**TECHNICAL NOTES**

**USDA-Natural Resources Conservation Service**

**Boise, Idaho**

***TN -- Water Quality No. 4 August 2005***

**NITROGEN TRANSPORT RISK ASSESSMENT**

This Nitrogen Transport Risk Assessment is a 5 x 5 matrix that uses a
limited number of site and management characteristics to determine the
probability of off-site transport of nitrogen. Off-site transport refers
primarily to transport below the crop root zone, although other
mechanisms include transport in overland flow and gaseous losses. The
assessment is part of an overall planning process imbedded within the
ONEPLAN Nutrient Management Planner program. The assessment, together
with a nutrient management plan, is used as a tool for understanding the
contributions that individual landform and management parameters have on
nitrogen transport and the potential for applied conservation practices
(Best Management Practices) to mitigate situations where transport/loss
can occur.

## 

## Nitrogen Concerns in the Environment

Concerns about agriculture's role in nitrogen (N) delivery to the
environment have increased over the past decade. Nitrogen is a major
input to crop and livestock production, and industrial production of N
fertilizers has resulted in increased yields and more intensive
agricultural operations. However, nitrogen use efficiency of most
agricultural systems is currently estimated at only 30 - 50% worldwide,
leading to nitrogen losses that degrade air and water quality.

One of the most widespread contaminants in Idaho ground water related to
land use is nitrate. This is a major concern, since more than 90% of
Idahoans get their drinking water from ground water sources. Twenty-five
nitrate priority areas have been designated by the Idaho Department of
Environmental Quality. Of those areas with sufficient data for trend
analysis, 35% showed long-term increases in nitrate concentration and
40% demonstrated short-term increases. The southern portion of the state
is especially impacted, where contamination is correlated with large
nitrogen inputs and the vulnerability of the Snake River Plain Aquifer.
Vulnerability is determined by the intrinsic susceptibility of the
aquifer based on physical properties, coupled with management factors.

Water carrying nitrates and other contaminants can take decades to flow
through the soil substrate. Schumann et al. (2002) calculated nitrate
movement at 1 m/year through silt loam soils. ARS watershed studies in
Iowa found that nitrates applied to soil took nearly 30 years to reach a
70-ft deep water table (Pons 2003). The slow rate of movement and lack
of dilution in saturated zones means that contamination may persist for
a long time period, even with improvements in management. Surface waters
can also be degraded by nitrogen. The high flux of nitrates in streams
during irrigation season can result from both overland flow and from
groundwater inflow.

## Nitrogen Movement in the Landscape

Nitrogen is one the most dynamic and mobile nutrients in the
plant-soil-air continuum, with many pathways for loss (Figure 1). There
is a large reservoir of N in soil, but most of this is in the organic
form. Organic N is mineralized through microbial action under typical
soil conditions to ammonium. Oxidation by specialized bacteria rapidly
converts ammonium to nitrite and then nitrate (nitrification) under
optimum conditions of soil temperature, aeration and moisture. The
mineralized form of N (nitrate and ammonium) is readily available for
uptake by plants. It is estimated that only 2-3% of organic N is
mineralized annually. Therefore, intensive agricultural systems rely on
inputs of fertilizer N to meet crop and animal demands.

The N cycle is both spatially and temporally variable within
agricultural systems. Variability of soil properties impacts nitrogen
movement and loss within agricultural operations, including soil organic
matter, residual nitrate, crop residue amount, crop yield variability,
and changes in soil chemical and physical properties across the field.
Losses of nitrogen to the air can occur through denitrification of
nitrate or volatilization of ammonia. Nitrogen can also be lost in
solution, or attached to soil and organic matter, via overland flow. The
primary loss mechanism of nitrogen in agricultural systems, however, is
leaching of nitrate below the root zone. Nitrate is a negatively charged
ion that is highly mobile in the soil. The amount of water that
percolates through and below a crop's root zone is important in
determining the amount of nitrate leached. Soil, crop, climate and
management factors interact to determine the amount of percolation.

