United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

General Technical Report RMRS-GTR-271

March 2012

Hydrologic Processes in the 

Pinyon-Juniper Woodlands: 

A Literature Review
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Ffolliott, Peter F.; Gottfried, Gerald J. 2012. Hydrologic processes in the pinyonjuniper woodlands: A literature review. Gen. Tech. Rep. RMRS-GTR-271. Fort 

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 20 p.

Abstract

Hydrologic processes in the pinyon-juniper woodlands of the western region of the 

United States are variable because of the inherent interactions among the occurring 

precipitation regimes, geomorphological settings, and edaphic conditions that 

characterize the ecosystem. A wide range of past and present land-use practices 

further complicates comprehensive evaluations of these hydrologic processes. 

Heterogeneous vegetative covers make it even more difficult to generalize 

hydrologic processes and the effects of land management practices on the water 

balance. Nevertheless, estimates of interception, infiltration, evapotranspiration, 

soil moisture storage, and hillslope soil erosion of these on-site hydrologic 

processes have been obtained in plot studies. Estimates of off-site streamflow 

volumes, sediment yields and transport, and quality of streamflow water are 

available from the results of watershed-level investigations. Estimates of these 

respective hydrologic processes are presented in this general technical report.

Keywords: hydrologic processes, pinyon-juniper woodlands, plots studies, 

watershed-level investigations

The Authors

Peter F. Ffolliott is a professor with the School of Natural Resources and the 

Environment, University of Arizona, Tucson, Arizona.

Gerald J. Gottfried is a research forester with the Rocky Mountain Research Station, 

U.S. Forest Service, Phoenix, Arizona.

Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not contain 

recommendations for their use, nor does it imply that the uses discussed here have 

been registered. All uses of pesticides must be registered by appropriate State and/

or Federal agencies before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable 

plants, and fish or other wildlife—if they are not handled or applied properly. Use 

all pesticides selectively and carefully. Follow recommended practices for the 

disposal of surplus pesticides and pesticide containers.
Contents

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

General Hydrologic Characteristics..................................... 1

Hydrologic Processes........................................................... 2

Plot Studies .................................................................. 2

Watershed-Level Investigations..................................... 8

Conclusions ....................................................................... 15

Acknowledgments.............................................................. 16

References......................................................................... 16

USDA Forest Service RMRS-GTR-271. 2012. 1

Introduction

Pinyon-juniper woodlands, the largest forest type 

in the United States, lie adjacent to and surround the 

montane forests of the western region. These woodlands—one of the most xeric forest types in the United 

States—occupy about 48 million acres primarily in 

Nevada, Utah, Arizona, New Mexico, and Colorado 

(Larson 1980). Extensive stands are also found in 

California and Texas with a few stands in southern 

Idaho, southern Wyoming, and western Oklahoma. 

Several associations of pinyon (Pinus spp.) and 

juniper (Juniperus spp.) trees comprise the pinyonjuniper woodlands throughout its range including 

pure or nearly pure stands of either pinyon or juniper trees and varying mixtures of the two tree genera 

(Tueller and others 1979, Meeuwig and Bassett 1983, 

Gottfried and others 1995, West 1999, Jacobs 2008, 

Pieper 2008). Complete crown closure of overstory 

trees is rare, but tree roots causing a suppression of 

understory vegetation often occupy the soils underneath well-developed stands.

Occurring at lower elevations and generally with 

less annual precipitation and higher evaporation 

rates than the montane forests, the pinyon-juniper 

woodlands (seemingly) possess a lower potential 

to increase streamflow volumes through vegetative 

management practices when compared to the higherelevation montane forests (fig.1). However, because 

of the wide distribution of the woodlands, early investigators felt that vegetative management practices 

that increase the inherently low streamflow volumes 

might also increase water supplies to downstream users (Barr 1956, Dortignac 1960). The management 

practices considered focused largely on clearing (removal) of the comparatively high water-demanding 

overstory trees by mechanical, chemical, or burning 

treatments with the conversion of these sites to less 

water-demanding herbaceous covers. It was thought 

that reductions in water consumption by herbaceous 

plants might become recoverable water.

Estimates of the on-site hydrologic processes of 

interception, infiltration, evapotranspiration, soil 

moisture storage, and hillslope soil erosion and the 

off-site processes of streamflow volumes, sediment 

yields and transport, and quality of streamflow water 

can be obtained in plot studies and watershed-level 

investigations, respectively. A review of the literature 

reporting estimates of these on-site and off-site processes is presented in this report. General hydrologic 

characteristics of the pinyon-juniper woodlands are 

characterized initially to present these estimates in 

some perspective.

General Hydrologic 

Characteristics

Pinyon-juniper woodlands are found on diverse 

landscapes including sloping mesas and escarpments, 

steep canyon, and valley bottoms at middle elevations 

within semi-arid climates (Tueller and others 1979, 

Larson 1980, Gottfried and others 1995). Older stands 

of trees often occupy rocky hillslopes with sparse understories of herbaceous plants (Swetnam and Brown 

1992). Developed from a variety of parent materials, 

soils supporting the woodlands are shallow to moderately deep and well drained with often low fertility 

levels (Evans 1988, Jacobs 2008, Laycock 1999). Most 

of the pinyon-juniper woodlands on the Coconino 

National Forest in north-central Arizona are found on 

soils belonging to the Alfisol, Mollisol, or Inceptisol 

Soil Orders (G. Robertson, 2011, personal conversation). Erodability of these soils is a general function 

of the variable vegetative cover, surface topography, 

and soil texture and structure encountered. High levels of naturally occurring hydrophobicity in some of 

these soils (Robinson 2009) can inhibit or even impede the infiltration process and, as a consequence, the 

linked processes of evapotranspiration. The amount 

of organic materials on the mineral soil surface also 

influences the hydrologic characteristics of the woodlands. Infiltration and absorption rates of the soils are 

affected by the spatial orientation and distribution of 

surface particles.

