Xixi Wang

EVALUATION OF THE SWAT MODEL'S SNOWMELT HYDROLOGY IN A NORTHWESTERN MINNESOTA WATERSHED

Snowmelt hydrology is a very important component for applying SWAT (Soil and Water Assessment Tool) in watersheds where the stream flows in spring are predominantly generated from melting snow. However, there is a lack of information about the performance of this component because most published studies were conducted in rainfall-runoff dominant watersheds. The objective of this study was to evaluate the performance of the SWAT model’s snowmelt hydrology by simulating stream flows for the Wild Rice River watershed, located in northwestern Minnesota. Along with the three snowmelt-related parameters determined to be sensitive for the simulation (snowmelt temperature, maximum snowmelt factor, and snowpack temperature lag factor), eight additional parameters (surface runoff lag coefficient, Muskingum translation coefficients for normal and low flows, SCS curve number, threshold depth of water in the shallow aquifer required for return flow to occur, groundwater “revap” coefficient, threshold depth of water in the shallow aquifer for “revap” or percolation to the deep aquifer to occur, and soil evaporation compensation factor) were adjusted using the PEST (Parameter ESTimation) software. Subsequently, the PEST-determined values for these parameters were manually adjusted to further refine the model. In addition to two commonly used statistics (Nash-Sutcliffe coefficient, and coefficient of determination), a measure designated “performance virtue” was developed and used to evaluate the model. This evaluation indicated that for the study watershed, the SWAT model had a good performance on simulating the monthly, seasonal, and annual mean discharges and a satisfactory performance on predicting the daily discharges. When analyzed alone, the daily stream flows in spring, which were predominantly generated from melting snow, could be predicted with an acceptable accuracy, and the corresponding monthly and seasonal mean discharges could be simulated very well. Further, the model had an overall better performance for evaluation years with a larger snowpack than for those with a smaller snowpack, and tended to perform relatively better for one of the stations tested than for the other.

Influences of potential evapotranspiration estimation methods on SWAT's hydrologic simulation in a northwestern Minnesota watershed

The Soil and Water Assessment Tool (SWAT), a widely used watershed hydrology and water quality model, provides three different methods (Hargreaves, Priestley-Taylor, and Penman-Monteith) for estimating potential evapotranspiration (PET) and the corresponding actual evapotranspiration (AET). Although these methods have been extensively tested, the effects of using them within SWATs framework are largely unknown. The objective of this study was to test the three PET methods within SWATs framework using data collected in the Wild Rice River watershed, located in northwestern Minnesota. The performance of the SWAT models was measured using three statistics: the Nash-Sutcliffe coefficient (Ej2), coefficient of determination (R2), and performance virtue (PVk). The three models were independently calibrated and validated using the observed daily stream flows at two USGS gauging stations. The simulated stream discharges were compared with the corresponding observed values and the estimated evapotranspiration examined in accordance with the wet-environment areal evapotranspiration (ETW) derived from the evaporation data for Williams Lake, located about 100 km southeast of the study watershed. The use of the three PET methods resulted in different values for two calibration parameters, namely the soil evaporation compensation factor and SCS curve number. At the lower station, which is near the watershed outlet, the observed annual mean discharge (8.33 m3/s) during the model validation period was predicted to be 10.25, 10.87, and 9.69 m3/s by SWAT-Penman, SWAT-Priestley, and SWAT-Hargreaves, respectively. The annual mean discharge (10.83 m3/s) was more accurately predicted during the model calibration period, with an absolute error of less than 0.5 m3/s. The prediction errors for the upper station were comparable with those for the lower station. In addition, all three models exhibited good performance when simulating the monthly, seasonal, and annual mean discharges (Ej2 >0.75 and PVk >0.80) and satisfactory performance when predicting the daily stream flows (Ej2 >0.36 and PVk >0.70). In estimating evapotranspiration for the study watershed, SWAT-Hargreaves seemed to be slightly superior to the other two models, while SWAT-Priestley might be more appropriate for an ETW value greater than 8.0 mm/d. Nevertheless, the AET values estimated by the three models shared a concurrent spatial pattern and temporal trend, and were insignificantly different from each other at a 5% significance level (p-values > 0.05). The results indicated that after calibration, using the three ET methods within SWAT produced very similar hydrologic (AET and discharge) predictions for the study watershed.

