Established and Remnant Riparian Forests as Buffers of Concentrated Flow from Crop Fields

Thomas Isenhart, Richard Schultz, Leigh Ann Long, Nicholas Leete, Nicholaus Ohde, Kris Knight, and Keegan Kult

Iowa State University

Department of Natural Resource Ecology and Management

Agricultural Hydrology and Water Quality II

2013 AWRA Spring Specialty Conference

March 26, 2013

Before/After on Specific Sites (1990-1994)

Riparian Management Systems

Buffers/filters

Constructed Wetlands

Streambank Bioengineering

Channel Control Structures

Controlled Grazing

Multi-species Riparian Buffer

4 year old RFB4 year old RFB

Native Native GrassGrass

ShrubsShrubs TreesTrees

Buffer Design

Flexible to match: 1. Landowner objectives!! 2. Site characteristics 3. Management realities 4. Wildlife habitat 5. Cost-share requirements

Research Findings: 1. Soil quality is greater under buffers than row crops

• Infiltration rates increased 5x • Soil microbial biomass increased 2.5x • Perennial root biomass much greater • Denitrification rates increased 4x • Greenhouse gas emissions greatly reduced • Soil organic matter increased (It is all about the carbon!)

2. Buffers reduce stream bank erosion by 60%

3. Buffers provide wildlife habitat (4 times increase of bird species)

Sediment/nutrient retention within riparian buffers

• Simulated runoff • Rainfall simulation • Natural runoff • 8% field slope, 5% buffer slope • Buffer/source area ratio 0.32 for grass only, 0.74 for grass and trees

Reduction (%) Rainfall intensity/

Buffer type Total-N NO 3 -N Total-P PO 4 -P Sediment

2-h rainfall at 2.5 cm h -1

Switchgrass 64.3 61.1 67.6 43.7 70.2

Switchgrass /woody 89.7 87.8 93.1 85.3 94.4

Sediment/Nutrient Retention

Sheet and Rill Erosion Through Buffers

• Restored buffer infiltration rates 5X > crop field/pasture.

• 20 m wide grass/woody buffers remove >90% of sediment & > 80% of total N & P in surface runoff

• Switchgrass filters more effective than cool-season grass filters.

Lee et al. 1999 Agroforestry Systems Lee et al. 2000 JEQ Lee et al. 2003 JSWCS

Streambank Erosion and Riparian Research in Northeast Missouri

Have Observed Many Gullies Flowing Through Forest Buffers During Rain Events

• Ephemeral gullies can continue from crop fields to streams

• Gullies carry sediment or pollutants through buffers

• Excess flow and sediment can increase streambank erosion

Riparian Forests May Not Be Enough

One Landowners Way of Stopping A Gully Nick Point!

Note water flowing through debris – effective filtering

Field Border

Grass Filter Strip

USDA-FSA – CP 21 USDA-NRCS 393

USDA-FSA – CP 33 USDA-NRCS 386

Natural riparian forest buffers with and without planted grass filters as buffers of concentrated flow from row crop fields

Natural riparian forest buffers with and without planted grass filters as buffers of concentrated flow from row crop fields

Knight et al. 2010 JAWRA

Concentrated flow path before grass filter was installed – currently inactive Grass Filter

Knight et al. 2010 JAWRA

Results suggest that natural forest buffers are not as effective without a live ground cover under the trees or at the edge

Need a grass filter in most cases

Knight et al. 2010 JAWRA

Quantify sediment & nutrients moving in gullies through existing ‘native’ riparian forest buffers without zone 3 grass

Use instrumented flumes to monitor discharge & collect water samples

Lake Rathbun

Control

Grass 1

Grass 2

Instrumented 6 Gullies flowing into grass filters & forest buffers (2 replications into each)

Monitoring: Discharge Sediment load Total N & P Nitrate, Dissolved reactive P

Buffers perform well in low-intensity events, but are overwhelmed in large events

Kult et al. Unpublished

In-field and edge-of-field factors influencing ephemeral gully connectivity

Canopy cover only factor significantly correlated with likelihood of gully connection

Ohde et al. Unpublished

• Soil loss from ephemeral gullies ranged from 18 to 40 Mg per hectare (8 to 18 tons per acre)

• MUSLE under predicted the amount of sediment load

• WEPP underestimated annual soil loss by 60% on average

• Underestimation may be due to side-wall sloughing and headcut erosion

Current models do not adequately account for ephemeral gully erosion

Kult et al. Unpublished Ohde et al. Unpublished

Lessons Learned

• Riparian buffers can be very effective in removing sediment and nutrients in sheet flow

• Natural forest buffers may not be effective without a live ground cover under the trees or at the edge

• Buffers perform well in low-intensity events, but are overwhelmed in large events

• Canopy cover only factor significantly correlated with likelihood of gully connection

• Current models do not adequately account for ephemeral gully erosion

• Management alternatives?