Wide fluctuations in weather patterns, low precipitation amounts, and high evapotranspiration rates 

characterize the pinyon-juniper woodlands (Gottfried 

and others 1995, Monsen and Stevens 1999, West 

1999). Only during the coldest winter months is 

precipitation generally greater than the evapotranspiration rates. Streamflow is largely ephemeral with 

only a few permanent streams originating in the 

woodlands. Streamflow events coincide mostly with 

the occurrence of streamflow-generating precipitation events with many of these flows associated with 

high-intensity rainstorms, rapid melting of snow, or 

rain-on-snow events (Lopes and others 1996, 1999). 

Snowmelt-runoff produces the greatest amount of annual streamflow volume (fig. 1). Over 70 percent of 

the annual streamflow originating in the pinyon-juniper woodlands on the Beaver Creek watersheds in 
2 USDA Forest Service RMRS-GTR-271. 2012.

north-central Arizona (Ffolliott and Baker 1977) occurs as the result of snowmelt-runoff or the occasional 

rain-on-snow events (Clary and others 1974, Baker 

1984, Baker and Ffolliott 2000). Streamflow following 

torrential thunderstorms in summer months is generally low in volume, variable in velocity, and short in 

duration. However, summer streamflows frequently 

produce the highest peaks. Hillslope soil erosion rates, 

sediment yields and transport, and water quality characteristics are variable throughout the natural range of 

the woodlands.

Hydrologic Processes

Hydrologic processes in the pinyon-juniper 

woodlands are influenced by species compositions; 

structural development and density patterns of the tree 

overstories; the nature of precipitation events occurring; and decisions made by the people responsible for 

management of the woodlands. It is not possible to adequately describe the magnitudes and dynamics of these 

hydrologic processes for all conceivable situations encountered because descriptions for the full spectrum of 

hydrologic conditions are not available. Furthermore, 

there are no known studies or investigations where all 

of the components of the hydrologic cycle have been 

measured on a single site (Roundy and Vernon 1999). 

Nevertheless, estimates of the hydrologic processes in 

the pinyon-juniper woodlands can be obtained from 

plot studies and watershed-level investigations. Some 

of these estimates are presented below.

Plot Studies

Estimates of interception, infiltration rates, evapotranspiration, soil moisture storage, and hillslope soil 

erosion rates have been made in plot studies. While 

these on-site estimates were obtained mostly from individual studies conducted on different sites, they offer 

insight to these hydrologic processes in the pinyonjuniper woodlands.

Interception—Interception of precipitation is related largely to the composition, distribution, and density 

of trees in the overstory and the intensity, duration, and 

type of precipitation, that is, rain, snow, or a combination of both types of precipitation. Increases in tree 

canopy coverage reduce the amount of precipitation 

that eventually reaches the surface of mineral soils 

(fig. 2). Skau (1964a) estimated that an average of between 10 to 20 percent of the precipitation falling on 

the pinyon-juniper woodlands of northern Arizona is 

intercepted by Utah (J. osteosperma) and alligator (J. 

deppeana) juniper tree crowns. A comparable level of 

interception by trees was reported by Collings (1966) 

in the pinyon-juniper woodlands on the Fort Apache 

Reservation in eastern Arizona.

Interception losses ranging from 25 to 35 percent 

have been associated with the canopies of redberry 

(J. pinchotii) and Ashe (J. ashei) juniper trees in the 

woodlands of the Edwards Plateau in west-central 

Texas (Hester 1996 cited by Thurow and Hester 1997). 

Elsewhere, reported interception values of western juniper (J. occidentalis) trees in the Interior West were 

less than 10 to over 40 percent depending once again 

Figure 1—The potential to 

increase streamflow volumes 

in the pinyon-juniper 

woodlands on this typical 

site in the Coconino National 

Forest near Camp Verde, 

Arizona, is not great because 

of low annual precipitation, 

high evapotranspiration, and 

shallow soils.
USDA Forest Service RMRS-GTR-271. 2012. 3

on the extent of the canopy coverage (Young and Evans 

1987, Larsen 1993 cited by Kuhn and others 2007).

The only known attempt to evaluate the effects 

of clearing tree overstories on interception values 

was that of Gifford (1975a) who compared interception losses following chaining treatments, where the 

downed trees were either windrowed or left in-place, 

with an adjacent site of natural (untreated) woodlands. Gifford found that interception losses from 

the chained sites were 10 to 70 percent less than the 

value in the natural site. The differences in interception losses on the chained sites were attributed to the 

characteristics of the specific site and the year of the 

measurements.

Precipitation passing directly through tree canopies (throughfall) must also pass through the litter 

layer beneath the canopies before infiltrating into 

the mineral soil. Interception losses associated with 

the litter layers are variable with estimates depending largely on the depth of litter accumulations on 

the site (Jameson 1966b, Gifford 1970, Scholl 1971). 

Thurow and Hester (1997) estimated interception 

losses of throughfall by the litter layer in woodlands 

comprised of redberry and Ashe juniper trees on the 

Edwards Plateau exceeding 40 percent. These losses 

were higher than values reported earlier for western 

juniper trees (Young and Evans 1987). The differences in these estimates of interception losses were 

related to the greater buildup of the litter layers beneath the redberry and Ashe juniper trees.

Varying threshold amounts of precipitation are 

necessary for precipitation to reach the soil surface 

by flowing down the stems of trees, shrubs, or herbaceous plants (stemflow) during a rainstorm event 

(Skau 1964a, Collings 1966, Thurow and Hester 

1997). However, the magnitude of stemflow in pinyon-juniper woodlands is less than 5 percent.

