Simulating the Water Storage Benefits of Cover Crops Using the Gridded Surface Subsurface Hydrological Analysis (GSSHA) Model

By Salam Murtada, Daniel Reinartz, and Steve Kloiber

OVERVIEW

Cover crops provide many benefits that include improving soil health, providing storage to reduce runoff, preventing soil erosion, and protecting water quality. They benefit both the environment and farm operators by optimizing the use of fertilizers, allowing runoff treatment through the unsaturated soil, and preventing the loss of nutrients to the rivers.

Cover crops are typically applied between harvest and growing seasons. They can also be interseeded with cultivated crops during the growing season. However, they have the greatest potential benefit in early spring, before planting, when the ground is otherwise fallow and soil is vulnerable to intense rainfall events.

This study compares the effects of cover crops between growing and non-growing seasons and investigates the processes that drive them, using continuous and synthetic events simulations. These processes include infiltration, surface runoff, storage capacity, and soil moisture. This paper quantifies the benefits of cover crop application in the Shakopee watershed, Minnesota through hydrological modeling using the Gridded Surface Subsurface Hydrological Analysis (GSSHA) model.

Haruna et al., 2020, summarized studies published in the last 20 years addressing the benefits of cover crop on soil physical properties. According to these studies, cover crops significantly reduced soil density and increased soil organic content leading to reductions in soil loss and surface runoff, and increases in water holding capacity, infiltration and potential carbon sequestration. In another study, Basche and DeLong, 2017, presented the benefits of cover crops and other continuous living crops in combating rainfall infiltration variability, after statistically determining their effects on increasing porosity (8.0 + 2.2%) and soil water retention at field capacity (9.3 + 2.7).

The Gridded Surface Subsurface Hydrological Analysis (GSSHA) model is a physically-based, distributive model that simulates the interactions between the complex hydrological processes taking place on the surface and subsurface media at fine temporal and spatial scales. It uses finite difference in space to the second order, and forward difference in time to the first order. The model interfaces with the Watershed Modeling System (WMS) graphical user interface. GSSHA was developed by Dr. Charles Downer of the Environmental Research and Development Center (ERDC), United States Army Corps of Engineers (USACE) and is currently supported by Aquaveo, Inc.

The Shakopee watershed (323 mi2) extends across three counties: Kandiyohi, Chippewa, and Swift counties in central Minnesota (Figure 1). It is located in predominantly agricultural areas, where corn and soybean crops comprise approximately 68% of the total watershed. The soils are primarily fine-textured and poorly drained.

Figure 1: Shakopee Creek watershed land use and location.

METHODS

In this study, the effects of cover crops on the subsurface and surface processes were simulated to evaluate changes to surface water runoff, infiltration, and evapotranspiration. After calibrating and validating the model, cover crops were introduced as a hypothetical scenario in the form of small grains applied over all agricultural areas during growing and non-growing seasons. Examples of small grains are winter-hardy cultivars of rye, wheat, and triticale that can survive cold weather. Input climate variables were held constant in the model.

Large grid cells (9.88-acre) were used to accommodate the computation time to simulate the large, 323 square-mile watershed. The model consists of three layers to represent the upper tillage layer and lower soil layers as reported by the Soil Web. Key model assumptions and parameters are summarized in Table 1.

The simulation was broken down into two connected and continuous cycles, non-growing and growing, where different processes controlled the hydrology. This enabled us to compare the effects of cover crops between the two cycles in order to highlight their benefits more accurately (Figure 2).

Figure 2: Continuous simulation broken down into three cycles where different processes dominate. Note that an increase in interception was needed for the October event to represent the late growing season just before harvest. The model interception was based on early growing season values that overestimated the observed peak and volume.
Figure 2: Continuous simulation broken down into three cycles where different processes dominate. Note that an increase in interception was needed for the October event to represent the late growing season just before harvest. The model interception was based on early growing season values that overestimated the observed peak and volume.

The model simulated the effects of cover crops as follows:

Hydraulic Conductivity

Hydraulic conductivity is an important factor to characterize the effects of cover crops. Based on cover crop ability to decrease soil compaction and increase the organic content of the soil within its root zone, empirical relationships from the Water Erosion Prediction Project (WEPP) method (USDA-WEPP, 1995) were used to determine the saturated hydraulic conductivity. The WEPP method combines the effects of soil properties based on the Hydrologic Soil Group (HSG) and soil texture, as well as land use features based on a specific agricultural practice (e.g. conservation tillage) or the curve number (CN). The model used the hydraulic conductivity to simulate infiltration using the Green & Ampt method. Soil textures were obtained from the Soil Survey Geographic Database (SSURGO) to compute the other parameters using Saxton & Rawls equations (2006) and Brooks & Corey equations.

