Evaluating ephemeral gullies with a process-based topographic index model

Soil conservation practices have been implemented to control soil degradation from sheet and rill erosion, but excessive sediment runoff remains among the most prevalent water quality problems in the world. Ephemeral gully (EG) erosion has been recognized as a major source of sediment in agricultural watersheds; thus, predicting location and length of EGs is important to assess sediment contribution from EG erosion. Geomorphological models are based on topographic information and ignore other important factors such as precipitation, soil, topography, and land use/land management practices, whereas physically based models are complex, require detailed input information, and are difficult to apply to larger areas. In this study, an approach was developed to incorporate a process-based Overland Flow-Turbulent (OFT) EG model that contained factors accounting for drainage area, surface roughness, slope, soil critical shear stress, and surface runoff in the ArcGIS environment. Two hydrologic models, Soil Water Assessment Tool (SWAT) and ArcCN-Runoff (ACR), were adopted to simulate precipitation excess in Goose Creek watershed in central Kansas, USA. These two realizations of the OFT model were compared with the Slope-Area (SA) topographic index model for accuracy of EG location identification and length calculation. The critical threshold index in the SA model was calibrated in a single field in the watershed prior to EG identification whereas the OFT models were uncalibrated. Results demonstrated overall similar performance between calibrated SA model and uncalibrated OFT-SWAT model, and both outperformed the uncalibrated OFT-ACR model. In simulation of EG location, the OFT-SWAT model resulted in 12% fewer false negatives but 8% more false positives than the SA model, compared with 19% fewer false positive and 6% more false negatives than the OFT-ACR model. Greater errors in runoff estimation by ACR translated directly into errors in EG simulation. All models over-predicted EG lengths compared with observed data, though OFT-SWAT and SA models did so with better fit exceedance probability curves, about zero Nash-Sutcliff model efficiency and ≤40% bias compared to -3 model efficiency and >100% bias for OFT-ACR. Success of the uncalibrated OFT-SWAT model in producing satisfactory predictions of EG location and EG length shows promise for process-based EG simulation. The OFT-SWAT model used data and parameters also commonly used for SWAT model development, which should simplify its adoption to other watersheds and regions. Further testing is needed to determine the robustness of the OFT-SWAT model to dissimilar field and hydrologic conditions. It is expected that inclusion of more site-specific physical properties in OFT-SWAT would improve model performance in predicting location and length of EGs, which is essential for accurate estimation of EG sediment erosion rates

Pasture BMP effectiveness using an HRU-based subarea approach in SWAT

Citation: Aleksey Y. Sheshukov, Kyle R. Douglas-Mankin, Sumathy Sinnathamby, Prasad Daggupati, Pasture BMP effectiveness using an HRU-based subarea approach in SWAT, Journal of Environmental Management, Volume 166, 2016, Pages 276-284, ISSN 0301-4797, http://dx.doi.org/10.1016/j.jenvman.2015.10.023.Many conservation programs have been established to motivate producers to adopt best management practices (BMP) to minimize pasture runoff and nutrient loads, but a process is needed to assess BMP effectiveness to help target implementation efforts. A study was conducted to develop and demonstrate a method to evaluate water-quality impacts and the effectiveness of two widely used BMPs on a livestock pasture: off-stream watering site and stream fencing. The Soil and Water Assessment Tool (SWAT) model was built for the Pottawatomie Creek Watershed in eastern Kansas, independently calibrated at the watershed outlet for streamflow and at a pasture site for nutrients and sediment runoff, and also employed to simulate pollutant loads in a synthetic pasture. The pasture was divided into several subareas including stream, riparian zone, and two grazing zones. Five scenarios applied to both a synthetic pasture and a whole watershed were simulated to assess various combinations of widely used pasture BMPs: (1) baseline conditions with an open stream access, (2) an off-stream watering site installed in individual subareas in the pasture, and (3) stream or riparian zone fencing with an off-stream watering site. Results indicated that pollutant loads increase with increasing stocking rates whereas off-stream watering site and/or stream fencing reduce time cattle spend in the stream and nutrient loads. These two BMPs lowered organic P and N loads by more than 59% and nitrate loads by 19%, but TSS and sediment-attached P loads remained practically unchanged. An effectiveness index (EI) quantified impacts from the various combinations of off-stream watering sites and fencing in all scenarios. Stream bank contribution to pollutant loads was not accounted in the methodology due to limitations of the SWAT model, but can be incorporated in the approach if an amount of bank soil loss is known for various stocking rates. The proposed methodology provides an adaptable framework for pasture BMP assessment and was utilized to represent a consistent, defensible process to quantify the effectiveness of BMP proposals in a BMP auction in eastern Kansas

