Watershed-level Comparison of Predictability and Sensitivity of Two

Phosphorus Models

Sumit Sen IIT Roorkee

Puneet Srivastava Latif Kalin

Auburn University, AL Peter A. Vadas

USDA-ARS Dairy Forage Research Center, Madison, WI

Justification Poultry production is one of the

top industry in Alabama excessive land application of litter

on pastures contamination of water bodies

that support aquatic and human uses

Hydrologic and water quality models to simulate P loss from agricultural fields

Justification Models, such as SWAT,

AGNPS, EPIC, etc P sub-modules developed for

manure applications when manure is incorporated into soils

Is the use of these models scientifically or conceptually correct where manure is surface-applied and left unincorporated?

P Models Two P models Soil and Water Assessment Tool (SWAT) model: a

physically-based, continuous, GIS linked watershed model developed by USDA-ARS Simulates hydrology, sediment and nutrients (N and P)

SurPhos model, state-of-the-Art Manure P model: developed by Vadas et al. (2007) Simulate dissolved inorganic and organic P release from

surface-applied, unincorporated manures to soil or runoff

Objective To compare the predictability and sensitivity of

two P loss models (SWAT and SurPhos) for dissolved P transport from a poultry litter-applied pasture watershed

SWAT Soil P pools

SurPhos model Manure and Soil P Pools M

anur

e P

Pool

s So

il P

Pool

s

SWAT P vs SurPhos model SWAT: organic (and inorganic) P from surface

applied manure becomes part of six soil P pools in top 10 mm soil layer

SurPhos model: Manure P applied becomes one layer above soil surface, having four P pools 80% of manure P leached into soil remains in top 10 mm

SWAT: no dissolved organic P estimation, only dissolved inorganic P

SurPhos model: estimates both forms of P in runoff from manure and soil

Study Site A 336.7 ha agricultural

watershed, Rice Branch, in Randolph County, AL

Watershed was delineated into 7 subbasins, 268 HRUs landuse/soil/slopes

Study Site The watershed is 68%

hay/pasture and 31% forested

SWAT model was calibrated on daily time step for surface runoff, base flow, and stream flows for 2004 and 2005

Methods Calibrated model was run

for different poultry litter application rates: 1121 kg ha-1 (0.5 t ac-1),

2242 kg ha-1 (1 t ac-1), 3363 kg ha-1 (1.5 t ac-1), 4484 kg ha-1 (2 t ac-1),and 6725 kg ha-1 (3 t ac-1)

Methods To study the effect of initial soil solution P Three different initial solution P levels: 0.5 mg/kg

P, 5.0 mg/kg P, and 50.0 mg/kg P In addition, to focus on the effect of poultry litter

application rates on soil P solution pool, the SWAT model was used to estimate the soil P solution values in different soil layers

Methods Surface runoff generated at all HRUs was used in

SurPhos model Simulated dissolved inorganic P in runoff at

different poultry litter application rates using SurPhos model was compared with SWAT output

Results & Discussion Hydrology calibration was achieved Surface runoff (Nash-Sutcliffe) = 0.72

Results & Discussion SWAT simulated higher dissolved P at different

application rate, except 0.5 mg/kg SLP

SWAT SurPhos App. Rate

(kg.ha-1)

TDP (kg)

TDP (kg.ha-1.yr-1)

Increase (kg.ha-1.yr-1)

TDP (kg)

TDP (kg.ha-1.yr-1)

Increase (kg.ha-1.yr-1)

0.5 mg.kg-1 initial soil Solution P 6725 927 0.68 0.29 1857 1.35 0.65 4484 527 0.38 0.12 960 0.70 0.27 3363 359 0.26 0.10 589 0.43 0.20 2242 217 0.16 0.08 312 0.23 0.18 1121 109 0.08 73 0.05 0.05

0 55 Average 0.12 0 5.0 mg.kg-1 initial soil Solution P

6725 1595 1.16 0.33 1847 1.34 0.64 4484 1146 0.83 0.16 967 0.70 0.27 3363 919 0.67 0.17 591 0.43 0.20 2242 680 0.50 0.19 313 0.23 0.18 1121 431 0.31 74 0.05 0.05

0 151 Average 0.17 0 50.0 mg.kg-1 initial soil Solution P

6725 1800 1.31 0.32 2179 1.57 0.71

4484 1355 0.99 0.16 1203 0.86 0.31

3363 1128 0.82 0.18 771 0.55 0.19

2242 892 0.65 0.18 379 0.26 0.20

1121 646 0.47 105 0.06 0.05

0 364 Average 0.17 22 0.01

Results & Discussion Total dissolved P export were linear for SWAT

and nonlinear for SurPhos

0.50 t.ac-1 1.00 t.ac-1 1.50 t.ac-1 2.00 t.ac-1 3.00 t.ac-1

Year Precip. (mm)

SWAT SUR-PHOS

SWAT SUR-PHOS

SWAT SUR-PHOS

SWAT SUR-PHOS

SWAT SUR-PHOS

2003 1036.20 3.04 1.92 3.46 4.84 3.89 8.38 4.30 12.02 5.13 18.81

2004 1275.60 6.09 2.17 7.97 8.04 9.87 12.55 11.56 16.52 15.14 23.91

2005 1322.30 7.34 0.80 10.21 4.58 12.95 9.71 15.44 14.79 20.45 25.33

2006 1174.70 7.47 0.41 10.41 0.88 13.32 3.25 16.08 6.08 21.46 13.03

2007 565.60 1.03 0.00 1.46 0.01 1.87 0.29 2.28 0.67 2.99 1.65

2008 1250.20 8.68 0.28 12.86 1.99 16.87 6.85 20.76 13.45 28.31 29.89

Total 33.65 5.59 46.37 20.35 58.76 41.03 70.42 63.54 93.49 112.63

Simulated annual TDP exports (kg) from HRU #101 by the SWAT and SurPhos models for different poultry litter application rates at 50 mg.kg-1 initial soil Solution P level

Annual TDP exports (kg): HRU #101

Soil Solution P Analysis: Accumulation of P in Top Soil Layer

Soil Solution P Analysis: Accumulation of P in Second Soil Layer

Conclusions Both models were sensitive to different application

rates SWAT model simulated approx. 0.12 kg/ha/yr (at 0.5

mg/kg SLP) and 0.17 kg/ha/yr (at 5 and 50 mg/kg SLP) increase in dissolved P for every 1121 kg/ha at application rate

SurPhos model simulated an increase between 0.05 and 0.71 kg/ha/yr for every 1121 kg/ha

SurPhos model was not found to be sensitive to initial soil Solution P levels as compared to SWAT

Conclusions At HRU level showed that SurPhos model does

not accumulate soil Solution P at low poultry litter application rates

On the contrary, SWAT model tends to simulate excessive accumulation of soil Solution P in the top 10 mm soil layer from long-term poultry little application

Conclusions Overall, the study suggests that the SurPhos model

is more reflective of the observed TDP export and soil Solution P accumulation mechanisms

Acknowledgment Forestry Department, Auburn University

Limitations Though SurPhos model showed higher

sensitivity and P reduction at different application rates: Developed and verified using plot scale data Field & watershed scale testing has not been done No hydrology simulation No sediment-bound P estimation

SWAT P model incorporates applied manure in soil P pools