Infiltration Rates—Infiltration rates are typically 

greater beneath tree overstories than on sites supporting herbaceous plants because the trees reduce the 

erosive force of raindrops and the litter accumulation 

beneath the trees slows the overland flows of water 

from a site. Early plot studies established “benchmark 

values” for the range of infiltration rates that can be encountered in pinyon-juniper woodlands. For example, 

the infiltration rates of soils derived mainly from sedimentary rock, sandstones, and shale in central Utah 

ranged from 2.1 inches hr-1 in the initial 5 minutes following a precipitation event, decreasing to 1.5 inches 

hr-1 after 25 minutes (Williams and others 1969). The 

infiltration rates of soils developed (collectively) from 

sedimentary and volcanic rocks, sandstone, and shale 

in southern Utah decreased from 3.5 inches hr-1 in the 

initial 5 minutes to almost 2.0 inches hr-1 after 25 minutes (Gifford and others 1970). These results and 

findings of other plot studies (Smith and Leopold 1942, 

Figure 2—The relationship 

between woodland tree 

canopy and interception of 

precipitation was studied 

as part of the Beaver 

Creek Project within the 

Coconino National Forest, 

Arizona. The Beaver Creek 

watersheds were established 

in the late 1950s by the 

Rocky Mountain Forest 

and Range Experiment 

Station, the Coconino 

National Forest, and their 

cooperators to evaluate 

land management measures 

designed to increase water 

yields from pinyon-juniper 

woodlands and ponderosa 

pine (Pinus ponderosa) 

forests (Clary and others 

1974). Evaluations of the 

treatment impacts on other 

resources, such as wildlife, 

timber, and forage, were 

important components of 

the research.
4 USDA Forest Service RMRS-GTR-271. 2012.

Dortignac 1960, Gifford and Tew 1969a, Williams and 

others 1972, Blackburn and Skau 1974) indicate that 

infiltration rates vary greatly from site to site. Because 

of this variability, Gifford (1975b) concluded that in 

general it is difficult to “pinpoint” the factors that consistently influence the infiltration rates of a site.

Litter can store relatively large quantities of 

throughfall and, in doing so, can limit the amount of 

water infiltrating through the mineral soil surface. 

Nevertheless, the litter layers under pinyon-juniper 

trees can also contribute to the development of a better soil structure with large pores forming in the soil 

beneath the litter layers through which infiltrated water 

can pass (Thurow and Hester 1997). Hester and others 

(1997) found that the infiltration rates of soils on the 

Edwards Plateau were higher underneath the canopies 

of juniper trees than on grassland sites with the litter 

accumulations beneath the trees the presumed contributing factor. However, this latter situation might not be 

the case throughout the woodland ecosystems.

The clearing of pinyon-juniper woodlands in establishing herbaceous understories does not always alter 

infiltration rates. Of 14 sites in central Utah, Williams 

and others (1969) reported increased infiltration rates 

on two areas where comparisons were made between 

cleared and natural sites, while decreased infiltration 

rates were found on two other sites. There were no statistically significant differences in infiltration between 

cleared and natural areas on the other sites in the study. 

Elsewhere in Utah, Gifford and Tew (1969b) observed 

an increase in permeability of the surface soil on a site 

that had been cleared of trees six months previously. 

However, this trend in increased permeability was not 

observed on a nearby site. Largely similar conclusions 

were reported in other studies conducted in southern 

Utah by Gifford (1970) and Gifford and others (1970).

The microflora (crytogamic) crust that forms a cover 

in some soils in the pinyon-juniper woodlands on the 

Colorado Plateau can have an impact on hydrologically important soil properties including infiltration rates 

according to a study by Loope and Gifford (1972). 

These investigators found that plots with a microflora 

soil cover in southern Utah often have higher infiltration rates than plots that had been cleared of woodland 

trees. Loope and Gifford also indicated a possibility 

of increased soil erosion once the crust had been disturbed by clearing tree overstories.

Pierson and others (2008a) found higher infiltration 

rates on microsites beneath tree canopies than on the 

interspace areas situated between tree and large shrub 

canopies following simulations of artificial rainfallconcentration flows in the Interior West. This finding 

supported the general observations that infiltration 

rates are greater beneath tree overstories than on sites 

supporting herbaceous plants. The amount of plant 

cover on these study sites was reported to be the main 

causal factor for these observed differences in infiltration. Reid and others 1999) reported similar results in 

a pinyon-juniper woodland in the southwestern region. 

Cline and others (2010) working on rain-fall simulation plots in Utah reported that bare soil areas that 

were covered with juniper residues generated during a 

mastication treatment had higher infiltration rates and 

lower sedimentation rates than sites without a cover of 

residues.

The site-specific estimates of infiltration rates reported in the literature should be interpreted as largely 

localized results. Nevertheless, the results of the plot 

studies reviewed suggest that it is likely that there is 

neither a consistent increase nor decrease in infiltration 

rates on sites cleared of trees.

Evapotranspiration—Evapotranspiration is generally the largest route of water outflow from a site. 

There are no known estimates of the magnitude of 

evaporation of the precipitation that is intercepted by 

the trees in the pinyon-juniper woodlands. However, 

estimates of evaporation of the throughfall that is intercepted by litter layers can be inferred from studies 

of the depths to which throughfall penetrates into the 

litter layers.

Skau (1964b) found that the evaporation from soils 

in the pinyon-juniper woodlands of northern Arizona 

is generally confined to the upper 10 to 12 inches unless there was severe cracking of the basaltic soils on 

the site. In a study of the movement of water through 

the litter accumulations beneath tree overstories in the 

woodlands of southern Utah, Gifford (1970) found 

that 3.8 inches of water had penetrated only 1 inch 

into the litter layer that averaged 1.5 inches in depth 

beneath closed tree canopies with no water reaching 

the mineral soil surface. However, evaporation of 

comparable volumes of throughfall appeared less on 

sites with largely discontinuous tree canopies in the 

same area because water on these sites penetrated 59 

inches into the soil profile. Thurow and Hester (1997) 

speculated that the throughfall from low-intensity, 

short-duration rainstorms in the hill country of eastern Texas will not infiltrate into a soil body because 

of the high rates of evaporation of the rainfall intercepted by tree canopies and underlying litter.

Transpiration losses from overstory trees have 

been estimated to account for 80 to 95 percent of the 

precipitation input into the pinyon-juniper woodlands 

by other investigators (Lane and Barnes 1987, Weltz 
USDA Forest Service RMRS-GTR-271. 2012. 5

1987 cited by Thurow and Hester 1997, Carlson and 

others 1990). The magnitudes of the reported transpiration losses depended largely on the composition, 

structure, and densities of the tree overstory and the 

amount, intensity, and duration of the precipitation 

event occurring.

