Daycent5chem Manual

Natural Resource Ecology Laboratory Colorado State University IRC Model Project DayCent5Chem Manual

Contents 2 Introduction 2 See Also 3 Model Structure 3 Structure and Information Flow 4 Atmospheric deposition and fertilization 4 Plant dynamics 5 Soil and weathering dynamics 5 Surface water and snow dynamics 6 Dissolved gases 7 Simulation Configuration 7 Configuration Steps 7 Configuration Directory Structure 8 Configuration Files 8 geochemistry configuration file 8 initial soil data file 8 Initial snow file 9 dry deposition file 9 wet deposition file 9 See Also 10 Running a Simulation 10 Standard Command-Line Options 11 Debug Command-Line Options 11 Example command line 12 Simulation Results 12 Output Files 13 Examining Simulation Results 13 Comparing Two Sets of Simulation Results 14 Bibliography 16 Acknowledgments 17 Copyright and Disclaimer 17 Copyright 17 Disclaimer

Last updated: 2015-04-06 Introduction DayCent5Chem was created by coupling two models, DayCent version 5 (DayCent5) and PHREEQC (Hartman et al, 2007). DayCent is a model of daily biogeochemistry for forest, grassland, cropland, and savanna systems (Parton et al., 1998). PHREEQC is a model of soil and water geochemical equilibrium (Parkhurst and Appelo, 1999). The purposes of creating DayCent5Chem were to capture the biogeochemical responses to atmospheric deposition and to explicitly consider those biogeochemical influences on soil and surface water chemistry. The linked model expands on DayCent’s ability to simulate N, P, S, and C ecosystem dynamics by incorporating the reactions of many other chemical species in surface water.

DayCent5 is the daily timestep version of the CENTURY ecosystem model (version 5). DayCent5 is a non-spatial, lumped parameter model that simulates C, N, P, S, and water dynamics in the soil-plant system at a monthly timestep over time scales of centuries and millennia (Parton et al., 1987, 1994). DayCent5 can represent a grassland, crop, forest, or savanna system with parameters that describe the site-specific plant community and soil properties. DayCent adds layered soil temperature, a trace gas submodel, a more detailed soil hydrology submodel, and explicitly represents inorganic N as - + either NO3 or NH4 (Parton et al., 1998; Kelly et al., 2000; Del Grosso et al., 2001).

DayCent5 implements a layered soil structure and algorithms to manage soil layers (Hilinski, 2001). The model is initialized with an organic soil depth and up to ten soil layers where each layer has a specified thickness, texture, bulk density, field capacity, wilting point, and saturated hydraulic conductivity. Climate drivers required for DayCent5 are daily precipitation, and minimum and maximum air temperatures. DayCent5 output includes daily evapotranspiration; soil water content; outflow; inorganic and organic C, N, P, and S stream fluxes; C, N, P, and S contents in soil and plant pools; net primary production (NPP); nutrient uptake; trace gas flux; and heterotrophic respiration.

In addition to the DayCent5 output, DayCent5Chem output includes the solution chemistry for soil layers, groundwater, and stream at a daily timestep.

PHREEQC (Parkhurst and Appelo, 1999) is a model based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, exchangers, and sorption surfaces. The model has an extensible chemical data base used to compute aqueous speciation, ion-exchange equilibria, fixed-pressure gas-phase equilibria, dissolution and precipitation of mineral phases to achieve equilibrium, and irreversible aqueous mineral phase reactions. The aqueous model uses ion-association and Debye-Huckel expressions. Ion-exchange reactions are modeled with the Gaines-Thomas convention and equilibrium constants derived from Appelo and Postma (1993). We did not alter the PHREEQC model except to provide a function call interface from DayCent5Chem. DayCent5Chem currently incorporates PHREEQC version 2.18.

See Also Hartman, et al., 2007. Application of a coupled ecosystem-chemical equilibriummodel, DayCent-Chem, to stream and soil chemistryin a Rocky Mountain watershed.

