Location: National Soil Erosion Research Laboratory
2021 Annual Report
Objectives
Objective 1. Advance the knowledge and improve mathematical representation of processes affecting sediment, nutrient, and pesticide losses in surface and subsurface waters.
Subobjective 1.1. Quantify surface and subsurface hydrologic processes affecting transport and transient storage of sediments and chemicals.
Subobjective 1.2. Evaluate and improve scientific understanding of nutrient dynamics from the rhizosphere, upland areas, riparian zones, and drainage waterways.
Objective 2. Develop methods to reduce pollutant losses from agricultural fields and watersheds, thus protecting off-site water quality.
Subobjective 2.1. Develop removal strategies for dissolved phosphorus in drainage water.
Subobjective 2.2. Test the impact of established and new conservation practices at the field and watershed scale.
Subobjective 2.3. Determine optimal BMPs for control of runoff, sediment, and chemical losses from agricultural fields and watersheds, under existing and future climates.
Objective 3. Improve erosion and water quality modeling systems for better assessment and management of agricultural and forested lands.
Subobjective 3.1. Develop WEPP model code, including testing and scientific improvement.
Subobjective 3.2. Improve ARS soil erosion and water quality model software architectures, interfaces, and databases for end-user model delivery.
Objective 4. As part of the LTAR network, and in concert with similar long-term, land-based research infrastructure in the Midwest region, use the Eastern Corn Belt LTAR site to improve the observational capabilities and data accessibility of the LTAR network and support research to sustain or enhance agricultural production and environmental quality in agroecosystems characteristic of the Midwest region. Research and data collection are planned and implemented based on the LTAR site application and in accordance with the responsibilities outlined in the LTAR Shared Research Strategy, a living document that serves as a roadmap for LTAR implementation. Participation in the LTAR network includes research and data management in support of the ARS GRACEnet and/or Livestock GRACEnet projects.
Subobjective 4.1. Quantify the relationship between soil quality and water quality under different cropping and management scenarios at the CEAP and Eastern Corn Belt LTAR sites.
Subobjective 4.2. Develop techniques that enhance field-to-watershed scale parameterization for improved hydrologic model predictions at the CEAP and Eastern Corn Belt LTAR sites.
Subobjective 4.3. Provide data management and services for CEAP and LTAR research sites.
Approach
Lab experiments will be used to study topographic driven surface hydraulic processes and soil hydraulic gradient driven subsurface flow effects on sediment and chemical loading and transient storage. Landscape attributes will be used to confirm the lab findings on conditions for sediment and chemical transport processes and processes such as deposition and hyporheic exchange. Field rainfall simulation experiments will be conducted using pan lysimeters to collect leachate to assess the effect of fertilizer placement on phosphorus leaching to subsurface tile drains. Stable water isotopes will be measured at the outlet of a headwater watershed during storm events to identify potential flow pathways and infer potential nutrient sources. We will use lab prototypes to assess the efficiency of steel slag in three potential field-scale phosphorus removal structure configurations, i.e., blind inlet, cartridge, and in-ditch slag dam for testing, and information obtained will be used to design field-scale installations for testing in the St. Joseph River Watershed (SJRW). We will subject steel slag materials to anaerobic conditions, and determine the effects on P solubility, and also explore the feasibility of regenerating materials for P removal structures. Field- and watershed-scale studies will be conducted to assess the impact of conservation practices on water quality, and field-scale studies will be used to assess the impact of drainage design and drainage water management on water quality. The Water Erosion Prediction Project (WEPP), Agricultural Policy Extender (APEX), and Soil and Water Assessment Tool (SWAT) models will be applied to monitored fields and small catchments in the SJRW in northeastern Indiana. Data from General Circulation Models (GCM) will be downscaled to develop modified climate inputs which will allow examination of the impacts of projected future climate on flow and pollutant losses. WEPP development efforts will occur in: Atmospheric CO2 impacts on plant growth; Model response to subsurface tile drainage; Water quality components to simulate nutrient and pesticide pollutant losses. Model development and testing efforts will include maintenance of the WEPP model scientific code, development of user interfaces, model databases, and user support. The WEPP module in the NRCS Cloud Services Innovation Platform (CSIP) software architecture will be made available as an option in the NRCS Integrated Erosion Tool (IET). Additionally, a separate WEPP web-based interface is being developed that allows WEPP to be run using standard NRCS databases. Data will be collected from the new Eastern Corn Belt LTAR sites once they are identified. Real-time weather information, field-measured profile soil moisture data and remotely sensed surface soil moisture content from agricultural fields will be used to improve prediction of surface runoff and tile flow and better understand runoff generation mechanisms. Topographic attributes, soil profile characteristics, and land management will be used to quantify potential for runoff and tile flow (i.e., profile drainage).