![](media/image1.png){width="4.43125in" height="3.642361111111111in"}

***Figure 1. A simplified nitrogen cycle (Source: NRCS-NEDC 2001).***

Management plays a critical role in reducing N loss to the environment,
and management is [the]{.underline} dominant factor influencing
long-term nitrate leaching (Shaffer et al. 2002). Soil, climate,
watershed and aquifer characteristics must also be taken into account in
order to minimize nitrate leaching. Loss of nitrate from agricultural
systems can range from 0 - 60% of N applied. In grain production
systems, 10 - 30% was the average loss observed (Meisinger and Delgado
2002). Leaching loss is dependent on the concentration of N in soil
solution and the volume of water leached. Over-irrigation can lead to
nitrate leaching, especially with shallow rooted crops. Effective
management is therefore aimed at reducing transport through proper
irrigation water management, and optimizing N application amounts and
timing in concert with crop uptake. Crop type and cultivation are also
important considerations.

## Idaho's Nitrogen Transport Risk Assessment

# 

The purpose of the Nitrogen Transport Risk Assessment is to provide
field staffs, watershed planners, and land users with a tool to assess
the various landforms and management practices for potential risk of
nitrogen movement to aquifers and other water bodies.

Shaffer et al. (2002) describe the need for, and the basic elements of,
a national nitrate leaching assessment tool. The impacts of crop type,
fertilizer, manure and irrigation management, coupled with soils,
climate, and watershed factors, are essential parameters of this
leaching index. The index would utilize a tiered structure dependent on
potential risk:

-   Tier 1: Broad-based screening tool that identifies risk level based
    on controlling factors. Areas identified with higher risk levels
    would warrant further study (Tier 2, 3).

-   Tier 2: Larger-scale quantification of nitrate leaching using
    appropriate modeling tools.

-   Tier 3: Site-specific quantification of nitrate leaching based on
    current management and site conditions through field studies and
    research models.

The Idaho Nitrogen Transport Risk Assessment is a Tier 1 screening tool
that addresses the key factors identified by Shaffer et al. (2002). A
number of climate, soil, hydrology and aquifer site characteristics
describe the landform, along with management factors. The Nitrogen
Transport Risk Assessment (Table 1) is a simple 5 by 5 matrix utilizing
parameters that influence nitrogen availability, retention, management
and movement.

There are five site characteristics used in the assessment to evaluate a
particular site. Each site characteristic is rated VERY LOW/NOT
APPLICABLE, LOW, MEDIUM, HIGH or VERY HIGH by determining the range for
each category. A log base of 2 is used for the rating categories.
Therefore, a VERY LOW rating is assigned 0 points, while a VERY HIGH
rating is assigned 8 points. The higher the point value, the greater the
potential for significant problems related to nitrogen movement (Table
1). Particular site characteristics may be more prominent than others in
allowing potential nitrogen movement (primarily leaching) from the site.
There is scientific basis for concluding that these relative differences
exist; however, the absolute weighting factors assigned to site
characteristics to reflect these differences are currently based on
professional judgment.

The site characteristics and weighting factors are:

-   Deep percolation risk (2.00)

-   Irrigation efficiency (1.00)

-   N application rate (1.00)

-   N application timing (1.00 if non-irrigated, 0.75 if irrigated)

-   Water table depth/soil type (1.00 if irrigated, 1.5 if
    non-irrigated)

The sum of the site characteristic rankings provides an index of the
potential for off-site nitrogen transport, primarily leaching through
the root zone (Table 2). A description of each site characteristic and
the factors that are used in their determination follows.

