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Author(s)
First Name
Middle Name
Surname
Role
Erin
S.
Brooks
Author and presenter
Affiliation
Organization
URL
University of Idaho, Moscow, ID 83844-2060
Email
ebrooks@uidaho.edu
Author(s)
First Name
Jan
Middle Name
Surname
Role
Boll
Co-author
Affiliation
Organization
URL
University of Idaho, Moscow, ID 83844-2060
Email
jboll@uidaho.edu
Publication Information
Pub ID
711P0311cd
Paper #11077
Pub Name
International Symposium on Erosion and Landscape
Evolution CD-Rom Proceedings (18-21 September
2011, Hilton Anchorage, Anchorage Alaska) St. Joseph,
Michigan ASABE
Pub Date
19 September
2011
BUILDING PROCESS-BASED UNDERSTANDING FOR IMPROVED
ADAPTATION AND MANAGEMENT
E.S. Brooks and J. Boll 1
ISELE Paper Number 11077
Presented at the
International Symposium on Erosion and Landscape Evolution
Hilton Anchorage Hotel, Anchorage, Alaska
September 18-21, 2011
A Specialty Conference of the
American Society of Agricultural and Biological Engineers
Held in conjunction with the Annual Meeting of the
Association of Environmental & Engineering Geologists
September 19-24, 2011
1
Erin S. Brooks and Jan Boll, University of Idaho, Moscow, ID 83844-2060.
BUILDING PROCESS-BASED UNDERSTANDING FOR IMPROVED
ADAPTATION AND MANAGEMENT
Erin S. Brooks and Jan Boll 2
ABSTRACT
There is an increasing need for improved process-based decision support tools for watershed
management. Many of the tools available to assist managers in locating and selecting effective
management practices are either too general or too complex to be used practically. Empirical models
based on historic data may not be relevant to future climates. In this study, we present a simple webbased Hydrologic Characterization Tool (HCT) that can be used to analyze the effects of management
practices on the hydrology, erosion, and sediment delivery within specific landscapes. The HCT, based
fundamentally on the Water Erosion Prediction Project model, was developed as a simple tool to help
managers build a process-based understanding of the hydrologic flow paths and the processes driving
erosion for common management practices within a region. Users select a range of soil characteristics,
slope attributes, and crop rotations common for their region and then select management options which
are currently being applied in the region or are being considered as potential future conservation
options. The HCT then creates input files for all possible combinations of attributes and provides
annual and monthly hydrologic and soil erosion output for both within and at the outlet of each
hillslope. The output allows users to not only compare the effect of the management practices for a
single attribute, for example a reduction in soil erosion, but it provides them with an understanding of
the effect of the practices on the hydrologic flow paths generating and delivering the pollutant through
the hillslope. We illustrate characterizations from two regions of the US: Idaho and Iowa. Each of
these locations has a unique set of climatic, topographic, and soil characteristics resulting in much
different dominant hydrologic flow paths. We demonstrate that adoption of conservation/mulch tillage
practices in Idaho effectively converts the dominant runoff generating process from an infiltration
excess to saturation excess mechanism leading to a reduction in erosion. Understanding the interaction
between soil depth and topography is critical for identifying saturation-excess processes. In contrast,
since the rainfall intensities in Iowa are much greater than in Idaho, conservation tillage practices alone
are not as effective at reducing erosion in Iowa as in Idaho. Managers in Iowa often must use terracing
practices along with conservation tillage to successfully reduce erosion rates to acceptable levels. In
addition to sediment transport we demonstrate how the HCT can be used to identify sensitive areas for
soluble pollutants. Future versions of the tool will provide direct prediction of transport of nitrate,
phosphorus, and pesticides within each landscape.
KEYWORDS. Soil Erosion Control, WEPP, Conservation Practices
2
University of Idaho, Moscow, ID 83844-2060.
INTRODUCTION
Despite long-term adoption of soil and water conservation practices in many regions of the United
States throughout the last one hundred years, very few watershed studies show that sediment levels
have declined to acceptable levels (Mulla et al., 2008). In a few places we are beginning to measure
significantly declining trends in annual sediment loading at a watershed outlet (Brooks et al., 2010).
One of the most widespread management changes has been the adoption of conservation tillage
practices which reduce the number of tillage passes and increases surface residue cover (Kok et al.,
2009). The introduction of the Conservation Reserve Program and similar programs in other countries
that have taken highly erodible land out of production have also undoubtedly contributed significantly
to the reduction of soil erosion and sediment yield. Although these reductions are a good indicator for
improvement, further reductions in sediment and nutrient loads will come at an ever-increasing cost.
