Ecology and Resilient Infrastructure

Korean Society of Ecology and Infrastructure Engineering (응용생태공학회)
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Analysis of the Changes of the Vegetated Area in an Unregulated River and Their Underlying Causes: A Case Study on the Naeseong Stream

  • Lee, Chanjoo (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology) ;
  • Kim, Donggu (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology)
  • Received : 2018.11.28
  • Accepted : 2018.12.22
  • Published : 2018.12.31
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Abstract

This study aims to investigate the changes in the riparian vegetated area in the Naeseong stream, an unregulated river, in order to analyze the main factors leading to these changes. For this purpose, the land surface cover in the channel area of the Naeseong stream was classified into 9 categories using past aerial photographs collected between 1970 and 2016, which recorded the long-term changes of the Naeseong stream. The increase or decrease in the vegetated area was calculated for each category using a pair of before and after images. The changes in the vegetated area were divided into 6 periods: the unvegetated channel period (1970 - 1980), the first rapid increase (1980 - 1986), the period of decrease due to flood (1986 - 1988), the period of repetitive man-induced disturbance and vegetation increase (1988 - 2008), the period of gradual vegetation increase (2008 - 2013), and the period of second rapid increase (2013 - 2016). Multiple regression analysis was performed using independent variables representing hydrology, climate, and geomorphology. The major variables found to be involved in the changes in the vegetated area of the Naeseong stream were the discharge during June - July, channel width, and temperature during April - June. Among the three variables, discharge and temperature were respectively the main independent variables in the downstream and the upstream reaches as per a single variable model. Channel width was the variable that distinguished the upstream and downstream reaches of the stream. The implication of the long-term increase in the vegetated area in the Naeseong stream was discussed based on the result of this study.

Keywords

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Fig. 1. Change in land surface cover in the Naeseong stream during 1971 - 2013 (modified from Kim and Lee 2014).

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Fig. 2. Watershed and study reach of the Naeseong stream.

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Fig. 3. Annual mean temperature and precipitation at Yeongju during 1973 - 2016 (Source: http://www.weather.go.kr).

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Fig. 4. Daily discharge as computed using water level data at Wolpo station. Dashed line means a 2-year moving average.

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Fig. 5. Change in ENCR (red) and RECV (blue) according to year along the sub-reaches. Sub-reaches of no value indicate exclusion due to human disturbance.

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Fig. 6. A series of orthorectified aerial photos with layers of classified land surface cover of vegetation (VGH, VGT), cultivated land (HUC) and plantation or pasture (HUP) along sub-reach R07 - R08 during 1980 - 2016.

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Fig. 7. Temporal changes in the vegetated area for the whole study reach.

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Fig. 8. A uni-variable regression model plot between AC and TAJ for upstream sub-reaches, together with R02.

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Fig. 9. Mean temperature during April to June in Youngju. The red line indicates a 3-year moving average.

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Fig. 10. Relationship between annual rate of increase in VGH/VGT/HUC/HUP area and width of 1965.

Table 1. Drainage area and channel characteristics of the 13 sub-reaches

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Table 2. Topographic maps, aerial photos, and satellite images collected from various sources. Abbreviated sources are as follows: National Geographic Information Institute of Korea (NGII), Korea Forestry Promotion Institute (KOFPI), Samah Aerial Survey Co. (SAS)

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Table 3. Types of land surface cover. HUP represents both Plantation1, which includes Italian Poplar tree cultivation before the year 2000, and Pasture2, which means areas used for repetitive herb raising/cutting after 2000, as identified from field investigations

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Table 4. Definition of dependent variables

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Table 5. Independent variables used for sub-reach analysis. Units of discharge, width, rainfall, and temperature are m3/s, m, mm, and ℃, respectively

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Table 6. Regression models for sub-reach-based analysis. p-values of all the models were below 0.01. The prefix L means that the variable is log-transformed

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Table 7. Relative importance (RI) of regressors in the all sub-reach models (Eq. 2)

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Table 8. Multiple regression models for whole-reach based analysis. p-values of all the models were below 0.01. The t in variables means a whole reach analysis was applied

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