5. CONCLUSIONS
The long-term groundwater
abstraction and forest growth impact on the watershed hydrology was
evaluated using SWAT. Using groundwater use and forest growth database
from 1976 to 2015 that are prepared in 10-year interval
(1980s;1976~1985, 1990s; 1986~1995,
2000s; 1996~2005, 2010s;2006~2015) as
SWAT input data, SWAT was calibrated based on the daily observation data
at two multipurpose dams and three multifunction weirs in the study
area. Calibrated SWAT estimated the watershed hydrology of three
scenarios in terms of hydrological components. The hydrologic response
to GA and FG was measured by comparing estimated components.
The long-term change of GA and FG in the target watershed affected the
watershed hydrology and caused the spatial and temporal decrease of TR.
Temporally, the decrease of TR influenced by GA and FG showed 3.6%,
10.3%, and 14.9% in the 1990s, the 2000s, and the 2010s, respectively.
Spatially, the subbasins with small stream were appeared to be more
vulnerable to the loss of streamflow. Furthermore, the analysis on
flow-duration curve showed that the decadal change of GA and FG has
consistently worsened the annual streamflow condition and their impact
on the streamflow loss became stronger in lower flow rate.
The monthly analysis on the change of hydrological components explained
that the decadal change of GA and FG mainly influenced the hydrological
components in Summer due to the development of cultivation under
structure and the vegetation growth while GF showed the biggest
decrease. And decreasing GF in Winter came from water curtain
cultivation expanded for the last four decades. By the way, GA showed
bigger influence on the spatial vulnerability of TR loss than FG. Among
three types of GA, agricultural use from groundwater showed the highest
increase temporally and accordingly the fluctuation of GF in the monthly
hydrologic response was biggest in Summer, farming season in South
Korea. Therefore, we can infer that the agricultural water use pumped
from groundwater use imposes the biggest impact on the stream drying in
the Geum River basin.
In this study, groundwater abstraction and forest growth were selected
as the stream drying factors that possibly influence the watershed
hydrology. However, there are still many factors that are expected to
affect the hydrology including land use change, road network
development, and soil erosion. These factors have temporally developed
like groundwater abstraction and forest growth. Thus, further studies on
them are necessary to figure out comprehensive impact of stream drying
factors on the watershed hydrology and to determine the priority of
water resource management.