## Deep Percolation Risk

Deep percolation is dependent on numerous factors, including climate,
soil type and irrigation efficiency. The deep percolation factor for
sprinkler-irrigated fields is determined from daily evapotranspiration
(ET) rates for an individual crop type, totaled over the irrigation
season using local climate station data. Total deep percolation loss is
calculated from monthly deep percolation loss from a simple water budget
developed within ONEPLAN. Deep percolation risk for sprinkler irrigated
fields is then calculated as the ratio of deep percolation to total ET,
over the irrigation season. For surface-irrigated fields, deep
percolation risk is based on the highest monthly deep percolation loss
(a relative comparison of the percent water applied that percolates
below the root zone in any given month). For non-irrigated fields,
nitrogen loss risk is based on the New York Nitrogen Leaching Index
(Czymmek et al. 2003) which is essentially a water percolation index
based on soil water storage. Slight modifications were made to some of
the percolation index equations to adjust for low precipitation zones
found in areas of Idaho. Total annual precipitation for specific
locations is determined from local climate station data, as is winter
precipitation. The percolation index is based on precipitation and
hydrologic group. A seasonal index is calculated as the ratio of winter
precipitation to annual precipitation. The deep percolation risk is then
calculated as the product of the percolation index and seasonal index.

## Irrigation Efficiency

Managing irrigation water will minimize nitrogen losses from leaching
and surface runoff. Irrigation efficiency and irrigation water
management have significant impacts on water movement through the root
zone. Monthly NIR (net irrigation requirement) values are determined for
crop type based on ET estimates. For sprinkler-irrigated fields, total
irrigation water applied is adjusted for system efficiency and runoff to
determine season-long irrigation efficiency (NIR/net water applied). For
surface-irrigated fields, the lowest monthly irrigation efficiency for
the season is used as the index.

## N Application Index

Crop nitrogen requirement is determined based on crop yield and
University of Idaho fertilizer recommendations. Total available nitrogen
is determined from all sources, including prior year crops. The
application index is the ratio of the total N available (application N
plus surplus/residual N) to the crop nitrogen requirement.

## N Application Timing

Timing of N application directly influences potential transport due to
the high mobility of nitrate in soils. The appropriate timing of N
application is complicated by the soil processes of nitrification and
mobilization, which affect N plant availability. Split applications of N
better match crop growth requirements, reducing the likelihood of loss.
Fall application in most instances has the greatest potential for loss
prior to planting season, and then additional N applications are
required to meet crop demand.

## Water Table Depth/Soil Type

Soils can stop or slow nitrogen movement depending on their chemical and
physical characteristics. Depth of soils, depth to water tables and
limiting layers such as hard pans will influence rooting depth, nitrogen
movement, and leaching potential. Fine textured soils (Hydrologic Group
D) have a lower potential for leaching due to reduced hydraulic
conductivity, while coarse textured soils (Hydrologic Group A) have a
higher likelihood of nitrate leaching due to the rapid infiltration and
movement of water through the profile. If a water table is present
within five feet of the surface, the potential for ground water
contamination is high despite the soil type.

## Making an Assessment Using the Nitrogen Transport Risk Assessment Tool

It is recommended that assessments for nitrogen movement be done within
the context of nutrient management planning using the Idaho ONEPLAN. If
done manually, the user would need to obtain climatic data for the local
area and crop nutrient and water requirements, as well as irrigation
application information and soil and hydrologic characteristics.

## 

## References

Czymmek, KJ, QM Ketterings, HM van Es, and SD DeGloria. 2003. The New
York Nitrate Leaching Index. Cornell University, Department of Crop and
Soil Sciences, Extension Publication E03-2.

Delgado, JA. 2002. Quantifying the loss mechanisms of nitrogen. J. Soil
and Water Conservation 57: 389 -- 398.

Donato, MM. 2000. Probability of detecting atrazine/desethyl-atrazine
and elevated concentrations of nitrite plus nitrate as nitrogen in
ground water in the Idaho part of the Western Snake River Plain. USGS
Water Resources Investigations Report 00-4163.

Focazio, MJ, TE Reilly, MG Rupert, and DR Helsel. 2003. Assessing ground
water vulnerability to contamination: providing scientifically
defensible information for decision makers. USGS Circular 1224.

Follet, RF, and JA Delgado. 2002. Nitrogen fate and transport in
agricultural systems. J. Soil and Water Conservation 57: 402 -- 408.

