Taesoo Lee

Soil and Water Assessment Tool (SWAT) Hydrologic/Water Quality Model: Extended Capability and Wider Adoption

This article introduces a special collection of 16 research articles on new developments and applications of the Soil and Water Assessment Tool (SWAT) to address various environmental issues at a range of geographic and temporal scales. Highlights include addition of a subdaily erosion and sediment transport algorithm, a biozone module, and a new algorithm for shallow water table depth. Model applications include climate change impact assessments, model adaptation to regional environmental conditions, watershed-scale soil erosion assessments, and linkages to other models. A summary of reported model performance indicates that 85% of daily flow calibration statistics reported in this collection were satisfactory or better, with very good performance in four of the 20 calibration results and in three of the 19 validation results. Details of reported model parameters for calibration of flow and water quality constituents are provided for other SWAT modelers. This collection builds upon a previous ASABE 2010 SWAT Special Collection, demonstrating continued developments to enhance SWATs capabilities and highlighting SWATs continued expansion in international applications, especially in Asia.

Assessment of seasonal and spatial variation of surface water quality, identification of factors associated with water quality variability, and the modeling of critical nonpoint source pollution areas in an agricultural watershed

Surface water quality impairment in agricultural watersheds is a major environmental concern in the United States. To assess seasonal and spatial variability of surface water quality and identify factors associated with surface water quality variability, we monitored surface water quality at seven locations in Bayou Plaquemine Brule Watershed in Louisiana twice monthly from March of 2002 to February of 2008 and performed multivariate analyses of the dataset. Using the Soil and Water Assessment Tool (SWAT) model, we identified critical areas of nonpoint source pollution in the watershed. While temperature, turbidity, dissolved oxygen (DO), conductivity and pH were determined in the field using YSI Sonde (YSI Incorporated, Yellow Springs, Ohio), surface water samples were analyzed for total nitrogen (TN), total phosphorus (TP), nitrate/nitrite-N (NO3/NO2-N), soluble reactive phosphate (SRP), total suspended solids (TSS), and five-day biological oxygen demand (BOD5) in laboratory. The monthly water quality sampling included a regular sampling and an after-rain-event sampling. Average DO for the summer months, March through November, was 4.91 ± 0.08 mg L−1 (4.91 ± 0.08 ppm), while average DO for the winter months, December through February, was 8.32 ± 0.12 mg L−1 (8.32 ± 0.12 ppm). Dissolved oxygen was negatively correlated with TN (r = −0.22, p ≤ 0.001), SRP (r = −0.17, p ≤ 0.001), TP (r = −0.17, p ≤ 0.001), BOD5 (r = −0.25, p ≤ 0.001), and surface water temperature (r = −0.70, p ≤ 0.001). Turbidity was strongly correlated with TSS (r = 0.59, p ≤ 0.001), suggesting that most turbidity in the water body comes from suspended solids. Similarly BOD5 was significantly positively correlated with TN (r = 0.43, p ≤ 0.001), NO3/NO2-N (r = 0.26, p ≤ 0.001), TP (r = 0.25, p ≤ 0.001), and SRP (r = 0.18, p ≤ 0.001). Results of factor analyses showed sediment, phosphorus (P), nitrogen (N), surface water temperature, dissolved solids, and acidity/alkalinity as the most important factors associated with surface water quality variability in this watershed. Although relatively higher concentrations of sediments, TP, and TN were observed in the upper reaches of the watershed based on water quality monitoring, the SWAT simulation results showed the critical nonpoint source pollution areas of sediment, P, and N in the lower reaches of the watershed. Lower reaches of the watershed have mainly rice and crawfish production, while the upper reaches include primarily sugarcane, pasturelands, and soybean production. Information on seasonal variability of surface water quality, factors associated with surface water quality variability, and the critical areas for nonpoint source pollution will be valuable inputs for developing a watershed management plan for effective nonpoint source pollution control in an agricultural watershed.

Evaluation and Spatially Distributed Analyses of Proposed Cost-Effective BMPs for Reducing Phosphorous Level in Cedar Creek Reservoir, Texas

