A Procedure for the Determination of
Storage Period for Manure Storages in Oregon

Donald L. Stettler, Environmental Engineer
USDA, NRCS, National Water and Climate Center
Portland, Oregon

Bruce D. Wilson, Conservation Planning Engineer
USDA, NRCS, Portland, Oregon

Determining an appropriate storage period for sizing manure storage facilities is of great importance in planning and designing agricultural waste management systems that will successfully protect the resource base. However, because of the wide variation of environmental conditions throughout the United States, the Natural Resources Conservation Service practice standards and handbook guidance are necessarily vague on how storage period is determined. Oregon NRCS has adopted a procedure for determining storage period that takes into account the wide variation of Oregon climatic conditions as well as farm variables including the level of management, soils, field location, nutrient loading, and application method. This procedure may be adaptable to other regions the United States.

The NRCS Practice Standard 313, Waste Storage Facility, defines the storage period as "... the maximum length of time anticipated between emptying events." The criteria for its determination is that it "...shall be based on the timing required for environmentally safe waste utilization considering the climate, crops, soil, equipment, and local, state, and Federal regulations." The necessary vagueness of this criteria requires that individual States supplement the criteria or provide guidance on its determination based on the local conditions.

Oregon NRCS uses a procedure for determining storage period that considers the level of management, soils, location, nutrient loading, and method of application. Oregon's climate is highly varied from its coastal area with mild winters and high precipitation to its eastern high desert with cold winters and snow. As such, the Oregon procedure may serve as a model for other parts of the country.

Oregon NRCS's procedure begins with the determination of a basic storage period that is based on the period of time between when three-fourths of crop's evapotranspiration (ET) has occurred and when evapotranspiration become equal to or greater than precipitation the next spring. The premise of using three-fourths of the ET is that at this point the crop has absorbed the majority of the nutrients that it will use. Nutrients in manure applied after this time will be available for runoff and leaching. It is assumed that runoff ceases when ET becomes equal to precipitation the next spring. The procedure allows for adjustment of the basic storage period based on the assumption that management can take advantage of times within the basic storage period to apply wastes to reduce the storage requirement. The Oregon NRCS procedure involves methodology for evaluating first, the level of management, and then availability farm variables that are conducive to winter time manure applications. These include availability of suitable soils and buffered land application areas. Also evaluated are the method of application and nutrient loading in terms of percent of the crop requirement.

1. Determine the basic storage period (SP_b). Determine the day of the year (L) when 3/4 of annual evapotranspiration has accrued. Determine earliest day of the year (E) when evapotranspiration is equal to precipitation by interpolation of month averages SP_b = 365 - (L - E)

2. Determine adjustment factor for level of management (F_m). The basic storage period is adjusted on the assessed level of management that will be provided.

3. Determine adjustment factor for availability of suitable soils for land application (F_s). An adjustment to basic storage period based on soils can only be made if the application area does not have any of the severe limitations given in Table 5-3 of the NRCS Agricultural Waste Management Field Handbook.

4. Determine adjustment factor for availability of land application area buffered from water bodies and free of concentrated flow areas (F_l). Buffered application areas are not subsurface drained, have a 0 to 5 percent chance of flooding from adjacent water bodies, and are at least 50 feet from water bodies, ground water sources, and areas of concentrated flow.

5. Determine adjustment factor for nutrient loading (F_n). In order for the nutrient loading factor, Fn, to be less than 1, the location factor, F_l, must be 0.95 or less. The percent of annual nutrient requirement (%ANR) is determined by taking the total nutrients applied annually (TNA) and dividing it by the total nutrient requirement (TNR) of all the crops grown.

%ANR = TNA/TNR x 100%

7. Compute the adjusted storage period (ASP)

ASP = SP_b x F_m x F_s x F_l x F_n x F_a

8. Compute adjusted earliest calendar day for application (E_a)

E_a = ASP - (365 - L)

9. Check adjusted storage period, ASP, to ensure it spans seasonal periods of flooding. Also check to ensure the adjusted storage period spans frozen ground and snow conditions where the application areas slope is greater than 5% and buffer strip of 100 feet or more cannot be maintained between the application area and open water courses.

Determine the calendar day when seasonal frozen ground, snow or flooded conditions are first anticipated for application area (L_f).

If L_f comes before L (Step 1) then the ending date for manure application (L_sp) = L_f
If L_f comes after L or there is no seasonal frozen ground, snow or flooding then L_sp = L

Determine the calendar day when frozen ground, snow or flooding conditions are no longer anticipated for the application area (E_f).

If E_f comes after E_a (Step 8) then the beginning date for manure application (E_sp) = E_f
If E_f comes before E_a or there is no seasonal frozen ground, snow or flooding then E_sp = E_a

10. Compute Storage Period (SP)

SP = 365 - (L_sp - E_sp)

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