Modified Soil And Water Assessment Tool Model Research Articles

Quantifying factors influencing runoff in mining areas using the SWAT model – a case of the Kuye River in Northern Shaanxi, China

Abstract There has been a marked decrease in river runoff in northern China recently, particularly from coal mining areas, but few studies have been able to quantify the factors influencing this phenomenon. Some methods to calculate the reasons for the river runoff decrease are urgently needed, and the degree to guide the river ecology restoration. Taking the Kuye River as an example, this study established a reference period (1960–1979) for a Soil and Water Assessment Tool (SWAT) model, and simulated the runoff changes and distribution. Then, the degree of influence of weather, water and soil conservation, industrial and domestic water consumption, and coal mining on river runoff during a recent period (1999–2010) was quantified. Finally, recent changes in runoff and its distribution were simulated with a modified SWAT model. The results show that in comparison to the reference period, for the recent period 21,987 × 104 m3 per year of runoff from the Kuye River was attributable to weather, while 8,840 × 104 m3, 5,346 × 104 m3 and 13,788 × 104 m3 was attributable to soil and water conservation measures, industrial and domestic water consumption, and coal mining respectively. The distribution of the runoff in the recent period was similar to that in the reference period, although its value was only half of the latter. These results can serve as an important reference for the ecological restoration of the Kuye River basin.

Uncertainties of SWAT Model in Irrigated Paddy Field Watershed

Soil and Water Assessment Tool (SWAT) merupakan model hidrologi yang sangat berpotensi digunakan untuk memodelkan daerah aliran sungai yang didominasi lahan pertanian. Namun demikian, struktur model ini dapat menyebabkan ketidakpastian khususnya apabila diaplikasikan untuk lahan sawah beririgasi. Hal ini dikarenakan SWAT pada awalnya dikembangkan untuk memodelkan lahan pertanian yang tidak memiliki genangan sehingga asumsi ataupun struktur modelnya berbeda dibandingkan dengan konsep pemodelan yang biasa digunakan di lahan sawah. Namun demikian, tingkat pengaruh ketidakpastian ini terhadap performa model secara keseluruhan belum teridentifikasi secara detail. Penelitian ini bertujuan untuk menganalisa performa, kesesuaian aplikasi dan ketidakpastian SWAT (model awal dan modifikasinya) untuk memodelkan daerah aliran sungai berlahan sawah irigasi. Analisa dilakukan dengan mengevaluasi struktur model dan menganalisa ketidakpastian menggunakan metode Sequential Uncertainty Fitting (SUFI-2) pada beberapa tipe model, yaitu model orisinil dan termodifikasi. Berdasarkan hasil penelitian, dapat disimpulkan bahwa struktur model pada SWAT tidak mengakomodir proses genangan, rembesan, dan irigasi di lahan sawah. Pengaruh dari ketidaktepatan struktur model ini dapat dikurangi dengan melakukan kalibrasi sehingga menghasilkan indeks performa yang baik. Namun demikian, perbedaan performa secara signifikan dapat diamati setelah dianalisa lebih lanjut dengan memperhatikan ketidakpastian. Reliabilitas model termodifikasi lebih baik karena menghasilkan rentang ketidakpastian yang lebih sempit khususnya pada periode debit rendah. Hasil ini juga menunjukkan bahwa genangan, rembesan, dan irigasi merupakan proses yang sangat penting untuk pemodelan hidrologi di daerah aliran sungai berlahan sawah irigasi.

