Development of Field Pollutant Load Estimation Module and Linkage of QUAL2E with Watershed-Scale L-THIA ACN Model
The Long Term Hydrologic Impact Assessment (L-THIA) model was previously improved by incorporating direct runoff lag time and baseflow. However, the improved model, called the L-THIA asymptotic curve number (ACN) model cannot simulate pollutant loads from a watershed or instream water quality. In this study, a module for calculating pollutant loads from fields and through stream networks was developed, and the L-THIA ACN model was combined with the QUAL2E model (The enhanced stream water
... stream water quality model) to predict instream water quality at a watershed scale. The new model (L-THIA ACN-WQ) was applied to two watersheds within the Korean total maximum daily loads management system. To evaluate the model, simulated results of total nitrogen (TN) and total phosphorus (TP) were compared with observed water quality data collected at eight-day intervals. Between simulated and observed data for TN pollutant loads in Dalcheon A watershed, the R 2 and Nash-Sutcliffe efficiency (NSE) were 0.81 and 0.79, respectively, and those for TP were 0.79 and 0.78, respectively. In the Pyungchang A watershed, the R 2 and NSE were 0.66 and 0.64, respectively, for TN and both statistics were 0.66 for TP, indicating that model performed satisfactorily for both watersheds. Thus, the L-THIA ACN-WQ model can accurately simulate streamflow, instream pollutant loads, and water quality.     , have been used at the screening level. Among these models, L-THIA has been used in various studies and long-term watershed management plans because only daily rainfall data, in conjunction with a land use or soil dataset, are needed for daily direct runoff simulation and pollutant loads using the Natural Resources Conservation Services curve number (NRCS-CN) method and event mean concentration (EMC) databases for representative land uses [17,    . The ArcView/ArcGIS/Web/Web GIS interface to the L-THIA model has been developed and used for almost two decades [15, 16, 18, 20, 22, 23] . The Low Impact Development (LID) module was developed and integrated into L-THIA for environmentally friendly urban land use planning  . However, the current L-THIA model has limitations in estimating direct runoff during low-flow season because it does not compute direct runoff depending on the amount of rainfall [17, 25, 26] . In addition, the baseflow component for each hydrologic response unit (HRU) is not simulated  . When applying the L-THIA model to a watershed, lag time of the direct runoff component (to account for time needed to reach streams), the baseflow component contribution from unconfined aquifers, and flow routing through channel networks should all be considered. These limitations were addressed in the watershed-scale L-THIA asymptotic curve number (ACN) model developed by Ryu et al. . With these capabilities enhanced, the watershed-scale L-THIA ACN model could be used at ungauged watersheds for streamflow estimation. However, it does not simulate pollutant loads from HRUs, or evaluate the effects of watershed routing on water quality changes through stream networks. Thus, the objectives of this study are to (1) develop a module for estimation of pollutant loads from fields based on EMC databases for various land uses and amounts of rainfall; (2) to add the QUAL2E model (The enhanced stream water quality model) for stream water quality modeling to the watershed-based L-THIA model; and (3) to evaluate the accuracy of the watershed-scale L-THIA ACN model with water quality (watershed-scale L-THIA ACN-WQ model) by comparing estimated water quality with observed water quality. Materials and Methods Development of the Water Quality Simulation Module of the Watershed-Scale L-THIA ACN Model The watershed-scale L-THIA ACN model, developed by Ryu et al. , does not include a water quality component. Therefore, a field pollutant load estimation module and a stream water quality module were added to the L-THIA model (Figure 1 ). In the L-THIA model, pollutant loads for eight representative land uses were computed by multiplying EMC data collected in the USA by direct runoff volume [17, 28] .