Obtaining reliable estimates of transpiration losses 

under field conditions is a difficult task (Brooks and 

others 2003, Shuttleworth 2008). One approach to estimating these transpiration losses is by applying the 

evapotranspiration tent method outlined by Mace and 

Thompson (1969). Decker and Skau (1964) enclosed 

Utah and alligator juniper trees in a ventilated tent of 

transparent plastic sheeting to estimate transpiration 

losses from the sample trees. Increased humidity of 

the ventilation stream was considered to be a direct 

index of vapor production in this study and, therefore, 

transpiration rates through conversion. The estimates 

of transpiration observed by Decker and Skau were 

variable throughout a day as might be expected. 

Transpiration rates increased throughout the morning 

hours, peaked close to noon, and remained at this high 

level until the middle of the afternoon after which 

transpiration decreased rapidly into the early night. 

By linking these fluctuations of daily transpiration 

rates to the occurrence of trees in a “typical” pinyon-juniper woodland of northern Arizona that were 

similar in characteristics to the trees sampled, these 

researchers estimated water losses by transpiration 

approached the values reported by the investigators, 

cited in this section.

Dugas and others (1998) estimated a short-term 

decrease in evapotranspiration rates following the 

removal of Ashe juniper trees in central Texas. To 

place the reported reduction in evapotranspiration of 

only 0.003 inch per day into perspective, the average precipitation in the vicinity of this study was 26.5 

inches annually. The lack of a sustained reduction 

in evapotranspiration losses after the juniper trees 

were removed was attributed largely to the increased 

growth of herbaceous plants on the plot.

Estimates of evapotranspiration losses on a watershed-basis can be approximated by an analysis of a 

simplified water budget (Brooks and others 2003). 

Ignoring the other components of the hydrologic 

cycle in such an approximation, the estimates of 

evapotranspiration are expressed (simply) as the difference between precipitation inputs and streamflow 

outputs. On the Beaver Creek watersheds located in 

the pinyon-juniper woodlands, an estimate of annual 

precipitation falling on the watersheds (the input) is 

18.1 inches, while the annual streamflow from these 

watersheds (the output) averages about 1 inch (Clary 

and others 1974, Baker 1984). The 1 inch value is 

equivalent to a 1 inch depth of water covering the total 

acreage of the watershed. In reality, not all of the areas of a watershed produce the same amount of water. 

Some sites produce more runoff than others depending on location with respect to the channels, geology 

and soils and vegetation characteristics. The difference of 17.1 inches is considered an approximation 

of evapotranspiration. This value is at the high end 

of the estimates reported above. Evapotranspiration 

was likely over-estimated in this example, however, 

because the other components of the hydrologic cycle 

were not considered in the analysis (fig. 3).

In separating the components of evapotranspiration, evaporation of precipitation that is intercepted 

in tree crowns and transpiration losses from the 

Figure 3—Plot studies to determine the influence of pinyonjuniper woodlands and management treatments on 

evapotranspiration, soil moisture, and other attributes 

have been conducted throughout the West, such as this 

microclimatology study site in the Coconino National 

Forest.
6 USDA Forest Service RMRS-GTR-271. 2012.

trees themselves are decreased or eliminated (to unknown magnitudes) by clearing the tree overstories. 

However, evaporation of water from soil surfaces is 

likely to increase (also to unknown magnitudes) because the cleared site is exposed to increased solar 

radiation. Magnitude of the combined evapotranspiration processes following a clearing operation is 

dependent mostly on the characteristics of the site after the operation.

Soil Moisture Storage—Estimates of soil moisture storage in the pinyon-juniper woodlands vary 

depending largely on the infiltration characteristics 

of the soil; the magnitudes and sequencing of the 

recent precipitation events; whether the points of 

measurement were beneath tree canopies or situated 

in intercanopy spaces; the topographic position of 

the measurement point; and the instrumentation and 

methods applied and sampling protocols followed 

in obtaining the estimates. Only a few estimates of 

soil moisture storage have been obtained from plot 

studies, and, therefore, these estimates should be considered site-specific.

Intermittent measurements of soil moisture storage 

beneath natural and cleared stands of alligator and 

Utah juniper trees in northern Arizona were obtained 

by Skau (1964b) from the beginning of the summer 

monsoons (June 30) through the end of November in 

2 consecutive years with a Veihmeyer tube. He found 

more water held in storage in the upper 24 inches 

of clayey and clayey-loam soils on cleared plots in 

75 of the 90 comparisons made with plots supporting natural woodland conditions. The differences in 

the comparisons were considered small, however, 

ranging from -6.6 to +6.7 percent with only 15 comparisons exceeding ±4.0 percent. Skau concluded that 

the clearing of overstory trees might have little effect 

on streamflows insofar as these streamflows are influenced by soil water storage in the upper soil layers. 

He emphasized, however, that interpretations of the 

results of this study should be “conditional” because 

only five measurements were taken beneath each of 

the tree species on nine sampling dates in each year 

of the study and that only two types of soil were 

sampled.

Gifford and Shaw (1973) compared the soil moisture storage of plots on two study areas in natural 

pinyon-juniper woodland sites in Utah with plots 

where trees had been cleared by chaining and either 

windrowed or left in place. These researchers found 

that the greatest storage of soil moisture occurred on 

plots where the chained trees had been left in place 

with the least soil moisture storage on the natural 

woodland plots. Gifford and Shaw attributed the observed differences in soil moisture storage to changes 

in microclimate caused by the chaining treatment; 

differences in the rooting depths on the plots; and the 

mulching effect of litter on the debris-in-place plots. 

Also, there was no evidence of deep seepage of soil 

water on any of the plots in the study (see below).

Average volumetric water content (VWC) in the 

upper 4 inches of fluvial deposits on small (plot) 

watersheds of 2.5 to 3.2 acres in area in the pinyonjuniper woodlands of northern New Mexico ranged 

widely depending mostly on whether the averages calculated were obtained on sites beneath tree 

canopies or intercanopy sites on grass-dominated 

hillslopes or valley bottoms (Ochoa and others 2008). 

