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.