Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System

Abstract

Non-point source (NPS) runoff of pollutants is viewed as one of the most important factors causing impaired water quality in freshwater and estuarine ecosystems and has been addressed as a national priority since the passage of the Clean Water Act. To control NPS pollution, state and federal agencies developed a variety of programs that rely heavily on the use of watershed management in minimizing riverine and receiving water pollution. Watershed models have become critical tools in support of watershed management. Lumped, empirical models such as HSPF do not account for spatial heterogeneity within subwatersheds and the simulations of the actual processes are greatly simplified. This chapter describes a distributed water flow, sediment and nutrient dynamic modeling system developed at U.S. Army Engineer Research and Development Center. The model simulates detailed water flow, soil erosion, nitrogen (N) and phosphorus (P) cycling at the watershed scale and computes sediment transport across the landscape, nutrient kinetic fluxes for N and P species. The model consists of three distinct parts: (1) watershed hydrology, (2) soil erosion and sediment transport, and (3) nitrogen and phosphorus transport and cycling. The integrated watershed model was tested and validated on two watersheds in Wisconsin (French Run and Upper Eau Galle Watersheds). The model performed well in predicting runoff, sediment, nitrogen and phosphorus. This chapter presents the model development and validation studies currently underway in Wisconsin.

Appendix: Water Quality Parameters

orgN _frs :

concentration of soil layer fresh organic N pool [M/L³]

ON _min|imb :

net mineralization/immobilization rate of soil layer fresh organic N pool [M/L²/T]

ON _dec :

decomposition rate of soil layer fresh organic N pool [M/L²/T]

ON _frs,e :

net surface erosion/deposition rate of soil layer fresh organic N pool [M/L²/T]

ON _frs,s :

external sources [M/L²/T]

$orgN_{act}$ :

concentration of soil layer active organic N pool [M/L³]

ON _trn :

rate transferred between the active and stable organic N pools [M/L²/T]

ON _min :

mineralization rate of soil layer active organic N pool [M/L²/T]

ON _act,e :

net surface erosion/deposition rate of soil layer active organic N pool [M/L²/T]

ON _act,s :

external sources [M/L²/T]

ON _sta,e :

net surface erosion/deposition rate of soil layer stable organic N pool [M/L²/T]

ON _sta,s :

external sources added to the soil layer stable organic N pool [M/L²/T]

$NH_{4}^{ + }$ :

concentration of soil layer $NH_{4}^{ + }$ pool [M/L³]

NH _min :

total mineralization processes rate of soil layer organic N pools [M/L²/T]

NH _nit|vol :

net nitrification/volatilization processes rate in the soil layer [M/L²/T]

NH _up :

plant uptake rate of soil layer $NH_{4}^{ + }$ pool [M/L²/T]

R _NH4,e :

mass transfer rate of $NH_{4}^{ + }$ between the upper soil layer and surface runoff [M/L²/T]

NH _s :

external sources [M/L²/T]

$NO_{3}^{ - }$ :

concentration of soil layer $NO_{3}^{ - }$ pool [M/L³]

NO _dnit :

denitrification processes rate in the soil layer [M/L²/T]

NO _up :

plant uptake rate of soil layer $NO_{3}^{ - }$ pool [M/L²/T]

R _NO3,e :

mass transfer rate of $NO_{3}^{ - }$ between the upper soil layer and surface runoff [M/L²/T]

R _NO3,f :

infiltration rate of soil layer $NO_{3}^{ - }$ pool [M/L²/T]

NO _s :

external sources [M/L²/T]

PON _ov :

concentration of the overland flow PON [M/L³]

orgN :

total concentration of organic N in the upper soil layer [M/L³]

k _hn :

PON hydrolysis rate constant [1/T]

DON _ov :

concentration of DON in the overland flow [M/L³]

k _mn :

DON mineralization rate constant [1/T]

$NH_{4\;ov}^{ + }$ :

concentration of $NH_{4}^{ + }$ in the overland flow [M/L³]

k _en :

effective mass transfer rate constant [L/T]

k _nit :

nitrification rate constant [1/T]

R _NH4,up :

plant uptake rate of the overland flow $NH_{4}^{ + }$ [M/L³/T]

$NO_{3\;ov}^{ - }$ :

concentration of $NO_{3}^{ - }$ in the overland flow [M/L³]

R _NO3,up :

plant uptake rate of the overland flow $NO_{3}^{ - }$ [M/L³/T]

orgP _frs :

concentration of soil layer fresh organic P pool [M/L³]

OP _dec :

decomposition rate of soil layer fresh organic P pool [M/L²/T]

OP _min|imb :

net mineralization/immobilization rate of soil layer fresh organic P pool [M/L²/T]

OP _frs,e :

net surface erosion/deposition rate of soil fresh organic P pool [M/L²/T]

OP _frs,s :

external sources [M/L²/T]

orgP _act :

concentration of soil layer active organic P pool [M/L³]

OP _min :

mineralization rate of soil humic active organic P pool [M/L²/T]

OP _trn :

rate transferred between the active and stable organic P pools [M/L²/T]

OP _act,e :

net surface erosion/deposition rate of soil humic active organic P pool [M/L²/T]

OP _act,s :

external sources [M/L²/T]

orgP _sta :

concentration of soil layer stable organic P pool [M/L³]