Roughness

Roughness characterizes the ability of cover crops to slow water flow and create micro-storage on the surface. By increasing residency time due to roughness, cover crops increased storage and allowed more time for water to infiltrate. The model included roughness to simulate surface water flow using the Diffusive Wave equation.

Evapotranspiration (ET)

The model used the Penman-Monteith equation, where the canopy stomatal resistance and vegetation heights were adjusted to account for the effects of small grains. In addition, the evapotranspiration was adjusted seasonally in the model through a multiplier called the Canopy Resistance Amplification Factor that increased the stomatal resistance during the winter months and decreased it to 1.0 during the growing season.

Interception and Retention

These parameters were used in the late growing season to account for canopy effects in intercepting rain and retaining it on the surface.

RESULTS AND DISCUSSION

Runoff volume reduction

According to this study, the application of cover crops reduced discharge volume at the watershed outlet by an average of 11% and 41% for growing and non-growing seasons, respectively (Figure 3). Cover crop applications achieved maximum benefits during the non-growing season after they were compared with fallow ground conditions. This was attributed primarily to increases in the saturated hydraulic conductivity and roughness by 2 and 3 times, respectively. In the growing season, however, the cover crops were competing with the cultivated crops, showing improvements based on hydraulic conductivity, but not necessarily roughness. As a result, surface runoff reduction due to cover crop application was higher for the non-growing season than the growing season by a factor of 3.6 (Figure 3). Furthermore, the rate of infiltration increase due to cover crop application was higher for the non-growing season by a factor of 4.7 (Figure 3).

Figure 3: Comparing the benefits of crop cover for non-growing versus growing seasons.
Figure 3: Comparing the benefits of crop cover for non-growing versus growing seasons.

Effects of cover crops on storage

According to the model results, extensive cover crop application removed an average of approximately 17,500 acre-feet of runoff volume computed at the watershed outlet (Figure 4) (equal to approximately 1-inch of runoff over the watershed) during the non-growing season. Most of the net volume removed infiltrated through the soil during the fall and early spring when the ground was not frozen. Water storage was spatially distributed across the landscape, yielding significant cumulative benefits at the watershed outlet (Figure 5).

Figure 4: Effects of cover crops on net storage and infiltration volumes for the non-growing cycle.
Figure 4: Effects of cover crops on net storage and infiltration volumes for the non-growing cycle.
Figure 5: Watershed showing net gain in infiltration for most areas.
Figure 5: Watershed showing net gain in infiltration for most areas.

Peak flow and discharge volume sensitivity to model parameters

The simulation for the non-growing period was broken down further into separate simulations to investigate the influence of different model parameters on both peak flows and the discharge volume. Model parameters for hydraulic conductivity and roughness were analyzed independently using synthetic rainfall events. The results showed that peak flow was reduced by 30% from the combined effects of roughness and hydraulic conductivity. Discharge volume was reduced by 24%, but the relative importance and the interaction between hydraulic conductivity and roughness were different. Hydraulic conductivity and roughness were both important in reducing the volume of discharge when considered separately. However, their effects did not combine as they did for peak flow reduction. For peak flow reduction, the rate of infiltration alone is overwhelmed by the rate of precipitation. However, an increase in roughness would help slow the flow and give it more residence time for infiltration to occur. For volume reduction, both parameters are as important independently and in combination, because they affect storage on the surface as well as the subsurface in the form of recharge into the groundwater or plant uptake. In the end, the total volume at the watershed outlet will also include infiltrated flows that will ultimately exfiltrate into the stream network as groundwater baseflow.

Conclusion

Using the GSSHA model, this study quantified and characterized the water storage benefits of cover crop application for reducing both peak flow and volume discharge. It highlights the importance of cover crops as a potential tool for flood mitigation and watershed management.

The results of the analysis presented here show that the hydrologic benefits of cover crops are particularly important for the non-growing season. Cover crop applications in the non-growing season produced significant reductions in discharge volume of up to 41% when compared to fallow ground conditions. The GSSHA simulation also demonstrated volume reductions up to 11% in discharge volume for the growing season.

Both saturated hydraulic conductivity and surface roughness were the main drivers in controlling the peak flow and volume of discharge reductions, according to the simulation. Their combined effects amplified peak flow reductions.