PHREEQC version 2 User's Guide: online (a copy is provided in the source code directory)

Century5 and DayCent5 User's Guide (online) Model Structure

Structure and Information Flow DayCent5Chem is an ecosystem-geochemical model containing layered soil pools and properties including water fluxes and solute concentrations, and models exchanges between the plant-soil ecosystem and the geochemical system, The geochemical submodel defines soil layers and a groundwater pool. The submodel calculates daily wet deposition amounts from precipitation concentrations and dry deposition amounts from dry:wet ratios if dry amounts are not input explicitly. Surface water concentrations are simulated in a two step process where solutes are first transported, then PHREEQC calculates the solution reactions. At each timestep the model updates exchangeable cation pools (CaX2, MgX2, NaX, KX, NH4X, HX, FeX2, MnX2, AlX3, AlOHX2, where X− is a permanent negatively charged exchange site) and soil solutions in each soil layer, along with groundwater and stream solutions.

The following diagram shows the structure of the soil and water pools and flow of data between them.

DayCent5Chem invokes the PHREEQC model through a subroutine call, but the transfer of information to and from PHREEQC occurs through its text-format input and output files. For each daily timestep, the model writes the initial compositions of the soil layers, groundwater, and stream solutions to the PHREEQC input file, soln.pqi. When there are n soil layers, there are n + 2 solutions defined in the input file, plus a SELECTED OUTPUT block that tells PHREEQC what information to write to its output file. PHREEQC performs its calculations on each initial solution in soln.pqi, it writes its reacted solution results to a “selected output file”, soln.sel. The geochemical submodel then reads the file soln.sel, and updates its soil, groundwater, and stream pools with these values.

The following diagram shows the structure of the soil and water pools and flow of data between them.

DayCent5Chem simulates atmospheric deposition, plus snowpack, plant, soil, and stream dynamics (left side of the figure above) while utilizing PHREEQC’s soil and stream water reactions (right side of figure).

Atmospheric deposition and fertilization Atmospheric deposition occurs in wet and dry forms. If the air temperature is cold enough, deposition will be routed to the snowpack, otherwise it will be routed to the soil surface where it can infiltrate the soil, seep into groundwater, or run off directly to stream flow. Additional N, P, and S may be incorporated into the top soil layer through inorganic fertilizer or organic matter additions.

Plant dynamics Carbon is dynamically allocated to above and below ground plant parts according to nutrient availability, water stress, and - + vegetation type. Plants take up nutrients (NO3 , NH4 , P, and S) from soil as needed to maintain carbon:nutrient ratios in the range specified for the plant. Nutrients from scenescing leaves are translocated to plant storage at the end of the growing season. Dead plant material remains standing or is dropped onto the soil surface. Some plant types are able to reduce their mineral N demand by symbiotically fixing nitrogen.

Because the DayCent model simulates only C, N, P, and S dynamics, there is no assimilation of base cations (Ca2+, Mg2+, K+) by plants in the linked model; we assume that annual plant uptake of these three nutrients equals the amount released through decomposition of plant material. This assumption may hold for undisturbed ecosystems, but not for recently disturbed areas or relatively young forests. Net cation assimilation is small (25 meq m−2 year−1) in the old-growth forest of LVWS, but there may be spatial variation within the soil profile between the uptake of base cations and their release through decomposition of fine roots and litter (Arthur, 1990).

Soil and weathering dynamics Organic matter is incorporated into the soil through litter fall and death of roots. Decomposing organic matter is redistributed among active, passive, and slow pools while N, P, and S is either mineralized or immobilized. CO2 produced by heterotrophic respiration is dissolved in soil solutions. Rainfall and snowmelt infiltrating the litter leach dissolved organic C, N, P, and S (DOC, DON, DOP, and DOS, respectively). Both inorganic and organic species are transported downward with water that further leaches organic and mineral soil. Nitrification and denitrification produce N2, N2O, and + - NOx through oxidation and reduction of NH4 and NO3 , respectively. Cations in solution are adsorbed and desorbed from permanent negatively-charged exchange sites (X−). Dissolu- tion of primary and secondary minerals releases base cations, aluminum hydroxides, bicarbonate, metallic cations, hydrogen ions, silica, sulfate, and other inorganic species into soil layer, groundwater, and stream solutions. A portion of DOC is organic acid that reacts with other species in soil and stream solutions. Water exiting the soil profile enters either the stream or groundwater; a user-specified fraction of this groundwater storage is released to stream flow each day.