Simulated wetland conservation-restoration effects on water quantity and quality at watershed scale.

Wetlands are one of the most important watershed microtopographic features that affect hydrologic processes (e.g., routing) and the fate and transport of constituents (e.g., sediment and nutrients). Efforts to conserve existing wetlands and/or to restore lost wetlands require that watershed-level effects of wetlands on water quantity and water quality be quantified. Because monitoring approaches are usually cost or logistics prohibitive at watershed scale, distributed watershed models such as the Soil and Water Assessment Tool (SWAT), enhanced by the hydrologic equivalent wetland (HEW) concept developed by Wang [Wang, X., Yang, W., Melesse, A.M., 2008. Using hydrologic equivalent wetland concept within SWAT to estimate streamflow in watersheds with numerous wetlands. Trans. ASABE 51 (1), 55-72.], can be a best resort. However, there is a serious lack of information about simulated effects using this kind of integrated modeling approach. The objective of this study was to use the HEW concept in SWAT to assess effects of wetland restoration within the Broughtons Creek watershed located in southwestern Manitoba, and of wetland conservation within the upper portion of the Otter Tail River watershed located in northwestern Minnesota. The results indicated that the HEW concept allows the nonlinear functional relations between watershed processes and wetland characteristics (e.g., size and morphology) to be accurately represented in the models. The loss of the first 10-20% of the wetlands in the Minnesota study area would drastically increase the peak discharge and loadings of sediment, total phosphorus (TP), and total nitrogen (TN). On the other hand, the justifiable reductions of the peak discharge and loadings of sediment, TP, and TN in the Manitoba study area may require that 50-80% of the lost wetlands be restored. Further, the comparison between the predicted restoration and conservation effects revealed that wetland conservation seems to deserve a higher priority while both wetland conservation and restoration may be equally important.

Simulation of an Agricultural Watershed Using an Improved Curve Number Method in SWAT

The USDA Soil Conservation Service (SCS) curve number (CN) method has been the foundation of the hydrology algorithms in commonly used continuous simulation models, including the Soil and Water Assessment Tool (SWAT). This expanded use of the SCS-CN method has proven successful for many applications. However, because the SCS-CN method was originally developed to determine design peak discharges of synthetic storm events under an average antecedent moisture condition, research is needed to address the controversy over the use of this method to represent continuous precipitation runoff processes. In addition, poor results obtained for some conditions indicate the necessity to improve the method to provide a more realistic and accurate representation of water flow amounts, paths, and source areas upon which erosion and water quality predictions depend. Thus, the objectives of this study were to: (1) propose a modified curve number (MCN) method, and (2) assess the MCN method relative to the existing SWAT method with an Ia/S value either equal to 0.2 or 0.05. The equations that formulate the MCN method were coded into the SWAT 2005 framework. A SWAT model implementing the MCN method was evaluated along with the models implementing the existing SWAT method with Ia/S values of 0.2 and 0.05. The evaluation was conducted in the 870 km2 upper portion of the Forest River watershed located in northeastern North Dakota. The results revealed that the total streamflows predicted by the three models were comparable, as indicated by similar values for the Nash-Sutcliffe coefficient. However, the MCN approach resulted in the most accurate prediction of the streamflow components (i.e., baseflow versus direct flow) as well as water yields. For the study area, the MCN method was judged to be superior to the existing commonly used curve number methods in terms of mimicking the precipitation runoff processes.