Author Details

Salam Murtada is a civil and environmental engineer working as floodplain hydrologist for the Minnesota Department of Natural Resources. His job includes developing and reviewing hydrological and hydraulic models for watershed studies, FEMA and flood related projects, and geomorphic evaluation of culvert designs and stream restoration projects. He graduated from West Virginia University with a Master of Science degree in Civil and Environmental Engineering, and from the University of Texas at Austin in Bachelor of Science degrees in Civil Engineering and Petroleum Engineering.

Daniel Reinartz has worked for the Lake Ecology Unit of the Minnesota Department of Natural Resources for the past 9 years. He retired from the U.S. Army Corps of Engineers as a Hydrologic Engineer after 35 years. He has a total of 49 years in civil engineering with a BCE Degree from the University of Minnesota.

Steve Kloiber supervises the Lake Ecology Unit of the Minnesota Department of Natural Resources. He has over 30 years of experience in water resource science and environmental analysis. He received his masters and PhD from the University of Minnesota in environmental engineering with a minor in water resources science.

References

  1. Agricultural Research Science. (2009) Soil Water Characteristics (SWC). USDA.
  2. Agricultural Research Service. (1995). Water Erosion Prediction Project (WEPP). Chapter 7, Soil Component.
  3. Basche, A. D., DeLonge, M. (2017). The Impact of Continuous Living Cover on Soil Hydrologic Properties: A Meta-Analysis. Agronomy & Horticulture, Faculty Publications.
  4. Downer, C. W., Ogden, F. L., Byrd, A. R. (2008). GSSHAWIKI User’s Manual, Gridded Surface Subsurface Analysis Version 7.13 for WMA 10.1. ERDC Technical Report. Engineer Research and Development Center, Vicksburg, Mississippi.
  5. Haruna, S., Anderson, S. H., Udawatta, R. P., Gantzer, C. J., Phillips, N. C., Cui, S., Gao, Y. (2020). Improving soil physical properties through the use of cover crops: A review. Agrosystems, Geosciences & Environment.
  6. Singer, J., Kaspar, T., Pederson, P. (2005). Small Grains Cover Crops for Corn and Soybean. Iowa State University Extension and Outreach.
  7. Minnesota Geospatial Commons. (2013). https://gisdata.mn.gov/dataset/quick-layers
  8. US Army Corps of Engineers. HEC-DSSVue Version 1.2. (2006).
  9. University of Minnesota Extension. (2021). Cover Crops. https://extension.umn.edu/soil-and-water/cover-crops

United States Department of Labor O*Net Data Collection Program

Hydrologists and Hydrologic Technicians:

You have the opportunity to participate in this important initiative and your input will help ensure that the complexities of your profession are described accurately in the O*NET database for the American public.

You are considered an Occupation Expert if you: 

  • Fit the Department of Labor occupation description for Hydrologists: “Research the distribution, circulation, and physical properties of underground and surface waters; and study the form and intensity of precipitation and its rate of infiltration into the soil, movement through the earth, and return to the ocean and atmosphere.” 
  • Hydrologic Technicians: “Collect and organize data concerning the distribution and circulation of ground and surface water, and data on its physical, chemical, and biological properties. Measure and report on flow rates and ground water levels, maintain field equipment, collect water samples, install and collect sampling equipment, and process samples for shipment to testing laboratories. May collect data on behalf of hydrologists, engineers, developers, government agencies, or agriculture.” 
  • Have at least one year of professional experience working in the occupation. 
  • Have a minimum of five years combined professional experience practicing, training, teaching, and/or supervising others who perform in the occupation. 
  • Are currently actively involved in the field and based in the United States

How to Participate: If you meet these criteria, please contact Jahmilah Rodgers at RTI International, the O*NET Data Collection Contractor, at jrodgers@onet.rti.org or 919-926-6634. 

Please provide the following in your email: 

  • Full Name and Job Title 
  • Years of experience in occupation 
  • Indicate which occupation (Hydrologists or Hydrologic Technicians)
  • Mailing Address with City, State and Zip Code 
  • Daytime Phone Number 
  • Email Address

Process and Participation Incentive: A random sample of experts responding to this request will be invited to complete a set of questionnaires. Experts who are selected and agree to participate will receive $40.00 VISA gift code and an oak-framed certificate of appreciation from the U.S. Department of Labor. We encourage you to consider helping to keep information about your profession accurate and current for the benefit of our colleagues and the nation.

Thank you for your support. If you know anyone within your network who is also an expert and would be interested in participating, feel free to forward this information to them.