Annual denudation rates of common primary and secondary minerals are model inputs, and each mineral can be dictated to dissolve only, to precipitate only, or to do either. The maximum amount of any mineral that can be dissolved within a solution on a given day is calculated by dividing its annual denudation rate by 365. The user specifies the distribution of minerals among the soil layers, groundwater, and stream. For solutions where mineral dissolution occurs, the daily potential denudation rates of each mineral are listed in a PHREEQC EQUILIBRIUM PHASES reaction. This means minerals can dissolve until they reach equilibrium with the solution, or until the maximum daily amount has been dissolved, whichever comes first. No kinetically limited mineral dissolution reactions are considered. Minerals precipitate when their components are oversaturated in solution.

Surface water and snow dynamics Stream flow is controlled by a number of model parameters to obtain a watershed response. Rain and snow melt that do not infiltrate the soil surface carry precipitation solutes to stream flow along with dissolved organics leached from surface litter, although a fraction of surface runoff can be routed to groundwater for deferred release into the stream. Water that infiltrates the soil profile leaches organic and inorganic species. The soil hydrology algorithm directs a constant fraction of soil drainage to stream flow and to groundwater; a constant fraction of groundwater is routed to stream flow. Some of the CO2 carried from soil solutions and groundwater degasses as it encounters the lower partial pressure of the atmosphere. Geochemical reactions in stream flow and groundwater include the same aqueous reactions defined for soils, except for cation exchange.

Three parameters regulate snowpack accumulation, melt, and sublimation. Precipitation is added to the snowpack when maximum daily air temperature is below TMELT(1), and snow melts above this temperature. The rate of melt is a function of TMELT(2) and the difference between maximum daily air temperature and TMELT(1). When a snowpack is present, the maximum amount of sublimation is the product of parameter FSUBLIM and a potential evapotranspiration rate calculated by the model as a function of air temperature. FSUBLIM > 1 is used to account for sublimation by wind that can substantially increase snowpack loss from windy, high elevation environments (Hartman, 1999).

Elution causes early melt water to be the most chemically concentrated of the season. Even when ionic concentrations in precipitation are low, snowpacks may accumulate large quantities of ionic species and release these species in a strong ionic pulse (Campbell et al., 1995). The magnitude of the ionic pulse depends on snow depth, rate of melt, and number of melt/freeze cycles in the snowpack (Williams and Melack, 1991; Bales, 1992), and therefore may vary regionally, spatially within a watershed, and/or from year to year at the same site (Williams and Caine, 2001). The assumption in this model is that elution occurs beginning with the spring melt and continues into early summer. The effect of elution on ionic concentrations is calculated internally on a sine function of the fraction of the maximum number of elution days allowed. The peak of the elution effect is half-way through the elution time period.

Dissolved gases The total amount of dissolved inorganic carbon in soil, groundwater, and stream solutions is regulated in part by equilibrating solutions with CO2(g). The partial pressure of CO2(g) in soil and groundwater solutions, pCO2 soil was set to 10−2.5 atm (Lindsay, 2001). The stream water solution and the top soil layer are brought to equilibrium with pCO2(z), where z is the site elevation. Simulation Configuration

Configuration Steps 1. Create a directory to hold your simulation files. 2. Create a subdirectory to hold your simulation results. 3. Copy this INI file example and paste into a new text file. Save it to an INI file in your simulation directory. Suggest using the site file name as the name part, and ini as the file extension. 4. Create a file containing initial soil chemistry data, the "initSoil" file. For an example, see the file in the source code tree: Ecosystem/DayCent5Chem/tests/HJAndrews/initSoil.csv 5. Create a file containing initial snow chemistry data (optional). For an example, see the file in the source code tree: Ecosystem/DayCent5Chem/tests/HJAndrews/initSnow.csv 6. Create a file containing dry deposition chemistry. You can specify this as either deposition amounts or as dry:wet ratios. For an example, see the file in the source code tree: Ecosystem/DayCent5Chem/tests/HJAndrews/dryDep.txt 7. Create a file containing wet deposition chemistry. For an example, see the file in the source code tree: Ecosystem/DayCent5Chem/tests/HJAndrews/wetDepConc.txt 8. Add the DayCent5 site and management, and if needed, fixed and other parameter files. Put these files in your simulation directory.

