National Park Service

Department of the Interior

Natural Resource Stewardship and Science

Describing Trends in Past and Future Soil Moisture 

in the Pinyon-Juniper Woodland Ecosystem in Grand 

Canyon National Park

Natural Resource Report NPS/GRCA/NRR—2020/2115
ON THE COVER

TOP: Estimated past seasonal soil water patterns at Grand Canyon NP, described in detail in Figure 3. 

MIDDLE: Soil samples from different depths of a soil core taken from a long-term monitoring plot. NPS photo.

BOTTOM: Estimated current and future soil water patterns at Grand Canyon NP, described in detail in Figure 4.

BACKGROUND: Photo of Southern Colorado Plateau Network monitoring plot in the Grand Canyon NP pinyon-juniper 

woodland ecosystem. NPS photo.
Describing Trends in Past and Future Soil Moisture 

in the Pinyon-Juniper Woodland Ecosystem in Grand 

Canyon National Park

Natural Resource Report NPS/GRCA/NRR—2020/2115

Caitlin Andrews1, John Bradford1, Jodi Norris2, Jennifer Gremer3, Michael Duniway4, Seth Munson1, Lisa 

Thomas2

, and Megan Swan2

1 U.S. Geological Survey 

Southwest Biological Science Center

2255 N Gemini Dr. 

Flagstaff, AZ 86001

2

National Park Service

Southern Colorado Plateau Inventory and Monitoring Network

2255 N Gemini Dr. 

Flagstaff, AZ 86001

3

Department of Evolution and Ecology

University of California Davis

Davis, CA 95616

4

Southwest Biological Science Center

2290 SW Resource Blvd.

Moab, Utah 84532

April 2020

U.S. Department of the Interior

National Park Service

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii Describing Trends in Past and Future Soil Moisture in Grand Canyon National Park

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This report is available from the Southern Colorado Plateau Network, and the Natural 

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Please cite this publication as:

Andrews, C., J. Bradford, J. Norris, J. Gremer, M. Duniway, S. Munson, L. Thomas, and M. 

Swan. 2020. Describing trends in past and future soil moisture in the pinyon-juniper woodland ecosystem in Grand Canyon National Park. Natural Resource Report NPS/GRCA/

NRR—2020/2115. National Park Service, Fort Collins, Colorado.

NPS 113/168217, April 2020
 National Park Service iii

Contents 

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Acknowledgements . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Introduction . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Discussion . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Page
iv Describing Trends in Past and Future Soil Moisture in Grand Canyon National Park

Figures

Figure 1. Pinyon-juniper woodland ecosystem in Grand Canyon NP. NPS photo. . . . . . . . . . . 1

Figure 2. Map of Grand Canyon NP showing Southern Colorado Plateau Network 

monitoring plots in the pinyon-juniper woodland ecosystem. Plots from which soil 

cores were used are indicated as solid blue dots. NPS map. . . . . . . . . . . . . . . . . . . . . 2

Figure 3. Historical (A) and future projections (B) of soil water potential (SWP) in 

the pinyon-juniper woodland ecosystem in Grand Canyon NP. . . . . . . . . . . . . . . . . . . . 4

Figure 4: Panel A. Average seasonal patterns of climate in the pinyon-juniper 

woodland ecosystem in Grand Canyon NP. Panel B. Average daily SWP for the current period (1970–2010), near future (2020–2059), and long-term future (2060–2099). . . . . . . 5
 National Park Service 

v

Executive Summary

Variation in the structure and function of 

Colorado Plateau ecosystems is driven by 

seasonal moisture availability in the soil pro-

file; however, the availability of soil moisture 

is expected to change in the coming decades 

as a result of climate change. To understand 

how ecosystems will be affected by this, we 

need to understand in more detail how these 

soil moisture patterns relate to plant produc-

tivity, and what changes in soil moisture are 

expected, both seasonally and at the range of 

soil depths that plants are adapted to use.

