چکیده:
فرسایش خاک در حوزههای آبخیز باعث تضعیف پوشش اراضی و کاهش کمیت و کیفیت منابع آبی میشود. بنابراین ضرورت دارد که با شناسایی منابع تولید رسوب در مقیاس سطحی (دامنه) و خطی (آبراهه و رودخانه)، نوع و حجم عملیات آبخیزداری اصلاحی- تکمیلی به منظور کاهش فرسایش و رسوبدهی در مکانهای بحرانزا مشخص شود. هدف از تحقیق حاضر نیز ارزیابی توزیع مکانی برنامههای آبخیزداری اجرایی سالهای 74-1371 در حوزه آبخیز هفتان تفرش به منظور ارائه الگوی اصلاحی- تکمیلی این برنامهها است. بدین منظور امتیاز عاملهای مدل تجربی MPSIAC به کمک GIS برای شرایط قبل و بعد از عملیات آبخیزداری تعیین شد. سپس با مقایسه تغییر امتیاز عاملها، اثرگذارترین برنامههای آبخیزداری مشخص گردید. همچنین با مقایسه تغییر مساحت کلاسهای درجه رسوبدهی، ارزیابی از توزیع مکانی عملیات آبخیزداری انجام شد تا با تعیین نقاط ضعف پروژه، عملیات اصلاحی- تکمیلی پیشنهاد شود. نتایج تحقیق نشان داد امتیاز عامل فرسایش رودخانهای با 8/30 درصد، بیشترین تاثیر را در کاهش 1/4 درصدی رسوبدهی داشته است. همچنین از مساحت کلاس درجه رسوبدهی 75-50، مقدار 4/8 درصد کم شده است که عمدتا این شدت رسوبدهی مربوط به اراضی بالادست حوضه است. با توجه به معنیدار بودن تغییرات رسوبدهی زیرحوضهها در سطح 05/0، نتایج حاکی از کارا بودن برنامههای آبخیزداری در کاهش رسوبدهی است. به منظور مدیریت منابع آب و خاک و اثرگذاری بیشتر برنامه های آبخیزداری در کاهش فرسایش و رسوب حوزه آبخیز هفتان، الگوی اصلاحی- تکمیلی برنامهها شامل بانکت توام با بادامکاری در دامنههای بحرانی، بذرپاشی و کپه کاری در دامنههای با شیب تند و عمق کم خاک، تقویت پوشش گیاهی دامنههای مارنی و همچنین پایدارسازی کناره رودخانه پیشنهاد میشود.
1-Introduction
Soil erosion is a major environmental problem that has devastating effects on natural and
ecosystem. Although soil erosion has existed throughout history, but in recent years has
intensified related to inappropriate land use. So, due to the damages caused by the increase of
deposition in rivers, watershed management planning in area scale (slope) and linear
(channel) to reduce the soil erosion are applicable. Also the project evaluation for before,
during and after the implementation, according to the project objectives and environmental effects on watershed and its outside to verify the effectiveness of watershed management
programs is essential.
The more researchers aim is the investigation of effects watershed management measures to
reduce erosion and sedimentation and flooding potential in output of watersheds. However
assessment of spatial distribution programs in most of these studies has been ignored. With
attention to high cost in watershed management planning, these measured planning must be
on based priority of critical areas of erosion rates. Identify and priority, while will provide a
correction- supplementary pattern of watershed management programs, lead to cost management and also projects efficiency increasing in related to stakeholders welfare.
2- Methodology
In this study for a more investigation of spatial distribution the watershed programs, was
selected the Haftan watershed, because the diversity of measures in slope and channels and
also age 18 years of the project. Haftan watershed with 40.67 km2 is located in the Markazi
province, Tafresh city that has annual average precipitation equal to 304 mm and temperature
with 10.6 C˚.
For the estimation erosion and sediment the before of watershed practices, was used MPSIAC
model. In this research, nine factors the model was scored in ArcGIS 10 software. Then with
overlay of nine layers, was prepared sediment rate map by calculating the sediment degree of
each pixel. For after of watershed practices, with field survey, GPS and technical issues
Geography and Environmental planning, 27th year, vol. 61, No. 1, April 2016
reports, the condition of vegetation and erosion were reviewed. Then factors of land use, land
cover, runoff, and surface and channel erosion were updated. So erosion and sediment were
estimated on based updated distribution maps after of watershed practices. In finally, with
comparing the difference percent of sediment degree for before and after conditions using paired T-test in SPSS software was assessment the spatial distribution of watershed
management measures.
3– Discussion
The results showed that land cover and land use have been the least variability with 1.1 and
1.8 percent, respectively. However the most variability is for channel erosion factor with 30.8
percent. These results are entirely consistent with the type and volume of practices in the
Haftan watershed. So that biomechanical programs affecting on slope, has been implemented
only in 101 hectares; but check dams are large numbers in the streams.
According to results, sediment rate degree is focused in classes of 25-50 and 50-75. After of
watershed practices, the area class 50-75 is decreased equal to 8.4 percent. Also sediment
variability in sub-watersheds is 0.7 to 8.7 percent that with attention to T-test statistical method, difference for before and after practices is significant. Although the results showed
that watershed protection programs in slope scale of type almonds- contoured traces has high
performance, but the volume it is low. So sediment decreasing is only 4.1 percent for whole
the watershed.
4– Conclusion
Since biomechanical programs properly were implemented in slopes within sediment rate of
class 50-75, but only 3.6 percent of area is covered. Therefore it is necessary that the cover
protection program such as seeding and almost planting will become in the more area of class
50-75.
Because the most area of Haftan watershed is Eocene volcanic units with resistive category of
medium, it is necessary to implement of biomechanical project that slopes with high potential of erosion and sediment. Therefore other slopes will be under biological practices that has
lower cost.
With attention to high potential of erosion in marl unit, conservation practices in this zone
must be on based management and biological practices. Due to the impact of 30.8 percent
channel erosion factor, the protection and repair check dams is essential. Biological practices
such as land use change and cultivation pattern in buffers can be effective for channel stability and diversity increasing in ecosystem. Since stakeholders have a key role in land
management programs, it is suggested that will prepare further researches on based multiple objectives in a comprehensive and integrated watershed management.