺗﺤﺖ ﺗﺄﺛﯿﺮ ﺷﺪت ﺑﺎران ﻗﺮار ﮔﯿﺮد .از اﯾﻦ رو اﯾﻦ ﭘﮋوﻫﺶ ﺑﺎ ﻫﺪف ﺑﺮرﺳﯽ ﺗﻐﯿﯿﺮات زﻣﺎﻧﯽ ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري و ﺧﺼﻮﺻﯿﺎت ﺟﺮﯾﺎن در ﺧﺎك ﻣﺎرﻧﯽ ﺗﺤﺖ ﺑﺎرانﻫﺎي ﺑﺎ ﺷﺪت ﻣﺘﻔﺎوت اﻧﺠﺎم ﺷﺪ . مواد و روش ها : آزمايش در شش باران شبيه سازي شده با شدت متفاوت از10تا 60 ميلي ليتر بر ساعت به مدت ثابت ﯾﮏ ﺳﺎﻋﺖ ﺑﺎ ﺳﻪ ﺗﮑﺮار در ﺷﺮاﯾﻂ آزﻣﺎﯾﺸﮕﺎﻫﯽ اﻧﺠﺎم ﮔﺮﻓﺖ .ﻧﻤﻮﻧﻪ ﻫﺎي ﺧﺎك از ﺳﺎزﻧﺪﻫﺎي ﻣﺎرﻧﯽ در ﻏﺮب زﻧﺠﺎن برداشت شد و فلومي به طول 4متر و عرض 0/94 متر و با شيب 10درصد ريخته شد. فرسايش شياري در كنار ﺧﺼﻮﺻﯿﺎت ﺟﺮﯾﺎن( دﺑﯽ و ﻏﻠﻈﺖ ﺟﺮﯾﺎن) در ﻓﺎﺻﻠﻪ زﻣﺎﻧﯽ ﭘﻨﺞ دﻗﯿﻘﻪ از آﻏﺎز رواﻧﺎب در ﻫﺮ ﯾﮏ از ﺑﺎران ﻫﺎي ﺷﺒﯿﻪ ﺳﺎزي ﺷﺪه ﻣﻮرد ﺑﺮرﺳﯽ ﻗﺮار ﮔﺮﻓﺖ .ﺷﺪت ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري و ﺧﺼﻮﺻﯿﺎت ﺟﺮﯾﺎن در ﺑﺮاﺑﺮ زﻣﺎن ﺑﻪ دﺳﺖ آﻣﺪ و ﺗﻔﺎوت ﻫﺎ ﺑﯿﻦ ﺷﺪت ﻫﺎي ﺑﺎرﻧﺪﮔﯽ ﺑﺎ اﺳﺘﻔﺎده از روش ﺗﺠﺰﯾﻪ وارﯾﺎﻧﺲ ﺗﻌﯿﯿﻦ ﺷﺪ . ﯾﺎﻓﺘﻪ ﻫﺎ :ﻧﺘﺎﯾﺞ ﻧﺸﺎن داد ﮐﻪ ﺗﻔﺎوت ﻫﺎي اﺳﺎﺳﯽ از ﻧﻈﺮ زﻣﺎن آﻏﺎز ﺟﺮﯾﺎن، دﺑﯽ ﺟﺮﯾﺎن، ﻏﻠﻈﺖ ﺟﺮﯾﺎن و ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﺑﯿﻦ ﺷﺪت ﻫﺎي ﺑﺎرﻧﺪﮔﯽ وﺟﻮد دارد ( P >0/0001) ﺟﺮﯾﺎن ﺷﯿﺎري و ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﺑﺎ اﻓﺰاﯾﺶ ﺷﺪت ﺑﺎرﻧﺪﮔﯽ ﺳﺮﯾﻊ ﺗﺮ اﺗﻔﺎق اﻓﺘﺎد .ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﻃﯽ زﻣﺎن به شدت افزايش يافت و در زمان هاي پاياني ( حدود 45 دقيقه ) به حد تقريبا ﺛﺎﺑﺘﯽ رﺳﯿﺪ. ﻏﻠﻈﺖ ﺟﺮﯾﺎن ﻧﯿﺰ اﻟﮕﻮﯾﯽ ﻣﺸﺎﺑﻪ ﺑﺎ ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﻧﺸﺎن داد .ﺑﺎ اﯾﻦ ﺣﺎل دﺑﯽ ﺟﺮﯾﺎن در زﻣﺎن ﻫﺎي ﭘﺎﯾﺎﻧﯽ اﻓﺰاﯾﺶ ﭼﺸﻢ گير پيدا كرد. در طي زمان 45 دقيقه ، اغلب ذرات فرسايش پذير ، طي بارندگي دچار فرسايش شدند و پس از آن ، جريان هاي شديد اما با غلظت ( رسوب ) پايين در شيارها مشاهده شد. ﻧﺘﯿﺠﻪ ﮔﯿﺮي: ﺑﻪ ﻃﻮر ﮐﻠﯽ اﯾﻦ ﭘﮋوﻫﺶ ﻧﺸﺎن ﻣﯽ دﻫﺪ ﮐﻪ ﺑﺎ ﺗﻐﯿﯿﺮ ﺷﺪت ﺑﺎران، آﺳﺘﺎﻧﻪ وﻗﻮع ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري و اوج آن ﻃﯽ ﺑﺎرﻧﺪﮔﯽ ﺗﻐﯿﯿﺮ ﻣﯽﯾﺎﺑﺪ .ﻫﻢ ﭼﻨﯿﻦ روﻧﺪ ﺗﻐﯿﯿﺮات ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﻃﯽ ﺑﺎرﻧﺪﮔﯽ، ﺑﺎ اﻓﺰاﯾﺶ ﺷﺪت ﺑﺎرﻧﺪﮔﯽ ﺷﺪت ﭘﯿﺪا ﻣﯽ ﮐﻨﺪ . در زﻣﺎن ﻫﺎي اوﻟﯿﻪ ﺑﺎرﻧﺪﮔﯽ، واﺑﺴﺘﮕﯽ ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﺑﻪ دﺑﯽ ﺟﺮﯾﺎن ﺑﯿﺶ ﺗﺮ اﺳﺖ در ﺣﺎﻟﯽ ﮐﻪ در زﻣﺎن ﻫﺎي ﭘﺎﯾﺎﻧﯽ ﺑﻪ دﻟﯿﻞ اﻧﺘﻘﺎل ﺑﺨﺶ ﻋﻤﺪه ذرات ﻓﺮﺳﺎﯾﺶ ﭘﺬﯾﺮ در زﻣﺎن ﻫﺎي ﻗﺒﻞ، اﻟﮕﻮي ﺗﻐﯿﯿﺮات ﻓﺮﺳﺎﯾﺶ ﺷﯿﺎري ﺷﺒﯿﻪ ﺑﻪ دﺑﯽ ﺟﺮﯾﺎن ﻧﯿﺴﺖ