Progress Report
For Objective 1, high frequency measurements of discharge (10 min), nutrient concentration (daily; three hours during events), electrical conductivity, and stable water isotopes were collected for a third year on the Matson Ditch within the Upper Cedar Creek watershed in northeastern Indiana. These datasets are being used to understand the dominant components of stream discharge (e.g., new water vs. old water) and to quantify how and when nutrients such as nitrogen and phosphorus are transported from upland fields to the stream. Datasets will continue to be analyzed throughout the next year, with findings used to inform water and nutrient management strategies.
Climate and discharge datasets from edge-of-field monitoring sites in Ohio and Indiana have been analyzed, with results reported in several manuscripts. Results showed that both precipitation amount and antecedent wetness were important factors controlling surface runoff and subsurface tile discharge. Thus, changing rainfall patterns due to climate change that influence rainfall amount, timing, and antecedent wetness will be important for understanding and quantifying nutrient loads.
To assess the impacts of management on landscape processes and spatial heterogeneity, surface soil samples were collected annually, along a 50m x 50m spatial grid, in Long-term Agricultural Research (LTAR) fields, for a total of four years. Geospatial analysis was performed to identify dominate flow pathways and topographic characteristics (curvature). Surface soil samples are being analyzed for properties such as texture, nutrients, aggregate stability, and microbial biomass carbon. Spatial and temporal changes in soil properties, as well production (via grain yield monitor data), along the spatial grid, are being used to identify: (i) surface connectivity of the landscape; (ii) erosion/deposition processes; and (iii) yield gaps that arise as a result of the interplay of management and hydrologic processes.
For Objective 2, several new phosphorus (P) removal structures were constructed in cooperation with the Columbus, Ohio, ARS unit and other groups. In August of 2020, the first ever bottom-up P removal structure using activated alumina was constructed near Sunbury, Ohio. This unit is stacked in-line with a nitrogen (N) bioreactor and is currently being monitored. A video on the construction of this unit was made and posted on YouTube with permission of the USDA communications department. The video has been viewed many times and has served well to disseminate the technology. https://www.youtube.com/watch?v=LUfZo9zBk6I.
We completed a 1-year pilot study on P removal by an 8% metal turnings-gravel mixture. Approximately 300 lbs of the media was placed in a large flow-through box in which water was continuously fed to the media for approximately one year. Dissolved P concentrations were measured one to three times per day at the inlet and outlet for quantifying P removal and trace metal concentrations for safety. This experiment provided the “P removal curve” that was later used for designing field-scale units.
In cooperation with the Ohio State University, we designed and helped to construct a subsurface tile drain structure that utilized metal turnings mixed with gravel; this unit is being monitored. The unit contained two buried tanks (top-down flow), each possessing about nine tons of media. In May 2021 we designed a subsurface tile drain P removal structure for a large swine farm located near Holland, Michigan. This unit flows from the top-down and was designed for five year-40% removal of dissolved P and consists of about 30 tons of metal turnings mixed with gravel (8% metal turnings). This was the first ever buried bed structure for metal turnings not contained in a tank.
In June of 2021, we constructed the first ever bottom-up metal shavings tile drain P filter. This unit is located on a farm near Waterloo, Indiana, and was a rebuild of a previous slag filter at that site. About nine tons of filter media were placed in the buried tank. The structure was designed to remove 53, 35, and 25% of the one, two and three year dissolved P loads, respectively. All flow is monitored. This unit will serve as a pilot-field scale unit due to the ease of access into the tank containing the media, allowing new materials to be tested. In addition, this unit was constructed with an experimental aeration unit for displacing any sediment that may accumulate in the media.
Experiments were conducted for examining the solubility of P previously adsorbed onto three different P filter media, under anaerobic conditions: iron (Fe)-coated alumina, acid mine drainage treatment residuals, and metal turnings. Anaerobic conditions were achieved in a controlled temperature and gas environment that deprived the samples of oxygen. Redox potential was monitored along with pH, and after reducing conditions suitable for Fe reduction occurred, the solutions were collected and tested for dissolved P, Fe, and other elements.