Calculations of the average VWC values were based 

on measurements made with VWC reflectometers 

inserted horizontally to about 4 inches on each sampling site. The lowest average value of 0.1 (1 percent) 

was observed in the intercanopy of grass-dominated 

hillslopes with the highest average value of 0.18 (18 

percent) in the intercanopy of the valley bottoms. The 

reported differences in average VWC values were attributed mostly to the topography of the watersheds 

that included greater valley areas than hillslope sites. 

Differences in measurements taken beneath the tree 

canopies in relation to intercanopy locations were 

insignificant.

Deep drainage of soil water is generally limited in 

pinyon-juniper woodlands because of the relatively 

small portion of a precipitation input that reaches the 

soil surface and ability of the overstory trees to extract relatively large quantities of soil moisture held in 

storage (Thurow and Hester 1997). As a consequence, 

only small amounts of the water percolating through 

a soil profile have a chance to drain significantly beneath the rooting zone of the overstory trees.

Overland Flows of Water—With exception of the 

information gleaned from a few infiltrometer studies, 

there are few estimates of the magnitudes of overland 

flows of water from the hillslopes of pinyon-juniper 

woodlands in the literature. Qualitatively speaking, 

the results of infiltrometer studies suggest that there 

might be increases in overland flows of water following varying treatment combinations involving the 

clearing tree overstories, burning of the resulting debris, and seeding of herbaceous plants (Myrick 1969, 

Gifford 1975b). However, it was hypothesized that 

any increase in overland flows following the treatments would likely diminish once the seeded plants 

become established.
USDA Forest Service RMRS-GTR-271. 2012. 7

Bolton and others (1991) suggested that the vegetative arrangement and amount of litter that has 

accumulated on a hillslope in the pinyon-juniper 

woodlands have a “considerable influence” on the 

magnitude of overland flows of water. These investigators observed that most of the overland flows in the 

woodlands of the southwestern region originate in the 

interspace areas among tree canopies where litter accumulations were less (fig. 4). The litter buildups on 

a hillslope can also delay the onset of overland flows 

following a rainstorm. For example, Heede (1987) 

found that overland flows on “micro-watersheds” in 

Arizona were disrupted by almost 60 percent as a result of the hummocks of litter that surrounded pinyon 

and juniper trees.

That litter can hinder the overland flows of water 

was also reported by Pierson and others (2008a) who 

measured greater overland flows on plots established 

in interspace areas between tree and shrub canopies 

than on plots beneath tree canopies following (artificial) rainfall-concentration flow simulations in the 

pinyon-juniper woodlands of the Great Basin. Gifford 

(1970) found that the removal of overstory trees (the 

primary source of litter buildups) on plots located in 

the pinyon-juniper woodlands of Utah also tended to 

increase the overland flows of water in comparison to 

the flows on plots supporting a tree overstory.

It can be concluded generally, therefore, that on 

sites where the infiltration rates are “strongly related” to the densities of tree canopies and associated 

accumulations of litter, overland flows of water originating beneath the tree canopies would be “markedly 

less” than those on sites dominated by herbaceous 

plants (Gifford 1975a). This conclusion, if generally 

true, has implications for clearing woodland trees to 

increase streamflow volumes.

Hillslope Soil Erosion Rates—The soil particles 

eroded from one hillslope site are either deposited 

downslope on another hillslope site or transported into 

a stream channel to become entrained sediment when 

water flows in the channel. Unfortunately, limited 

quantitative information is available to characterize 

hillslope soil erosion in natural pinyon-juniper woodlands. It was suggested in an early study on the Fort 

Apache Reservation in eastern Arizona, however, that 

the rates of soil erosion would probably increase as a 

result of suppressing herbaceous plants beneath tree 

overstories (Arnold and Schroeder 1955). In another early study, Dortignac (1956) concluded that soil 

erosion in the Rio Grande River Basin was related to 

the condition, density, and extent of herbaceous plant 

covers with greater soil erosion originating on sites 

largely devoid of trees.

More recently, Bolton and others (1991) reported 

that because tree canopies are comparatively dense 

relative to a cover of scattered herbaceous plants, the 

denser tree canopies tended to protect the soil surfaces 

beneath the tree canopies from the erosive impacts of 

falling raindrops and, therefore, helped to mitigate the 

detachment of soil particles on a hillslope. Bolton and 

his colleagues also stated that hillslope soil erosion 

rates in the pinyon-juniper woodlands are dependent 

largely on the varying interactions among the vegetation, soil, and commonly encountered rainstorm 

characteristics.

A conceptual framework formulated by Wilcox 

and Breshears (1995) supported by plot studies at the 

Los Alamos National Laboratory in northern New 

Mexico provides an insight to the patterns of hillslope 

soil erosion on the landscape-heterogeneity of southwestern pinyon-juniper woodlands. On the basis of 

Figure 4—Most of the overland water flows in the pinyonjuniper woodlands originate in the interspace between tree 

canopies, such as on this hillslope in the Sevilleta National 

Wildlife Refuge in central New Mexico. The windmill 

pumps water into a tank for wildlife.
8 USDA Forest Service RMRS-GTR-271. 2012.

observations by these investigators, overland flows of 

water and concurrent soil erosion rates were greatest 

on patches of bare mineral soil with the entrained soil 

particles moving downhill through interconnected 

intercanopy areas to the canyons below. In a verification of this conceptual framework, Pierson and others 

(2008a) found that soil erosion following the clearing 

of pinyon and juniper trees in the Great Basin was 

greater on plots situated in the interspace among trees 

and shrubs than on plots beneath these canopies following rainfall-concentration flow simulations.

More quantitative estimates of hillslope soil erosion rates have been obtained in a few plot studies. The 

rates of soil erosion in the pinyon-juniper woodlands 

of central Utah ranged from 0.48 to 0.76 tons per acre 

annually (Williams and others 1969). Results from 

another plot study, also in Utah, indicated that annual 

soil erosion rates averaged 0.10 to 0.42 tons per acre 

(Gifford and others 1970). These results and those from 

other plots studies conducted in the woodlands of Utah 

and the Pacific Northwest (Gifford 1973, Blackburn 

and Skau 1974, Pierson and others 2007, 2008b) report 

that generally there is little or no consistent decrease 

or increase in the rates of hillslope soil erosion following the clearing of tree overstories and then seeding 

the cleared sites with herbaceous plants. However, estimates of soil erosion obtained from plot studies can 

preclude a “full expression” of the turbulent activity 

of overland flows of water on a larger hillslope, and, 

therefore, these estimates obtained from plot studies must be interpreted accordingly. Furthermore, the 

results obtained from plot studies do not necessarily reflect soil erosion rates for time frames, site conditions, 

or precipitation patterns other than those associated 

with these often short-term studies.

