Model Assessment of Cattle and Climate Impacts on Stream Fecal Coliform Pollution in the Salmon River Watershed, British Columbia, Canada

Abstract

A bacterial water quality model (BWQM) was developed and used to evaluate the impacts of cattle farming and climate change on the stream fecal coliform pollution in the Salmon River watershed in south-central British Columbia, Canada. The accuracy of the model simulation was evaluated using the Nash-Sutcliffe coefficient of efficiency (COE). The BWQM simulated the observed field data well, with the values of the COE ranging from 0.76 to 0.78 for the stream flow, from 0.55 to 0.60 for the fecal coliform (FC) concentration, and from 0.85 to 0.89 for the FC loading. The BWQM captured more than 79%, 66%, and 90% variation of the daily stream flow, FC concentration, and FC loading, respectively. The BWQM predicts that between 70% and 80% of the FC were transferred from the cattle farm to the Salmon River through the snowmelt-caused surface runoff during late winter and early spring, with the balance 20% to 30% coming from the soil-lateral flow and the groundwater return flow. The model also indicates that the stream FC concentration is sensitive to the distance of the cattle farm to the Salmon River. The model scenario analysis reveals that the climate change, at an assumed 1°C increment of daily air temperature, results in an increase in the stream FC concentration in the spring, fall, and winter, but there is also a decrease in the summer. The increased air temperature also changes the seasonal pattern of the stream FC concentration. Rainfall can reduce the stream FC concentration and mitigate the impact of the increased air temperature on the stream FC concentration as long as it does not result in a surface runoff or flooding event.

Appendix: Main symbols and definitions

B _ll :

Bacteria number in litter layer (CFU)

B _ms :

Capacity of riparian strip soil to adsorb bacteria (CFU)

B _mv :

Capacity of riparian strip vegetation to adsorb bacteria (CFU)

B _ro :

Bacteria in runoff water (CFU)

B _rss :

Bacteria adsorbed on riparian strip soil (CFU)

B _rsv :

Bacteria adsorbed on riparian strip vegetation (CFU)

B _s :

Soil bacteria number (CFU)

B _sr :

Rate of bacteria released by riparian strip soil (CFU d⁻¹)

B _ss :

Subsoil bacteria number (CFU)

B _st :

Bacteria in animal feces on soil top (CFU)

B _sw :

Stream water bacteria number (CFU)

B _va :

Rate of bacteria adsorbed by riparian strip vegetation (CFU d⁻¹)

B _vr :

Rate of bacteria released by riparian strip vegetation (CFU d⁻¹)

C :

 

C _s :

Soil clay percent content (%)

C _v :

Vegetation canopy closure representing the percentage of shaded area on a site (%)

D :

 

D _fs :

Distance from a livestock farm to a stream (m)

D _ll :

Bacteria die-off rate in litter later (CFU d⁻¹)

D _s :

Soil bacteria die-off rate (CFU d⁻¹)

D _ss :

Subsoil bacteria die-off rate (CFU d⁻¹)

D _st :

Bacteria die-off rate in animal feces on soil top (CFU d⁻¹)

D _sw :

Stream water bacteria die-off rate (CFU d⁻¹)

E :

 

E _i :

Livestock bacteria excretion rate at an animal farm i (CFU d⁻¹)

E _t :

Total livestock bacteria excretion rate in a watershed (CFU d⁻¹)

E _sb :

Bacteria entrainment rate from stream bottom (CFU d⁻¹)

E _wl :

Wildlife bacteria excretion rate in a watershed (CFU d⁻¹)

F :

 

F :

Stream flow rate (m³ s⁻¹)

F ₀ :

Threshold stream flow rate (m³ s⁻¹)

L :

 

L _sl :

Stream length (km)

M :

 

M _rss :

Riparian strip soil mass at depth of subsoil (t m⁻²)

M _rsv :

Riparian strip vegetation biomass (kg m⁻²)

N :

 

N _ij :

Livestock species (j) number at farm i (head)

P :

 

P _sb :

Bacteria precipitation rate on stream bottom (CFU d⁻¹)

S :

 

S _rs :

Riparian strip area (m²)

T :

 

T :

Temperature (°C)

T _gwr :

Rate of bacteria transferred from ground water to a stream (CFU d⁻¹)

T _in :

Rate of bacteria transferred to litter layer through infiltration (CFU d⁻¹)

T _lllf :

Rate of bacteria transferred to a stream through litter layer lateral flow (CFU d⁻¹)

T _llp :

Rate of bacteria transferred to soil through litter layer percolation (CFU d⁻¹)

T _max :

Maximum temperature for bacteria survival (°C)

T _min :

Minimum temperature for bacteria survival (°C)

T _opt :

Optimal temperature at which bacteria have the highest survival rate (°C)

T _ro :

Rate of bacteria transferred to a stream through runoff (CFU d⁻¹)

T _slf :

Rate of bacteria transferred to a stream through soil lateral flow (CFU d⁻¹)

T _sp :

Rate of bacteria transferred subsoil through soil percolation (CFU d⁻¹)

T _sslf :

Rate of bacteria transferred to a stream through subsoil lateral flow (CFU d⁻¹)

T _ssp :

Rate of bacteria transferred to groundwater through subsoil percolation (CFU d⁻¹)

W :

 

W _gwr :

Groundwater return flow rate (mm d⁻¹)

W _gw :

Depth of the saturated unconfined aquifer zone (m)

W _in :

Water infiltration rate (mm d⁻¹)

W _ll :

Litter layer water content (mm)

W _lllf :

Litter layer lateral flow rate (mm d⁻¹)

W _llp :

Litter layer percolation rate (mm d⁻¹)

W _ro :

Runoff rate (mm d⁻¹)

W _s :

Soil water content (mm)

W _slf :

Soil lateral flow rate (mm d⁻¹)

W _sp :

Soil percolation rate (mm d⁻¹)

W _ss :

Subsoil water content (mm)

W _sslf :

Subsoil lateral flow rate (mm d⁻¹)

W _ssp :

Subsoil percolation rate (mm d⁻¹)

W _st :

Soil surface water from precipitation and snowmelt (mm)