Identifying Potential Locations for Grassed Waterways using Terrain Attributes and Precision Conservation Technologies

Grassed waterways (GWWs) are an effective conservation practice for preventing ephemeral gully erosion resulting from channelized surface runoff in agricultural fields. However, field reconnaissance to identify areas of channelized erosion within a watershed can be time-consuming and labor-intensive. Recent advances in precision conservation and light detection and ranging (LiDAR) technologies can provide valuable information on environmentally sensitive areas that cause soil degradation. The objective of this study was to demonstrate that a compound topographic index (CTI) model supplemented with LiDAR data can be used to identify potential GWW locations and inform design recommendations. A LiDAR digital elevation model with a spatial resolution of 3 m was used to derive terrain attributes (slope, drainage area, and plan curvature). The GWW identification and design process was automated in the ArcGIS Python environment. The plan curvature identified erosion channels, but discontinuity in the model output was observed. The CTI model was calibrated to a field with GWWs installed under USDA-NRCS guidelines, which yielded a CTI threshold of 30. The calibrated model (CTI = 30) was able to identify all 14 existing GWWs in the watershed. Field surveys were conducted in the watershed, and areas exhibiting evidence of channelized erosion were identified by the model for GWW placement. Furthermore, the CTI model overestimated (PBIAS = -23.34%) the lengths of predicted GWWs, suggesting a need to further extend the existing GWWs. The total surface area of the predicted GWWs was 29.3 ha in the study watershed, with depth of GWWs reaching 0.3 m. The design process provides an estimate of land to be set aside for conservation practices. The terrain analysis was effective in targeting conservation practice placement and improves the accuracy of field assessments

Introducing a new open source GIS user interface for the SWAT model

The Soil and Water Assessment Tool (SWAT) model is a robust watershed modeling tool. It typically uses the ArcSWAT interface to create its inputs. ArcSWAT is public domain software which works in the licensed ArcGIS environment. The aim of this paper was to develop an open source user interface for the SWAT model. The interface, QSWAT, is written in the Python programming language and uses various functionalities of the open source geographic information system, QGIS. The current interface performs similar functions to ArcSWAT, but with additional enhanced features such as merging small subbasins and static and dynamic visualization of outputs. The interface is demonstrated through a case study in the Gumera watershed in the Lake Tana basin of Ethiopia, where it showed a successful performance. QSWAT will be a valuable tool for the SWAT scientific community, with improved availability and functionality compared with other options for creating SWAT models

Spatio-Temporal Impacts of Biofuel Production and Climate Variability on Water Quantity and Quality in Upper Mississippi River Basin

Impact of climate change on the water resources of the United States exposes the vulnerability of feedstock-specific mandated fuel targets to extreme weather conditions that could become more frequent and intensify in the future. Consequently, a sustainable biofuel policy should consider: (a) how climate change would alter both water supply and demand; and (b) in turn, how related changes in water availability will impact the production of biofuel crops; and (c) the environmental implications of large scale biofuel productions. Understanding the role of biofuels in the water cycle is the key to understanding many of the environmental impacts of biofuels. Therefore, the focus of this study is to model the rarely explored interactions between land use, climate change, water resources and the environment in future biofuel production systems. Results from this study will help explore the impacts of the US biofuel policy and climate change on water and agricultural resources. We used the Soil and Water Assessment Tool (SWAT) to analyze the water quantity and quality consequences of land use and land management related changes in cropping conditions (e.g., more use of marginal lands, greater residue harvest, increased yields), plus management practices due to biofuel crops to meet the Renewable Fuel Standard target on water quality and quantity