Configuration Directory Structure The following is an example set of configuration files:

Configuration Files These configuration files are in addition to the DayCent5 configuration files, which include site parameters, management schedule, "fixed" parameter set, and climate data. See the Century5 and DayCent5 User's Guide for details. geochemistry configuration file default file name: none

The following is an example of a geochemistry configuration file. You can copy this block from "START" to "END" and save it to a text file with the file name extension "ini" (e.g., problem42.ini).

When running a simulation, put this file name as the option to the command line argument −chem, for example, −chem problem42.ini view: INI file example initial soil data file default file name: initSoil.csv

The initial soil data file contains:

1. initial soil solution concentrations; 2. exchangeable cations in each soil layer; 3. potential annual denudation rates for each mineral phase that could be dissolved in the soil, groundwater, or stream solutions.

The number of soil layers in the initial soil data file == number of soil layers in the site.100 file + 2.

The file format is CSV without quotes around text fields. view: Initial Soil data file example

Initial snow file default file name: initSnow.csv

Initial snow pack water content and chemical composition.

The file format is CSV without quotes around text fields. view: Initial Snow data file example dry deposition file default file name: dryDep.txt or dryDepRatios.txt

Daily dry deposition amounts or dry:wet ratios for all precipitation species. view: Dry Deposition Ratio data file example wet deposition file default file name: wetDepConc.txt

Daily atmospheric wet deposition concentrations for precipitation species Ca2+ Cl− K+ Mg2+ Na+ NH4+ NO3− SO42− H+ view: Wet Deposition Concentration data file example

See Also Century5 and DayCent5 User's Guide Running a Simulation The simulation can be run as either DayCent5 without the geochemistry model, or with geochemistry. Use the option -chem to run with geochemistry.

DayCent5Chem is a command-line program. To run a simulation enter the following command in a console window, or from your shell prompt:

DayCent5Chem [options] where options is an optional list of parameters (do not enter the brackets) from the list of standard options, and for further information, the list of debug options. For examples of configuring the environment to find the executable files, see below.

For file names, conform to your operating system's conventions. For example, in Microsoft Windows systems, sometimes strings should be enclosed in double quotes, such as when file names and paths contain spaces. In Linux (and other *nix systems), if a file name or path contains spaces, enclose it in single quotes.

Standard Command-Line Options -s filename (Required) Name of site parameter file. To import a Century 4 site file, add file name extension ".100".

-m filename (Required) Name of management file. To import a Century 4 schedule file, add the file name extension ".sch".

-o filename Name of output file (omit the file name extension). This name will have a suffix and and extension added to it.

-tn, -ts or -t0 Output file type: tn = netCDF ts = ASCII comma-separated values format for input into a spreadsheet (default). t0 = do not produce output files. This option turns off writing all output variable to the output files.

-td0 Do not produce daily output files. This option does not affect monthly output files. The default is to produce daily output files.

-an, -ar, -aa Output file access: an = new (default); ar = replace; aa = append (not currently implemented.)

-f filename Use the file name containing the fixed parameters (with optional path) supplied by the user.

-fm filename Use the fixed parameters in this file name supplied with the model. For example, you may want to use the file gfix.100 for a temperate grassland.

-pp path Specify a directory path for searching for parameter files (crop.100, etc.). This path is searched before the default path for the parameter files. The default path is a subdirectory of the installation path for the model. Parameter sets from this path are read first, then any sets in the default parameter file which have a different name are loaded next.

-mf name=value Modify a parameter or management value by a fractional value. The parameter specified is multiplied by this value.

-mf? View a list of names that can be specified in the -mf option.

-shp Calculate the soil hydrologic parameters at initialization. This includes wilting point, field capacity, and Ks. Calculations are based on soil texture, so sand, clay, and bulk density values per layer must be reasonable.

-chem filename Run DayCent5Chem model, where filename is the name of the geochemistry configuration file. If this option is not specified, DayCent5 (no geochemistry) is run.

-u Your name (in quotes if username contains spaces). This is written to the output files, and to the 'username&a output site file, if any. pos;

-i filename Specify the Century initialization file name and path.

-sf filename Write new site parameters file at end of simulation with this name. This file will be in NetCDF format, and will have " -ST.nc " added to filename.

-V Display software copyright and version information.

-d? Display debugging options. (See Debugging Command-Line Options below for the list.)