A model proposed by Baker and others (1995) 

describes the combined impacts of water and wind erosion of soils in the pinyon-juniper woodlands and their 

dependency on annual precipitation amounts. This 

model was developed initially by Heathcote (1983) 

with emphasis placed on the linkage of plant cover to 

the respective rates of soil erosion. Where both water and wind erosion occur under a vegetative cover, 

there is a compensating effect between the two erosion 

agents while the erosional effects are additive on bare 

soil. This model demonstrated that the clearing of trees 

is likely to increase both water and wind erosion. It 

also highlighted the relative significance of wind erosion in the pinyon-juniper woodlands—an erosional 

process that has been largely ignored in the past. Baker 

and others (1995) report on a study that demonstrated 

that wind can move significant amounts of soil and 

ash in a pinyon-juniper woodland site where harvesting slash was burned. Approximately 238 pounds per 

square foot of soil and ash were collected in a sampler 

located 1.6 ft above the soil surface in this study.

Watershed-Level Investigations

Expansion of pinyon-juniper woodlands onto 

landscapes dominated by herbaceous plants impedes 

streamflow for off-site (downstream) uses according to 

many investigators (Hawkins 1987, Gottfried and others 1995, Roundy and Vernon 1999, Kuhn and others 

2007, Pierson and others 2008a, Tennesen 2008). It has 

often been thought, therefore, that the clearing of tree 

overstories might reverse this process and, in doing so, 

increase downstream flows of water. The replacement 

of deep-rooted trees and shrubs with shallower-rooted herbaceous species that consume less water would 

(theoretically) make more water available for off-site 

use (Clary and others 1974, Ffolliott and Thorud 1975, 

Davis and Pase 1977). While there was some evidence 

in the early literature (prior to 1956) that the clearing 

of woodland trees and replacement with a herbaceous 

cover might not produce additional water, a recommendation of Barr (1956) and others was that evaluations 

of alternative conversion treatments to enhance streamflow regimes in the pinyon-juniper woodlands should 

be made.

A number of methods have been used to increase 

herbage (forage) production through clearing the 

competing tree overstories in the pinyon-juniper tree 

woodlands: mechanical removals, chemical control, 

and the use of fire. It was suggested by several early 

investigators (Skau 1964a, Collings 1966, Gifford and 

others 1970, Gifford 1973, Clary and others 1974) that 

the same methods of clearing tree overstories might 

also be suitable for streamflow enhancement. A review of watershed-level investigations to evaluate the 

effects of clearing tree overstories on streamflow volumes, sediment yields and transport, and the quality of 

streamflow water by applying these methods follows.

Streamflow Volumes—Extensive areas of pinyonjuniper woodlands have been cleared in the past by 

chaining or cabling in which a heavy anchor chain 

or cable was dragged between two tractors (Cotner 

1963, Arnold and others 1964, Warskow 1967, Clary 

and others 1974). Pushing (bulldozing) has also been 

used to remove the larger trees. However, both of these 

mechanical methods of pulling trees from the ground 

leave pits (root cavities) where the trees formally stood 

(fig. 5). Skau (1961) determined that these pits reduce 

overland flows of water and, in doing so, streamflow 
USDA Forest Service RMRS-GTR-271. 2012. 9

volumes from the cleared watersheds. However, an alternative approach to clearing tree overstories by this 

heavy equipment is hand-clearing with ax or saw, by 

using shears mounted on light tractors, or by mechanical mastication to grind up the trees without displacing 

the base. These latter methods would (in theory) have 

a minimal impact on reducing the consequent overland flows of water because pitting is eliminated and 

the increased soil erosion that often occurs with chaining, cabling, or pushing would be minimized. Tree 

branches left on the site would slow erosion processes. 

Mastication, depending on the equipment used, may 

cause some localized soil compaction and reduced infiltration (Cline and others 2010).

Early studies of the effects of clearing pinyon-juniper 

woodlands on streamflow volumes were undertaken in 

the basin of the adjacent Corduroy and Carrizo Creeks 

in eastern Arizona. The tree overstory on Corduroy 

Creek was cleared on 34,000 acres (25 percent of the 

basin) by chaining, while the shrubs and litter accumulations beneath the higher-elevation ponderosa 

pine (Pinus ponderosa) trees were burned on 18,000 

acres (13 percent of the basin). Carrizo Creek was 

left undisturbed to serve as a control to evaluate these 

treatments. Because evaluation of the two treatments 

on Corduroy Creek considered only their total (overall) 

effect on streamflow volumes (LeCrone 1959, Collings 

and Myrick 1966), conversion of the tree overstory to 

a herbaceous cover could not be isolated as the “sole 

influence” on the resulting streamflow volumes. This 

problem was not considered to be important by the investigators, however, because they concluded that the 

clearing treatment would probably produced no significant changes in streamflow volumes.

Two watersheds less than 100 acres in area on 

Cibecue Ridge in the same general vicinity as 

Corduroy and Carrizo Creeks were selected to investigate the possibility that converting tree overstories 

in the pinyon-juniper woodlands to herbaceous plants 

would increase streamflow volumes on a smaller-scale. 

Chaining cleared the tree overstory on one of the watersheds, the resulting slash was burned, and the treated 

watershed was fenced to exclude livestock and seeded 

with a mixture of grasses. The other watershed was 

designated a control. A “parameter model” to predict 

how components of the hydrologic cycle might change 

as a consequence of this conversion treatment was used 

to evaluate this experiment (Robinson 1965, Myrick 

1971). The investigators found that streamflow volumes 

increased significantly on the converted watershed in 

the first 2 post-treatment years, but then decreased to 

below the expected (projected) streamflow volumes on 

the untreated watershed in the following 2 years. One 

assumed reason for this decrease in streamflow volumes was an increase in transpiration rates attributed 

to the perennial grasses seeded as a part of the treatment prescription.