Meisinger, JJ, and JA Delgado. 2002. Principles of managing nitrogen
leaching. J. Soil and Water Conservation 57: 485 - 498.

Mosier, AR, JW Doran, and JR Freney. 2002. Managing soil
denitrification. J. Soil and Water Conservation 57: 505-513.

Neely, K. 2002. Nitrate results from the statewide program, 1991-2000.
IDWR Technical Summary (IDWR website).

NRCS-NEDC (National Employee Development Center). 2001. Nutrient and
pest management considerations in conservation planning: course
materials.

Parliman, DJ. 2002.Analysis of nitrate concentration trends in 25 ground
water quality management areas, Idaho. USGS Water Resources
Investigations Report 02-4056.

Pons, L. 2003. Old soil study uncovers value of long-term nitrate
research. ARS News Service, December 10, 2003.

Rupert, MG. 1996. Major sources of nitrogen input and loss in the upper
Snake River Basin, Idaho and western Wyoming, 1990. USGS Water Resources
Investigations Report 96-4008.

Rupert, MG. 1998. Probability of detecting atrazine/desethyl-atrazine
and elevated concentrations of nitrite plus nitrate as nitrogen in
ground water in the Idaho part of the upper Snake River Basin. USGS
Water Resources Investigations Report 98-4203.

Shaffer, MJ, and JA Delgado. 2002. Essentials of a national nitrate
leaching index assessment tool. J. Soil and Water Conservation 57: 327 -
335.

van Es, HM, KJ Czymmek, and QM Ketterings. 2002. Management effects on
nitrogen leaching and guidelines for a nitrogen leaching index in New
York. J. Soil and Water Conservation 57: 499 - 504.

Williams, JR, and DE Kissel. 1991. Water percolation: an indicator of
nitrogen-leaching potential. pp 59-83 *[IN]{.underline}*: RF Follet, DR
Keeney, and RM Cruse (eds). Managing nitrogen for groundwater quality
and farm profitability. Soil Science Society of America, Inc. Madison,
WI

**Table 1. Nitrogen Transport Risk Assessment. Sum of all weighted
rating values is used to determine the site vulnerability.**