Frequently Asked QuestionsO*NET Brochure

2021 World Rivers Day

World Rivers Day takes place on the fourth Sunday of September. While it is important to take that day and celebrate local rivers, it is also important to work for their protection throughout the year.

Waterkeeper Alliance: Get involved with your local waterkeeper organization, or streamkeeper group, to protect your stream or river (https://waterkeeper.org/get-involved/). If none are in your area, please consider starting your own group. Look at ways you can clean up the river channel, plant vegetation to secure eroded banks, and work with local government to protect and preserve streams in your area.

Storm Drains: Most storm drains are linked to local waterways and so it is important to avoid washing your car near these drains and to avoid putting any kind of chemical, soap or other pollutant in the drains.

#WorldRiversDay

AIH Call for Articles

The next issue of the AIH Bulletin is scheduled to be published in the winter of 2021, for which the editorial team invites contributions from members.

Original articles on any aspect of hydrology (e.g., administrative, technical, socioeconomic) will be considered for publication. It is not required that the article be based on academic or scientific work; however, it should not be published elsewhere. Book reviews may also be submitted under this category.

  • Please provide an un-formatted word document of your story without embedded images. You can signify where you’d like a submitted image using brackets.
  • Images you wish to be included with your article must not be embedded in the Word document; send them separately and labeled with names corresponding to where you’d like them used in the Word document.
  • Articles must have a brief title and a byline.
  • Supply a high-resolution head-shot of the author.
  • Article length must be between 500 – 1000 words.
  • Please include an “About the Author” post script, to provide our audience with the context of your perspectives. Include how you would like your name and title to be presented.
  • Avoid using too many bulleted lists, diagrams or graphs in your article.

Beside original articles, members may also submit leads to items of interest to the hydrologists’ community. Such items may include news related to the field of hydrology, conferences, new publications, etc.

If you are interested in contributing, please send articles or other items of interest via the Dropbox link below by Friday, October 15, 2021. Please ensure submissions are identified properly (example: TitleofArticle-FirstLastName.doc) and that supporting graphics/images are of the highest possible quality. Be sure to include your contact information within your submission as well.

Should you have any questions, do not hesitate to contact our office at admin@aihydrology.org.

AIH Executive Committee Elections!

Please cast your vote online by September 24, 2021.

AIH is pleased to present the ballot to fill the Vice President for Policy and Advocacy and Secretary-Membership Liaison positions on the AIH Executive Committee. Please check your email for a link to cast your vote by September 24, 2021!

AIH appreciates your participation in this important process. If you have any questions, please contact the AIH Executive Office at (916) 231-2149 or admin@aihydrology.org.

Call for Volunteers - Board of Examinations and Board of Registration

AIH is seeking volunteers in AIH’s four specialty certification areas (groundwater, hydrologic technician, surface water, or water quality) for positions on the Board of Registration (BOR) and the Board of Examinations (BOE). Volunteer positions on the BOR or BOE are not elected positions. Certified members are selected by the Executive Committee to join the BOR and BOE. Members on either the BOR or BOE are requested to serve terms of no less than three years.

Members of the Board of Examinations, under the leadership of the Vice President of Academic Affairs, support updates to AIH certification examinations, along with study guides and reference materials for certification examinations.

Members of the Board of Registration, under the leadership of the Chairman, evaluate membership and examination application packages for AIH.

If you are interested in volunteering for the BOR or BOE please email admin@aihydrology.org. In the subject line indicate: “AIH Volunteer Position – (BOR or BOE)”. In your submission, please include a statement of interest (maximum 150 words), contact info, your specialty area, and a professional biography (maximum 150 words).

Congratulations to New Members

Congratulations to those who have been recently certified as Professional members of the American Institute of Hydrology!

Matthew Burnette – PH Surface Water

David Ho – PH Surface Water

Bill Szafranski – PH Surface Water

Megan Gehrke – PH Water Quality

Ji Qi – PH Surface Water

Vignon Houenou – PH Surface Water

Justin Coffman – PH Surface Water

Andrew Earles – PH Surface Water

Robert Parrish – PH Surface Water

Sarah Harris – PH Surface Water

Sean Aucion – Hydrologic Technician I

Nikolaos Apsilidis – PH Surface Water

John Ramirez-Avila – PH Surface Water 

Call for Photos

Our industry-diverse membership often finds themselves in a variety of interesting locations either performing research, working on a project, or attending a conference. And, with some of us now working from home our additional ‘hydro-office’ serves as another “interesting” location to add to the collection.

