Concentrations and Loads of Suspended Sediment and Nutrients in Surface Water of the Yakima River Basin, Washington, 1999-2000--With an Analysis of Trends in Concentrations
By James C. Ebbert, Sandra S. Embrey, and Janet A. Kelley
|Water-Resources Investigations Report 03-4026 |
|USGS National Water-Quality Assessment Program|
Spatial and temporal variations in concentrations and loads of suspended sediment and nutrients in surface water of the Yakima River Basin were assessed using data collected during 1999–2000 as part of the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program. Samples were collected at 34 sites located throughout the Basin in August 1999 using a Lagrangian sampling design, and also were collected weekly and monthly from May 1999 through January 2000 at three of the sites. Nutrient and sediment data collected at various time intervals from 1973 through 2001 by the USGS, Bureau of Reclamation, Washington State Department of Ecology, and Roza-Sunnyside Board of Joint Control were used to assess trends in concentrations.
During irrigation season (mid-March to mid-October), concentrations of suspended sediment and nutrients in the Yakima River increase as relatively pristine water from the forested headwaters moves downstream and mixes with discharges from streams, agricultural drains, and wastewater treatment plants. Concentrations of nutrients also depend partly on the proportions of mixing between river water and discharges: in years of ample water supply in headwater reservoirs, more water is released during irrigation season and there is more dilution of nutrients discharged to the river downstream. For example, streamflow from river mile (RM) 103.7 to RM 72 in August 1999 exceeded streamflow in July 1988 by a factor of almost 2.5, but loads of total nitrogen and phosphorus discharged to the reach from streams, drains, and wastewater treatment plants were only 1.2 and 1.1 times larger.
In years of ample water supply, canal water, which is diverted from either the Yakima or Naches River, makes up more of the flow in drains and streams carrying agricultural return flows. The canal water dilutes nutrients (especially nitrate) transported to the drains and streams in runoff from fields and in discharges from subsurface field drains and the shallow ground-water system. The average concentration of total nitrogen in drains and streams discharging to the Yakima River from RM 103.7 to RM 72 in August 1999 was 2.63 mg/L, and in July 1988 was 3.16 mg/L; average concentrations of total phosphorus were 0.20 and 0.26 mg/L.
After irrigation season, streamflow in agricultural drains decreases because irrigation water is no longer diverted from the Yakima and Naches Rivers. As a result, concentrations of total nitrogen in drains increase because nitrate, which constitutes much of total nitrogen, continues to enter the drains from subsurface drains and shallow ground water. Concentrations of total phosphorus and suspended sediment often decrease, because they are transported to the drains in runoff of irrigation water from fields. In Granger Drain, concentrations of total nitrogen ranged from 2-4 mg/L during irrigation season and increased to about 6 mg/L after irrigation season, and concentrations of total phosphorus, as high as 1 mg/L, decreased to about 0.2 mg/L.
In calendar year 1999, Moxee Drain transported an average of 28,000 lb/d (pounds per day) of suspended sediment, 380 lb/d of total nitrogen, and 46 lb/d of total phosphorus to the Yakima River. These loads were about half the average loads transported by Granger Drain during the same period. Average streamflows were similar for the two drains, so the difference in loads was due to differences in constituent concentrations: those in Moxee Drain were about 40-60 percent less than those in Granger Drain.
Loads of suspended sediment and total phosphorus in Moxee and Granger Drains were nearly four times higher during irrigation season than during the non-irrigation season because with increased flow during irrigation season, concentrations of suspended sediment and total phosphorus are usually higher. Loads of nitrate in the drains were about the same in both seasons because nitrate concentrations are higher during the non-irrigation season.
Loads of nutrients in the Yakima River at Kiona were similar during irrigation and non-irrigation season, generally differing by less than 20 percent. This is because average streamflows and concentrations of nutrients were similar between the two seasons. Average streamflow during the irrigation season was within about 6 percent of the average streamflow during the non-irrigation season and average nutrient concentrations differed by less than 13 percent. Loads of suspended sediment were about 60 percent larger during irrigation season. Large fractions of the total annual loads were transported during the first half of the irrigation season. For example, 38 percent or more of the annual loads of suspended sediment, total nitrogen, ammonia-plus-organic nitrogen, total phosphorus, and orthophosphate were transported from March through June.
Departure from relations between average daily loads and annual mean streamflow suggests some reduction in loads of suspended sediment, ammonia-plus-organic nitrogen, and total nitrogen in the Yakima River at Kiona in water years 1999 and 2000. The relation between loads and streamflow was established using loads previously estimated for water years 1974, 1977, 1980, 1988, and 1989 and loads estimated by this study for other water years from 1974 through 1994.
The summary of trend statistics for concentration data collected from 1991–2000 indicates that some of the factors affecting constituent transport to surface water may be common to many agricultural subbasins in the Yakima Basin. For example, concentrations of nitrate and total nitrogen increased in five of six streams and drains while concentrations of suspended solids and total phosphorus either did not change or decreased. This difference is consistent with no change, or a decrease, in amounts of suspended solids and phosphorus transported to surface water in runoff and an increase in the amount of nitrate transported to streams and drains from ground-water discharges and subsurface drains. Decreased transport of sediment and associated phosphorus to streams and drains likely results from increased use of agricultural best management practices that reduce runoff from cropland. Nitrate is less affected because much of it enters from subsurface drains and shallow ground water. Increasing concentrations of nitrate in some drains over the period 1991–2000 suggest that concentrations of nitrate in ground water are increasing.
Data collected from 1997–2001 also indicate a decrease in concentrations in suspended solids and total phosphorus in some drains and streams receiving agricultural return flows. Turbidity has decreased in drains where concentrations of suspended solids have decreased. The data also indicate that concentrations of nitrate in some streams and drains have leveled off and may be decreasing. This observation does not conflict with the trend statistics for 1991–2000, but suggests that a slow increase over a decade may be leveling off.
Trend statistics computed using both unadjusted and flow-adjusted data indicate small, but significant, increases in turbidity and concentrations of total phosphorus in the Yakima River at Kiona from 1991 through 2000. There was also an increase in flow-adjusted concentrations of nitrate, which is consistent with increasing concentrations of nitrate in some streams and drains flowing to the Yakima River. Although nitrogen discharged from streams and drains is a large fraction of the nitrogen load in the river, a relation between increasing concentrations of nitrate in streams and drains and increasing concentrations in the Yakima River at Kiona was not established. The trend test using flow-adjusted concentrations showed no trend in the concentration of suspended solids. This seems reasonable because concentrations of suspended solids correlated with streamflow, and higher values of both were observed after 1995.
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