The effectiveness of converting tree overstories to 

herbaceous covers to increase streamflow volumes in 

the pinyon-juniper woodlands by mechanical methods 

was also evaluated on Beaver Creek (Clary and others 1974, Baker 1984, Baker and Ffolliott 2000). A 

clearing treatment similar to that used extensively to increase herbage (forage) production in the southwestern 

Figure 5—Mechanical methods 

of removing pinyon and 

juniper trees to increase 

forage and water yields 

were common throughout 

the West. This is a typical 

chaining operation on the 

Coconino National Forest.
10 USDA Forest Service RMRS-GTR-271. 2012.

region was applied on a 323-acre watershed. Trees 

were uprooted by cabling; the smaller trees missed by 

the cable were felled by power saws. The larger trees 

were then burned and the cleared watershed seeded 

with a mixture of forage species. A hand-clearing treatment with the trees felled by power saws was applied 

later on a second watershed that was 363 acres in area. 

Pitting and other soil disturbances created by uprooting trees by cabling were largely eliminated by this 

method (fig. 6). The stumps of alligator juniper, the 

principal tree species on the watershed, were treated 

with polychlorinated-benoic acid to reduce subsequent 

sprouting. Shrub live oak (Quercus turbinella) clumps 

were treated initially with fenuron and later with 

Figure 6—Felling trees 

in pinyon-juniper 

woodlands with (a) power 

saws (Beaver Creek), 

(b) mechanical shears 

mounted on light tractors 

(Greene Ranch, Torrance 

County, New Mexico), or 

(c) the use of mastication 

machinery (Dolores Public 

Lands, Dolores, Colorado),

eliminate pitting and other 

disturbances caused by 

uprooting trees by cabling.
USDA Forest Service RMRS-GTR-271. 2012. 11

picloram to reduce their occurrence. Gambel oak (Q. 

gambelii) sprouts were sprayed with 2,4,5-T in the dormant season to control possible re-emergence of these 

species. No seeding of forage species was done.

The results of investigations in the basin of 

Corduroy and Carrizo Creeks, at Cibecue Ridge, and 

on the two Beaver Creek watersheds indicated that the 

conversion of pinyon-juniper woodlands to herbaceous 

covers by mechanical means has little long-term effect 

on streamflow volumes (Clary and others 1974, Baker 

1984, Baker and Ffolliott 2000, Ffolliott and Stropki 

2008). The reduced overland flows of water caused by 

pitting where chaining or cabling methods were applied likely compensated for any potential increases in 

streamflow brought about by the reductions in transpiration losses by the tree removals. Trees in the more 

open stands on the Beaver Creek watershed converted 

to a herbaceous cover by hand-clearing were probably 

too few in number and too scattered in distribution for 

the treatment to impact the overall loss of water to the 

transpiration process.

Killing trees with herbicides is another option for 

converting tree overstories to herbaceous plants. In 

testing this option to increase streamflow volumes, 

an herbicidal treatment consisting of a mixture of 

21⁄2 pounds of picloram and 5 pounds of 2,4-D was 

applied by helicopter to a Beaver Creek watershed of 

363 acres (Clary and others 1974, Baker 1984, Baker 

and Ffolliott 2000). This aerial application was sprayed 

on 281 acres of the watershed with the remaining 

82 acres either untreated or where individual trees were 

hand-treated with the herbicide using a mist-blower. 

The purpose of this treatment was to lower transpiration losses by killing the trees while leaving the dead 

trees standing to reduce desiccating windspeeds and 

impinging solar radiation to control evaporation losses 

(fig. 7). The treatment also avoided trapping overland 

flow in pits formed by uprooting trees. The dead trees 

were harvested for firewood 8 years after the initial 

treatment; the resulting slash was piled and burned the 

following year.

Figure 7—Streamflow increased significantly after the aerial application of herbicides on a pinyon-juniper woodland 

watershed at Beaver Creek. However, the increases were lost once the dead trees, which reduced evaporation, were 

harvested.
12 USDA Forest Service RMRS-GTR-271. 2012.

Application of the herbicides on the Beaver Creek 

watershed was the only known conversion treatment 

tested to significantly increase streamflow volume. 

However, while this increase averaged 60 percent by 

the time when the dead trees were harvested on the watershed, it was less than 1⁄2-inch in absolute terms (Clary 

and others 1974, Baker 1984, Baker and Ffolliott 2000, 

Ffolliott and Stropki 2008). Furthermore, it was determined that this increase in streamflow volume could 

be expected to occur only when winter precipitation 

amounts equaled or exceeded the average (at the time 

of the experiment) of 9.4 inches (Baker 1984). It must 

be stated that the large scale application of herbicides 

on public lands is limited in the southwestern region 

at this time because many people oppose the use of 

chemicals for almost any natural resources management purpose, and, importantly, their use is controlled 

by environmental regulations.

Broadcast burning has been successful in killing 

trees in pinyon-juniper woodlands for rangeland improvement purposes where trees were dense enough 

to carry a fire (Cotner 1963, Arnold and others 1964). 

Individual tree burning (Cotner 1963, Jameson 1966a) 

has also been applied on a limited scale. From a hydrological standpoint, pits are not created and increases 

in soil erosion are minimal when these burning treatments are applied carefully. It is unknown, however, 

if streamflow volumes might increase on a watershedbasis by either broadcast burning or the burning of 

individual trees. It also is not known if streamflow 

volumes would change as a result of prescribed burning treatments. While the effects of prescribed burning 

on some of the hydrologic processes characterizing 

the pinyon-juniper woodlands have been estimated 

(Roundy and others 1978, Miller and Tausch 2001, 

Rau and others 2005, Pierson and others 2008b), this 

conversion method has not been adequately evaluated 

in terms of altering streamflow volumes. It is likely 

that the effects of prescribed burning treatments on 

streamflow volumes are “hydrologically less” than the 

effects of a large wildfire, however, because the tree 

overstories, herbaceous vegetation, and litter are only 

partially burned with the prescribed low severity fire 

(Baker 1990, DeBano and others 1995).