A P uptake experiment was completed on three different soybean cultivars using the indoor growth room. The experimental setup is one-of-a-kind in that it allows soybean (and corn) to be grown to full maturity with artificial media, under controlled conditions with no sunlight. Being able to grow 96 plants in this environment offers the advantages of field, greenhouse, growth chamber, and hydroponics studies without the disadvantages. Soybean plants were grown at eight different P concentrations, harvested, and analyzed for yield and nutrient partitioning. The data is still being processed.
A study was conducted at the Davis Purdue Agricultural Center (DPAC), Farmland, Indiana to assess the impact of gypsum (G) and chicken litter (CL) on phosphorus losses in surface runoff water. Twenty-one rainfall simulations were conducted, including the following treatments: (1) G and CL mixed before application, (2) CL applied on top of G, (3) G applied on top of CL, (4) G application, then 25 mm rain event, then CL; (5) CL only, (6) G only, and (7) control. All G and CL treatments were surface-applied. Runoff was collected every 10 min for 50 minutes. Runoff samples will be analyzed for sediment, P, N, calcium (Ca), magnesium (Mg), potassium (K), dissolved organic carbon (DOC), pH, and electrical conductivity. Also, we will analyze the runoff samples for heavy metals in order to assess any possible sources of heavy metals in gypsum or chicken litter. This study was conducted together with a scientist from the ARS National Soil Dynamics Laboratory in Auburn, Alabama.
Work continues to assess biochar produced from used creosote-treated railroad ties. Two additional temperatures (500 and 600 degrees C) were used to produce biochar and recover creosote. The biochars and creosote will be analyzed for residual creosote in biochar and composition of the creosote fractions. Also, sorption isotherms will be conducted to assess the efficiency of these biochars in removing contaminants from water.
Project Objective 3 efforts continue with the USDA Natural Resources Conservation Service (NRCS) on Water Erosion Prediction Project (WEPP) model science, interfaces, and databases to allow implementation and use by their field, state, and technical center users. The web-based interface and databases for NRCS hillslope profile applications are complete, and major efforts currently underway are towards development of web-based watershed and field-scale interfaces and databases, along with improved and validated channel erosion predictions.
To allow the WEPP and Wind Erosion Prediction Systems (WEPS) to better meet the needs of NRCS work on a common web-based user interface was initiated with the ARS Wind Erosion group in Fort Collins, Colorado. Updates done by the Wind Erosion group have used the existing WEPP web application as a baseline adding features to allow WEPS model simulations. Both the WEPP and WEPS preprocessing to setup model simulations use the Cloud Services Innovation Platform (CSIP) framework which allows a standard approach to be used in communicating with the model related web services. In addition to user interface work, support software was developed to summarize and compare WEPP and WEPS model parameters to insure consistency.
Work continues on updating the WEPP Windows desktop program. This software is being updated to use the same web services, based on the CSIP, as the NRCS web interface for WEPP.
A large-scale evaluation of recent (1970-2013) National Weather Service precipitation data resulted in an updated erosivity map for the conterminous United States, that included comparisons to other existing maps. Substantial differences were noted. For example, for stations across the state of Iowa, newly calculated erosivities increased at all locations, by up to 38%. These results are reported in a peer-reviewed journal article, and the detailed weather station data and the detailed spatial erosivity maps are available for download.
Work continues on development of a WEPP-Water Quality (WEPP-WQ) model, that can be applied to multiple spatial elements on a hillslope profile, as well as to a watershed with multiple hillslopes and channels. Validation data sets for multiple overland flow elements (OFEs) have been obtained, and work is underway on testing and application of the water quality routines.
As part of project Objectives 2 and 4, field and ditch runoff monitoring and water quality sample collection continues at our St. Joseph River Watershed Conservation Effects Assessment Project (CEAP) locations. Additional cameras were installed at field sites to provide visual verification of site conditions. In addition, remote access to site flow logging devices were configured which allowed automated downloads of flow data to the National Soil Erosion Research Laboratory (NSERL). The automated downloads are then imported into the Aquarius data management system.
Accomplishments
1. An updated erosivity map for the United States. Erosivity is a value associated with the power of rainstorms that cause water erosion. There are reported discrepancies between erosivity values calculated from weather station records and values in the current database that are used for erosion estimates and conservation planning at some locations in the US. ARS scientists in West Lafayette, Indiana, and Purdue University cooperators utilized data from 3,400 15-minute, fixed-interval National Weather Service (NWS) precipitation gauges from 1970 to 2013 to determine more up-to-date and accurate rainfall erosivity values across the conterminous United States. Substantial differences were noted. For example, for stations across Iowa, newly calculated average annual erosivities increased at all locations, by up to 38% compared to the current database values. In conservation planning, it is important to have the most accurate climate inputs such that proper conservation practices or design can be implemented to minimize soil erosion.