Impacts of hydrological model calibration on projected hydrological changes under climate change—a multi-model assessment in three large river basins

This study aimed to investigate the influence of hydrological model calibration/validation on discharge projections for three large river basins (the Rhine, Upper Mississippi and Upper Yellow). Three hydrological models (HMs), which have been firstly calibrated against the monthly discharge at the outlet of each basin (simple calibration), were re-calibrated against the daily discharge at the outlet and intermediate gauges under contrast climate conditions simultaneously (enhanced calibration). In addition, the models were validated in terms of hydrological indicators of interest (median, low and high flows) as well as actual evapotranspiration in the historical period. The models calibrated using both calibration methods were then driven by the same bias corrected climate projections from five global circulation models (GCMs) under four Representative Concentration Pathway scenarios (RCPs). The hydrological changes of the indicators were represented by the ensemble median, ensemble mean and ensemble weighted means of all combinations of HMs and GCMs under each RCP. The results showed moderate (5–10%) to strong influence (> 10%) of the calibration methods on the ensemble medians/means for the Mississippi, minor to moderate (up to 10%) influence for the Yellow and minor (< 5%) influence for the Rhine. In addition, the enhanced calibration/validation method reduced the shares of uncertainty related to HMs for three indicators in all basins when the strict weighting method was used. It also showed that the successful enhanced calibration had the potential to reduce the uncertainty of hydrological projections, especially when the HM uncertainty was significant after the simple calibration. © 2020, The Author(s)

Difference in the bed load transport of graded and uniform sediments during floods: An experimental investigation

The objective of this study was to experimentally evaluate the difference in the transport of uniform (5.17, 10.35, 14, 20.7 mm) and graded sediment (mixture of these rounded particles with equal weight proportions) under different unsteady flow hydrographs in a 12 m long, 0.5 m wide and deep glass-walled flume. There was a lag time between fractions and uniform particles, such that peaks of coarser and finer fraction particles occurred before and after the peak of uniform sediment with the same size, respectively. Comparison between uniform particles and fractions in graded sediment showed that the sediment transport rate of fine and coarse fractions was lower and higher than their counterpart uniform particles, respectively. Overall, the uniform particles demonstrated a clockwise hysteresis loop and graded sediment had a counterclockwise hysteresis loop. The mobility of coarser fractions increased during the rising limb of hydrograph, whereas the mobility of finer fractions increased during the falling limb. In general, the mobility of coarse fractions increased and that of fine fractions reduced. Result of transported sediment showed that average particle size collected in traps (Db50) was coarser than bed material (Ds50) on both limbs. The relative transport ratio for uniform and graded sediment is higher and lower than 1, respectively

Uniform and graded bed-load sediment transport in a degrading channel with nonequilibrium conditions

Bed-load transport plays a critical role in river morphological change and has an important impact on river ecology. Although there is good understanding of the role of the variation of river bed grain size on transport dynamics in equilibrium conditions, much less is understood for non-equilibrium conditions when the channel is either aggrading or degrading. In particular, the relative role of different grain sizes in the promotion and hindering of the transport of coarse and fine fractions in a degrading channel has yet to be investigated. The current study attempts to provide new understanding through a series of flume experiments done using uniform and graded sediment particles. The experiments revealed coarser grain-size fractions for a poorly sorted sediment, relative to uniform-sized sediment, reduced the transport of finer grains and finer fractions enhanced the transport of coarse grains. This hindering-promotion effect, caused by relative hiding and exposure of finer and coarse fractions, increased with bed slope and decreased with relative submergence. In particular, as relative submergence increased, the graded fractions tended towards behaving more like their uniform-sized counterparts. Also, the bed-load parameter of the graded fractions increased more with a rise in bed slope than observed for the uniform-sized counterparts. These results revealed, for degrading channel conditions, such as downstream of a dam, bed-load equations developed for uniform bed sediment are inappropriate for use in natural river systems, particularly in mountain streams. Furthermore, changes in river bed composition due to activities that enhance the input of hill-slope sediment, such as fire, logging, and agricultural development, are likely to cause significant changes in river morphology