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\begin{document}

\title{Catchment scale observations at the Niwot Ridge Long-Term Ecological
Research site}



\author[1]{Nels Bjarke}%
\author[2]{Ben Livneh}%
\author[2]{Sarah Elmendorf}%
\author[2]{Noah Molotch}%
\author[2]{Eve-Lyn Hinckley}%
\author[2]{Nancy Emery}%
\author[2]{Pieter Johnson}%
\author[2]{Katherine Suding}%
\affil[1]{University of Colorado Boulder}%
\affil[2]{University of Colorado at Boulder}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
The Niwot Ridge and Green Lakes Valley (NWT) long-term ecological
research (LTER) site collects environmental observations spanning both
alpine and subalpine regimes. The first observations began in 1952 and
have since expanded to nearly 300 available datasets over an area of 99
km2 within the north-central Colorado Rocky Mountains that include
hydrological (n = 101), biological (n = 79), biogeochemical (n = 62),
and geographical (n = 56) observations. The NWT LTER database is well
suited to support hydrologic investigations that require long-term and
interdisciplinary data sets. Experimentation and data collection at the
NWT LTER are designed to characterize ecological responses of
high-mountain environments to changes in climate, nutrients, and water
availability. In addition to the continuation of the many legacy NWT
datasets, expansion of the breadth and utility of the NWT LTER database
is driven by new initiatives including (a) a catchment-scale sensor
network of soil moisture, temperature, humidity, and snow-depth
observations to understand hydrologic connectivity and (b) snow-albedo
alteration experiments using black carbon to evaluate the effects of
snow-disappearance on ecosystems. Together, these observational and
experimental datasets provide a substantial foundation for hydrologic
studies seeking to understand and predict changes to catchment and
local-scale process interactions.%
\end{abstract}%



\sloppy


Abstract

The Niwot Ridge and Green Lakes Valley (NWT) long-term ecological
research (LTER) site collects environmental observations spanning both
alpine and subalpine regimes. The first observations began in 1952 and
have since expanded to nearly 300 available datasets over an area of 99
km\textsuperscript{2} within the north-central Colorado Rocky Mountains
that include hydrological (n = 101), biological (n = 79), biogeochemical
(n = 62), and geographical (n = 56) observations. The NWT LTER database
is well suited to support hydrologic investigations that require
long-term and interdisciplinary data sets. Experimentation and data
collection at the NWT LTER are designed to characterize ecological
responses of high-mountain environments to changes in climate,
nutrients, and water availability. In addition to the continuation of
the many legacy NWT datasets, expansion of the breadth and utility of
the NWT LTER database is driven by new initiatives including (a) a
catchment-scale sensor network of soil moisture, temperature, humidity,
and snow-depth observations to understand hydrologic connectivity and
(b) snow-albedo alteration experiments using black carbon to evaluate
the effects of snow-disappearance on ecosystems. Together, these
observational and experimental datasets provide a substantial foundation
for hydrologic studies seeking to understand and predict changes to
catchment and local-scale process interactions.

Full Text

1. Introduction

The Niwot Ridge and Green Lakes Valley (NWT) long-term ecological
research (LTER) site encompasses several catchments spanning alpine and
subalpine zones in the Southern Rocky Mountains. Such catchments provide
a vital natural reservoir for mountain source waters and snowmelt
generated runoff (Cayan, 1996; Mote, 2006; Mote et al., 2018). High
elevation regions are among the most sensitive to climate change and
present a valuable testbed for evaluating climate impacts on hydrology
and ecology (Barnett et al., 2005; Cayan, 1996; Clow, 2010; Livneh et
al., 2015; Livneh \& Badger, 2020; Pepin \& Losleben, 2002; Sexstone et
al., 2018; Vano, 2020), but comprehensive in situ observations in these
extreme environments are rare. Hydrologically relevant observations
collected at the NWT LTER include: 68 years of daily and hourly
observations of meteorology, 30+ years of snow depth and distribution
observations, multiple surveys of soil characteristics, a spatially
dense sensor array of collecting sub-hourly observations of catchment
soil moisture and temperature, stream gauge observations, and several
experiments designed to evaluate the drivers and consequences of
long-term ecological and biogeochemical trends. The unique
spatiotemporal density of hydrological and ecological observations
paired with multi-decadal climate records make the data collected at the
NWT LTER particularly well suited for research that seeks to understand
catchment-scale ecohydrological processes in alpine and subalpine
environments.