With many other competing economic issues future government-subsidized money must be spent
efficiently (i.e. achieve the biggest bang for the buck). In addition, some future climate change
scenarios suggest that rainfall intensities could increase and undoubtedly increase erosion. We believe
future reductions in the loading of sediment, nutrients and other agri-chemicals in the decades to come
will require spatially-explicit, process-based tools which capture variability in soil, climate, and
topography across a region and focus management on the most sensitive areas.
Mulla et al. (2008) attributed the lack of evidence of significantly reduced pollutant loads in many
watersheds to insufficient water quality monitoring records, failure to implement best management
practices (BMPs) that correct the most importance sources of pollution, and failure to implement
BMPs in the most critical areas of the watershed. It was noted that there is a great need for better tools
to identify these critical management zones and guide managers in the selection of appropriate BMPs.
These tools should be simple, requiring minimal input data and minimal calibration (Mulla et al.,
2008). However, the selection and location of BMPs must be based on fundamental understanding of
the timing and location of dominant hydrologic flow paths and runoff generating mechanisms within a
landscape. These tools must move beyond simple empirical tools based on excessively calibrated
parameters to process-based models that provide managers with a deeper understanding of the
hydrology of a landscape.
Many of the tools available to watershed managers, such as the Natural Resources Conservation
Service electronic Field Office Technical Guides are often too general to be used to identify the most
critical management areas within a particular landscape. Detailed soil erosion models such as RUSLE2
(Foster, 2003), which neglects subsurface hydrology, the Soil Water Assessment Tool (Arnold et al.,
1998), which requires calibration and is best suited for large watersheds, and the Water Erosion
Prediction Project (WEPP) (Flanagan and Nearing, 1995), which is often considered too complex, have
not been widely used to assist managers for selecting and locating BMPs in critical management areas.
In this paper, we present the development of a simple web-interface tool, based fundamentally on the
WEPP model, to guide managers in the selection and location of BMPs within a landscape. This
Hydrologic Characterization Tool (HCT) was developed to provide managers with answers to some of
the fundamental hydrologic questions for selecting and locating effective management practices.
These questions include “What is the dominant water flow path (percolation, infiltration, runoff, or
lateral subsurface flow)?”, “How much water is being transported in the dominant flow path (in
mm/year, or as percent of total precipitation)?”, “When does the transport occur (month, or season)?”,
and “What are the controlling factors (infiltration capacity, or soil storage capacity)?”.
Requirements for the HCT include that it should be able to evaluate site-specific information such as
local climate, soils, topography, crop rotation, and management, that it incorporates major flow paths
of water such as infiltration, runoff, subsurface lateral flow, and percolation, and that the user should
2
not have to learn a complex model. Instead of simulating all sites within a watershed specifically, the
spatial scale of interest is the hillslope, and the temporal scale is monthly to yearly, providing average
results based on 10 to 30-year simulations.
Development of the HCT was motivated by a NIFA funded synthesis project, which has the primary
goal to synthesize lessons learned from 13 Conservation Effectiveness Assessment Program (CEAP)
watersheds on assessing the effectiveness of suites of conservation practices on pollution reduction in
agriculturally-dominated watersheds. The main goal of CEAP is to build understanding of how best to
schedule and locate conservation efforts within a watershed to achieve locally defined water quality
goals. This paper provides an example application to the Iowa and Idaho CEAP watershed studies.
METHODS
The HCT is available on the internet (http://wepp.ag.uidaho.edu/cgi-bin/HCT.pl/) and relies
exclusively on readily available data and user specified information for representative hillslopes in the
watershed of choice. An introduction screen (Figure 1) provides the user the option to select a specific
watershed, and the next screen allows the user to select the appropriate state and climate station, preselected slope configurations, soil types, and management practices (Figure 2). The default slope in the
HCT is 300 m long, with three slope segments of 100 m each. Custom slopes and soils also can be
created. Management practices currently include conventional tillage, conservation tillage, and notillage, grass cover, and buffer strips on the lowest slope segment.
Users select multiple combinations of soils, management practices, and topography to be considered in
the analysis. The user also selects how many simulation years should be included. Pre-processing
algorithms in the HCT use the selected input information and creates input files for every possible
combination of soil, topography, crop rotation, and management practice selected by the user and feeds
this information into the WEPP model running on a server at the University of Idaho. After the WEPP
model run(s) are completed, post-processing algorithms provide the user with a table of yearly output
for the entire slope including the amount of percolation, lateral flow, runoff, and erosion. Users can
also view monthly details or details by slope segment (Figures 3 and 4).