Wetland Restoration Response Analysis using MODIS and Groundwater Data

Vegetation cover and groundwater level changes over the period of restoration are the two most important indicators of the level of success in wetland ecohydrological restoration. As a result of the regular presence of water and dense vegetation, the highest evapotranspiration (latent heat) rates usually occur within wetlands. Vegetation cover and evapotranspiration of large areas of restoration like that of Kissimmee River basin, South Florida will be best estimated using remote sensing technique than point measurements. Kissimmee River basin has been the area of ecological restoration for some years. The current ecohydrological restoration activities were evaluated through fractional vegetation cover (FVC) changes and latent heat flux using Moderate Resolution Imaging Spectroradiometer (MODIS) data. Groundwater level data were also analyzed for selected eight groundwater monitoring wells in the basin. Results have shown that the average fractional vegetation cover and latent heat along 10 km buffer of Kissimmee River between Lake Kissimmee and Lake Okeechobee was higher in 2004 than in 2000. It is evident that over the 5-year period of time, vegetated and areas covered with wetlands have increased significantly especially along the restoration corridor. Analysis of groundwater level data (2000-2004) from eight monitoring wells showed that, the average monthly level of groundwater was increased by 20 cm and 34 cm between 2000 and 2004, and 2000 and 2003, respectively. This change was more evident for wells along the river.

GIS-Based Integration of SWAT and REMM for Estimating Water Quality Benefits of Riparian Buffers in Agricultural Watersheds

The Soil and Water Assessment Tool (SWAT) is a process-based, watershed-scale model with a hydrologic response unit (HRU) as the basic computation element, which makes it difficult to accurately represent riparian buffers using their physical parameters (e.g., vegetation structure). On the other hand, the field-scale Riparian Ecosystem Management Model (REMM) provides the opportunity to consider details of hydrologic processes within a riparian buffer zone. However, the runoff and its associated constituents from the upland area that is hydraulically connected to the riparian buffer zone must be provided as inputs into REMM. The rationale proposed here is that the integration of SWAT and REMM would improve the assessment of riparian buffers, which is vital to watershed management but which has not been described in the literature. The objective of this study was to develop a GIS interface that integrated SWAT and REMM for estimating water quality benefits of riparian buffers in agricultural watersheds. For modeling purposes, the interface subdivided a watershed into a number of sub-basins, each of which was further subdivided into drainage areas of isolated impoundments (e.g., wetlands), concentrated flow, and riparian buffers using available GIS data. As a result, riparian buffers received runoff and associated pollutants from corresponding contribution areas to mimic actual field conditions. The interface facilitated transferring the SWAT outputs into REMM and computing the site characteristic parameters (e.g., length and width) of the riparian buffers. The outputs from subsequent REMM runs were in turn taken as inputs into SWAT for channel routing and further simulation. The interface was used to assess water quality benefits of riparian buffers in the Lower Canagagigue Creek watershed located in southern Ontario, Canada. The results indicated that the existing riparian buffer system achieved a 27.9% abatement in sediment and a 37.4% reduction in total phosphorus. The model runs demonstrated that the GIS interface was easy to use and could serve as a protocol for integrating models with distinctly different spatial scales.

Impacts of hydrodynamic disturbance on sediment resuspension, phosphorus and phosphatase release, and cyanobacterial growth in Lake Tai

The objective of this study was to link hydrodynamic disturbance with sediment resuspension, phosphorus release, and algal growth in Lake Tai, a typical shallow lake located in the south of the Yangtze River Delta in China. With this regard, a sediment–water-algae laboratory experiment was conducted and extrapolated to the real situation in terms of field observations. The results show that the algal growth rate synchronically increased with dissolved total phosphorus (DTP) release rate. The DTP decreased with increase of bottom flow velocity, indicating that the phosphorus release rate was lower than its transfer rate into algal biomass. While all levels of hydrodynamic disturbances could increase sediment resuspension and phosphorus release, a low to moderate disturbance was beneficial, but a strong disturbance was harmful for algal growth. Also, a low to moderate disturbance caused the dissolved alkaline phosphatase activity (DAPA) to increase with time, which provided the enzyme for hydrolyzing a variety of organic phosphorus compounds from bed sediment into algae-needed nutritional DTP. The experiment proved to be an efficient means to understanding eutrophication mechanisms of large shallow lakes such as Lake Tai.