We used a soil water model (SOILWAT2) to 

estimate historical, current, and future soil 

water availability in 300 vegetation monitor-

ing plots in 13 ecosystems in the Southern 

Colorado Plateau Network of the National 

Park Service. In Grand Canyon National 

Park, this included 29 vegetation monitoring 

plots within the pinyon-juniper woodland 

ecosystem. The SOILWAT2 model used 

plot-specific vegetation and soil core data 

collected as part of the Southern Colorado 

Plateau Network’s long-term upland vegeta-

tion monitoring program to represent the 

environment.

Model simulations were run using climate 

data from the past (1916–2011) to provide 

historical context, the present (1980–2015) 

to create a current baseline for compari-

son. Model simulations were also run for 

two time periods in the future: a near-term 

period (2020–2059) and a long-term pe-

riod (2060–2099). The future projections 

were made using nine general circulation 

models under two possible greenhouse gas 

emissions pathways: the 8.5 representative 

concentration pathway (RCP), and the 4.5 

RCP. Results of the 8.5 RCP simulations are 

presented here; this pathway corresponds to 

a comparatively high greenhouse gas emis-

sions scenario. 

The soil-water model results provide a de-

tailed look at historical patterns of soil water 

availability, and how soil water availability is 

projected to change in the future, both sea-

sonally and at different soil depths. Model 

output included daily soil water potential 

estimates at multiple depths within the soil 

profile. This report describes results at inter-

mediate (20–50 cm) soil depths.

A second goal of this work was to better 

understand how changes in seasonal soil 

water availability might affect vegetation. To 

do this, we initially examined how vegeta-

tion today is affected by seasonal differences 

in soil moisture by calculating correlations 

of annual NDVI (2001-2014) with soil water 

potential averaged over a range of moving 

windows. However, recent work (Norris 

and Walker, in review) has shown that the 

NDVI values in most pinyon-juniper eco-

systems do not correspond with vegetation 

greenness. Therefore those analyses were 

discarded.

Key findings for the Grand Canyon NP 

pinyon-juniper woodland ecosystem include 

the following: ● All of the climate models predicted 

increases in mean monthly and annual 

temperature in the near-term future 

(+2.5°C on average), and further increas-

es in the long-term future (+5.0°C on 

average). Models had greater variation 

in their predictions of both the direction 

and the amount of future precipitation, 

except for the late spring months, for 

which low precipitation is consistently 

predicted.

● Future soil water availability is projected 

to decline in the fall/winter and spring. 

By the end of the 21st century, average 

spring soil water potential is expected to 

drop below -3.0MPa, indicating condi-

tions of extreme water scarcity.

● The drying of soils in spring is predicted 

to occur earlier in both near and longterm scenarios, and will shorten the pe

-

riod when soils are wet during the spring 

growing season. This will likely have a 

substantial impact on plant species that 

rely on spring soil moisture.
vi Describing Trends in Past and Future Soil Moisture in Grand Canyon National Park

Acknowledgements

This report benefited from editorial improvements from Jean Palumbo and from 

reviews by David Thoma and Kathryn 

Thomas. Thanks to multiple years of Southern Colorado Plateau Network upland monitoring crews for vegetation and soils data 

collection. This work was supported by the 

National Park Monitoring Program of the 

U.S. Geological Survey and by the National 

Park Service Inventory and Monitoring Program, Southern Colorado Plateau Network.
 National Park Service 1

Introduction

In the semi-arid environment of the Colorado Plateau, soil water availability is an 

important driver of vegetation dynamics. 

Recent drought-associated tree mortality 

in the Southwest has highlighted the potential vulnerability of ecosystems to changing 

water availability. Climate change models 

predict continued warming and seasonally 

drier conditions over the next century.

How will changing soil water availability 

impact vegetation dynamics in these already 

water-limited ecosystems? To answer this 

question the U.S. Geological Survey (USGS) 

and National Park Service (NPS) have 

teamed up to model soil water availability 

under projected climate conditions in nine 

Southern Colorado Plateau national park 

units where long-term vegetation monitoring 

is underway.

