Jaehak Jeong

EPIC and APEX: Model Use, Calibration, and Validation

The Environmental Policy Integrated Climate (EPIC) and Agricultural Policy/Environmental eXtender (APEX) models have been developed to assess a wide variety of agricultural water resource, water quality, and other environmental problems. The EPIC model is designed to be applied at the field scale. APEX is a direct extension of EPIC that can also be applied to fields as well as to more complex multi-subarea landscapes, whole farms, and watersheds. This article describes key model components of EPIC and APEX, including different options for simulating surface runoff, evapotranspiration, soil erosion, and other processes. Field-scale calibration and validation procedures are then described for both models, with an emphasis on important calibration parameters and guidance regarding logical sequences of calibration steps. Additional calibration and validation guidance is further provided for applications of APEX at the landscape and watershed scales. Two calibration and validation case studies are presented: one for an EPIC plot study and one for an APEX study of a 35 ha field in north-central Missouri. Research and development needs for both models are also discussed.

Assessment of Input Uncertainty in SWAT Using Latent Variables

Applications of the Soil and Water Assessment Tool (SWAT) require a large amount of input data to perform model simulations. Consequently, uncertainty in input data tends to influence the accuracy of SWAT hydrologic and water quality outputs. It has been shown that input uncertainty can be quantified explicitly during model calibration with latent variables. In this study, latent variables were explored to examine their sensitivity to SWAT outputs and further the potential impact of input uncertainty to model predictions. Results show that the increases in the range of latent variables pose a significant influence to streamflow and ammonia predictions while the impact was less significant in sediment responses. The performance of SWAT in predicting streamflow and ammonia declined with wider ranges of latent variables. In addition, the increase in the range of latent variables did not present noticeable effect on the corresponding predictive uncertainty in sediment predictions. In this study, the calibration results did not improve significantly with the applications of wider ranges of latent variables which are different from the findings in previous research work. The use of latent variables to incorporate input uncertainty may not be the proper alternative choice in terms of generating better results and should be carefully evaluated in the implementations of complex watershed simulation models.

Transferability of SWAT Models between SWAT2009 and SWAT2012

In recent years, the Soil and Water Assessment Tool (SWAT) has experienced upgrades with enhanced functionalities and modeling capacities as it gets to the current version, SWAT2012. Changes in the SWAT code on a specific process may result in propagating influences in the output of other related processes. In this study, the characteristic significance of the enhancements in SWAT code was investigated using the two recent versions, SWAT2009 and SWAT2012. Using a global optimization technique, each model was calibrated for flow, sediment, and nutrient and then tested for transferability of parameters between the models. Results indicate that flow and water quality output were well calibrated with both models. However, the calibrated parameters determined by SWAT2009 and SWAT2012 were noticeably different, due mostly to the enhancements made in SWAT2012. Our results indicate that only the stream flow result was reliable when the models were upgraded or downgraded between the two versions after calibration. Sediment prediction was marginally reliable. SWAT parameters were nontransferrable if nutrient was the main output. The differences are due to various reasons, such as disparities in algorithms at the process level and propagation of the resulting uncertainty into higher-order processes.

Hydrologic Modeling of a Canal-Irrigated Agricultural Watershed with Irrigation Best Management Practices: Case Study

Simulating irrigation systems by accounting for various water loss rates is necessary while modeling the hydrology of cultivated canal-irrigated watersheds. The existing approaches to modeling canal irrigation use situation-specific optimization procedures. In addition, they are focused on a water management perspective rather than a hydrologic perspective. In this study, an approach is developed to model canal irrigation systems and irrigation best management practices (BMPs) to adequately simulate the water balance of irrigated watersheds. The approach is based on the water requirement of crops, number and frequency of irrigation, and critical crop water requirement stages. Two irrigation BMPs are modeled as water savers rather than physical changes in irrigation appurtenances. Land leveling is modeled by changing model parameters and water management by changes in frequency, timing, and magnitude of irrigation with respect to cumulative precipitation. The developed approach was tested with a 1,692  km2...

Modeling Sedimentation-Filtration Basins for Urban Watersheds Using Soil and Water Assessment Tool

AbstractSedimentation-filtration (SedFil) basins are one of the storm-water best management practices (BMPs) that are intended to mitigate water quality problems in urban creeks and rivers. A new physically based model of variably saturated flows was developed for simulating flow and sediment in SedFils within the Soil and Water Assessment Tool (SWAT). The integrated SWAT-SedFil model allows for simulation of unsaturated flow in the filtration basin during small storms and fully saturated flow. Unsaturated flow was modeled using a modified Green and Ampt equation, and saturated flow was simulated with Darcy’s Law. Unsaturated flow comprises only a small fraction of large storm events; however, many regular storms are small and may not generate sufficient runoff to create a saturated flow in the filtration basin. Therefore, the combined unsaturated/saturated flow approach for modeling SedFils improved the accuracy of the model, especially in long-term evaluations. The model performs well with respect to es...

Development of Algorithms for Modeling Onsite Wastewater Systems within SWAT

Onsite wastewater systems (OWSs) are a significant source of nonpoint-source pollution to surface and groundwater in both rural and suburban settings. Methods to quantify their effect are therefore important. The mechanics of OWS biogeochemical processes are well studied. However, tools for their assessment, especially at the watershed scale, are limited. As part of this work, modeling capabilities were developed within the Soil Water Assessment Tool (SWAT) such that OWSs and their subsequent environmental impacts can be evaluated A case study was initiated on the Hoods Creek watershed in North Carolina to test the new SWAT algorithms. Included were: (1) field-scale simulations of groundwater quantity (water table height) and quality (N, P), (2) Monte Carlo evaluations of OWS service life to evaluate suggested calibration parameters, and (3) assessments of watershed-scale pollutant loadings within the model. Results were then analyzed at both the field and watershed scales. The model performed well in predicting both site groundwater table levels (R2 = 0.82 and PBIAS = -0.8%) and NO3-N concentration in the groundwater (R2 = 0.76, PBIAS = 2.5%). However, the performance for PO4-P simulations was less reliable due to difficulty in representing the mobility of soluble P in the soil. An advanced P algorithm is recommended to address the sophisticated physiochemical properties of soil particles and improve the models performance.

