Management Submodel

Introduction

WEPS is expected to reflect effects of various management practices upon wind erosion. The diversity of current practices applied to crop land by land managers makes this a daunting task, but WEPS must adequately simulate typical cultural practices to accurately assess their effects upon wind erosion control. The MANAGEMENT submodel is assigned the task of handling the cultural practices that affect the soil/surface "state" within WEPS.

Purpose

All cultural practices are by definition "human initiated". These human-controlled processes affecting the soil and field surface "state" are initiated by typical management practices such as tillage operations, planting, harvesting, irrigation, etc. Therefore, the purpose of the MANAGEMENT submodel is to model what are considered the major human-controllable actions that can affect the "system state" within WEPS, in particular the system state variables defining the temporal soil and surface conditions.

Objectives

The MANAGEMENT submodel objectives are:

1.	To model the primary human-initiated processes that can affect a site's susceptibility to wind erosion.

2.	To provide the framework necessary to process a list of specified human-initiated actions (i.e., the cultural practices applied to a field such as a tillage/crop rotation sequence).

Keeping with the WEPS philosophy, The MANAGEMENT submodel simulates processes via a physical basis if possible, incorporates the conservation of mass and energy concepts, and uses a minimum number of parameters with readily available and/or attainable values.

Assumptions and Limitations

Several assumptions and limitations have been imposed on the MANAGEMENT submodel. The reasons range from simply limiting the scope of the submodel, to inadequate knowledge of specific processes that may have a significant impact on the soil and/or surface. Here is the list of current assumptions and limitations, provided in no particular order, that impact the MANAGEMENT submodel:

1.	Total soil water content within the current tillage zone is assumed to be unaffected by a tillage operation. The HYDROLOGY submodel is expected to handle changes in surface water content and, therefore, appropriately represent the usual rapid drying of the surface layer after tillage.

2.	Tillage depth is assumed not to influence how a tillage operation affects the soil and surface, except for determining which soil layers are directly affected by a tillage operation.

3.	Effects of tillage operations on soil layers below the tillage depth are not considered, (i.e., subsoil compaction below the tillage zone due to tillage).

4.	Effects of a management operation are assumed homogeneous within a subregion. Effects due to tractor tires will not be considered (except where they may knock down a significant proportion of standing residue). Certain zone-related tillage operations, such as row cultivator, will be treated in a manner such that the result will be "averaged" or "equivalent" values that represent the homogeneous region.

5.	Ridge and dike geometric specifications (oriented roughness) will be provided by the user. If the tillage depth specified is not sufficient to create or destroy them (for a particular tillage operation that does so), the MANAGEMENT submodel will modify the tillage depth accordingly to obtain the desired ridge and/or dike specifications. Tillage operations that do not modify the current ridge and/or dike specifications will not do so (i.e., ridge tillage equipment).

6.	Soil tillage depths will be adjusted to the nearest soil layer boundary. This will ensure that the most recent tillage operation modifications on the soil "state" are adequately represented.

7.	Aggregate stability and aggregate density are assumed to be unaffected by tillage operations. This decision is based on limited field data analysis. Future research may provide statistically significant affects that could then be modeled. These properties may still change among soil layers within the tillage zone due to aggregate mixing among layers caused by tillage operations.

Submodel Description

The approach taken within the MANAGEMENT submodel to deal with the variety of land management actions was to:

1.	Identify the primary physical processes involved.

2.	Represent individual management operations as a sequence of those primary physical processes.

3.	Develop a MANAGEMENT file format allowing the input of user-specified sequences of management operations, (i.e., a management practices/crop rotation file).

All operations modeled within the MANAGEMENT submodel fall within the defined management categories as listed in Table 1.1.

When a management or tillage operation is performed, it is simulated through a group of individual physical processes that represent the total effects of that operation. The basic individual physical processes to be modeled within the MANAGEMENT submodel of WEPS have been grouped according to the target of their actions and outlined in Table 1.2.

The underlying philosophy behind the MANAGEMENT submodel was to attempt to develop representations based on physical law, if possible, for each of the chosen physical processes. These processes are assumed to be independent with respect to each other and are to be simulated sequentially, even though many of them occur simultaneously in the real world. The order they are initiated in the submodel is dependent upon the specific operation.

The list of management operations performed for a given management plan (crop rotation or cyclical management practices) is specified in a MANAGEMENT input file. The MANAGEMENT submodel checks on a daily basis for any operations to be performed on that day. If operations are needed, the MANAGEMENT submodel will execute the specified routines required to simulate the effects of those operations as instructed in the MANAGEMENT input file. When the last operation is performed for that particular crop rotation cycle, the same sequence will be repeated for the next year(s) of simulation.

A single MANAGEMENT input file may include multiple management operation lists, one for each subregion being simulated.