+----------+----------+--------+--------+--------+--------+--------+
| **[Site  | *        | **[    |        |        |        |        |
| Characte | *[Factor | Rating |        |        |        |        |
| ristic]{ | Weight]{ | and    |        |        |        |        |
| .mark}** | .mark}** | Weig   |        |        |        |        |
|          |          | ht]{.m |        |        |        |        |
|          |          | ark}** |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
|          |          | Very   | Low    | Medium | High   | Very   |
|          |          | Low or |        |        |        | High   |
|          |          | N.A.   | 1      | 2      | 4      |        |
|          |          |        |        |        |        | 8      |
|          |          | 0      |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
| Deep     | S        | \< 5   | 5 - 10 | 10 -   | 20 -   | \> 40  |
| Per      | prinkler |        |        | 20     | 40     |        |
| colation | I        |        |        |        |        |        |
|          | rrigated |        |        |        |        |        |
|          |          |        |        |        |        |        |
|          | 2.0      |        |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
|          | Surface  | \< 10  | 10 -   | 20 -   | 30 -   | \> 40  |
|          | I        |        | 20     | 30     | 40     |        |
|          | rrigated |        |        |        |        |        |
|          |          |        |        |        |        |        |
|          | 2.0      |        |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
|          | Non-I    | 0      | 0 -- 2 | 2 -- 5 | 5 --   | \>10   |
|          | rrigated |        |        |        | 10     |        |
|          |          |        |        |        |        |        |
|          | 2.0      |        |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
| Ir       | S        | \> 90  | 80 -   | 60 -   | 50 --  | \< 50  |
| rigation | prinkler |        | 90     | 80     | 60     |        |
| Ef       | I        |        |        |        |        |        |
| ficiency | rrigated |        |        |        |        |        |
|          |          |        |        |        |        |        |
|          | 0.75     |        |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
|          | Surface  | \> 90  | 75 -   | 60 -   | 40 -   | \< 40  |
|          | I        |        | 90     | 75     | 60     |        |
|          | rrigated |        |        |        |        |        |
|          |          |        |        |        |        |        |
|          | 0.75     |        |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
| N        | 1.0      | \< 80  | 80 -   | 120 -  | 140 -  | \> 180 |
| App      |          |        | 120    | 140    | 180    |        |
| lication |          |        |        |        |        |        |
| Rate-    |          |        |        |        |        |        |
|          |          |        |        |        |        |        |
| \% of    |          |        |        |        |        |        |
| Crop     |          |        |        |        |        |        |
| Req      |          |        |        |        |        |        |
| uirement |          |        |        |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
| N        | Non-I    | None   | Split  | Split  | Pr     | Pr     |
| App      | rrigated | a      | appli  | appli  | eplant | eplant |
| lication |          | pplied | cation | cation | appli  | appli  |
| Timing   | 1.0      |        | with   |        | cation | cation |
|          |          |        | n      |        | in     | in     |
|          | I        |        | itrifi |        | spring | fall   |
|          | rrigated |        | cation |        |        |        |
|          |          |        | inh    |        |        |        |
|          | 0.75     |        | ibitor |        |        |        |
+----------+----------+--------+--------+--------+--------+--------+
| Water    | Non-I    | Water  | Water  | Water  | Water  | Water  |
| Table    | rrigated | table  | table  | table  | table  | table  |
| Depth    |          | \> 5   | \> 5   | \> 5   | at     | at     |
| and Soil | 1.0      | feet   | feet   | feet   | su     | su     |
| Type     |          | from   | from   | from   | rface, | rface, |
|          | I        | su     | su     | su     | p      | p      |
|          | rrigated | rface, | rface, | rface, | onded, | onded, |
|          |          | Hydr   | Hydr   | Hydr   | \< 5   | \< 5   |
|          | 1.5      | ologic | ologic | ologic | feet   | feet   |
|          |          | Group  | Group  | Groups | to     | to     |
|          |          | D      | C      | A, B   | su     | su     |
|          |          |        |        |        | rface, | rface, |
|          |          |        |        |        | Hydr   | Hydr   |
|          |          |        |        |        | ologic | ologic |
|          |          |        |        |        | Groups | Groups |
|          |          |        |        |        | C, D   | A, B   |
+----------+----------+--------+--------+--------+--------+--------+

**Table 2. Nitrogen Transport Risk Assessment Index rating and site
vulnerability.**

+-----------+--------+------------------------------------------------+
| *         | **T    | **Site Vulnerability Chart**                   |
| *Nitrogen | otal** |                                                |
| Transport |        |                                                |
| Risk      |        |                                                |
| Ass       |        |                                                |
| essment** |        |                                                |
|           |        |                                                |
| **Index   |        |                                                |
| Rating**  |        |                                                |
+-----------+--------+------------------------------------------------+
| LOW       | \< 9   | Low potential for nitrogen loss if current     |
|           |        | farming practices are maintained.              |
+-----------+--------+------------------------------------------------+
| MEDIUM    | 9 - 16 | Medium potential for nitrogen loss. Some       |
|           |        | remediation measures should be undertaken to   |
|           |        | minimize the probability of loss.              |
+-----------+--------+------------------------------------------------+
| HIGH      | 16 -   | High potential for N loss and adverse effects  |
|           | 25     | on ground water. Soil and water conservation   |
|           |        | measures and nitrogen management plans are     |
|           |        | needed to reduce the probability of loss.      |
+-----------+--------+------------------------------------------------+
| VERY HIGH | \>25   | Very high potential for nitrogen loss and      |
|           |        | adverse effects on ground water. All necessary |
|           |        | soil and water conservation measures and a     |
|           |        | nutrient management plan must be implemented   |
|           |        | to minimize loss from this field               |
+-----------+--------+------------------------------------------------+