Effects of urbanization and climate change on stream health in north-central Texas.

Estimation of stream health involves the analysis of changes in aquatic species, riparian vegetation, microinvertebrates, and channel degradation due to hydrologic changes occurring from anthropogenic activities. In this study, we quantified stream health changes arising from urbanization and climate change using a combination of the widely accepted Indicators of Hydrologic Alteration (IHA) and Dundee Hydrologic Regime Assessment Method (DHRAM) on a rapidly urbanized watershed in the Dallas-Fort Worth metropolitan area in Texas. Historical flow data were split into pre-alteration and post-alteration periods. The influence of climate change on stream health was analyzed by dividing the precipitation data into three groups of dry, average, and wet conditions based on recorded annual precipitation. Hydrologic indicators were evaluated for all three of the climate scenarios to estimate the stream health changes brought about by climate change. The effect of urbanization on stream health was analyzed for a specific subwatershed where urbanization occurred dramatically but no stream flow data were available using the widely used watershed-scale Soil and Water Assessment Tool (SWAT) model. The results of this study identify negative impacts to stream health with increasing urbanization and indicate that dry weather has more impact on stream health than wet weather. The IHA-DHRAM approach and SWAT model prove to be useful tools to estimate stream health at the watershed scale.

Development of Sub-Daily Erosion and Sediment Transport Algorithms for SWAT

New Soil and Water Assessment Tool (SWAT) algorithms for simulation of stormwater best management practices (BMPs) such as detention basins, wet ponds, sedimentation filtration ponds, and retention irrigation systems are under development for modeling small/urban watersheds. Modeling stormwater BMPs often requires time steps as small as minutes to realistically capture the instantaneous flow and sediment load coming from upland areas. SWAT2005 uses the Modified Universal Soil Loss Equation (MUSLE) for modeling upland erosion and sediment load. The MUSLE model is an empirical soil loss equation, which was formulated based on field observations rather than theoretically derived relationships to predict long-term average soil loss. This article presents modified physically based erosion models in SWAT for seamless modeling of erosion processes with the recently developed sub-hourly flow models. In the new algorithms, splash erosion is calculated based on the kinetic energy delivered by raindrops, adapted from the European Soil Erosion Model, and overland flow erosion is estimated using a physically based equation adapted from the Areal Nonpoint-Source Watershed Environment Response Simulation (ANSWERS) model. The Yang model and the Brownlie model were also modified for in-stream sediment routing. The SWAT model with the modified sub-daily sediment algorithms was calibrated and validated each for a one-year period at 15 min intervals with measured data from the USDA-ARS Riesel watersheds in Texas. Results show that SWAT with the sub-daily algorithms performed as well or better in terms of sediment yield prediction than SWAT with the current daily output structure. In addition, SWAT (sub-daily) was able to adequately represent the timing, peak, and duration of sediment transport events. Thus, this initial evaluation indicates that the new sub-daily flow and sediment structure in SWAT is a promising tool for water quality assessment studies in small watersheds or urban watersheds where sub-daily process are so important to quantify.

Quantifying the Contribution of On-Site Wastewater Treatment Systems to Stream Discharge Using the SWAT Model.

In the southeastern United States, on-site wastewater treatment systems (OWTSs) are widely used for domestic wastewater treatment. The degree to which OWTSs represent consumptive water use has been questioned in Georgia. The goal of this study was to estimate the effect of OWTSs on streamflow in a gauged watershed in Gwinnett County, Georgia using the Soil and Water Assessment Tool (SWAT) watershed-scale model, which includes a new OWTS algorithm. Streamflow was modeled with and without the presence of OWTSs. The model was calibrated using data from 1 Jan. 2003 to 31 Dec. 2006 and validated from 1 Jan. 2007 to 31 Dec. 2010 using the auto-calibration tool SWAT-CUP 4. The daily and monthly streamflow Nash-Sutcliffe coefficients were 0.49 and 0.71, respectively, for the calibration period and 0.37 and 0.68, respectively, for the validation period, indicating a satisfactory fit. Analysis of water balance output variables between simulations showed a 3.1% increase in total water yield at the watershed scale and a 5.9% increase at the subbasin scale for a high-density OWTS area. The percent change in water yield between simulations was the greatest in dry years, implying that the influence of OWTSs on the water yield is greatest under drought conditions. Mean OWTS water use was approximately 5.7% consumptive, contrary to common assumptions by water planning agencies in Georgia. Results from this study may be used by OWTS users and by watershed planners to understand the influence of OWTSs on water quantity within watersheds in this region.

Evaluation of new farming technologies in Ethiopia using the Integrated Decision Support System (IDSS)

Highlights • We propose a modeling framework that assesses environmental and economical consequences of agricultural intensification.• Agricultural interventions were evaluated using IDSS in two study sites in the Amhara region of Ethiopia.• IDSS analyses indicate that a significant improvement in family incomes and nutrition can be achieved through the adoption of farming technologies such as irrigation technologies and nutrient management.