Indicator
Nitrogen Load (Tributaries)
2015 Evaluation
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2015 Evaluation
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Status
Insufficient Data to Determine Status or No Target Established
Trend
Little or No Change
Confidence
Moderate
Evaluation Map
Nitrogen Tributaries Map.JPG
Description

The seven streams routinely monitored for total nitrogen load includes two streams in Nevada: (1) Third Creek, and (2) Incline Creek; and five streams in California: (3) Trout Creek, (4) Upper Truckee River, (5) General Creek, (6) Blackwood Creek, and (7) Ward Creek

Rationale Details
No target established. Established numerical standards (targets) for dissolved and total nitrogen load have been identified for the pelagic and littoral zones of Lake Tahoe. However, the comparative basis for these standards (i.e., average loads from the 1973 to 1981 period) from the various sources identified above are not available; thus, no determination of status can be rendered, and status is unknown. The annual total nitrogen load is strongly influenced by the volume of total annual runoff, perhaps more than the other constituents (i.e., suspended sediment and total phosphorus). The combined total annual total nitrogen load for the seven monitored streams averaged (1989 to 2014) 38,556 kilograms/year, with a median value of 35,051 kilograms/year, and a range from 12,268 kilograms/year in 1994 to 89,030 kilograms/year in 1995. Note that 1995 had both the highest total annual total nitrogen load, and the highest total annual runoff in the data record (1989 to 2014).

Blackwood Creek and the Upper Truckee River continue to be the most important source of total nitrogen entering the lake. Blackwood Creek has the greatest yield per unit area. The Upper Truckee River had a lower total nitrogen yield per unit area, but its large size makes it the greatest contributor of total nitrogen load. Trout Creek basin has a much lower yield than the other monitored watersheds, due perhaps to the relatively gentle slope of the extensive flood plain in the lower part of its basin.
Little or no change. No significant trend for change in total nitrogen load over time was detected. Highly significant downward trends (P < 0.003) in nitrate (NO3-N) loads were found for all seven streams with Alley's Adjusted Variable Kendall Test (Alley, 1988; Helsel and Hirsch, 2002).

In order to test for trends in total nitrogen, we added the nitrate (NO3-N) loads and total Kjeldahl nitrogen loads for each of the seven streams by year for 1989 to 2014. With total Kjeldahl nitrogen included, only Third Creek showed a significant downward trend (a 51.1 percent reduction in load over 24 years; less than 0.002). And with total nitrogen loads and flows aggregated across watersheds, no time trend was detected (see Figure 1). The apparent lack of a trend in total Kjeldahl nitrogen masks the basin-wide downward trend in nitrate-N.

Highly significant downward trends (P < 0.003) in nitrate (NO3-N) loads were found for all seven streams with Alley's Adjusted Variable Kendall Test (Alley, 1988; Helsel and Hirsch, 2002). Table 2 shows the percent changes in regression-estimated nitrate (NO3-N) loads for given annual hydrologic conditions. For example, at a given total annual and maximum annual discharge in Blackwood Creek, the estimated total annual nitrate (NO3-N) load in 2014 was (on average) 55.8 percent less than it would have been for the same hydrologic conditions (had they occurred) in 1975. Note that the annual nitrate (NO3-N) loads were estimated with the period-weighted sample method, and the residuals for trend analysis were taken from the regression of total annual load versus total annual and maximum annual daily discharge.
Confidence Details
Low. Where insufficient data exists to determine status, confidence in the status determination is low. The confidence in an estimate of total nitrogen annual load depends on the number of samples, and on the variance of the daily loads that are sampled to derive the annual load. (Coats and Lewis, 2014b, 2014a) presented tables that can be used to estimate the confidence intervals for average conditions in Tahoe Basin streams, for total nitrogen. Currently the LTIMP stream sampling program collects about 25 to 30 samples per year at each station. With the method used here to estimate total nitrogen loads based on 28 samples per year, we can be 95 percent sure that the true load is within +/- 20 percent of the estimated load. Since much of the total nitrogen load is dissolved, continuous turbidity monitoring may not provide much additional confidence in the estimates, but this has yet to be determined.
Low. The Kendall test reports the P value for the slope of a trend, that is, the probability that the slope is not different from zero. The trend test for total nitrogen had a p-value = 0.86, tau = -0.028.
Low.
Outcomes
Programs and Actions Implemented to Improve Conditions
Stream environment zone (SEZ) restoration and enhancement, urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, retrofit regulations for private and commercial property BMPs, reducing private automobile use through improvements to public transit and alternative transportation modes (with the goal of reducing air pollution and the subsequent deposition of nitrogen and fine sediment), and ongoing allocation of water quality mitigation funds to support erosion control and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have:

• Restored or enhanced 27,150 linear feet of stream channel
• Retrofitted 120.55 miles of road and decommissioned an additional 7.4 miles of road
• Restored or enhanced 120 acres of disturbed forested uplands
• Inspected 108.72 miles of unpaved non-urban roads and maintained 98.2 miles
• Issued 18,076 BMP certificates to commercial, multifamily and single family residential properties
Effectiveness of Programs and Actions
Quantitative evaluation of the effectiveness of any individual policy, program or action implemented to improve the tributary water quality is challenging because of the diversity of contributing factors. High inter-annual variability in loads, which is thought to be primarily driven by variability in annual precipitation, complicates the determination of overall effectiveness of the TRPA Regional Plan and actions taken by Regional partners. Further, there is no significant long-term trend in total nitrogen load coincident with TRPA Regional Plan implementation.
Interim Target
Because this is a management standard with no defined numerical targets, one cannot reasonably establish an interim target.
Target Attainment Date
Because this is a management standard with no defined numerical targets, one cannot reasonably establish an interim target.
Recommendations
Analytic Approach
The load calculations used in this indicator sheet was derived from procedures developed in an applied science project for the USDA Forest Service, with funding from the Sierra Nevada Public Land Management Act (Coats and Lewis, 2014b, 2014a). Software compatible with the updated water quality data base also were provided. It is recommended that the revised methods for calculating total loads continue in use for now, but that they be revised and updated as improvements become available. With each updating, annual loads should be recalculated for the entire period of record using the improved method.

Although primary productivity in Lake Tahoe is now P-limited at times, the warming of the lake may (by the end of this century) increase the internal supply of phosphorus, and thus shift the lake back to a condition of nitrogen limitation (N-limitation). Further investigation of the bioavailabity of different nitrogen forms could improve our understanding of the conditions under which N-limitation occurs.
Monitoring Approach
Total nitrogen loads should continue to be monitored to track long-term trends; however, coordinated additional work such as focused studies and effectiveness monitoring, are needed to assess the causes of those changes, or lack of changes. This includes assessing the effectiveness of watershed restoration projects and understanding the effects of uncontrollable drivers such as weather and climate change. As turbidity data become available, its efficacy in estimating total Kjeldahl nitrogen loads should be evaluated.
Modification of the Threshold Standard or Indicator
The load reduction needed to attain any of the three standards is not provided, which precludes objective evaluation of standard attainment. Objective determination of “attainment” status for standards without a specific target is a recurrent challenge both in the Region and in the larger field of monitoring and evaluation (M&E). The standard should be assessed against best practice for the establishment of standards and indicators for M&E, and amended as necessary to improve the evaluability of the standard and the information it provides for management. Any quantitative standards for pollutant loads would need to take into account the large natural variation in yearly stream inflow within and among watersheds. Development of any new standards should be consistent with the standards and management strategies implemented through the Lake Tahoe Total Maximum Daily Load (TMDL) program.
Attain or Maintain Threshold
Continue to pursue the strategies and actions identified in the Lake Tahoe TMDL with a goal of reducing tributary loading of sediment and nutrients, and achieving the interim target for Lake Transparency by 2031.
Additional Figures and Resources

No documents available.