**ATTACHMENT 1: Example for Conservation Planning**

+------------------------------------------------+---------------------+
| Surface-irrigated crop: sugarbeet -- onion --  |                     |
| small grain rotation on silty clay loam, water |                     |
| table at 10 feet \[Field has a ground water    |                     |
| resource concern as defined in ONEPLAN\]       |                     |
+------------------------------------------------+---------------------+
| Site Characteristic and Rating Value           | Factor Weighting X  |
|                                                | Rating Value        |
+------------------------------------------------+---------------------+
| Deep Percolation Risk is 35                    | 2.0 x 4 = 8.0       |
|                                                |                     |
| = HIGH (value = 4)                             |                     |
+------------------------------------------------+---------------------+
| Irrigation Efficiency is 35                    | 0.75 x 8 = 6.0      |
|                                                |                     |
| =VERY HIGH (value = 8)                         |                     |
+------------------------------------------------+---------------------+
| N Application Index is 160                     | 1.0 x 4 = 4.0       |
|                                                |                     |
| =HIGH (value = 4)                              |                     |
+------------------------------------------------+---------------------+
| N Application Timing is Split Application      | 0.75 x 2 = 1.5      |
|                                                |                     |
| =MEDIUM (value = 2)                            |                     |
+------------------------------------------------+---------------------+
| Water Table Depth and Soil Type                | 1.5 x 1 = 1.5       |
|                                                |                     |
| =LOW, (value =1)                               |                     |
+------------------------------------------------+---------------------+
|                                                |                     |
+------------------------------------------------+---------------------+
| **Sum total of all weighted values = 21.0**    |                     |
+------------------------------------------------+---------------------+

Site Vulnerability is **HIGH**

> **HIGH** - This site has a high potential for N loss and adverse
> effects on ground and/or surface waters. Soil and water conservation
> measures and nitrogen management plans are needed to reduce the
> probability of nitrogen loss.

[Using the individual site characteristics, identify some factors of
concern and management options that could be used to reduce this site
vulnerability:]{.underline}

**Deep Percolation Risk** -- The deep percolation risk is
[HIGH]{.underline} -- there is a high potential for nitrate leaching to
occur. Apply irrigation water according to crop requirements. Do not
apply nitrogen prior to leaching events. Water logging and poor soil
aeration may negatively affect crop yields in some areas of field.

**Irrigation Efficiency** -- The irrigation efficiency index under
furrow-irrigation with siphon tubes is [VERY HIGH]{.underline}
(inefficient). Careful management of soil moisture with irrigation
scheduling is needed. Be sure that the right amount of irrigation water
is applied as uniformly as possible to meet crop needs and minimize
leaching from the root zone -- consider converting to surge or sprinkler
irrigation. Check with irrigation professional to assure that crop
growth requirements are being adequately met.

**Nitrogen Application Index** -- The total nitrogen application was
[HIGH]{.underline}. The potential for nitrogen leaching exists if excess
water is applied from irrigation and/or precipitation events. There is
potential for detrimental effects of high nitrogen on crop production
and quality. Use soil and/or plant tissue tests and appropriate
fertilizer recommendations to determine nutrient application rates,
taking into account residual N.

The example described above has a high probability for an adverse impact
to ground water quality if existing management is not adjusted to reduce
the site vulnerability. Sites with a vulnerability rating greater than
LOW (especially those in the HIGH and VERY HIGH category) have the
greatest potential to adversely impact ground water quality. The
assessment can also be used to identify management options available to
land users and will allow them flexibility in developing remedial
strategies. The first step is to determine the management options
appropriate for sites with different N vulnerability assessments. N
management is very site-specific and requires a well-planned,
coordinated effort between the farmer, extension agronomist and soil
conservation specialist. The risk level can be reduced by planning
conservation practices and management techniques which will mitigate
leaching of nitrate. For example, a particular field has an irrigation
efficiency risk rating of VERY HIGH. To correct the problem, the
producer applies irrigation water management practices coupled with
conversion to surge irrigation to provide more uniform soil moisture to
the crop, based on crop demand. With these changes, a MEDIUM rating of 2
is now used to describe the overall risk due to irrigation efficiency.