Precipitation parameters of stochastic climate models for a changing climate

Changing climate can impact erosion directly by increasing or decreasing the rainfall depth and intensity and indirectly by influencing the vegetative cover on landscapes. Stochastic climate models are increasingly being used to allow the assessment of erosion to be done using ensemble statistics. Precipitation is mostly widely represented by first determining the state of the day is wet or dry using discrete transitional probabilities and then, for a wet day, determining the precipitation depth. The parameters for stochastic models are based on the statistical analysis of observed data for the current conditions. Simple methods to modify these stochastic parameters under different climate scenarios are desired to easily simulate their impact on erosion. Modifying parameters for precipitation depth needs to be considered carefully because changes in depth can be achieved by varying the statistics of daily precipitation, by changing the number of wet days, or by a combination of these statistics. A framework for modifying the precipitation depth for new climate conditions is developed in the study. In addition to the stochastic climate parameters related to the moments of probability density functions and transitional probabilities under the current conditions, the proposed framework requires the user to specify the fractional changes in the total precipitation depth and the mean daily precipitation depth. Relationships are developed to determine indirectly the proper number of wet days from transitional probabilities for a first-order Markov chain. These relationships are dependent on a user-specified parameter of the ratio of the mean number of wet-wet day sequences of the current and new climate conditions. The sequence of wet-wet days is important in modeling soil erosion. The framework is applied to 80 years of precipitation data for Stillwater, OK. The implications of assuming no change in the wet-wet-day ratio with new climate conditions is compared to the results obtained assuming a ratio equal to the fractional change in the mean number of wet days. If the new climate condition corresponds to an increase in the number of wet days, the assumption of an unchanged ration corresponds to new storm patterns that develop more often on dry days and dissipate more rapidly on wet days. The opposite trend occurs if the new climate condition corresponds to a decrease in the number of dry day. Under this scenario, storm patterns tend to dissipate more slowly resulting, on average, in more frequent consecutive days with precipitation. A wet-wet day ratio equal to the fractional change in the mean number of wet days corresponds to no change between the current and new climate conditions of the transitional probability of wet-given- wet-day. This result suggests that the persistence of storm systems do not change under the new climate conditions. The proposed framework is useful and easy to implement in stochastic climate models

Effects of Intra-Storm Soil Moisture and Runoff Characteristics on Ephemeral Gully Development: Evidence from a No-Till Field Study

Citation: Karimov, V.R.; Sheshukov, A.Y. Effects of Intra-Storm Soil Moisture and Runoff Characteristics on Ephemeral Gully Development: Evidence from a No-Till Field Study. Water 2017, 9, 742.Ephemeral gully erosion, prevalent on agricultural landscapes of the Great Plains, is recognized as a large source of soil loss and a substantial contributor to the sedimentation of small ponds and large reservoirs. Multi-seasonal field studies can provide needed information on ephemeral gully development and its relationship to physical factors associated with field characteristics, rainfall patterns, runoff hydrograph, and management practices. In this study, an ephemeral gully on a no-till cultivated crop field in central Kansas, U.S., was monitored in 2013 and 2014. Data collection included continuous sub-hourly precipitation, soil moisture, soil temperature, and 15 field surveys of cross-sectional profiles in the headcut and channelized parts of the gully. Rainfall excess from a contributing catchment was calculated with the Water Erosion Prediction Project (WEPP) model for all storm events and validated on channel flow measurements. Twelve significant runoff events with hydraulic shear stresses higher than the critical value were identified to potentially cause soil erosion in three out of fourteen survey periods. Analysis of shear stress imposed by peak channel flow on soil surface, antecedent soil moisture condition, and channel shape at individual events provided the basis on which to extend the definition of the critical shear stress function by incorporating the intra-storm changes in soil moisture content. One potential form of this function was suggested and tested with collected data. Similar field studies in other agriculturally-dominated areas and laboratory experiments can develop datasets for a better understanding of the physical mechanisms associated with ephemeral gully progression.

Hydrologic Alterations Predicted by Seasonally-Consistent Subset Ensembles of General Circulation Models

Citation: Sheshukov, A.Y.; Douglas-Mankin, K.R. Hydrologic Alterations Predicted by Seasonally-Consistent Subset Ensembles of General Circulation Models. Climate 2017, 5, 44.The Missouri River system has a large water storage capacity, where baseflow plays an important role. Understanding historical baseflow characteristics with respect to climate and land use impacts is essential for effective planning and management of water resources in the Missouri River Basin (MORB). This study evaluated statistical trends in baseflow and precipitation for 99 MORB minimally disturbed watersheds during 1950–2014. Elasticity of baseflow to climate variability and agricultural land use change were quantified for the 99 studied watersheds. Baseflow was derived from daily streamflow records with a recursive digital filter method. The results showed that baseflow varied between 38 and 80% (0 and 331 mm/year) of total streamflow with an average of 60%, indicating that more than half of streamflow in the MORB is derived from baseflow. The trend analysis revealed that precipitation increased during the study period in 78 out of 99 watersheds, leading to 1–3.9% noticeable increase in baseflow for 68 of 99 watersheds. Although the changes in baseflow obtained in this study were a result of the combined effects of climate and land use change across the basin, upward trends in baseflow generally coincide with increased precipitation and agricultural land use trends in the basin. Agricultural land use increase mostly led to a 0–5.7% decrease in annual baseflow in the basin, except toward east of the basin where baseflow mostly increased with agricultural land use increase (0.1–2.0%). In general, a 1% increase in precipitation and a 1% increase in agricultural land use resulted in 1.5% increase and 0.2% decrease in baseflow, respectively, during the study period. These results are entirely dependent on the quality of data used; however, they provide useful insight into the relative influence of climate and land use change on baseflow conditions in the Great Plains region of the USA