Development of a robust runoff-prediction model by fusing the Rational Equation and a modified SCS-CN method

Abstract The objective of this study is to develop a Modified Rational Equation (MoRE) that combines the advantages of the Rational Equation (e.g. simplicity and global acceptance) and those of the standard US Department of Agriculture (USDA) Soil Conservation Service (SCS) curve number (CN) method (e.g. easy parameterization and extensive verification across the world). Herein, the hypothesis is that the MoRE is more accurate, consistent and robust than the SCS-CN method and its improved versions in predicting runoff in watersheds with limited data. The MoRE was designed to have a simple structure that is described by four intrinsic parameters: CN, permanent wilting point, field capacity and saturation soil moisture, and does not include initial abstraction as a variable. An evaluation of 77 USDA small agricultural watersheds indicated that CN of the MoRE has different physical meanings from CN of the SCS-CN method. The MoRE (mean Nash-Sutcliffe coefficient, E > 0.73) performed better than the SCS-CN (mean E < 0.32) and the four improved models (mean E < 0.56) in reproducing the runoff of the study watersheds. Performance of all six models varied greatly between watersheds, as well as between events, but was independent of watershed drainage area. However, the model performances tend to be better for watersheds and/or events with a runoff-to-rainfall ratio of between 0.1 and 0.3 than for those with a ratio outside this range. The MoRE has the most consistent and robust performance. Editor D. Koutsoyiannis; Associate editor I. Nalbantis Citation Wang, X., Liu, T., and Yang, W., 2012. Development of a robust runoff-prediction model by fusing the rational equation and a modified SCS-CN method. Hydrological Sciences Journal, 57 (6), 1118–1140.

Experiment study of the effects of hydrodynamic disturbance on the interaction between the cyanobacterial growth and the nutrients

The eutrophication of shallow lakes is sensitive to dynamic currents (i.e., disturbances) because of their shallow depths and high contents of nutrients in bed sediments. The relation between the sediment resuspension and the algae bloom is not well understood in the field scale because the interwoven influencing factors cannot be examined individually. By combining the laboratory experiment and the field observation, this paper proposes a sediment-water-algae concept to simulate the effects of hydrodynamic disturbances on the algae growth in the Taihu Lake located in east China. The sediments are sampled from the Taihu Lake while the Microcystis aeruginosa (M. aeruginosa) algae is cultured in the laboratory and then transplanted into the experiment cylinders. The temperature and the light intensity in the experiment are adjusted to be similar with the prevalent in situ conditions. The results indicate that the M. aeruginosa populations under the disturbance condition of the rotational speed ≤ 300 rad/min in the experiment (corresponding to the bottom velocity flow ≤ 0.059 m/s, the shear stress ≤ 0.069 N/m2, or the wind speed ≤ 4 m/s in the field) are higher than those under the disturbance condition of the rotational speed is 400 rad/min (corresponding to the bottom flow velocity 0.079 m/s, the shear stress 0.124 N/m2). It is suggested that a low to moderate disturbance prompts the release of the nitrogen as well as the phosphate nutrients from the bed sediments, amplifying the eutrophication of the Taihu Lake.

Development of a Modified Rational Equation for Arid-Region Runoff Estimation

In practice, the United States (U.S.) Soil Conservation Service (SCS) curve number (CN) method is commonly used in distributed, continuous-time hydrologic models to estimate direct runoff. However, for the standard SCS-CN method, the determination of initial abstraction (I a ) as a fraction of potential maximum retention (S) after runoff starts is very subjective and thus highly debatable. The objective of this study was to develop a Modified Rational Equation (MoRE) that combines the advantages (e.g., simplicity and global acceptance) of Rational Equation and those (e.g., easy parameterization and extensive verification in U.S.) of the standard SCS-CN method as well as that can be used to more accurately predict direct runoff depth. The MoRE has two variables, rainfall intensity and relative saturation, but it does not require I a be determined. A verification using laboratory and field data indicated that the MoRE can more accurately reproduce the observed direct runoffs than the standard SCS-CN method as well as its improved version.