The resulting projections of soil water availability will allow us to develop hypotheses 

about how the dominant species in plant 

communities will respond to changing soil 

water conditions. This project will help park 

managers today by identifying the most sensitive ecosystems and informing strategies to 

improve long-term ecosystem resistance and 

resilience. Through continued monitoring 

over the coming decades we will also be able 

to compare observed vegetation dynamics 

to hypothesized responses, thus furthering our understanding of Colorado Plateau 

ecosystems.

For this project, USGS and NPS scientists 

used the range of variation in historical 

climate data to provide context for assessing the relative impact of projected future 

climate on water availability. We used a soil 

water model, SOILWAT2, to translate climate 

variability into site-specific estimates of 

soil water conditions that may drive plant 

responses. One of the valuable outputs of 

the model is daily soil water potential (SWP), 

which provides an estimate of water available 

for plants that reflects soil conditions. 

This report provides the results of modeled 

SWP at intermediate soil depths (20–50 cm) 

generated for the pinyon-juniper woodland 

ecosystem in Grand Canyon NP, Arizona 

(Figure 1). 

Figure 1. Pinyon-juniper 

woodland ecosystem in 

Grand Canyon NP. NPS 

photo. 
2 Describing Trends in Past and Future Soil Moisture in Grand Canyon National Park

Study Area

The Southern Colorado Plateau Network 

(SCPN) has been monitoring vegetation 

and soil stability in grasslands, woodlands 

and forests in network parks since 2007. 

In Grand Canyon NP, SCPN monitors the 

pinyon-juniper woodland ecosystem. The 

plant community in this ecosystem is characterized by dense pinyon-juniper overstory 

with understory shrubs including Stansbury 

cliffrose (Purshia stansburiana) and big 

sagebrush (Artemisia tridentata). Perennial 

grasses, forbs and cacti are also represented. 

Soils in this ecosystem are typically classified 

as clay loam. We used data from soil cores 

taken from 29 plots in this ecosystem as inputs to the SOILWAT2 model (Figure 2).

Figure 2. Map of Grand Canyon NP showing Southern Colorado Plateau Network monitoring 

plots in the pinyon-juniper woodland ecosystem. Plots from which soil cores were used are indicated as solid blue dots. NPS map. 
 National Park Service 3

Methods

Data were collected from the following 

sources for input into the SOILWAT2 model:

● texture and depth data from core samples that were taken during the establishment of SCPN upland monitoring plots 

(supplemented with NRCS SSURGO 

soil texture data when the soil cores did 

not extend to the full depth of the soil 

profile)

● vegetation species and functional group 

cover data from SCPN monitoring plots

● daily weather data estimates from 1920 

to the present 

■ Livneh (long-term historical weather; 1916–2011; 1/16th resolution)

■ DayMet (near-term historical 

weather; 1980–2015; 1 km × 1 km 

resolution)

● projected daily weather for 9 General 

Circulation Models (GCMs) for the periods 2020–2059 and 2060–2099, under 

representative concentration pathways 

(RCP) 8.5. RCPs, adopted by the International Panel on Climate Change, 

characterize potential future climate 

based on the hypothetical trajectories 

in the concentrations of greenhouse 

gases, with 8.5 demonstrating the highest 

potential concentrations. All projected 

climate data were downloaded from the 

Green Data Oasis (https://hpc.llnl.gov/

data-vis/green-data-oasis).

SOILWAT2 integrated these climate data, 

along with the soils and vegetation data, to 

generate the following simulations:

● long-term historical daily soil water potential (1915-2010) 

● near-term historical daily soil water 

potential (1980-2015)

● near-term future daily soil water potential (2020-2059)

● long-term future daily soil water potential (2060-2099)

We used box plots to represent annual variation in each of the seasonal soil moisture 

projections for the respective time periods. 

For future time periods, we used the mean 

projected soil moisture from the median 

ranked GCM.