2. Less phosphorus applied in 2019 resulted in less dissolved phosphorus transported to Lake Erie. Increased phosphorus (P) loading to the Western Basin has caused several dangerous algal blooms over the past decade. 2019 was an extremely wet year in the Western Basin and as a result, an appreciable area was not planted or fertilized. In cooperation with Heidelberg University and National Oceanic and Atmospheric Administration (NOAA), ARS scientists in West Lafayette, Indiana, studied the nutrient transport pattern to the Maumee River for the 2019 growing season compared to previous years. From this study, we determined that a 29% decrease in dissolved P losses from agricultural land in 2019 occurred with a 62% reduction in applied P. The results emphasize the impact of both climate and management practices on P transport and suggest that an appreciable portion of P transported to the Western Basin can be immediately reduced through proper fertilizer application timing and incorporation. These results impact farmers, extension agents, natural resource agency personnel, and other involved in efforts to reduce P losses in runoff water, as well as the general public and those associated with industries in Lake Erie affected by poor water quality. This information can help conservationists and farmers better choose best management practices that will be most effective in reducing P transport.
3. Development of a feasible and economical procedure for regenerating phosphorus filter media. Use of phosphorus (P) filter media in P removal structures is an effective management practice for trapping P in non-point drainage water for improving water quality. Phosphorus lost from agricultural lands can enter streams and lakes, resulting in hazardous algal blooms. Excess dissolved P transport has also led to other major water quality impairments such as eutrophication of Lake Erie. While several manufactured filter media exist, most are cost prohibitive. ARS scientists in West Lafayette, Indiana, developed a cost-effective technique for re-generating iron (Fe)-oxide filter media that is used in P removal structures, which can cut the cost of the practice in half or more. Briefly, the media is treated with a flowing solution of hydroxide, stripping the P from spent media in-situ. The captured P can then be safely land applied to growing crops that require P. Results showed that the same media can be stripped of P at least three consecutive times. These results impact farmers, natural resource agency personnel, drainage system installers, and other involved in efforts to reduce P losses from croplands. The general public and industries (recreation, tourism, municipal drinking water agencies) also benefit. The cost of P removal can be reduced to less than half compared to the cost of purchasing new filter media, which makes the conservation practice more affordable when conservationists are implementing P removal structures. At least one private company has already adopted this technique.
4. Development of a new software for designing phosphorus removal structures. A new software, P-Trap (phosphorus transport reduction app) was released. With dissolved P as the main culprit in causing algal blooms and ponds and lakes filling with green algae, the use of P removal structures provides a management practice for trapping dissolved P before it reaches a water body and causes damage. While the P removal structures are being promoted by the Natural Resources Conservation Service, there was little design guidance. This new software developed by ARS scientists in West Lafayette, Indiana, and software engineers allows for the lay conservationist to design a site-specific structure using any P filter media that they have available, and for achieving the desired goals. After input of site characteristics such as annual flow volume, flow rate, and dissolved P concentrations, filter media choice, and desired P removal goal and system lifetime, the P-Trap software provides exact design specifications for the user. This allows non-engineers to design a P removal structure and examine several possible scenarios for assessing the costs of various system and their expected performance and life.
5. Changing rainfall patterns due to climate change is increasing nutrient losses from croplands. In agricultural watersheds, shifting climate patterns present an immediate and localized risk to both farm productivity (i.e., too much or too little water) and downstream water quality. ARS researchers in West Lafayette, Indiana, and Columbus, Ohio, evaluated rainfall patterns and their influence on water quantity and quality across the Maumee River Basin, the largest contributor of nutrient loads to Lake Erie, from 1975-2017. Heavy (daily rainfall between 1-3 inches) and very heavy (daily rainfall >3 inches) increased by 45% over the 40-year study period, with increases primarily occurring during the spring and summer. Rainfall patterns were strongly tied to nutrient loadings at both the field and watershed scales. For example, edge-of-field monitoring showed that 80% of annual nutrient loadings occurred during storm events occurring during only a small fraction of the year (one week to two months). These findings indicate that innovative water and nutrient management practices (i.e. controlled drainage, cover crops, etc.) in the Maumee River Basin will be needed for agricultural production to sustainably adapt to climate change.
