Abstract
Water supply reliability in the Truckee River basin stands to
substantially benefit from forecast-informed reservoir operations(FIRO),
especially given expected increases in rain: snow ratios and a
transition to earlier runoff under warming climate that current
infrastructure and operational rules were not designed for. However, in
its position on the lee side of the Sierra Nevada mountains (CA/NV,
USA), several unique forecast uncertainties exist that must be
considered to mitigate against increased flood risk potential. Both
water supply and floods are strongly linked to wintertime atmospheric
rivers (AR) but despite improvements in forecasting these events at long
lead times, the timing and amount of spillover precipitation onto the
lee side remains a key uncertainty. In addition, storm runoff volumes in
this basin are highly sensitive to rain-snow elevation, which is also
difficult to forecast. Finally, antecedent snowpack and soil conditions
have the potential to modulate runoff volumes but factors controlling
the strength of these modulations are incompletely understood and
monitored. In this study, we assess streamflow forecast skill in the
Truckee River to provide a preliminary understanding of potential
forecast-related challenges and opportunities for FIRO. To accomplish
this, we used an archive of readily available short-range Hydrologic
Ensemble Forecast System winter (Oct-Apr) streamflow forecasts for water
years 2015-2020 and compared these to observed3-day flows at lead times
of 0 to 15 days. We subset the data into AR days, non-AR days and top
10% flow days examined the variance explained between the ensemble
median and observed 3-day flows as a function of lead time. We also
examined how the observed 3-day flows rank in relation to the ensemble
members for each day. We found that forecast accuracy improves
considerably starting at a 7-day lead time but tends to be lower for
high-flow and AR events relative to non-ARs. We also found the ensembles
to have a slight bias toward underprediction and tendency toward
under-dispersion (i.e. observed flows were sometimes outside the
ensemble range) with this being the case for AR and high flow days for
some but not all sites.