-? Displays this help screen.

Debug Command-Line Options -d1 Display total execution time.

-d2 Display time to the screen at each write to output.

-d3 Echo the command-line arguments.

-d4 Display simulation time limits.

-d5 Display verbose messages.

-da Turn on all the debugging options.

Example command line DayCent5Chem.exe -s HJA2.100 -m HJA.sch -o results/HJA2 -f HJA_fix.100 -pp . -ts -ar -td0 -d1 -d3 -chem HJA2_chem.ini Simulation Results These output files are generated by the Geochemistry submodel, in addition to the DayCent5 simulation output files. All files are text files. CSV files are in text format, and can be read in R (using "read.csv") or viewed in a spreadsheet program. For details on the DayCent5 simulation output files, see the Century5 and DayCent5 User's Guide. All output files except the PHREEQ log file are stored in the output directory. The PHREEQ log file is stored in the same directory from which the simulation is run.

Output Files In the following file names, your output file name root will be substituted for "SITE".

File Default File Name Description monthly fluxes SITE_chem_monthly.csv annual fluxes SITE_chem_annual.csv snow chemistry SITE_chem_snowComp.csv composition soil chemistry composition SITE_chem_soilComp1.csv file 1 soil chemistry composition SITE_chem_soilComp2.csv file 2 streamflow chemistry SITE_chem_streamFlow.csv solution simulation SITE_chem_summary.pqi Contains a record of all soil solution states during the summary simulation. This file is created if the configuration option saveSoilSolutions == 1. atmospheric deposition SITE_chem_totalDep.csv

Concatenated daily selected SITE_chem_summary.sel output geochemistry report 1 SITE_chem_report1.csv Contains monthly imports and exports from system, presented as a time-series of monthly tables. geochemistry report 2 SITE_chem_report2.csv Contains monthly imports and exports from system, in a time-series table format. final snow SITE_chem_finalSnow.csv Contains the chemical state of snow, same format as initSnow.csv, and can be used as input to a subsequent simulation. final soil SITE_chem_finalSoil.csv Contains the chemical state of soil, same format at initSoil.csv, and can be used as input to a subsequent simulation.

Examining Simulation Results

Comparing Two Sets of Simulation Results To compare DayCent5 (not geochemistry) simulation results from 2 different simulations, use the script Graph2Sims located in the source code directory

Ecosystem/Century5/scripts

Run the script inside of an R shell or session. For example, to compare the results from the test examples in the DayCent5Chem test directory:

cd Ecosystem/DayCent5Chem/tests/HJAndrews R --quiet # start an R session; the following are R commands source("../../../Century5/scripts/Graph2Sims.R") simName <- "HJA2" path1 <- "soil-MH/results-NoChem" path2 <- "soil-profile21/results-NoChem" Graph2Sims( simName, path1, simName, path2, "pdf" )

To compare DayCent5Chem soil geochemistry results from 2 different simulations use the script CompareSoilChem2Sims located in the source code directory

Ecosystem/DayCent5Chem/scripts

For example, to compare the soil chemistry results from the test examples in the DayCent5Chem test directory:

cd Ecosystem/DayCent5Chem/tests/HJAndrews R --quiet # start an R session; the following are R commands source("../../../Century5/scripts/CompareSoilChem2Sims.R") simName <- "HJA2" path1 <- "soil-MH/results" path2 <- "soil-profile21/results" CompareSoilChem2Sims( simName, path1, simName, path2, "pdf" ) Bibliography Appelo, C.A.J., Postma, D., 1993. Geochemistry, Groundwater and Pollution. A.A. Balkema, Rotterdam, p. 536.

Arthur, M.A., 1990. The Effects of Vegetation on Watershed Biogeochemistry at Loch Vale Watershed, Rocky Mountain National Park, Colorado. Ph.D. Dissertation, Cornell University, Ithaca, NY, 179 pp.

Bales, R.C., 1992. Snowmelt and the ionic pulse. In: The Encyclopedia of Earth Science, vol. 1. Academic Press, Orlando, FL, pp. 199–207

Campbell, D.H., Clow, D.W., Ingersoll, G.P., Mast, M.A., Spahr, N.E., Turk, J.T., 1995. Processes controlling the chemistry of two snowmelt-dominated streams in the Rocky Mountains. Water Resour. Res. 31, 2811–2822.

