Mel Kunkel & Jen Pierce Boise State University Climatic Indices: Predictors of Idaho's Precipitation and Streamflow

Outline Reconstructing Timing of Past Snowmelt Linking Climate Indices to the timing of snowmelt, snow accumulation and total precipitation The next steps…….

Reconstructing Timing of Past Snowmelt Purpose: Develop a repeatable technique for reconstructing past timing of snowmelt in Idaho to assess how past snowpacks have responded to oceanic and atmospheric influences --- this requires long term records.

Reconstructing Timing of Past Snowmelt SNOTEL/Snow Course:  Daily SNOTEL and monthly snow course SWE data is readily available from the NRCS online. NRCS snow course data (many from the 1930s) provides long-term monthly SWE readings while SNOTEL data provides detailed daily snow depth and SWE measurements, but in most cases these records only exist since the early 1980s.  There are over 350 SNOTEL and snow course (current and discontinued) sites located within the Idaho watershed.

Reconstructing Timing of Past Snowmelt SNOTEL/Snow Course:  Recognizing that the NRCS emplaced SNOTEL and snow course sites specifically to assist in forecasting the water supply of the western United States, we acknowledge that they may not be representative of the entire watershed.

Reconstructing Timing of Past Snowmelt Hydro-Climatic Data Network (HCDN): The U.S. Geological Survey (USGS) maintains the largest collection of stream gages in the United States; these gage records form the basis of the HCDN. Use of the HDCN for climate studies is well established (e.g. Dettinger and Cayan 1995; Cayan et al. 2001; Stewart et al. 2005; Jefferson et al. 2006; Barnett et al. 2008; Levi 2008). Since HCDN streamflow records are relatively free of anthropogenic influences (Slack and Landwehr 1992), variations in streamflow in the HCDN basins are likely caused by changes in climate or basin characteristics rather than dam-related flow regulation or other anthropogenically-induced change

Reconstructing Timing of Past Snowmelt Selecting suitable paired stream gage and SNOTEL site was critical to accurately reconstructing the timing of past snowmelt. We selected USGS gage stations and SNOTEL sites by spatially overlaying gage positions, SNOTEL positions, and stream/river information. We then identified gage sites and SNOTEL sites that were within the same drainage basin, and relatively close together.

Reconstructing Timing of Past Snowmelt Through this selection process, we identified 28 unregulated streams as candidates for use in extending the SNOTEL data sets. Other unregulated streams with gage sites lacked SNOTEL sites within their immediate drainages and were not used for this study.

Reconstructing Timing of Past Snowmelt Conducted a Short Term Fourier Transform on selected site discharge. STFT transforms the annual discharge data into a spectral signal that varies in shape and magnitude over time, reflecting changes in discharge. We developed procedures to use the results of the STFT to reconstruct the timing of snowmelt based on the temporal distance of final snowmelt from existing SNOTEL data from the spectral peak (peak spring discharge).

An example of a single spectral peak where the final snowmelt date always appears two timing points right of the spectral peak. (Trinity Mountain SNOTEL and Boise River)

An example of twin spectral. The timing point for this site always reads three timing points to the left of the left hand center of timing. (Meadow Lake SNOTEL and Lemhi River)

Reconstructing Timing of Past Snowmelt

Results: Reconstructions of snowmelt dates from these 19 sites extend records of the estimated timing of snowmelt back 42–97 years, >1100 reconstructions. None of the 419 compared reconstructed final snowmelt dates deviated from the actual snowmelt date by more 4 days ∼ 95% of the time and ±7 days 100% Assuming that the relationship between discharge and timing of snowmelt established using known data holds true for the earlier part of the record, we therefore assume reconstructions of the timing of snowmelt using this method are also within ±7 days.

Linking Climate Indices to the timing of snowmelt, snow accumulation and total precipitation

Linking Climate Indices Use past records of the timing of snowmelt, max snow accumulation, dates of max accumulation and total precipitation Indices of interest – ENSO (TNI/MEI) – Arctic Oscillation (AO) – Pacific Decadal Oscillation (PDO) – And others

Linking Climate Indices 1999 –La Nina and Positive AO

Linking Climate Indices Max SWE Total Precip Max SWE date Final Snowmelt date Date more than 1.5 weeks later than normal Date more than 1.5 weeks earlier than normal % or greater above normal 10% or greater below normal

Linking Climate Indices 1999 –Greater than normal precipitation Later than normal final snowmelt La Nina and Positive AO 1987 –El Nino and Negative AO

Linking Climate Indices Max SWE Total Precip Max SWE date Final Snowmelt date Date more than 1.5 weeks later than normal Date more than 1.5 weeks earlier than normal % or greater above normal 10% or greater below normal

Linking Climate Indices 1999 –Greater than normal precipitation Later than normal final snowmelt La Nina and Positive AO 1987 –Lower than normal precipitation Earlier than normal final snowmelt El Nino and Negative AO 1991 – El Nino and Positive AO

Linking Climate Indices Max SWE Total Precip Max SWE date Final Snowmelt date Date more than 1.5 weeks later than normal Date more than 1.5 weeks earlier than normal % or greater above normal 10% or greater below normal

Linking Climate Indices 1999 –Greater than normal precipitation Later than normal final snowmelt La Nina and Positive AO 1987 –Lower than normal precipitation Earlier than normal final snowmelt El Nino and Negative AO 1991 – Regional variations in precipitation Regional variation in final snowmelt El Nino and Positive AO 1998 – La Nina and Negative AO

Linking Climate Indices Max SWE Total Precip Max SWE date Final Snowmelt date Date more than 1.5 weeks later than normal Date more than 1.5 weeks earlier than normal % or greater above normal 10% or greater below normal

Linking Climate Indices 1999 –Greater than normal precipitation Later than normal final snowmelt La Nina and Positive AO 1987 –Lower than normal precipitation Earlier than normal final snowmelt El Nino and Negative AO 1991 – Regional variations in precipitation Regional variation in final snowmelt El Nino and Positive AO 1998 – Regional variations in precipitation Regional variation in final snowmelt La Nina and Negative AO

The Next Step…… Continue to use developed Empirical Rules of Thumb on how regional snow and precipitation responds under similar combined conditions to enhance understanding of physical processes driving Idaho weather Continued development of mixed statistical and physically based models for Idaho’s regional and watershed level snow, precipitation and streamflow

Thanks!!