We illustrate the use of the HCT for two CEAP watersheds: Paradise Creek watershed in Idaho
(Brooks et al., 2010) and Walnut Creek in Iowa (Schilling, 2001). In the Idaho example we first
explore the effect of soil depth on the hydrology, erosion, and sediment yield for a steep slope
configuration (5%, 35%, 5%). Three silt loam soils were selected having soil depths of 20 cm, 97 cm,
and 150 cm. All hillslopes were assumed to be managed identically in a three year winter wheatbarley-pea rotation following conservation tillage. Secondly, in the Iowa CEAP watershed, we examine
the effect of slope steepness on the hydrology, erosion, and sediment yield for a deep silt loam soil
without any restrictive soil layers in a corn-soybean rotation using conservation tillage. Two slope
configurations (2%, 2%, 2%, and 2%, 8%, 2%) were used in the analysis. Thirdly, we use the HCT to
compare the effects of tillage management and buffer strips on hydrologic flow paths and sediment
yield for both the Idaho and Iowa CEAP watershed. For each CEAP watershed, we choose one slope
configuration and soil type, and varied management by selecting conventional tillage, conservation
tillage (or mulch tillage), no-till (each with and without a 10 m buffer), and grass.
RESULTS AND DISCUSSION
Importance of topography and soil depth
The depth to a hydrologic restrictive layer (e.g. bedrock or argillic/fragipan soil horizon) and the
topography of a hillslope will greatly control the dominant hydrologic flow paths and transport of
3
Figure 1. Introduction screen of Hydrologic Characterization Tool on the internet.
Figure 2. Data selection screen of Hydrologic Characterization Tool.
pollutants within the landscape. Soil depth controls the amount of water that can be stored in a soil
profile before saturation occurs while topography provides the driving hydraulic gradient for
subsurface lateral flow through a perched water table that develops above the restrictive soil layer.
Downslope convergence of subsurface lateral flow yields increased saturation-excess runoff at toe
slope positions. The importance of soil depth and topography is clearly demonstrated in the Idaho
example (Figure 5). For the shallow (20 cm) soil runoff is the dominant flow path. For a medium soil
4
Figure 3. Example of average monthly output for the entire slope configuration.
Figure 4. Example of detailed model output for each slope segment. The green line represents the
topography of the hillslope.
depth (97 cm), lateral flow is the dominant flow path on the steepest slope section, however the
convergence of lateral flow at the toe slope results in runoff being the dominant flow path in the lowest
segment. In deep soils which do not have a restrictive soil layer, percolation is the dominant flow path.
The most erosive conditions in the Idaho CEAP watershed occur on the steep slope sections in shallow
soils. Although toe slope positions generate more runoff than midslope positions, the net erosion is
greater on midslope positions. Management of soluble pollutants therefore should focus on toe slope
positions whereas management of particulate-bound pollutants should focus on steep midslope
positions. In the Iowa CEAP site, soils are deep and therefore the dominant flow path is vertical
percolation. Slope steepness does not affect the amount of runoff or percolation however the amount of
erosion is highly sensitive to slope steepness (Figure 6).
5
140
60
120
40
100
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80
0
60
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40
‐40
‐60
40
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20
‐60
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1
160
Water Depth (mm/yr)
60
2
Overland Flow Element (OFE)
80
5b. Net Erosion (Tonnes/ha)
120
160
‐80
1
3
2
Overland Flow Element (OFE)
3
80
5c. 140
60
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‐20
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Net Erosion (Tonnes/ha)
Water Depth (mm/yr)
140
80
Water Depth (mm/yr)
Runoff (mm)
Lateral Flow (mm)
Percolation (mm)
Net Erosion
5a. Net Erosion (Tonnes/ha)
160
‐80
1
2
Overland Flow Element (OFE)
3
Figure 5. Average annual runoff (mm), lateral flow (mm), percolation (mm) and net erosion
(tonnes/ha) for three Idaho soils managed under mulch tillage in a 3-year winter wheat, barley, pea
crop rotation for (a) 20 cm, (b) 97 cm, and (c) 150 cm deep soils, respectively. The slope steepness for
segments 1, 2, and 3 are 5%, 35%, 5%, respectively.