An improved representation of geographically isolated wetlands in a watershed‐scale hydrologic model

AbstractGeographically isolated wetlands (GIWs), defined as wetlands surrounded by uplands, provide an array of ecosystem goods and services. Within the United States, federal regulatory protections for GIWs are contingent, in part, on the quantification of their singular or aggregate effects on the hydrological, biological, or chemical integrity of waterways regulated by the Clean Water Act (CWA). However, limited tools are available to assess the downgradient effects of GIWs. We constructed a Soil and Water Assessment Tool (SWAT) model with improved representations of GIW hydrologic processes for the approximately 1700 km2 Pipestem Creek watershed in the Prairie Pothole Region of North Dakota, USA. We then executed a series of novel modifications on the Pipestem Creek SWAT model. We (1) redefined the model's hydrologic response unit spatial boundaries to conform to mapped GIWs and associated watershed boundaries, (2) constructed a series of new model input files to direct the simulation of GIW fill–spill hydrology and upland flows to GIWs, and (3) modified the model source code to facilitate use of the new SWAT input files and improve GIW water balance simulations. We then calibrated and verified our modified SWAT model at a daily time step from 2009 through 2013. Simulation results indicated good predictive power (the maximum Nash–Sutcliffe Efficiency statistic was 0.86) and an acceptable range of uncertainty (measured using the Sequential Uncertainty Fitting v.2 uncertainty statistics). Simulation results additionally indicated good model performance with respect to GIW water balance simulations based on literature‐based descriptions of regional GIW hydrologic behaviour. Our modified SWAT model represents a critical step in advancing scientific understandings of the watershed‐scale hydrologic effects of GIWs and provides a novel method for future assessments in different watersheds and physiographic regions. Copyright © 2016 John Wiley & Sons, Ltd.

Assessing potassium environmental losses from a dairy farming watershed with the modified SWAT model

Potassium (K) was intensively used to optimize agricultural crop yield. Potassium losses should be accurately quantified for efficient nutrient management. However, no hydrologic model had been developed yet to quantify daily K losses at watershed scale. The Soil and Water Assessment Tool (SWAT) model was modified (named SWAT-K) by including the main K dynamic processes (solid–liquid distribution in soil and stream, plant uptake, and transportation with water flow and soil erosion) to simulate stream K load and K budget at the watershed scale. The SWAT-K was tested on the dairy farming Shibetsu River Watershed (672km2), Eastern Hokkaido, Japan. The solid–liquid distribution coefficient for K in suspended sediment was 10mlg−1. Langmuir equation was fitted to describe the solid–liquid distribution of K in soil, which derived an affinity constant of 0.046LmgK−1 and was used directly in SWAT-K. The fitted Langmuir equation also derived an adsorption maximum for K in soil. The adsorption maximum for K in soil normalized for clay content ranged from 4 to 20gKkg−1, and the fitted value of 15.5g Kkg−1 was used in SWAT-K. The SWAT-K satisfactorily predicted the daily dissolved K load at watershed outlet, and estimated an annual dissolved K load of 27.3kgKha−1year−1. The simulated pasture K yield of 36.1 (±2.5) kgKha−1year−1 was close to the observed data of 38.0 (±3.1) kgKha−1year−1. Then the model was used to quantify K budget at watershed scale. The simulated dissolved K leaching was 15.1kgKha−1year−1, and simulated soil K surplus of 75.1kgKha−1year−1 was much higher than plant uptake of 28.4kgKha−1year−1. The large amount of leaching and soil storage indicated that agricultural K input might be excessive and reducing the K application was recommended.

Predicting Streambed Sediment and Water Column Escherichia coli Levels at Watershed Scale

AbstractA sub‐model for the Soil and Water Assessment Tool (SWAT) is developed to predict Escherichia coli levels in the streambed sediment as well as in the water column. New formulations to estimate the levels of E. coli in streambed sediment and the water column are derived. These equations include calculations of E. coli resuspension from the streambed sediment to the water column, E. coli deposition from the water column to the streambed sediment, E. coli growth in the streambed sediment and the water column, and instream E. coli routing. These formulations were programmed in FORTRAN and integrated into SWAT. The modified SWAT model was applied to Squaw Creek Watershed, Iowa, to predict E. coli levels in the stream. Escherichia coli concentrations in the streambed sediment and the water column were monitored extensively in this watershed, and observations were used to verify the model predictions. The model proposed here can predict E. coli concentrations in streambed sediment as well as in the water column. Approximately 58% of the predictions of E. coli levels in the bed sediment were within 1 order of magnitude from the observed value, and in the water column 83% of the predictions of E. coli levels were within 1 order of magnitude. Results suggest that the proposed model will help predictions of instream bacterial contamination.