2. Site Description

The NWT LTER site is located in the Southern Rocky Mountains of North
America (40.05\selectlanguage{ngerman}°N, 105.59°W) (Figure 1). The sub-alpine Niwot Ridge
Saddle catchment (Area: 0.6 km\textsuperscript{2}; Elevation: 3528 m),
located 5.6 km east of the continental divide, receives a majority
(\textasciitilde{}80\%) of its approximately 1035 mm of annual
precipitation as snow (Bowman et al., 2001; Caine, 1995; Greenland,
1989; Jennings et al., 2019). The alpine Green Lakes Valley catchment
(Area: 2.3 km\textsuperscript{2}; Elevation: 3745 m) receives slightly
more annual precipitation than the Saddle catchment, 1200 mm, which
feeds into the Green Lakes and Lake Albion (Jennings et al., 2019).
Average daily temperatures of -8.8\selectlanguage{english} in the winter season and -0.5\selectlanguage{english} in
the summer season (Jennings et al., 2019) support persistent snowpacks
in the NWT LTER catchments through much of the year and a short growing
season for the alpine vegetation (1-3 months) (Jones et al., 2001).
Annual records of daily gauged streamflow begin each year with the onset
of snowmelt in the mid to late spring and extend back nearly 40 years.

3. Available Datasets

Hydro-climate observations were first collected at the NWT LTER in 1952
and have since expanded to nearly 300 available datasets over an area of
99.125 km\textsuperscript{2} within the Southern Rocky Mountains that
include hydrological (n = 101), biological (n = 79), geographical (n =
56), and biogeochemical (n = 62) observations (Figure 2). Here we
emphasize active datasets identified as being the most spatio-temporally
unique and relevant to hydrological catchment research, but encourage
exploration and use of the hundreds of current and legacy datasets
collected and stored on the publicly accessible NWT LTER database
(\emph{https://nwt.lternet.edu/data-catalog}). Emphasized hydrological
datasets and other major datasets relevant to catchment hydrology are
described briefly in Table 1.

3.1 Stream Discharge and Chemistry

Gauged stream observations of discharge are collected at daily
resolution for four major streams (Albion, Green Lake 4, Martinelli, and
Saddle) in the NWT LTER alongside weekly or monthly observations of
water chemistry. These long records of stream observations are valuable
for the validation of the performance hydrological (Biondi et al., 2012;
Du et al., 2014) and hydrochemical (Molotch et al., 2008) model
simulations interested in alpine and sub-alpine catchment behavior.
Furthermore, the combination of chemical and volumetric streamflow
observations has been used to understand changes to the contribution of
glacial melt to surface runoff (Barnes et al., 2014; Leopold et al.,
2011) and changes in carbon respiration and nitrification (Barnes et
al., 2014; Blanken et al., 2009; Knowles et al., 2015; Liu et al., 2004)
for alpine tundra under a changing climate.

3.2 Long-Term Climate Observations

Daily observations of precipitation and temperature are available from
1952 to present for the D1 meteorological station that is located at the
head of the Green Lakes Valley catchment and the C1 sub-alpine
meteorological station located below tree-line. In addition to the raw
observations, temporally infilled and continuous records of daily
temperature (Kittel et al., 2019a, 2019b) and precipitation (Kittel et
al., 2019c, 2019d) following methods described in (Kittel, 2009) are
available on the NWT LTER data catalog. Daily precipitation and
temperature observations beginning in 1981 for the Saddle meteorological
station, located at the head of the Niwot Ridge Saddle catchment, were
updated to hourly frequency in 2009. Continuous observations of relative
humidity, wind speed and direction, atmospheric pressure, and solar
radiation are available at daily or hourly frequencies at the
aforementioned meteorological stations in addition to more recently
developed sites (Table 1).

4. High-Resolution Observations

Within the last five years, two high-resolution observation arrays have
been established and produce observations relevant to catchment-scale
hydrology research: the Saddle Catchment Sensor Array and the Green Lake
4 Sensor Array. These datasets are highlighted here due to the uniquely
high spatial and temporal density of observations collected as part of
the research framework.

4.1 Saddle Catchment Sensor Array

A high-density array of sensor nodes, installed in 2017, collects
observations of soil moisture and temperature within the Niwot Ridge
Saddle catchment at a 10-minute frequency. Sixteen sensor nodes are each
equipped with a three-part transect of sensors that measures soil
temperature and moisture at 5 cm and 30 cm depths, for a total of six
observation points at each node, following methods outlined in (Kerkez
et al., 2012). These high spatiotemporal resolution soil observations in
the Saddle catchment can help constrain uncertainties in the
distribution of subsurface water (Grayson et al., 1997; Western et al.,
2004) and can be used to further understand the role of catchment-scale
soil moisture variability as it relates to runoff generation and
hydrologic connectivity (Western et al., 2004; M. W. Williams et al.,
2015).