The assessment of BMP (best management practice) impacts using a watershed model has helped to establish a watershed conservation and protection plan that is projected to be required by government and decision makers. Cedar Creek watershed, located southeast of Dallas, Texas, was included in the 303(d) list as an impaired watershed due to high pH values. A number of efforts have been made to develop watershed protection plans by the North Central Texas Water Quality (NCTXWQ) project team. Tarrant Region Water District (TRWD) has monitored the water quality in the reservoir and found that chlorophyll-a has been increasing at an annual rate of 3.85%. Chlorophyll-a is a good indicator of algae growth, and TRWD, with 18 years of monitoring, revealed that the increase of chlorophyll-a needs to be a primary focus of the watershed protection plan. A stakeholder group and the project team suggested that total phosphorous (TP) reduction from the watershed should be targeted at 35% of current loading in order to preserve the water quality in the reservoir. In previous studies, flow and nutrients in the watershed were calibrated using SWAT (Soil and Water Assessment Tool). In addition, sensitivity analyses for each BMP were conducted such that each BMP was simulated in the model at a 100% adoption rate. The cost-effectiveness of each BMP was estimated and ranked by TP reduction. In this study, using the calibrated model and the cost-effectiveness analyses of the BMPs, the initially selected BMPs were simulated in SWAT to identify the reduction rate at the watershed outlet (reservoir) using a marginal adoption rate and to illustrate the spatially distributed impacts of each BMP at the subbasin scale. The results show that simulation of the eight selected BMPs in subbasins with higher TP loading can achieve the 35% reduction goal at the reservoir.

Technical Note: Estimation of Fresh Water Inflow to Bays from Gaged and Ungaged Watersheds

The long-term estimation of fresh water inflow to coastal bays is important for understanding and managing estuarine coastal ecosystems. The Texas Water Development Board (TWDB) has estimated the total fresh water inflow to bays in Texas using the TxRR (Texas Rainfall-Runoff) model, which is a simple rainfall-runoff relation model. Recently, TWDB requested to develop and apply the SWAT model using up-to-date technologies for estimating inflow to the bays.

Simulating sediment loading into the major reservoirs in Trinity River Basin

The Upper Trinity River Basin supplies water to about one-fourth of Texass population. The anticipated rapid growth of North Central Texas will certainly increase regional demands for high-quality drinking water. This has increased concerns that sediment and nutrient loads received by drinking water reservoirs are reducing and will continue to reduce reservoir volumes and water quality. The objectives of this study are to calibrate and validate the Soil and Water Assessment Tool (SWAT) model for streamflow and sediment to assess current rates and sources of sediment loadings to 12 major reservoirs in the Upper Trinity River Basin (in 7 eight-digit watersheds) and to use the calibrated model for assessing the effects of upland ponds. SWAT performed well for streamflow, as evidenced by r2 values ranging from 0.55 to 0.95. Nash-Sutcliffe efficiency values ranged from 0.5 to 0.9 based on monthly streamflow comparisons between simulated and observed values for calibration, and r2 values ranged from 0.58 to 0.95 for validation. SWAT simulated sediment loads reasonably well, as evidenced by the percentage of errors within 11%. Streamflow and sediment loading were quite diverse across the Trinity River Basin, resulting in a multitude of parameter adjustments during calibration. Long-term predictions indicate that the Richland-Chambers, Ray Hubbard, and Lavon watersheds have significant channel contribution to sediment loading reaching the reservoirs. Pond removal scenario analysis shows a 4% to 48% reduction in sediment loadings to reservoirs via pond detention of overland flow. This wide range is mainly due to the vastly different proportion of land area draining to ponds, the locations of ponds, and the basins erosion (upland and channel) characteristics within each study watershed. The results indicate that in addition to implementing conservation practices such as ponds in upland areas, it is also necessary to have conservation practices in channels to further reduce erosion and subsequent loss to reservoirs. One limitation of this study is the lack of site-specific management information, and it is known that poor management practices at the field level can dramatically elevate sediment loads from an area. In this study, reasonable management operations were applied mainly at the county conservation district level. Opportunity exists for further data collection, including detailed data of field management and channel dimensions, which will allow the model to provide greater insight in identifying sensitive areas and reaches for stabilization and restoration. Opportunity also exists for further evaluation of the effects of optimizing pond size and placement to minimize reservoir sediment loading.

Estimating Sediment and Nutrient loads of Texas Coastal Watersheds with SWAT

Texas Water Development Board (TWDB) has been studied and estimated the total fresh water inflow to bays in Texas using TxRR (Texas Rainfall Runoff) model. Recently, TWDB requested to develop and apply the SWAT model using up to date technologies for estimating inflow, sediment and nutrient loads to the bays. In 2011, SWAT (Soil and Water Assessment Tool) model was developed and applied for estimating inflow to the bays over TxRR by Lee et al. In this study, SWAT model was used to estimate sediment and nutrient loading from a coastal watershed based on the developed model in 2011.

Deriving the Spatial Distribution of Irrigated-croplands in Texas using MODIS Satellite Imageries

Spatial distribution of irrigated areas and their temporal variations are a prerequisite for effective planning, management, and resource conservation. The estimation of irrigated agriculture water-use depends on several factors including crops, cropped acreage, and climatic conditions. Crop types and acreage data are based on the reported information by the producers. The data reporting process might be time-consuming and inconsistent causing delay in producing the required annual water-use summary reports. Moreover, it does not provide the spatial pattern of the irrigated areas for any administrative unit.