Runoff simulation using a modified SWAT model with spatially continuous HRUs

The Soil and Water Assessment Tool (SWAT) model is a well-established eco-hydrologic model that employs the hydrological response unit (HRU) as the basic unit. Land surface patches within one HRU have identical hydrological properties (e.g., land use, soil, slope and management) and thus have similar hydrological responses. The non-spatial aspects of HRUs, however, are considered a key weakness of the SWAT model because it is difficult to determine the spatial locations and describe the interactions between different HRUs. Here, a new method to produce continuous HRUs with a clear spatial position for SWAT using Geographic Information System tools is proposed and then tested in a small catchment of the Taihu Basin, China. The SWAT model was then modified based on spatial continuous discretized HRUs accounting for the surface runoff lag difference of HRUs in one sub-basin. The results showed that the modified model was more sensitive to the lag in runoff processes and thus had better simulation accuracy.

Simulating Landscape Sediment Transport Capacity by Using a Modified SWAT Model.

Sediment delivery from hillslopes to rivers is spatially variable and may lead to long-term delays between initial erosion and related sediment yield at the watershed outlet. Consideration of spatial variability is important for developing sound strategies for water quality improvement and soil protection at the watershed scale. Hence, the Soil and Water Assessment Tool (SWAT) was modified and tested in this study to simulate the landscape transport capacity of sediment. The study area was the steeply sloped Arroio Lino watershed in southern Brazil. Observed sediment yield data at the watershed outlet were used to calibrate and validate a modified SWAT model. For the calibration period, the modified model performed better than the unaltered SWAT2009 version; the models achieved Nash-Sutcliffe efficiency (NSE) values of 0.7 and -0.1, respectively. Nash-Sutcliffe efficiencies were less for the validation period, but the modified model's NSE was higher than the unaltered model (-1.4 and -12.1, respectively). Despite the relatively low NSE values, the results of this first test are promising because the model modifications lowered the percent bias in sediment yield from 73 to 18%. Simulation results for the modified model indicated that approximately 60% of the mobilized soil is deposited along the landscape before it reaches the river channels. This research demonstrates the modified model's ability to simulate sediment yield in watersheds with steep slopes. The results suggest that integration of the sediment deposition routine in SWAT increases accuracy in steeper areas while significantly improving its ability to predict the spatial distribution of sediment deposition areas. Further work is needed regarding (i) improved strategies for spatially distributed sediment transport measurements (for improving process knowledge and model evaluation) and (ii) extensive model tests in other well instrumented experimental watersheds with differing topographic configurations and land uses.

Development and Application of Algorithms for Simulating Terraces within SWAT

<abstract><title><italic>Abstract.</italic></title> Terraces have been proven to be an effective conservation practice for controlling high soil loss. In large hydrological programs such as the Soil and Water Assessment Tool (SWAT), terrace effects are simulated by adjusting the slope length and the USLE P-factor. In this study, a process-based terrace algorithm was developed and incorporated into SWAT (version 2009) to simulate the environmental effects of different kinds of terraces, i.e., normal and bench terraces. The terrace algorithm was activated at the hydrological response unit (HRU) level. Terrace description, storage effects, and the flow interaction between the terraces and the HRU were also introduced in the method. The modified SWAT model was evaluated using a four-year, six-plot event runoff and sediment data set with five years of plant yield data collected on a natural rainfall terraced field in southeast Franklin County, Kansas. Results indicated that the model’s performance was satisfactory in simulating single and average plot runoff as well as average plot sediment yields, with Nash-Sutcliffe efficiencies always greater than 0.5 and often greater than 0.7. The model’s performance was less consistent in simulating sediment yields from the no-till plots. The development and incorporation of the terrace algorithm provide a process-based alternative to the use of the P-factor in representing the effectiveness of terraces.

