Adapted from Dillaha, T. A., M. L. Wolfe, A. Shirmohammadi, F. W. Byne. 2001. ANSWERS-2000. In Non-Point Source Water Quality Models: Their Use and Application.  Final Report of USDA-CSREES Southern Region Research Project S-273 "Development and Application of Comprehensive Agricultural Ecosystems Models".

Beasley and Huggins (1982) developed the original ANSWERS (Areal Nonpoint Source Watershed Environment Response Simulation) model in the late 1970s. The model was based on one of the first true distributed parameter hydrologic models (Huggins and Monke, 1966). The original ANSWERS was a distributed parameter, event-oriented, planning model developed to evaluate the effects of BMPs on surface runoff and sediment loss from agricultural watersheds. ANSWERS subdivides the watershed into a uniform grid of square cells. Land use, slopes, soil properties, nutrients, crops, and management practices are assumed uniform within each cell. Differences between cells allow the model to consider the heterogeneous nature of watersheds and the site specific effectiveness of individual BMPs. Typical cell sizes range from 0.4 to 1 ha with smaller cells providing more accurate simulations. Ten to twelve parameter values must be provided for each homogeneous cell. Within each cell, the model simulates interception, surface retention/detention, infiltration using Holtan's method (Holtan, 1961), surface runoff, percolation through the infiltration control zone, sediment detachment and sediment transport. Flow was from routed downslope to adjacent overland flow cells or in channel cells. The model could simulate BMPs such as conservation tillage, ponds, grassed waterways, tile drainage (Bottcher et al., 1981) and other practices whose effects on the physically based model input parameters could be described. An original weakness of the ANSWERS model was its erosion model, which was largely empirical and simulated only gross sediment transport. The model was modified in the early 1980s to simulate the particle size distribution of eroded sediment (Dillaha and Beasley, 1983) using Yalin's method (Yalin, 1961) to estimate sediment transport. Rewerts and Engel (1991) developed GIS interfaces for this version of the model. In the late 1980s, phosphorus (Storm et al., 1988) and nitrogen (Dillaha et. al., 1988) transport versions of the event-oriented model were developed. They considered the transport of dissolved and adsorbed orthophosphorus, nitrate and dissolved and adsorbed ammonium and TKN.

The current version of the model, ANSWERS-2000, is a continuous simulation model that was developed in the mid 1990s (Bouraoui and Dillaha, 1996). In this version, the nutrient submodels were overhauled and improved infiltration (Green and Ampt), soil moisture and plant growth components were added to permit long-term continuous simulation. Bouraoui (1994) describes the current version of the model in detail. ANSWERS-2000 simulates transformations and interactions between four nitrogen pools including stable organic N, active organic N, nitrate and ammonium. Transformations of nitrogen include mineralization simulated as a combination of ammonification and nitrification, denitrification, and plant uptake of ammonium and nitrate. The model maintains a dynamic equilibrium between stable and active organic N pools. Four phosphorus pools are simulated: stable mineral P, active mineral P, soil organic P and labile P. Equilibrium is maintained between stable and active mineral P and between active mineral P and labile P. Plant uptake of labile P and mineralization of organic P are also simulated.

The continuous version, ANSWERS-2000, was tested on two watersheds in Watkinsville, Georgia, and performed well in predicting runoff, sediment, nitrate, dissolved ammonium, sediment-bound TKN, and dissolved phosphorus losses from both watersheds (Table 1 and Figures 2 to 8 for the P2 watershed). The model did not predict sediment-bound ammonium losses from either watershed well. The model was also tested on the 1153 ha Owl Run watershed in Virginia. The model performed well for the largest storms, and cumulative predictions of runoff volume, sediment yield, nitrate, ammonium, sediment-bound TKN, and orthophosphorus were within 40% of the measured values (Table 2). In a practical application of ANSWERS-2000, the model was run for eight years on a 225 ha subwatershed in the Nomini Creek watershed in Virginia. Potentially critical source areas were selected, and BMPs (conservation tillage) were then implemented on 10, 20, 30 and 100% of the cropland in the watershed. Targeting was found to significantly increase the efficiency of BMP application with respect to water quality (Table 3). For example, nutrient reductions were 2.4 and 2.1 times as great on a per hectare basis for nitrogen and phosphorus, respectively, when BMPs were targeted to 10% of the cropland with the highest sediment losses as compared to application of BMPs to all cropland. The model also showed that fields that had very high sediment and nutrient yields did not always have large impacts on yields at the watershed level.

To facilitate use of the model, the ANSWERS-2000 NPS decision support system was developed (Wolfe et al. 1995). This ArcInfo based decision support system has a knowledge-based system to advise the user on parameter selection and database creation. The user interface also keeps track of BMP scenarios evaluated and controls model output. The user interface links components and functions of the ANSWERS-2000 modeling system in a manner that is reasonably transparent to the user. A set of menus, prompts, helps, and rule-based functions guide the user, but does not eliminate interaction with the system. Procedures are incorporated to flag problems associated with incorrect data and report when system constraints have been violated and provide appropriate corrective action. The user interface consists of a main menu and secondary menus. The menus allow the user to provide site-specific information for input parameters for the ANSWERS-2000 model. The modeling-related tasks facilitated by the interface include the following: selection of the land area of interest (watershed, farm, or field) from a GIS coverage; automatic access to a soils parameter file with options for replacing general parameter values with site-specific information; developing the grid overlay of the watershed; calculation of topographic characteristics; generating synthetic weather data if desired; assigning crop and tillage parameter values; creation of the ANSWERS-2000 input data file; running the model; displaying model output; applying alternative practices to user-specified land areas; and comparing predicted output values for alternative scenarios. In addition, a help function in the interface includes specific information for the user on how to proceed with the different menu selections.  The ArcInfo based decision support system was converted to ArcView in 2000 and 2001.  The new decision support system is called QUESTIONS and ANSWERS.

In a recent development, a groundwater component was added to ANSWERS-2000. This version of the model was validated at the local, field and watershed scales. At the local and field, it accurately predicted drainage below the root zone and evaporation for different vegetative covers. At the watershed scale, it accurately reproduced piezometric levels and trends across the watershed (Bouraoui et al., 1997). It is not known if this version of the model simulates nutrient transport.


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Bottcher, A. B., E. J. Monke, and L. F. Huggins. 1981. Nutrient and sediment loadings from a subsurface drainage system. Trans. Of the ASAE 24(5):1221-1226.

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Wolfe, M. L., W. D. Batchelor, T. A. Dillaha, C.D. Heatwole and S. Mostaghimi. 1995. Modeling the effects of farm management practices on off-site water quality. Proceedings of the International Symposium on Water Quality Modeling, April, 1995. Kissimmee, Florida.

Yoon, K. S., A. Shirmohammadi, W. J. Rawls. 1995. Application of Continuous, Distributed Watershed/water Quality Model (ANSWERS) on a Mixed Landuse Watershed ASAE Paper No. 952402, ASAE, St. Joseph, MI.   



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