EPIC/APEX Literature Database for Peer-Reviewed Journal Articles

Title:Evaluating the Impact of Climate Change on Paddy Water Balance Using APEX-Paddy Model 
Authors:Mohammad Kamruzzaman , Syewoon Hwang , Soon-Kun Choi , Jaepil Cho , Inhong Song , Jung-hun Song , Hanseok Jeong , Taeil Jang and Seung-Hwan Yoo 
Volume:12 (3) 
URL (non-DOI journals): 
Broad Application Category:hydrologic only 
Primary Application Category:hydrologic assessment 
Secondary Application Category:climate change 
Watershed Description:The experiment was conducted at the Seoul National University research field (35.9016◦N, 127.0331◦E), located near Ikasn city of the Jeonbuk province in South Korea, as shown in Figure 1. The predominant soils at the Iksan experimental site are Jeonbug series. The series Jeonbug is a poorly drained silty loam soil series built on the fluvio-marine plain. The upper layer of the soil’s physical properties (~20 cm) contains 11.1% sand, 71.1% silt, and 17.8% clay. The daily metrological records of this study were retrieved from the Jeonju metrological station. Total rainfall in the area during the cropping season (May to October) ranged from 707.1 mm to 1860 mm in 1976–2005, and the average annual rainfall of 30 years was around 1300 mm, whereas the average maximum and minimum temperatures were around 27 ◦C and 17 ◦C, respectively. Seasonally, the monsoon, a rainy season in Korea between July and August, is highly influenced by precipitation at the study sites. Approximately 52% of the annual rainfall occurs in this region during this time, and part of the increasing rice season includes the monsoon period. 
Calibration Summary:The APEX-Paddy model was calibrated using the daily surface discharge data obtained from the paddy fields with conventional management during the cropping period of 2013 and 2014. The statistical measures of the R2 , NSE, and PBIAS downscale dynamical for the quantitative test of the model performance. The results of model performance are displayed in Table 4. The hydrology sub-module performed well with the statistics of 0.78, 0.65, and 5.41% for R2 , NSE, and PBIAS, respectively, for the calibration period. The hydrological yield simulation model proved to be strong enough for the long-term application according to Moriasi et al.’s [41] decision criteria. The detailed calibrated results are found in the previous study [43]. 
Validation Summary:The calibrated APEX-Paddy model was validated using the daily surface discharge data obtained during the cropping season of 2015 from the paddy fields under traditional management. The R2 , NSE, and PBIAS statistical tests were used to quantitatively assess the model results. Figure 3 displays the yield of the simulated surface discharge against the values observed for the validation periods. 
General Comments:The APEX-Paddy model is expected to be used as a tool for water management plans considering climate change and for vulnerability assessment of rice productivity. This study has forecast changes in the demands of agricultural water from climate change and has provided an index to quantitatively reflect the water vulnerability of paddy rice. In addition, it has given policy baseline data and adaptive solutions for agricultural water resources. Nonetheless, climate change projections include some degree of uncertainty depending on the downscaling techniques or spatial resolution used [49,50]. Therefore, sensitivity analysis may produce different results. This study refers directly to the Iksan area in South Korea, but indirectly to comparable regions due to the advantages mentioned above in methodological terms. As a follow-up, APEX-Paddy will continue to be used to demonstrate the feasibility of implementing best management practices (BMPs) for sustainable water management in paddy fields at Korea’s national scale, and the results will be incorporated into agricultural policy development. 
Keywords:water balance; paddy field; APEX-Paddy model; climate change; irrigation water demand