The SOILWAT2 model, along with the soil 

moisture simulation for this report, is available at https://doi.org/10.5066/F7D50K6S.
4 Describing Trends in Past and Future Soil Moisture in Grand Canyon National Park

Results

The outputs from our model show SWP for 

the past, present, and future, and highlight 

relatively moist and dry periods. These data 

can be considered in a variety of ways, including climate variability, variability among 

soil profiles, and relationships with plotbased vegetation monitoring data. Here we 

present seasonal and temporal patterns in 

soil water availability in the pinyon-juniper 

woodland ecosystem in Grand Canyon NP.

Long-term trends in past and 

future water availability 

● Past SWP fluctuations highlight relatively 

moist and dry periods and allow us to 

compare the past variability in SWP to 

future projections (Figure 3A).

● Future projections (Figure 3B) suggest 

declining water availability in the fall/

winter and spring. By the end of the 21st 

century (L), average spring SWP is expected to drop below -3.0MPa, indicating conditions of extreme water scarcity.

● Future soil moisture in the summer will 

remain relatively unchanged. However, 

spring SWP will likely be lower than 

summer, a pattern not observed in the 

20th century.

Historical and future climate 

patterns

● Currently, mean annual temperature 

(MAT) is 11.5°C. In the future, MAT is 

expected to increase under all climate 

models (near future +2.5°C, long-term 

future +5.0°C), underscoring the consistency of these projected rising temperatures across climate models (variation in 

temperature projections among models 

is reflected by the variation in orange and 

red values in Figure 4A).

● Mean annual precipitation (MAP) is 430 

mm. The precipitation is bimodal, falling during the monsoon and in winter 

(Figure 4A). While future changes in 

precipitation are less consistent across 

models than temperature, the substantial 

projected increases in temperatures will 

likely result in decreased soil moisture, 

especially during spring, and longer 

summer dry periods (Figure 4B). 

● Periods of meteorological drought occur 

when the precipitation line is below the 

temperature line in Figure 4A. The current period of meteorological drought 

spans from mid-April to mid-July. In 

the future, the duration of these dry 

conditions will likely start earlier and 

last longer, primarily as a result of rising 

temperatures.

Figure 3. Historical (A) and future projections (B) of soil water potential (SWP) at intermediate soil depths (20–50 cm) in the

pinyon-juniper woodland ecosystem in Grand Canyon NP for three seasons: fall/winter (October–February), spring (March–

June), and summer (July–September). Panel A shows historical annual variability (fine lines) and moving 10-year averages 

(thick lines) of drying and wetting from 1915–2010. Panel B compares variability amongst years for three time periods, current (C; 1970–2010), near future (N; 2020–2059), and long-term future (L; 2060–2099), grouped by season. Boxplot whiskers 

are 1.5 times the inter-quartile range of values, while dots are values outside this range. Dashed lines in both panels highlight plant-relevant SWP levels: traditional wilting point (-1.5 MPa) and a drylands wilting point (-3.0 MPa; Kolb and Sperry 

1999a, 1999b).
 National Park Service 5

Future soil moisture

● Future soil moisture is predicted to be 

lower in the early part of the year, particularly in March through June (Figure 4B).

● Soil moisture from July to October will 

remain relatively unchanged or increase, 

but will decrease from October onwards.

● This earlier seasonal onset of dry soils 

will shorten the period when soils are wet 

during the fall/winter growing season, 

with likely substantial impacts on vegetation reliant on this soil moisture.

Figure 4: Panel A. Average seasonal patterns of climate in the pinyonjuniper woodland ecosystem in Grand Canyon NP. Climate diagram 

(Walter and Leith 1967) shows mean monthly temperature (°C) and 

precipitation (mm) in dark red and blue lines, respectively, in the current 

period (1970–2010). Shaded red and blue areas indicate the range of 

potential values in the near (darker area; 2020–2059) and long-term 

(lighter area; 2060–2099) future periods. Mean annual temperature 

(MAT) and mean annual precipitation (MAP) are derived from the mean 

monthly values. Panel B. Average daily SWP for the current period 

(1970–2010; black), near future (2020–2059; yellow), and long-term 

future (2060–2099; purple). For future time periods, thick lines represent 

the median value among climate models, while shaded areas represent 

the range of values from the 11th to 89th percentile, removing both the 

highest and lowest model values.
6 Describing Trends in Past and Future Soil Moisture in Grand Canyon National Park

Discussion

These results present a long-term historical and future perspective on soil moisture 

in the pinyon-juniper woodland ecosystem 

in Grand Canyon NP. Predicted changes 

in SWP are likely to affect vegetation community dynamics in the future. Future dry 

spring-season periods are expected to 

become drier and longer as winter precipitation decreases and snowmelt events occur 

earlier. This change in the timing of soil 

moisture will leave this community vulnerable to declines as a result of climate change. 