6. Subsurface tile drainage is largely comprised of old water. Old water is defined as water stored in the soil profile or groundwater prior to a rainfall event. Understanding how water moves through the soil to subsurface tile drains in agricultural fields is important for assessing nutrient loss and implementing conservation practices, especially as rainfall patterns continue to shift due to climate change. ARS researchers in West Lafayette, Indiana, used stable water isotopes to assess water storage, mixing, and fluxes in a tile-drained field. Rainfall and mobile soil water within the top eight inches of the soil profile were well mixed throughout the year, but groundwater stored below the tile depth was primarily recharged during winter months. Tile discharge from the field had an average age of 245 days and was comprised of both groundwater and water stored in the soil profile above the tiles. Tracing water through the soil profile provides critical information on the processes controlling nutrient transport and for developing sustainable water and nutrient management plans in tile-drained landscapes. For example, knowing the partitioning of the water going into the tile flow, and the soluble P concentrations in the groundwater (often near zero) and in the soil profile above the tile allows estimation of P losses from the tile outflow to ditches and downstream water bodies.
Review Publications
Flanagan, D. C., Wagner, L. E., Cruse, R. M., and Arnold, J. G. 2020. Chapter 24. Modeling soil and water conservation. In: Delgado, J. A., Gantzer, C. J., Sassenrath, G. F., Editors. Soil and Water Conservation: A Celebration of 75 Years. Ankeny, Iowa: Soil and Water Conservation Society. pp. 255-269.
Revuelta-Acosta, J.D., Flanagan, D.C., Engel, B.A., King, K.W. 2020. Improvement of the Water Erosion Prediction Project (WEPP) model for quantifying field scale subsurface drainage discharge. Agricultural Water Management. 244. Article 106597. https://doi.org/10.1016/j.agwat.2020.106597.
Guo, T., Johnson, L., Lebarge, G., Penn, C.J., Stumpf, R., Baker, D., Shao, G. 2020. Less agricultural phosphorus applied in 2019 led to less dissolved phosphorus transported to Lake Erie. Journal of Environmental Science and Technology. 55(1):283-291. https://doi.org/10.1021/acs.est.0c03495.
Aboelnour, M.A., Engel, B.A., Frisbee, M.D., Gitau, M.W., Flanagan, D.C. 2021. Impacts of watershed physical properties and land use on baseflow at regional scales. Journal of Hydrology: Regional Studies. 35. Article 100810. https://doi.org/10.1016/j.ejrh.2021.100810.
Guo, T., Srivastava, A., Flanagan, D.C. 2021. Improving and verifying channel erosion simulation in the Water Erosion Prediction Project (WEPP) Model. Journal of Environmental Management. 291. Article 112616. https://doi.org/10.1016/j.jenvman.2021.112616.
Hanrahan, B.R., King, K.W., Williams, M.R. 2020. Controls on subsurface nitrate and dissolved reactive phosphorus losses from agricultural fields during precipitation-driven events. Science of the Total Environment. 754. Article 142047. https://doi.org/10.1016/j.scitotenv.2020.142047.
Penn, C.J. 2021. The past, present, and future of phosphorus removal structures. Water. 13(6). Article 797. https://doi.org/10.3390/w13060797.
Penn, C.J., Frankenberger, J.R., Livingston, S.J. 2021. Introduction to P-TRAP software for designing phosphorus removal structures. Agricultural and Environmental Letters. 6(1). Article e20043. https://doi.org/10.1002/ael2.20043.
Penn, C.J., Scott, I., Huang, C. 2020. Development of a regeneration technique for aluminum-rich and iron-rich phosphorus sorption materials. Water. 12(6). Article 1784. https://doi.org/10.3390/w12061784.
Penn, C.J., Livingston, S.J., Shedkar, V., King, K.W., Williams, M.R. 2020. Performance of field-scale phosphorus removal structures utilizing steel slag for treatment of subsurface drainage. Water. 12(2). Article 443. https://doi.org/10.3390/w12020443.
Pignotti, G., Cherkauer, K., Crawford, M., Williams, M.R., Chaubey, I. 2021. Evaluating soil water routing approaches in watershed-scale, ecohydrologic modelling. Hydrological Processes. 35(3). Article e14034. https://doi.org/10.1002/hyp.14034.
Scott, I.C., Penn, C.J. 2021. Estimating the variability of steel slag properties and their influence in phosphorus removal ability. Chemosphere. 276. Article 130205. https://doi.org/10.1016/j.chemosphere.2021.130205.