Background and Objectives: Rill erosion is a major factor of soil loss in the marl formations. The marl formations are very susceptible to water erosion processes and cover a wide area in some watersheds in arid and semi-arid regions. Knowledge of temporal variation of rill erosion and effect of during a rainfall event can provide information on the mechanism of rill erosion in the hillslopes. The rate of temporal variation of flow characteristics and rill erosion can be affected by the rainfall intensity. Therefore, this study was conducted to the study of temporal variation of flow characteristics and rill erosion in a marl soil under different simulated rainfalls. Materials and Methods: A laboratory experiment was carried out using six simulated rainfall intensities ranging from 10 mm h-1 to 60 mm h-1 with three replications. Soil samples were collected from the marl formations in west of Zanjan and separately purred to a flume with 4 m in length and 0.94 m in width putted on 10% slope. Rill erosion and flow characteristics (discharge and concentration) were measured at 5-min from starting flow/ runoff in each rainfall intensity. Rate of rill erosion and flow characteristics versus rainfall duration was obtained and differences among the different rainfall intensities were computed using the variance analysis method. Results: Results indicated that there are substantial differences in the flow starting time, flow concentration and rill erosion among the rainfall intensities (P<0.001). Rill flow and erosion rapidly occurred with increasing rainfall intensity. Rill erosion increased speedily during rainfall and reached to approximately constant value in the last times (about 45 min). The flow concentration appeared also a similar trend with the rill erosion, while flow discharge showed an increasing trend in the last times. Most of erodible particles were eroded during 45 min from rainfall and after this time, large flows containing lower concentration/sediment were observed in the rills. Conclusion: The study revealed that the threshold and pick time of rill erosion were strongly varied during rainfall. The variation trend of rill erosion during rainfall increases with increasing the rainfall intensity. Rill erosion temporally varies during rainfall. Rill erosion is strongly dependent on the flow discharge in initial times of rainfall, while in the last times its trend is very different from flow discharge.