Simulation of rock infiltration systems

Rain gardens, infiltration trenches, and dry detention ponds are widely used to treat runoff from construction site and urban watersheds. These infiltration systems have the potential to remove contaminants by filtration, but they also may become clogged with deposited sediment and other debris. An overview of studies done at the University of Minnesota over the past 10 years to investigate the trapping of sediment and the corresponding changes in permeabilities is presented. They include experimental data collected from the laboratory cores, a prototype dry detention basin, and field studies as well as the development of algorithms used in the WATER (Watershed Assessment Tool for Environmental Risk) model

Integrating Watershed Management Across the Urban–Rural Interface: Opportunities for Extension Watershed Programs

Urban–rural partnerships are increasingly viewed as a critical component of efforts to improve water quality at the watershed scale. We present an opportunity for such partnerships, using an off-site best management practice (BMP) program developed between the City of Wichita and agricultural producers in the Little Arkansas River Watershed of south-central Kansas as an example. We highlight the critical role of Extension specialists in developing this and similar programs, the success of which hinges on targeted BMP implementation and relationships with agricultural producers

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

Annual baseflow variations as influenced by climate variability and agricultural land use change in the Missouri River Basin

The Missouri River system has a large water storage capacity, where baseflow plays an important role. Understanding historical baseflow characteristics with respect to climate and land use impacts is essential for effective planning and management of water resources in the Missouri River Basin (MORB). This study evaluated statistical trends in baseflow and precipitation for 99 MORB minimally disturbed watersheds during 1950–2014. Elasticity of baseflow to climate variability and agricultural land use change were quantified for the 99 studied watersheds. Baseflow was derived from daily streamflow records with a recursive digital filter method. The results showed that baseflow varied between 38 and 80% (0 and 331 mm/year) of total streamflow with an average of 60%, indicating that more than half of streamflow in the MORB is derived from baseflow. The trend analysis revealed that precipitation increased during the study period in 78 out of 99 watersheds, leading to 1–3.9% noticeable increase in baseflow for 68 of 99 watersheds. Although the changes in baseflow obtained in this study were a result of the combined effects of climate and land use change across the basin, upward trends in baseflow generally coincide with increased precipitation and agricultural land use trends in the basin. Agricultural land use increase mostly led to a 0–5.7% decrease in annual baseflow in the basin, except toward east of the basin where baseflow mostly increased with agricultural land use increase (0.1–2.0%). In general, a 1% increase in precipitation and a 1% increase in agricultural land use resulted in 1.5% increase and 0.2% decrease in baseflow, respectively, during the study period. These results are entirely dependent on the quality of data used; however, they provide useful insight into the relative influence of climate and land use change on baseflow conditions in the Great Plains region of the USA

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

Reservoir Sedimentation and Upstream Sediment Sources: Perspectives and Future Research Needs on Streambank and Gully Erosion

Citation: Fox, G.A., Sheshukov, A., Cruse, R. et al. Environmental Management (2016) 57: 945. doi:10.1007/s00267-016-0671-9The future reliance on water supply and flood control reservoirs across the globe will continue to expand, especially under a variable climate. As the inventory of new potential dam sites is shrinking, construction of additional reservoirs is less likely compared to simultaneous flow and sediment management in existing reservoirs. One aspect of this sediment management is related to the control of upstream sediment sources. However, key research questions remain regarding upstream sediment loading rates. Highlighted in this article are research needs relative to measuring and predicting sediment transport rates and loading due to streambank and gully erosion within a watershed. For example, additional instream sediment transport and reservoir sedimentation rate measurements are needed across a range of watershed conditions, reservoir sizes, and geographical locations. More research is needed to understand the intricate linkage between upland practices and instream response. A need still exists to clarify the benefit of restoration or stabilization of a small reach within a channel system or maturing gully on total watershed sediment load. We need to better understand the intricate interactions between hydrological and erosion processes to improve prediction, location, and timing of streambank erosion and failure and gully formation. Also, improved process-based measurement and prediction techniques are needed that balance data requirements regarding cohesive soil erodibility and stability as compared to simpler topographic indices for gullies or stream classification systems. Such techniques will allow the research community to address the benefit of various conservation and/or stabilization practices at targeted locations within watersheds

Stability analysis of gravity-driven infiltrating flow

[1] Stability analysis of gravity-driven unsaturated flow is examined for the general case of Darcian flow with a generalized nonequilibrium capillary pressure-saturation relation. With this nonequilibrium relation the governing equation is referred to as the nonequilibrium Richards equation (NERE). For the special case where the nonequilibrium vanishes, the NERE reduces to the Richards equation (RE), the conventional governing equation for describing unsaturated flow. A generalized linear stability analysis of the RE shows that this equation is unconditionally stable and therefore not able to produce gravity-driven unstable flows for infinitesimal perturbations to the flow field. A much stronger result of unconditional stability for the RE is derived using a nonlinear stability analysis applicable to the general case of heterogeneous porous media. For the general case of the NERE model, results of a linear stability analysis show that the NERE model is conditionally stable, with lower-frequency perturbations being unstable. A result of this analysis is that the nonmonotonicity of the pressure and saturation profile is a requisite condition for flow instability