When compared to other woodland ecosystems examined in the SCPN, Grand Canyon 

receives the most overall precipitation, as 

well as winter precipitation. Understanding 

how winter moisture will change is critical to 

understanding this community’s future.

The projections of future soil moisture 

provided here can be used in conjunction 

with species-specific vegetation monitoring 

data collected by the SCPN to (1) contribute 

to hypotheses about how individual species 

within an ecosystem may respond to changing soil water availability and (2) inform 

future management decisions in the face of 

changing and increasingly variable climate. 

Furthermore, the SWP estimates here have 

been published as a data release (Andrews 

and Bradford 2017) and can be used to 

inform future research. A recent example of 

this is a study by Gremer and others (2019) 

that used these SWP values to model the balance between grasses and shrubs in SCPN 

parks. The model created in that study predicted that in the future, climate change will 

favor greater relative abundance of shrubs 

across most Southern Colorado Plateau 

Network parks, including for the long-term 

future predictions (2060–2099) for the 

Grand Canyon NP pinyon-juniper woodland ecosystem described in this report. 

The modeled soil moisture values are influenced by both the logic embedded within the SOILWAT2 model and site-specific 

weather, soil, and vegetation. This approach 

provides a consistent, process-based framework for assessing climate change impacts 

on soil moisture, and projections of change 

in SWP are robust. The variation among climate projections displays the uncertainty in 

the magnitude of SWP change, but modeled 

SWP patterns consistently point to earlier 

and drier spring periods. These changes in 

both the timing and amount of soil moisture 

are certain to affect the communities that 

rely on them. 
 National Park Service 7

Literature Cited

Andrews, C., and J. Bradford, 2017. Simulated Soil Water Potential in National Parks and 

Monuments of the Southern Colorado Plateau, 1915–2099—Data: U.S. Geological 

Survey data release, https://doi.org/10.5066/F7D50K6S. 

Gremer, J. R., C. Andrews, J. R. Norris, L. P. Thomas, S. M. Munson, M. C. Duniway, and 

J. B. Bradford, 2018. Increasing temperature seasonality may overwhelm shifts in soil 

moisture to favor shrub over grass dominance in Colorado Plateau drylands. Oecologia, 

188.4:1195-1207. 

Kolb, K. J., and J. S. Sperry. 1999a. Differences in drought adaptation between subspecies of 

sagebrush (Artemisia tridentata). Ecology, 80:2373–2384.

Kolb, K. J., and J. S. Sperry. 1999b. Transport constraints on water use by the great basin 

shrub, Artemisia tridentata. Plant, Cell and Environment, 22:925–935.

Livneh B., E. A. Rosenberg, C. Lin, B. Nijssen, V. Mishra, K. M. Andreadis, E. P. Maurer, and 

D. P. Lettenmaier. 2013. A Long-Term Hydrologically Based Dataset of Land Surface 

Fluxes and States for the Conterminous United States: Update and Extensions, Journal 

of Climate, 26, 9384–9392.

Norris J. R. and J. J. Walker. In Review. Solar and sensor geometry, not vegetation response, 

drive satellite NDVI phenology in widespread ecosystems of the western United States. 

Walter, H., and H. Leith. 1967. Climate diagram world atlas. Fischer Verlag, Jena, Thuringia, 

Germany. Oakley, K. L., L. P. Thomas, and S. G. Fancy. 2003. Guidelines for long-term 

monitoring protocols. Wildlife Society Bulletin 2003, 31(4):1000–1003.

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