We want to broadcast the diversity of the hydrology industry and specifically showcase our AIH certified members. Take a moment to snap a few photos of your surroundings so we can paint a clearer picture of what hydrology really looks like. Are your cats or family part of your work from home life? Include them! Do you spend time surrounded by nature and breathtaking environments? We want to see it all!

Upload your photos to this link [https://www.dropbox.com/request/jLQB3udMBGFaAJLw0LEB] and label the file with your name, the location, and your agency.

Thank you for assisting us as we enhance and strengthen the standing of hydrology as a science and profession.

AIH Meet & Greet Event

AIH is planning a Meet and Greet interactive virtual event on September 9, 2021 from 3:00 to 4:00 PM (PDT). The event is designed for AIH’s members to meet and interact with AIH’s Executive Committee, Executive Director, and management office. We will feature opening remarks by AIH President, Jamil Ibrahim, and Executive Director, Sarah Erck, and introductions of AIH’s Executive Committee members. The agenda will also include an overview of AIH’s current initiatives and future activities. We look forward to meeting you and hearing your feedback!

2021 AIH Membership Report

By: Jolyne Lea, Acting Secretary

The 2021 American Institute of Hydrology (AIH) currently has 410 members. An overwhelming majority of the members specialize in surface water hydrology, which is nearly an equal mix of longtime members (who have been certified for over 20 years) and newer members. AIH currently has 15 Hydrologist-in-Training members. There are currently 14 Certified Hydrologic Technician members. Additionally, there are 23 Emeritus members who support the Institute and their profession. Figure 1 shows the Professional, student, and emeritus members by certification identification.

[Figure 1. AIH members by membership category.]

It is also interesting to look at the longevity of our members. The AIH certification identification can be determined when each person was originally certified by AIH. Members range from those who were founding members at the forming of AIH in 1982, to new members. Figure 2 shows current members’ length of membership. There has been a steady influx of new members since 2005, where over ten new members, per year, were certified.

[Figure 2. AIH membership year of certification.]

In figure 3, the membership is grouped in ten-year increments. Over half of the current members (233) have joined within the last twenty years. Long time members of 20-40 years of membership number 151. In the last ten years, AIH has added 112 new members.

[Figure 3. AIH membership grouped by length of membership.]

Lastly, AIH membership is widely distributed across the U.S., Mexico, and Canada. The largest number of members are located in California, Colorado and Texas. In addition, the Institute has ten international members: three from Mexico and seven from Canada.  Figure 4 shows the members by state/country.

[Figure 4. AIH member location]

In summary, based on a review of membership in 2021 compared to historical numbers, the AIH membership numbers appear to be strong and continue to expand with new certified members. The AIH Executive Committee is committed to improving the membership certification process, increasing membership benefits, and expanding student membership to keep the Institute strong for the future. But, we urge our members to contact the AIH Executive Committee regarding input on how AIH membership can continue to be improved and to be active by volunteering for roles where you can.

Interview with Amesha Morris, DEI Committee

Interviewed by: Jule Rizzardo, President-Elect

I sat down with Amesha Morris over a virtual cup of coffee.  Amesha has submitted her application to obtain her Professional Hydrologist certification with AIH, and she serves on the newly formed AIH Diversity, Equity and Inclusion (DEI) Committee. Amesha is currently the stormwater program manager for the City of McKinney’s Stormwater Management Program in North Central Texas.

What is the most challenging thing about your job?

The most challenging part of my job is communication. My work requires coordination with 10 different departments and every year our permit has new requirements. It can be hard to juggle communicating with multiple personnel about changing parameters and changing program requirements.

Describe the most fun project team you’ve been part of at work?

Our department has been upgrading our data to be visualized using GIS. I have been nerding out, because we now have so many more options to display and analyze stormwater data.

What’s something people would be surprised to find out about you outside of work?

I started watching kdramas while I was in graduate school.  It was the perfect way to forget about my thesis for a few hours.

What is one thing you’re glad you tried but would never do again?

The Portland Saint Patty’s Fest Celebration – the event started with running a marathon and ended with lots of singing, dancing, and people celebrating in green.

What’s your favorite hydrologic feature and why?

Honestly, I enjoy a well-designed bioswale with diverse landscaping. Working in stormwater, I’ve learned that green infrastructure can be functional and aesthetically pleasing.

What is the best vacation you’ve taken?

I’m currently on a getaway vacation in Paso Robles California with my best friends, but the best vacation I’ve ever taken was visiting my dad stationed in Korea.

Where in the world do you want to travel next?

I really want to go to Portugal!  It seems like the perfect blend of metro and nature. If I could, I would love to retire there when the time comes.