Scott, I., Huang, C., Bowling, L.C. 2020. The use of electrical conductivity to develop temporally precise breakthrough curves in tracer injection experiments. Journal of Hydrology. 588. Article 124998. https://doi.org/10.1016/j.jhydrol.2020.124998.
Shedekar, V., Penn, C.J., Pease, L., King, K.W., Kalcic, M., Livingston, S.J. 2020. Performance of a ditch-style phosphorus removal structure for treating agricultural drainage water with aluminum-treated steel slag. Water. 12(8). Article 2149. https://doi.org/10.3390/w12082149.
Wacha, K.M., Hatfield, J.L., O'Brien, P.L., Dold, C. 2021. Short-term effects of nitrogen source on soil properties and plant growth. Agrosystems, Geosciences & Environment. 4(2). Article e20176. https://doi.org/10.1002/agg2.20176.
Wacha, K.M., Huang, C., O'Brien, P.L., Papanicolaou, T.N., Hatfield, J.L. 2021. Quantifying the time-specific kinetic energy of simulated rainfall using a dynamic rain gauge system. Agricultural & Environmental Letters. 6(1). Article e20042. https://doi.org/10.1002/ael2.20042.
Wang, S., Flanagan, D.C., Engel, B.A., Zhou, N. 2020. Impacts of subsurface hydrologic conditions on rill sediment transport capacity. Journal of Hydrology. 591. Article 125582. https://doi.org/10.1016/j.jhydrol.2020.125582.
Wang, S., Flanagan, D.C., Engel, B., Zhou, N. 2021. Prediction of rill sediment transport capacity under different subsurface hydrologic conditions. Journal of Hydrology. 598. Article 126410. https://doi.org/10.1016/j.jhydrol.2021.126410.
Wang, B., Li, P., Huang, C., Liu, G., Yang, Y. 2020. Effects of root morphological traits on soil detachment for ten herbaceous species in the Loess Plateau. Science of the Total Environment. 754. Article 142304. https://doi.org/10.1016/j.scitotenv.2020.142304.
Wang, E., Cruse, R.M., Sharma-Acharya, B., Herzmann, D.E., Gelder, B.K., James, D.E., Flanagan, D.C., Blanco-Canqui, H., Mitchell, R., Laird, D.A. 2020. Strategic switchgrass (Panicum virgatum) production within row cropping systems: Regional soil erosion loss and water runoff impacts. Global Change Biology Bioenergy. 12:955–967. https://doi.org/10.1111/gcbb.12749.
Wang, L., Penn, C.J., Huang, C., Livingston, S.J., Yan, J. 2020. Using steel slag for dissolved phosphorus removal: Insights from a designed flow-through laboratory experimental structure. Water. 12(5). Article 1236. https://doi.org/10.3390/w12051236.
Williams, M.R., McAfee, S.J. 2021. Water storage, mixing, and fluxes in tile-drained agricultural fields inferred from stable water isotopes. Journal of Hydrology. 599. Article 126347. https://doi.org/10.1016/j.jhydrol.2021.126347.
Yousaf, N., Khan, B., Flanagan, D.C., Mian, I.A. 2020. Effect of soil infiltration capacity, soil texture and rainfall on soil erosion occurring under different land use land cover (LULC) in Miandam Valley, Swat Pakistan. Journal of Engineering and Applied Science. 39(2):135-142. https://doi.org/10.17582/journal.jeas/39.2.135.142.
Yousaf, N., Khan, B., Flanagan, D.C., Mian, I.A., Iqbal, J., Rashid, I., Khalid, S., Ghaffar, N., Manzoor, S., Bilal, M. 2021. Estimation of erosion induced nutrients loss under different land uses: A case study in Miandam Valley, Swat Pakistan. International Journal of Agricultural and Biological Engineering. 14(2):135-141. https://doi.org/10.25165/j.ijabe.20211402.5704.
Zhang, X., Lopes, I., Ni, J., Yuan, Y., Huang, C., Smith, D.R., Chaubey, I., Wu, S. 2021. Long-term performance of three mesophilic anaerobic digesters to convert animal and agro-industrial wastes into organic fertilizer. Journal of Cleaner Production. 307. Article 127271. https://doi.org/10.1016/j.jclepro.2021.127271.
McGehee, R.P., Flanagan, D.C., Srivastava, P., Engel, B.A., Huang, C., Nearing, M.A. 2021. An updated isoerodent map of the conterminous United States. International Soil and Water Conservation Research. https://doi.org/10.1016/j.iswcr.2021.06.004.
