|Abstract or Summary
- Runoff from agricultural lands into Upper Klamath Basin rivers and lakes can
cause water quality problems affecting fish and wildlife. Excessive eutrophication
in Upper Klamath Lake is linked to high nutrient input (particularly phosphorus)
stemming from both lake sediments and watershed tributaries.
On a unit area load basis the Wood River Valley contributes a much greater load
of phosphorus to Upper Klamath Lake than other regions. The purpose of this
study was to measure the export of nutrients from flood irrigated cattle pasture in
the Wood River Valley to; 1) compare irrigation water quality with background
sources, 2) determine the sources and transport mechanisms of sediment and
nutrients on flood irrigated pastures and 3) consider opportunities for water
Subsurface and surface water quality samples and water flow measurements
were taken on two flood irrigated and grazed pastures. Limited samples were
also collected at two additional pasture sites and during a storm. Water samples
were analyzed for concentrations of sediment, dissolved organic carbon (DOC),
total dissolved nitrogen (TDN), total dissolved phosphorus (TDP),
orthophosphate (OP), ammonia (NH₄⁺-N) and nitrate (NO₃⁻-N). Nutrient and
sediment loads were calculated and flow weighted concentrations were
compared. A nutrient and sediment budget was estimated during two irrigations.
On a 70 ha study plot, the TDP and TDN concentrations were highest when
cattle were causing disturbance in actively flowing irrigation ditches which had
mean values of 0.50 mg TDP L⁻¹ and 2.55 mg TDN L⁻¹. When cattle were not
present the dissolved nutrient concentrations in tailwater ditches were lower with
mean values of 0.07 mg TDP L⁻¹ and 0.85 mg TDN L⁻¹. An irrigation headwater
canal had a seasonal flow weighted concentration (FWC) of 0.03 mg TDP L⁻¹ and
0.22 mg TDN L⁻¹ while seasonal FWC for tailwater ditches was 0.84 mg TDN L⁻¹
and 0.06 mg TDP L⁻¹.
On a 2 ha study plot, irrigation tailwater samples were collected directly from
pasture runoff. DOC, TDN, TDP and OP concentrations were 2.8, 6.3, 2.8 and
3.2 times greater, respectively, in tailwater than in headwater. A first flush was
evident as greatest dissolved nutrient concentrations occurred early in the
irrigation runoff period. The mean net export for two irrigations was 24 kg
sediment ha⁻¹, 7.4 kg DOC ha⁻¹, 0.43 kg TDN ha⁻¹ and net accumulation of 0.02
kg TDP ha⁻¹. TDN and DOC export progressively decreased during runoff but
showed significant net export throughout the entire irrigation. TDP was more
variable with an export rate of 0.2 kg TDP ha⁻¹ during the first flush followed by
insignificant export and accumulation later during the irrigation runoff period.
Most dissolved-N in surface water was in the organic form, NH₄⁺-N was low and
NO₃⁻-N was undetected. Sediment concentration in irrigation runoff did not follow
the first flush pattern and was dependent on original background levels or on
Shallow groundwater had low mean concentrations of TDP (<0.06 mg TDP L⁻¹)
and high mean concentrations of dissolved-N (0.65 mg TDN L⁻¹ and 0.13 mg
NH₄⁺-N L⁻¹). Stormwater runoff flow volume was low from the 2 ha pasture and
contained low sediment and high dissolved nutrient concentrations.
Cattle disturbance to canals and nutrient flushing during irrigation were important
transport mechanisms for all nutrients. Management considerations for decreasing nutrient export to sensitive waterways in the Wood River Valley
include reducing irrigation surface runoff, enhancement of riparian wetlands and
vegetation and limiting and excluding livestock access to waterways.
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