Twin Watersheds Summer Rains

Only one rainfall large enough to produce significant runoff from the Twin Watersheds occurred annually during the 1994 – 1997 sampling period. The number of samples and time of sampling during these events varied, with some events providing more in-depth information than others. The July 2,1997 event provides a good example of sample collection across the hydrograph.

Rainfall runoff event differences in the peak concentrations of nutrients and sediments between the east and west watersheds varied from year to year.

Rainfall Runoff Nutrient and Sediment Data at the Twin Watersheds

In contrast to snowmelt runoff, the rainfall runoff contribution of particulate nutrients constitutes a major portion of the total nutrient runoff and in many cases it exceeds the dissolved contribution. This was particularly evident with phosphorus and carbon when sediment concentration levels reached 300 mg/L.

When the 1994 -1997 nutrient and sediment concentrations and flow rate results were evaluated for a relationship, none were evident at either of the Twin Watershed sites during these events, this is in contrast to the HWY 240 and Miami main channel sites.

(Phos Concentration-V-Flow)

NFR Concentration - V-Notch

When comparing yearly nutrient export coefficients (kg/ha/yr) the two watersheds generally follow a similar trend. The exception to this would be 1996, where unusual flow rates were recorded during the year and need to be taken into consideration.

Average (1993 – 1997) Annual Export Coefficient – Twin Watersheds

(Export coefficient = kg/ha/yr.)

Twin Watersheds Comments

Nitrogen

• Spring runoff contributed most of the annual nitrogen load in 1993 and 1995 from both watersheds
• In 1994, more than 70% of the annual nitrogen load occurred during the October rainfall runoff event.
• In 1996 the majority of annual nitrogen load for the east watershed was contributed by rainfall runoff, while spring runoff contributed more in the west watershed.
• In 1997, the first year of no-till, the July rainfall event contributed over 60% of the nitrogen from the east watershed, but only 30% from the west. This may be partially explained by different spring nitrogen application methods, west was spring banded and the east was spring surface broadcast with no-tillage.

Phosphorus

• In 1993 and 1997, spring runoff contributed most of the annual phosphorus load from both watersheds.
• In 1994 and 1995, spring runoff contributed equally to the annual phosphorus load.
• In 1996, spring and rainfall runoff contributed roughly equally to the annual phosphorus load in the west watershed, however the April rainfall runoff event produced the majority of the annual phosphorus load in the east watershed.

Organic Carbon

• Organic carbon loads were heaviest during rainfall runoff events in October 1994 and April 1996.
• In 1995 and 1997, spring snowmelt and rainfall runoff contributed equally to annual organic carbon loading.

Sediment

• From 1994 to 1997, rainfall runoff events contributed the overwhelming majority of sediment loading.
• In 1993, spring snowmelt runoff contributed more to annual sediment loading than rainfall runoff.