1. INTRODUCTION
Stream drying phenomena, which are often defined as long-term declines
in water levels caused by sustained factors, are key issues associated
with groundwater use, and they define the reduction in streamflow rates
caused by various factors that are hydraulically related to a given
stream. With watershed development and increasing groundwater use,
studies evaluating these phenomena have recently gained interest.
However, there are many difficulties in investigating continuous stream
drying and studying the phenomena from a quantitative perspective. The
difficulties come from complex factors that are not clearly understood
or defined (Jung & Kim, 2017) and the lack of hydrological data in
small rivers where stream drying usually occurs. Jung, Lee, Lee and Kim
(2019) considered stream drying phenomena by applying five different
factors, groundwater abstraction, afforestation, watershed development,
road development, and soil erosion, and ranked their contribution rates
to the loss of runoff. The results demonstrated that groundwater
abstraction and forest growth had a relatively higher influence on
stream drying than did other factors.
Increased population numbers and increased amounts of harvest trigger
increases in water demand. Although the demand can be fulfilled using
surface water sources such as lakes, rivers, and reservoirs, heavy rain
in South Korea is concentrated in Summer; thus, the water demand in
other seasons cannot be met from surface water stored in the sources.
Therefore, groundwater is often used solely or with surface water to
satisfy insufficient surface water supplies. However, when groundwater
abstraction exceeds groundwater recharge for a long time, groundwater
depletion can occur (Gleeson et al., 2010), and the lowering of
groundwater levels can result in serious effects on natural streamflow
and related ecosystems (Kirk & Herbert, 2002). Many studies have tried
to determine the hydrological impacts of groundwater abstraction. Wen
and Chen (2006) explored the spatial distribution of streamflow trends
for and climatic impacts on the watershed and concentrated on the
analysis of streamflow residuals from gauging stations to determine the
decrease in baseflow caused by groundwater withdrawals. Kim, Lee, Lee
and Won (2012) quantified the streamflow depletion from groundwater
pumping for the target watershed. Jung and Kim (2017) identified stream
drying by tracing the flow decrease from cell-based hydrological routing
under different land use and groundwater use conditions.
Forest growth changes the net loading of streamflow by intercepting
precipitation, controlling the rate of evapotranspiration, and
extracting groundwater from vegetation roots. Thus, quantifying the
influences of forest growth on watershed hydrology is crucial for
planning forest or land management and adaptation strategies for
watershed ecosystem sustainability. The relationship between forest
growth and water cycle in watersheds has been studied for a long time,
and several studies have demonstrated that forest changes can
significantly affect streamflow and watershed hydrology by altering its
pattern, magnitude, frequency, and quality. Mackay and Band (1997)
showed that the canopy distribution has significant effects on simulated
hydrological outputs where evaporative demand exceeds available water.
Birkinshaw, Bathurst, and Robinson (2014) made full use of the unique
45-year dataset over the entire cycle from the original upland grassland
vegetation through plowing the catchment and through forest growth up to
mature trees. The results showed clear changes in the nonstationary
nature of the catchment, with an annual increase in intercepted
evaporation and a decrease in discharge as the trees grew. Yue and
Hashino (2004) assessed the impact of forest growth on the streamflow of
the basin using statistical trend analysis. The results demonstrated
that forest growth was responsible for the decrease in all regimes and
that the increase in evapotranspiration due to forest growth resulted in
a decrease in both total runoff and low flow.
This study applied the periodic changes in groundwater abstraction and
forest growth to the hydrological model and quantified their influences
on watershed hydrology. It is believed that considering the two
effective stream drying factors and defining their relative
contributions to watershed hydrology will improve previous studies that
did not consider them together. The study was performed using the Soil
and Water Assessment Tool (SWAT), and the applicability of SWAT in
simulating watershed scale hydrology and water quality has been proven
around the world (Shi et al., 2011; Zhang, Srinivasan, Arnold,
Izaurralde & Bosch, 2011; Luo, Arnold, Allen & Chen, 2012; Lee, Shin
& Jung, 2014; Lee, Jung, Kim & Kim, 2019; Woo, Jung, Lee & Kim,
2019). SWAT was calibrated to verify its applicability in simulating the
watershed hydrology of the Geum River basin. The calibrated SWAT
simulated the watershed hydrology under three scenarios, and the results
were analyzed to determine the contributions of water loss factors to
the hydrologic variations.