Water Tower Index for Saddle Catchment, 2000 - 2018.

The Saddle Catchment of the Niwot Ridge LTER experiences significant
spatial variation regarding snow distribution and, in turn, the timing
and amount and surface water input generation (i.e., rainfall and
snowmelt). In winter months and in areas that receive a high amount of
redistributed snow (via wind), snow accumulation is large and the
snowpack persists until the snowmelt season, creating a lag between
the timing of precipitation (as snow) and the timing of surface water
inputs (as snowmelt). Alternatively, areas that are scoured of snow,
retain little snow. In these areas, rain is the primary source of
surface water inputs, and thus there is little/no lag in the timing of
precipitation and surface water inputs (as rain). The lag in the
timing of precipitation and surface water inputs in the wind
deposition zones, however, is critical to providing water to the
surrounding and downstream environments later in the year. The snow in
these areas thus act as a natural water tower to retain water (as
snow) until the snowmelt season. To capture the timing and magnitude
of the delay between precipitation and surface water inputs, a Water
Tower Index (WTI) was generated. The WTI uses equations (see Methods)
to fit a sine curve to annual precipitation (P) and annual surface
water inputs (SWI). A third equation then compares to the phase and
amplitude of the two sine curves, generating a metric between -1 and
1. Positive WTI values signify P and SWI out of temporal alignment
(where WTI = 1 indicates strong (i.e., high amplitude) temporal
misalignment between P and SWI). Negative WTI values signify P and SWI
in temporal alignment (where WTI = -1 indicates strong (i.e., high
amplitude) temporal alignment between P and SWI). P and SWI data were
taken from DHSVM output run by Nels Bjarke, which included 19 years of
information (WY2000-WY2018). The methodology was applied, spatially,
across the Saddle Catchment of the Niwot Ridge LTER. In turn, this
dataset reveals, importantly, the areas that store significant water
as snow for later in the year (high WTI), a phenomenon that may
diminish in a warming climate.