BACKGROUND

Water quality models in their common context were primarily developed from the middle 1970’s to the current times. Many tools, such as the Universal Soil Loss Equation (USLE), are models per se, but do not have the complexity of the water quality tools developed in the past twenty-five years. A common feature of most of the water quality models is that they are driven by a hydrology component linked to an erosion and sediment component. Pollutants are partitioned by their affinity to move with the water and sediment phases of the system.

Water quality models in use today generally produce results that are most useful for comparative rather than predictive purposes. Models are physically based in that mathematical equations and "algorithms" are used to define complex natural relationships. As the complexity of the natural relationship increases, the ability to mathematically "model" it decreases. Without careful calibration the model results are usually of an appropriate magnitude, but should not be used for present or future predictions. They can be used to compare one alternative to another and provide a mechanism to qualitatively rank one alternative against another. As a general rule, the predictive capability of a model increases in proportion to its complexity of simulation and the amount of input data required.

The term "water quality model" is a misnomer in most cases. Many of the models called "water quality models" are actually pollutant loading models in that they determine the quantity and timing of a pollutant being delivered to a water body (or below the root zone), but do not show the impact of the pollutant being assimilated in the water body or aquifer. At least one of the newer watershed scale tools does include an assimilation process and could correctly be referenced as a water quality tool. For the purposes of this discussion, the term "water quality model" will be used to describe pollutant loading models as well as actual water quality models.

NRCS (SCS) commitment to water quality models began in the late 1970’s, primarily at the National Office level (NHQ and the National Technology Center (NTC’s)). Specialists in hydrology, geology, agronomy, and environmental engineering were involved with their ARS counterparts as the first models such as Chemicals, Runoff and Erosion from Agricultural Management Systems (CREAMS), Erosion Productivity Impact Calculator (EPIC), Simulation for Water Resources in Rural Basins (SWRRB), and Agricultural Non Point Source Model (AGNPS) were assembled and tested. The NTC specialists linked the model developer to NRCS offices and provided assistance to states interested in applying the tools.

NRCS liaison positions were established at selected ARS locations to insure the tools being developed would meet the needs of NRCS and to act as keys in the technology transfer program. Liaisons were located at Morris, Minnesota (MN) (AGNPS), Tifton, Georgia (GA) (CREAMS and Groundwater Loading Effects of Agricultural Management Systems (GLEAMS)), and Temple, Texas (TX) (EPIC and SWRRB). The liaisons at Tifton, GA, and Morris, MN, were relatively short term, but the liaison position at Temple, TX was sustained for over a decade. The liaisons were useful to both ARS and NRCS and the workshops and training sessions they organized resulted in many NRCS offices making use of modeling tools as part of their conservation programs. During some of this time there was also an NRCS position located with ARS in Fort Collins, Colorado (CO), working on a watershed scale model for the Tucannon River in Washington.

Numerous inter-agency and intra-agency workshops over the years have enabled NRCS and our research partners to communicate water quality modeling needs and developments in modeling technology. Some of these workshops and other efforts are included here to give the flavor of the methods used by NRCS to acquire and transfer water quality model technology.

• 1985 Water Quality Modeling Workshop in Lincoln, Nebraska (NE) – Organized by the Midwest NTC, this was one of the first workshops to actively involve university research and modeling efforts in an NRCS water quality technology forum. The workshop consisted primarily of discussions of different models followed by opportunities for hands-on demonstrations of the tools.
• 1992 Water Quality/Quantity Work Conference in Lakewood, CO – National effort to develop integrated proposals for short and long range development of water quality and quantity technology to insure NRCS adequate tools to address mandated water resource issues.
• 1993 Water quality model evaluation – First comprehensive effort by NRCS to evaluate water quality models developed by ARS and adapt them where needed for NRCS use. Effort involved many disciplines at the National Office level (NHQ and NTC) primarily looking at five models (SWRRB, EPIC, GLEAMS, AGNPS, and Nitrate Leaching and Economic Analysis Package (NLEAP) and evaluating their utility and usability, as well as verifying and validating the models. States were funded to take the tools and use them in a planning setting. This effort resulted in the five models being recognized as "NRCS approved models."
• 1994 – 1995 Water quality model technology transfer efforts – The technology in the five models mentioned above were transferred to the states in a series of videoconferences and workshops. The videos provided an explanation of the technology within the models, and the hands-on training was offered to teach the use of the tool. The ARS scientists were full partners in this effort.
• Hydrologic Unit/Water Quality (HU/WQ) project – A UNIX based platform and software was developed by NRCS to meld Geographic Resources Analysis Support System (GRASS) Geographical Information Systems (GIS) with the data input for four water quality models; AGNPS, SWRRB, EPIC, and GLEAMS. The concept was the user could enter one data set and run any of the four models. The operational version of the tool was limited to SWRRB and AGNPS.
• Soil and Water Assessment Tool (SWAT) technology – Concurrent with the development of HU/WQ, the ARS and Texas A&M at the Blacklands Research Center improved the technology in the SWRRB watershed scale model and combined it with GRASS GIS to form the SWAT tool. SWAT was designed to function in a Disk Operation Systems (DOS)/WINDOWS environment. The Texas NRCS State Office assigned a team of specialists to the Blacklands Center to use the SWAT technology to address Texas water resource issues and to provide some limited technology transfer to the remainder of NRCS.
• 1997 Modular Modeling System Workshop in Fort Collins, Colorado - Representatives from several agencies met to discuss a modular modeling concept for federal hydrology and water quality model development, following the work of George Leavesley's group at the United States Geological Survey (USGS).
• 1998 Federal Interagency Hydrologic Modeling Conference in Las Vegas, Nevada. - This was a forum for many federal agencies and others to share information on their activities in hydrologic model development. There are many opportunities to collaborate with others and to benefit from their experiences, which we should consider as we pursue any efforts in model development.
• Water Erosion Prediction Project (WEPP) Project - This was partially funded by the NRCS. It is more physically based than other USDA erosion models, but it does not have water quality components other than sediment delivery. It is applicable to small areas, not large watersheds. If other water quality components and some additional hydrology components were added, however, it could become the model of choice for small watersheds.
• AGNPS98 Development – NRCS specialists have worked directly with ARS to convert AGNPS from a single event tool to one with continuous simulation and to add a GIS component. The improved AGNPS tool and other components have been presented to NRCS in a series of workshops.
• Watershed Ecosystem Nutrient Dynamics (WEND) and Systems Thinking Experimental Laboratory (STELLA) programming – The Watershed Ecosystem Nutrient Dynamics (WEND) tool was the first water quality model used by NRCS programmed with STELLA. WEND is supported by the Watershed Sciences Institute. STELLA, which stands for Systems Thinking Experimental Laboratory, allows the user to graphically represent or "map" the natural system as a series of related inputs, outputs, and storages. When the "map" is defined in terms of mathematical relationships, the software develops the model equations or code.

There continues to be a great deal of interest in water quality models, and the technology continues to evolve. New issues such as total maximum daily loads (TMDL’s) and the joint USDA - EPA Clean Water Action Plan point to the increased use of water quality models in the future. Where does NRCS put its limited resources? Which models are most appropriate? Which are easiest to use? These questions prompted the Water Resources Group to form the team mentioned above, and to charge the team with beginning an evaluation of the current suite of tools.