4.2 GL4 Lake Sensor Array

To better document spatial and temporal variation in alpine lake
systems, both within and among seasons (including under ice), an
instrumented buoy line was deployed with sensors at fixed depths in
Green Lake 4 (Figure 1) in early summer 2018. The sensor line was
positioned in the area of deepest depth and contained eight RBR soloT
sensors to record water temperature, three PME miniDOT sensors to record
dissolved oxygen, one PME miniPAR sensor to record photosynthetically
available radiation (PAR) and a Cyclops 7 from Turner Designs to
optically measure in situ chlorophyll-a. Available data from summer 2018
and winter 2018-2019 have revealed the establishment and erosion of
stratification in GL4, which previous `snapshot' sampling did not
detect. Given that alpine lakes such as GL4 are typically frozen for
\textgreater{}200 days per year, this depth-stratified sensor array
provides high-resolution limnological data and valuable insights into
lake temporal dynamics during a period often difficult to sample
directly.

5. Extended Summer Experiments

Plot scale experiments, established in 2018, are conducted annually at
the NWT LTER to evaluate the hydrologic and ecologic impacts of
accelerated snowmelt and warmer alpine temperatures driven by climate
change.~ Modulations of albedo and organic composition are applied to
snow/ice located at several terrestrial and aquatic sites, paired with
high-resolution observations of soil characteristics, snow depth, and
biological composition.

5.1 Terrestrial Snowmelt Timing Manipulations

Beginning in 2018, ``black sand'' experiments were conducted to
reproduce and observe the effects of changing snow albedo on snowmelt
timing and subsequent soil moisture by applying varying volumes of black
sand to snow-covered plots equipped with soil moisture sensors
(Blankinship et al., 2014; Bueno de Mesquita, 2019). Hourly observations
of soil moisture, soil temperature, and electrical conductivity are
collected at four locations for each of the five habitat sites along
with sub-weekly (\textasciitilde{} every 3 days) observations of snow
depth for each of the sites. These terrestrial experiments are motivated
in part by observed changes in snow albedo associated with dust-on-snow
events, resultant from human activities (Neff et al., 2008), shown to
impact the timing of peak snowmelt in the Western US (Deems et al.,
2013; Livneh et al., 2015; T. H. Painter et al., 2010; Thomas H. Painter
et al., 2012; Skiles et al., 2012).

5.2 Mesocosms: Experimental manipulation of ice cover and dissolved
organic matter

In September 2019, an experimental array of 20 large-volume
(2,600-liter) mesocosms were positioned at a high-elevation site in
Boulder Watershed (10,800 feet above sea level). This represents the
highest established mesocosm experiment and provides an aquatic analog
to the Black Sand experiments that are conducted in the terrestrial
plant communities. Half of the mesocosms are made of a plastic, light in
color (control), while half are black (warmed), creating an albedo
difference that is expected to drive variation in both ice-off date and
water temperature (based on pilot experiments). The experiment also
manipulates dissolved organic material (DOM) through the addition of
willow-leaf packs, thereby mimicking forecasted advances in terrestrial
vegetation around alpine lakes---which is expected to help mitigate the
extreme UV effects common to such high-elevation environments. Each of
the four treatments (light vs. dark tanks, enhanced vs. ambient DOM) are
replicated five times in all factorial combinations (i.e., a 2 x 2
manipulation with five replicates per condition = 20 mesocosms).
Mesocosms were seeded with lake sediment and zooplankton and are
intended to run for two years. It is expected that higher temperatures,
longer growing seasons, and higher DOM will function to `soften' the
harsh abiotic limitations inherent to alpine systems, leading to greater
planktonic production (chl-a and zooplankton biomass) and lower water
clarity. Additionally, mesocosm experiments are designed to observe
expected shifts in zooplankton community composition and body size with
warmer water and lessened UV stress, although whether these will be
additive or interactive is uncertain.

6. Data Availability Statement

The data collected at the Niwot Ridge and Green Lakes Valley is stored
and publicly accessible through the Niwot Ridge LTER data catalog
(\emph{https://nwt.lternet.edu/data-catalog}). All datasets include
documentation of data collection practices, instrumentation, collection
site information, and quality control information. In addition to the
raw data, multiple temporally infilled and quality-control filtered
datasets exist alongside documentation of the methods used to generate
those datasets.

7. Acknowledgements

Logistical support and/or data were provided by the NSF-supported Niwot
Ridge LTER program (NSF DEB -- 1637686). We acknowledge the
contributions of the data collection and management teams within the NWT
LTER and the CU Boulder staff, faculty, and students who have
contributed to the continued quality of observations collected and
research produced as part of the NWT LTER mission.

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