SWAT plant growth modification for improved modeling of perennial vegetation in the tropics

The Soil and Water Assessment Tool (SWAT) has been used for assessing the impact of land cover and land management changes on water resources for a wide range of scales and environmental conditions across the globe. However, originally designed for temperate regions, SWAT must be critically examined for its appropriate use in tropical watersheds. One major concern is the simulation of perennial tropical vegetation due to the absence of dormancy. While for temperate regions SWAT uses dormancy to terminate growing seasons of trees and perennials, seasonality in the tropics (wet and dry season) can only be represented by defining date or heat unit specific “plant” and “kill” operations which are fixed for every year of simulation. In this paper, we discuss these shortcomings and present an alternative approach to automatically initiate annual growing cycles based on changes in soil moisture. Furthermore, we propose a logistic leaf area index (LAI) decline function which approaches a user-defined minimum LAI instead of using the default function, which is not considering the minimum LAI. The modified SWAT model was tested based on MODIS LAI and evapotranspiration data for the Santa Maria/Torto watershed in Central Brazil, covered mostly by Cerrado (savanna) vegetation. Our model results show that the modified model can reasonably represent seasonal dynamics of the Cerrado biome. However, since the proposed changes are process-based but also allow flexible model settings (e.g. the beginning of growing cycles based on a soil moisture threshold adjustable for plant/land cover types), the modified plant growth module should be useful for large parts of the model community.

Evaluating hydrology of the Soil and Water Assessment Tool (SWAT) with new tile drain equations

Although subsurface drainage is a water management system widely used to maximize crop production in regions with seasonal high water tables, such as the midwestern United States, it is also a major source of nutrients into water bodies. Recently, physically based Hooghoudt and Kirkham tile drain equations were incorporated into the Soil and Water Assessment Tool (SWAT) model (herein referred to as Modified SWAT) as alternative tile flow simulation methods and a tool to design cost-effective and environment-friendly tile drain water management systems. The goal of this study was to determine a range of values for the new tile drain parameters and to use measured streamflow data from the South Fork Watershed (SFW) in Iowa to evaluate the capability of the Modified SWAT to simulate water balance components for this tile-drained watershed. This was accomplished by reviewing literature of tile drainage studies and by comparing measured streamflow with that predicted by the Modified SWAT using the Nash-Sutcliffe efficiency (NSE) and percent bias (PBIAS [%]) statistical methods in addition to hydrographs. During the calibration period, the Modified SWAT simulated streamflow very well (monthly NSE = 0.85 and PBIAS = ±2.3%). During the validation period, the Modified SWAT model simulated streamflow well (monthly NSE = 0.70 and PBIAS = ±2.5%). Simulated water balance results indicated that the soil water with tile drainage (260 mm [10 in]) was significantly ( p -value = 0.00) lower than soil water without tile drainage (355 mm [14 in]), while streamflow with (205 mm [8 in]) tile drainage was significantly ( p -value = 0.03) greater than streamflow without (128 mm[5 in]) tile drainage. This shows that the Hooghoudt steady-state and Kirkham tile drain equations are potential alternative tile flow simulation methods and tile drainage design tools in SWAT.

The modified SWAT model for predicting fecal coliforms in the Wachusett Reservoir Watershed, USA

This study assessed fecal coliform contamination in the Wachusett Reservoir Watershed in Massachusetts, USA using Soil and Water Assessment Tool (SWAT) because bacteria are one of the major water quality parameters of concern. The bacteria subroutine in SWAT, considering in-stream bacteria die-off only, was modified in this study to include solar radiation-associated die-off and the contribution of wildlife. The result of sensitivity analysis demonstrates that solar radiation is one of the most significant fate factors of fecal coliform. A water temperature-associated function to represent the contribution of beaver activity in the watershed to fecal contamination improved prediction accuracy. The modified SWAT model provides an improved estimate of bacteria from the watershed. Our approach will be useful for simulating bacterial concentrations to provide predictive and reliable information of fecal contamination thus facilitating the implementation of effective watershed management.