OP _sta,e :

net surface erosion/deposition rate of soil humic stable organic P pool [M/L²/T]

OP _sta,s :

external sources [M/L²/T]

P _sol :

concentration of soil layer soluble P pool [M/L³]

IP _min :

total mineralization processes rate of soil layer organic P pools [M/L²/T]

IP _sol|act :

net sorption rate transferred between the soluble P pool and active inorganic P pool [M/L²/T]

IP _up :

plant uptake rate of soil layer soluble P pool [M/L²/T]

R _DIP,e :

mass transfer rate of soluble P between the upper soil layer and surface runoff [M/L²/T]

IP _s :

external sources [M/L²/T]

minP _act :

concentration of soil layer active inorganic P pool [M/L³]

IP _act|sta :

net slow sorption transfer rate between the active inorganic P pool and the stable inorganic P pool [M/L²/T]

IP _act,e :

surface erosion/deposition rate of soil active inorganic P detachment [M/L²/T]

IP _act,s :

external sources [M/L²/T]

minP _sta :

concentration of soil layer stable inorganic P [M/L³]

IP _sta,e :

surface erosion/deposition rate of soil stable inorganic P detachment [M/L²/T]

IP _sta,s :

external sources [M/L²/T]

POP _ov :

concentration of POP in the overland flow [M/L³]

orgP :

total concentration of organic P in the upper soil layer [M/L³]

k _hp :

POP hydrolysis rate constant [1/T]

DOP _ov :

concentration of DOP in the overland flow [M/L³]

k _mp :

DOP mineralization rate constant [1/T]

DIP _ov :

concentration of DIP in the overland flow [M/L³]

k _ep :

DIP mass transfer rate between the upper soil layer and overland flow [L/T]

R _DIP,up :

plant uptake rate of the overland flow DIP [M/L³/T]

PON _ch :

concentration of in-stream PON [M/L³]

k _dp :

temperature-dependent phytoplankton death rate [T⁻¹]

k _db :

temperature-dependent bottom algae death rate [T⁻¹]

A _p :

stream phytoplankton concentration [M/L³]

A _b :

stream bottom algae concentration [M/L²]

k _hn :

temperature-dependent PON hydrolysis rate coefficient [T⁻¹]

DON _ch :

concentration of in-stream DON [M/L³]

k _mn :

temperature-dependent DON mineralization rate coefficient [T⁻¹]

F _oxmn :

DON mineralization attenuation due to low oxygen

$TNH_{4\;ch}^{ + }$ :

Total concentration of in-stream $NH_{4}^{ + }$ [M/L³]

k _rp :

temperature-dependent phytoplankton respiration rate [T⁻¹]

F _oxna :

nitrification attenuation due to low oxygen

k _nit :

temperature-dependent $NH_{4}^{ + }$ nitrification rate coefficient [T⁻¹]

P _ap :

preference coefficient of phytoplankton for $NH_{4}^{ + }$

P _ab :

preference coefficient of bottom algae for $NH_{4}^{ + }$

$NO_{3\;ch}^{ - }$ :

concentration of in-stream $NO_{3}^{ - }$ [M/L³]

K _scdn :

DOC half-saturation constant for denitrification [gC/m³] [M/L³]

k _dnit :

temperature-dependent $NO_{3}^{ - }$ denitrification rate coefficient [T⁻¹]

F _oxdn :

effect of low oxygen on denitrification

POP _ch :

concentration of in-stream POP [M/L³]

DOP _ch :

concentration of in-stream DOP [M/L³]

TIP _ch :

total concentration of in-stream inorganic P [M/L³]

k _hp :

temperature-dependent POP hydrolysis rate coefficient [T⁻¹]

k _mp :

temperature-dependent DOP mineralization rate coefficient [T⁻¹]

F _oxmp :

DOP mineralization attenuation due to low oxygen

POC _ch :

concentration of in-stream POC [M/L³]

DOC _ch :

concentration of in-stream DOC [M/L³]

DIC _ch :

concentration of in-stream DIC (mole/L) [M/L³]

k _hc :

temperature-dependent POC hydrolysis rate coefficient [T⁻¹]

F _oxmc :

DOC mineralization attenuation due to low oxygen

k _mc :

temperature-dependent DOC mineralization rate [T⁻¹]

k _ac :

0.923 k _a  = temperature-dependent CO₂ deaeration coefficient [T⁻¹]

k _H :

Henry’s constant [mole/(L atm)]

p _CO2 :

partial pressure of carbon dioxide in the atmosphere [atm]

α₀ :

fraction of total inorganic carbon in carbon dioxide

DO _ch :

concentration of in-stream DO [M/L³]

k _a :

temperature-dependent oxygen reaeration coefficient [T⁻¹]

DO _s :

saturation concentration of oxygen [mgO₂/L]

S _SOD :

sediment oxygen demand rate [M/L³]

A _b :

stream bottom algal concentration [M/L²]

$\mu_{b}$ :

benthic algal photosynthesis rate [T⁻¹]

F _oxb :

attenuation due to low oxygen

k _rb :

temperature-dependent benthic algal respiration rate [T⁻¹]

k _db :

temperature-dependent benthic algal death rate [T⁻¹]