FOTG: Section II-iii-L - Water Quality and Quantity ...



Section II

Water Quality and Quantity Interpretations

Soil-Pesticide Interaction Ranking for Pesticide Leaching and Runoff

Soil-pesticide interaction rankings help determine the potential for pesticide loss from surface runoff and from leaching below the root zone, when a specific pesticide is used on a specific soil.

Soil and pesticide ranking

Soils and pesticides are ranked according to criteria used to develop the Windows Pesticide Screening Tool (WIN-PST) computer software.

Soils and pesticides are rated for leaching potential, solution runoff potential, and adsorbed runoff potential (pesticides attached to soil particles). Rating classes are very low, low, intermediate, and high.

Note: The WIN-PST computer software should be used by field offices to evaluate the potential for pesticide leaching and surface runoff. The software and user information is available on the internet at wcc.nrcs.pestmgt

Soil Rating for Nitrate and Soluble Nutrients

This section provides a way to determine the degree to which water percolates below the root zone. Percolating water containing dissolved nitrates or other soluble nutrients could be a hazard to ground water. The method presented is based on the Leaching Index (LI). The Leaching Index was developed by J.R. Williams and D.E. Kissel and is published in the book “Managing Nitrogen for Groundwater Quality and Farm Profitability” edited by R.F. Follet.

For areas with ground water concerns, the LI should be determined to evaluate the potential for contaminating the ground water with soluble nutrients. The LI uses annual precipitation, hydrologic soil group, and rainfall distribution data.

Leaching index

A Leaching Index map of Oregon was developed for each hydrologic soil group. The LI maps follow this section. The hydrologic groups are described in Section II-iii-A – Cropland Interpretations in the FOTG. Some soils have dual hydrologic ratings because of differences in drainage. Soils with hydrologic rating such as A/D should be evaluated on the basis of the current drainage status. If the soil has a high LI and it is over a shallow aquifer, soluble nutrients—especially nitrates—may contaminate the water.

The LI does not account for irrigation. If irrigation is applied only to supply plant needs, there will be little additional loss below the root zone. The additional loss would be relative to the precipitation events after the soil profile is saturated or nearly saturated because of irrigation.

In areas of marginal water quality, the amount of irrigation water applied includes a leaching fraction to insure that salts do not build up in the soil. If a leaching fraction is applied, this amount of water must be added to the LI. For example, if the leaching fraction is 1.2 and irrigation is applied to make up a 4 inch soil-water deficit, a 4.8 inch (1.2 x 4.0 in) irrigation would be applied. The LI should be increased by 0.8 inches. The same calculation must be made for each irrigation.

Procedure

Follow these steps to determine the leaching index of a certain soil:

1. Find the soil’s hydrologic group.

2. Locate the iso-leaching map for that group.

3. From the map, based on the soil location, determine the LI. (Note: For counties in which the LI iso-leaching lines are close together and thus difficult to interpret, the field office has the option to develop a county LI map. The map needs to be developed using the calculated LI values for the weather stations in the area. The calculated values and list of weather stations are available in the state office soils section. All LI maps need to be reviewed and approved by the state office.)

Guidelines for recommendations

A LI below 2 inches would probably not contribute to soluble nutrient leaching below the root zone. Eastern Oregon cropland areas generally have LI’s below 2 inches because of low rainfall. The main concern in eastern Oregon is where irrigation water application plus rainfall exceed the crop requirements. Where this is true, the LI needs to be modified (see guide in this section for modifying LI). Effective leaching can also be increased in summer fallow rotations.

A LI between 2 and 10 inches may contribute to soluble nutrient leaching below the root zone and nutrient management should be considered. A LI larger than 10 inches will contribute to soluble nutrient leaching below the root zone. Intensive nutrient management should be practiced, or soluble nutrients should not be applied. Also, consider using conservation practices that minimize infiltration, such as strip cropping rather than pipe outlet terraces.

Note: Insert Leaching Index Maps following this page.

[pic]

[pic]

[pic]

Leaching Index Map for Hydrologic Group D

[pic]

The Phosphorus Index

The Phosphorus Index for Oregon was developed by a nutrient management advisory group comprised of members from the Natural Resources Conservation Service in Oregon and Washington, Oregon State University, Washington State University, and agricultural industry.

The Phosphorus Index is a field-level assessment tool designed to evaluate the relative potential for off-site movement of phosphorus from the landscape. The purpose of the Phosphorus Index is to provide field personnel, watershed planners, and land managers with a tool to assess various soils, landforms, and management practices for potential risk of phosphorus movement into surface waters. Conservation planners can use the Phosphorus Index to develop alternatives that include management and conservation practices that will reduce the potential for phosphorus transport to surface waters.

Agronomy Technical Note 26 – Revised includes the Oregon Phosphorus Index and was issued October 2001. It can be accessed at the following address:



Water Quality Indicator Tools

This Oregon Technical Note provides information on water quality indicator tools to be used by NRCS field office personnel. The tools presented provide a general background into the pollution process for various water quality parameters. The technical note can be found at the following address:



Water Quantity Interpretations

Soil interpretations, specifically for water quantity, have not been developed. However, several soil properties and soil interpretations relate to water quantity. These include runoff, flooding, ponding, seasonal water table, available water capacity, infiltration and moisture deficiency. Most of these are located in the soil database or are in other subsections of Section II. Some are located elsewhere and reference is made to the location of each.

Soil Properties and Interpretations Affecting Water Quantity

Runoff: Refer to the Hydrologic Group designations in the Water Features Table of soil survey.

Flooding: Refer to flooding information in the Water Features Table.

Ponding: Refer to water table information in the Water Features Table.

Seasonal Water Table: Refer to water table information in the Water Features Table.

Available Water Capacity: Refer to the Physical Properties Table.

Infiltration: Refer to intake families in the Oregon Irrigation Guide.

The National Engineering Handbook contains tools for water quantity evaluations.



................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download