240
210
60
210
60
180
40
180
40
150
20
150
20
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0
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Overland Flow Element (OFE)
3
‐20
‐40
80
6b. Runoff (mm)
Lateral Flow (mm)
Percolation (mm)
Net Erosion
90
60
‐60
30
‐80
0
Net Erosion (Tonnes/ha)
Runoff (mm)
Lateral Flow (mm)
Percolation (mm)
Net Erosion
90
Water Depth (mm/yr)
80
6a. Net Erosion (Tonnes/ha)
Water Depth (mm/yr)
240
‐20
‐40
‐60
‐80
1
2
Overland Flow Element (OFE)
3
Figure 6. Average annual runoff (mm), lateral flow (mm), percolation (mm) and net erosion
(tonnes/ha) for a deep Iowa soil managed under mulch tillage in a 2-year corn-soybean crop rotation
for (a) uniform 2% slope and (b) slopes of 2%, 8%, and 2% for segments 1, 2, and 3, respectively.
6
Practice Effectiveness
The HCT quantified the effect of single and/or multiple management practices on hydrologic flow
paths and sediment transport within different land types. We found that effectiveness of management
practices is site specific, and that ideal practices in one location (e.g., Idaho) may not be ideal in
another location (e.g., Iowa). In Idaho, the mulch tillage practice is more effective than 10 m grass
buffers at reducing sediment load (Figure 7), while this practice only slightly reduced runoff for
shallow and intermediate soils. In Iowa, a 10 m grass buffer is more effective than mulch tillage at
reducing sediment loads, but mulch tillage is very effective at reducing runoff (i.e., slope steepness
does not affect runoff volume, Figure 7).
70
7a. 50
40
30
20
90
7b. 80
70
60
50
40
30
20
10
10
0
70
Sediment Yield (Tonnes/ha/yr)
100
Runoff (mm/yr)
Sediment Yield (Tonnes/ha/yr)
60
Shallow Soil (20 cm)
Intermediate Soil (97 cm)
Deep Soil (152 cm)
60
50
40
30
20
10
0
7c. Flat Slope (2%)
Mod. Slope (2% 8% 2%)
90
80
7d. 70
)r
y/60
m50
m
(f
f 40
o
n
u30
R
20
10
0
0
Figure 7. Average annual sediment yield (tonnes/ha/yr) and runoff (mm/yr) for cropping systems
managed under conventional (CNV), mulch (MT) and no-till (NT) management practices in Idaho (7a
and 7b) and Iowa (7c and 7d) both with and without 10 m grass buffers (Buffer).
The primary difference between the Iowa and Idaho sites is climate. According to the CLIGEN
weather generator in WEPP, the maximum 30 min rainfall intensity for Knoxville, IA (43.43 mm/hr) is
over 3 times greater than the maximum 30 min rainfall intensity for Moscow, ID (13.46 mm/hr).
Conversion from conventional tillage to mulch tillage in Idaho increases the infiltration rate of the soil
to the point that the dominant runoff generation process converts from infiltration-excess to saturationexcess processes. This transition decreases the runoff on steep portions of the slope and increases the
runoff on flat toe slope positions which are less likely to erode. In Iowa, despite the increase in the
infiltration rate under mulch tillage, the rainfall intensity exceeds the infiltration capacity often enough
that erosion on steep slopes remains a problem. As a result one of the common management practices
7
in Iowa, especially in regions where slope steepness exceeds 5%, is to build terraces which minimize
the runoff velocity in addition to converting to a more conservation tillage practices. Grass buffer strips
are likely more effective in Iowa than in Idaho since the runoff velocity of the high intensity, short
duration storms typical in Iowa will be greatly reduced by the grass leading to greater deposition. In
Idaho grass buffer strips during runoff events will often be saturated preventing any infiltration of
runoff through the buffer area.
We are in the process of linking the HCT to a water quality algorithm which will provide daily nitrate,
phosphorus and pesticides delivery through each hydrologic flow path and OFE within a hillslope.
CONCLUSION
The HCT presented in this paper, while still under development, shows great promise as a management
tool for placement of conservation practices. This tool uses a physically-based model but does not
require the user to deal with the model complexities such as developing all input parameters and
climate input files. Users build a fundamental understanding of the effectiveness of management
practices not only on the delivery of a pollutant but on the mechanisms controlling the generation and
transport of pollutants through a landscape. Future versions of the tool could potentially provide
managers with a valuable decision support tool for selecting and locating management practices both
within a watershed and within a particular hillslope.
Acknowledgements
This study was funded through the USDA-NIFA Funded CEAP Synthesis Project NO: 2007-5113003992 and a USDA-Forest Service Round 7 Southern Nevada Public Land Management Act (2A11)
grant through the Rocky Mountain Research Station (08-JV-11221665-050).
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