Century5 and DayCent5 User's Guide (web link).

Del Grosso, S.J., Parton, W.J., Mosier, A.R., Hartman, M.D., Brenner, J., Ojima, D.S., Schimel, D.S., 2001. Simulated interaction of carbon dynamics and nitrogen trace gas fluxes using the DAYCENT model. In: Shaffer, M.J., Ma, L., Hansen, S. (Eds.), Modeling Carbon and Nitrogen Dynamics for Soil Management. Lewis Publishers, Boca Raton, Florida, pp. 303–332.

Hartman, M.D., Baron, J.S., Lammers, R.B., Cline, D., Band, L.E., Liston, G.L., Tague, C., 1999. Simulations of snow distribution and hydrology in a mountain basin. Water Resour. Res. 35, 1587–1603.

Hartman, Melannie D., Jill S. Baron, Dennis S. Ojima, 2007. Application of a coupled ecosystem-chemical equilibrium model, DayCent-Chem, to stream and soil chemistry in a Rocky Mountain watershed. Eco. Mod. 200, 493-510.

Hilinski, T.E., 2001. Implementation of an Algorithm for a Layered Soil Submodel with Application to the CENTURY Model. (web link).

Kelly, R.H., Parton, W.J., Hartman, M.D., Stretch, L.K., Ojima, D.S., Schimel, D.S., 2000. Intra- and interannual variability of ecosystem processes in shortgrass steppe. J. Geophys. Res. 105 (D15), 20093–20100.

Lindsay, W.L., 2001. Chemical Equilibria in Soils. The Blackburn Press, Caldwell, NJ, p. 449.

Parkhurst, D.L., Appelo, C.A.J., 1999. User’s Guide to PHREEQC (Version 2)—A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations, U.S. Geological Survey Water- Resources Investigations Report 99-4259, Denver, CO, 312 pp.

Parton, W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Sci. Soc. Am. J. 51, 1173–1179.

Parton, W.J., Schimel, D.S., Ojima, D.S., Cole, C.V., 1994. A general model for soil organic matter dynamics: sensitivity to litter chemistry, texture and management. In: Bryant, R.B., Arnold, R.W. (Eds.), Quantitative Modeling of Soil Forming Processes. Soil Science Society of America, Madison, WI, pp. 147–167.

Parton, W.J., Hartman, M., Ojima, D., Schimel, D., 1998. DAYCENT and its land surface submodel: description and testing. Global Planet. Change 19, 35–48.

PHREEQC version 2 Manual (PDF, web link).

Williams, M.W., Caine, N., 2001. Hydrology and hydrochemistry. In: Bowman, W.D., Seastedt, T.R. (Eds.), Structure and Function of an Alpine Ecosystem: Niwot Ridge, Colorado. Oxford University Press, New York, NY, pp. 75–96. Williams, M.W., Melack, J.M., 1991. Solute chemistry of snowmelt and runoff in an alpine basin, Sierra Nevada. Water Resour. Res. 27, 1575–1588. Acknowledgments Funding for the initial production of the DayCent5Chem model was provided by the US Geological Survey Western Mountain Initiative.

Incorporation of the DayCent-Chem code into the latest source code of the IRC/DayCent5 model, and updating the PHREEQC model to the latest available of the version 2 series, was done by Tom Hilinski at the Natural Resource Ecology Laboratory, Colorado State University.

Funding for the original DayCent-Chem model was provided by Environmental Protection Agency Clean Air Markets Division, National Park Service Air Resources Division, and US Geological Survey Western Mountain Initiative. The model was developed with the assistance of Melannie Hartman, Cindy Keough, Lee Casuto, Tom Hilinski, Bill Parton, Dennis Ojima, Jill Baron. Copyright and Disclaimer

Disclaimer Neither the Natural Resource Ecology Laboratory (NREL) nor Colorado State University (CSU) nor any of their employees nor any of the participating organizations nor any of the funding agencies make any warranty or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference to any special commercial products, processes, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute or imply endorsement, recommendation, or favoring by the NREL or CSU. The views and opinions of the authors do not necessarily state or reflect those of NREL or CSU and shall not be used for advertising or product endorsement.