Effect of streambed bacteria release on E. coli concentrations: Monitoring and modeling with the modified SWAT

Streambed sediment has been attracting attention as a reservoir for bacteria, including pathogenic strains. Soil and Water Assessment Tool (SWAT) has been augmented with a bacteria transport subroutine in SWAT2005 in which bacteria die-off is the only in-stream process. The purpose of this study was to develop the partial model of sediment-associated bacteria transport in stream and to evaluate the potential significance of streambed Escherichia coli ( E. coli) release and deposition within the SWAT microbial water quality simulations. Streambed E. coli release and deposition were simulated based on the sediment resuspension and deposition modules in SWAT. The modified SWAT was applied to the Little Cove Creek watershed, Pennsylvania, which has forestry and dairy pasture landuses. Temporal changes in sediment E. coli concentrations were derived from monitoring data rather than from a streambed bacteria population model. Sensitivity analyses and calibrations were separately conducted for both hydrologic and bacteria parameters. Hydrologic calibration characterized soils in the watershed as pervious and thus the surface runoff was only moderately contributing to the streamflow. However, the surface runoff carried large numbers of E. coli to the stream, and sediment resuspension contributed to the persistent concentration of E. coli in stream water. Although the uncertainty of E. coli concentrations in streambed sediments and from wildlife probably affected the performance of the modified SWAT model, this study qualitatively confirmed the significance of modeling E. coli release from streambed and deposition for the SWAT microbial water quality simulations. Further developments should include modeling dynamics of bacteria populations within streambeds.

DEVELOPMENT AND APPLICATION OF SWAT TO LANDSCAPES WITH TILES AND POTHOLES

SWAT (Soil and Water Assessment Tool) is a watershed model that has been incorporated into USEPA’s modeling framework called BASINS used for total maximum daily load (TMDL) analysis. It is thus important that SWAT realistically simulate tile flow and pothole landscapes that are common in much of the Corn Belt and Great Lakes states. In this study, SWAT was modified to simulate water table dynamics and linked with a simple tile flow equation. Algorithms were also added to SWAT to include simulation of potholes (closed depressions), surface tile inlets, and aeration stress on plants. Flow interaction between HRUs was introduced in order to simulate pothole water. The modified SWAT model (SWAT-M) was evaluated using eight years of measured flow data from Walnut Creek watershed (WCW), an intensively tile-drained watershed in central Iowa. The model was calibrated during the period of 1992 to 1995 and validated during the period of 1996 to 1999. In addition, comparisons between the enhanced version (SWAT-M) and older version (SWAT2000) of SWAT were conducted. For assessing overall performance of the SWAT models, predictions were compared to data measured at stream sites approximately at the midpoint of the watershed (site 310) and at the outlet (site 330). Nash-Sutcliffe E values for the simulated monthly flows during the calibration/validation periods were 0.88/0.82 and 0.84/0.72 at sites 310 and 330, respectively. The relative mean errors (RME) of the simulated monthly flows during the calibration/validation periods were -2% / -1% and -18% / 10%, respectively, for the same two sites. These statistical values indicate that SWAT-M estimated both pattern and amount of the monthly flows reasonably well for the large flat landscape of WCW containing tile drains and potholes. SWAT-M needs to improve in its daily prediction because of its lower E values (-0.11 to 0.55), compared to the monthly results. In applying the model to a third site (site 210) that was predominantly influenced by tile drainage, it was concluded that the pattern and amount of simulated monthly subsurface flows (E values of 0.61 and 0.70 and RME values of 10% and -9% for the calibration and validation periods, respectively) were relatively close to the measured values. Nevertheless, SWAT-M simulation of daily subsurface flows was less accurate than monthly results. In general, the pattern and amount of monthly flow and subsurface tile drainage predicted by SWAT-M has been greatly improved as compared to SWAT2000.