Kuhn and others (2007) combined available research findings relating to the impacts of removing 

juniper trees in the Klamath River Basin of northern 

California with average precipitation and vegetation 

information to evaluate the possibilities of increasing 

streamflow volumes by clearing the tree overstories on 

a large-scale. Kuhn and his colleagues applied this information to tributaries in the Klamath River Basin that 

had “significant areas” supporting juniper trees by identifying the areas dominated by these trees; determining 

the areas with average annual precipitation amounts 

above an assumed (but not specified) threshold value 

thought necessary to increase streamflow volumes; 

and identifying the extent of the overlap between areas 

dominated by juniper trees and those receiving greater 

than the threshold precipitation. These investigators 

found that only 4,438 acres within the 520,000 acres of 

one of the tributaries in the basin met their feasibility 

criteria for evaluating streamflow enhancement. No areas in the other three tributaries studied satisfied these 

requirements. It was determined that there was “no 

strong evidence” that streamflow volumes would increase “substantially” by even the complete removal of 

overstory trees. It was further concluded that opportunities for small-scale increases in streamflow volumes 

could not be adequately evaluated with the available 

data.

Partial harvesting of pinyon-dominated woodlands 

is a recent management activity. The objectives of this 

harvesting are to increase growth of high quality and 

healthy residual trees, to limit insect caused mortality 

by reducing stand densities and removing poor quality 

trees, to provide tree regeneration sites, and to enhance 

pinyon seed production (Gottfried 2008, Page 2008). 

The population of pinyon trees throughout the West 

has declined because of the interaction of drought, 

overstocked stands, and infestations of the engraver 

bark beetle (Ips confusus). A survey in 2003 and 2004 

of five National Forests in Arizona reported that pinyon mortality caused by the beetle ranged from zero 

to 48 percent on a Forest level and that mortality was 

positively correlated to stand densities and negatively 

correlated to elevation on most Forests (McMillan and 

others 2008). Many pinyon-juniper stands are being 

converted to essentially pure juniper stands.

The available literature does not include much information from plot or watershed—level research on 

the impacts of partial harvesting or insect mortality on 

hydrologic processes in the pinyon-juniper woodlands. 

However, we know from studies in the higher elevation 

ponderosa pine watersheds at Beaver Creek (Gottfried 

and others 2008) and mixed conifer stands at Workman 

Creek in central Arizona (Rich and Gottfried 1976) 

that it is necessary to reduce stand densities and to create openings to significantly increase in water yields. 

If stand densities are reduced significantly by harvesting or insects, it could be possible to increase moisture 

availability and affect surface runoff at higher (moister) elevations. If dead trees are left standing, it might 

be possible to duplicate the water yield results from the 
USDA Forest Service RMRS-GTR-271. 2012. 13

herbicide treated watershed at Beaver Creek (Ffolliott 

and Stropki 2008). However, if significant numbers of 

juniper trees survive, they would likely consume any 

additional moisture that was previously used by pinyon. Potential surplus moisture is less likely on drier, 

lower elevation sites where mortality appears to be 

greater.

Sediment Yields and Transport—Knowledge of 

sediment accumulations in a stream channel is applied 

occasionally as a proxy for estimating the magnitudes of hillslope soil erosion on a watershed-basis. 

Watersheds concentrating large quantities of sediment 

in their stream channels are generally the watersheds 

experiencing high rates of soil erosion (Brooks and 

others 2003). It must be emphasized, however, that not 

all of the soil particles eroded on a hillslopes reach a 

stream channel to become sediments. As mentioned 

earlier, some of the soil particles will be deposited 

on other hillslope sites before reaching the channel to 

become sediment. Keeping this point in mind, a few 

measurements of sediment yields in the southwestern 

pinyon-juniper woodlands are available.

Total sediment yields have been measured on some 

of the Beaver Creek watersheds by means of catchment basins to collect bedload materials and a splitting 

device to sample suspended sediments on the watersheds (Brown and others 1970). Progressively smaller 

fractions of streamflow are diverted by the splitter to 

a collection tank where samples are taken for analysis 

of suspended sediments (fig. 8). Total sediment yields 

from watersheds supporting natural pinyon-juniper 

woodlands (based on 13 station-years of measurements) ranged from 0.01 to 0.31 tons per acre annually 

with an average of 0.10 tons per acre (Clary and others 

1974, Ffolliott and Thorud 1975). In contrast, the total 

sediment yields following the chaining of one of the 

Beaver Creek watersheds ranged from a trace to 1.1 

tons per acre annually with the largest total sediment 

yields following the historic 1970 Labor Day rainstorm 

(Thorud and Ffolliott 1973). To place the larger value 

in perspective, the recurrence interval of this unprecedented rainstorm event was estimated to be 100 to 150 

years.

A catchment basin and splitting device had not been 

installed on the Beaver Creek watershed where trees 

were felled by power saws, precluding measurements 

of total sediment yields from this watershed. However, 

samples of suspended sediment concentrations measured before and after the hand-clearing treatment 

were not significantly different (Clary and others 1974, 

Ffolliott and Thorud 1975). The researchers concluded, therefore, that there likely was “no meaningful” 

change in sediment yields following the mechanical 

conversion treatments evaluated on the Beaver Creek 

watersheds.

There were no significant changes in total sediment 

yields measured with a catchment basin and a splitting device following application of herbicides on one 

of the Beaver Creek watersheds in comparison to the 

pre-treatment measurements (Clary and others 1974, 

Ffolliott and Thorud 1975). While there “appeared” 

to be a decrease in total sediment yields following the 

Figure 8—Bedload materials 

are collected in a catchment 

basin and suspended 

sediments are collected by a 

splitter at the weir site on an 

experimental pinyon-juniper 

watershed at Beaver Creek.
14 USDA Forest Service RMRS-GTR-271. 2012.

application of this treatment, the evaluation period was 

“unusually” dry according to the investigators, and, as 

a consequence, sediment yields from the watershed 

were likely reduced significantly. The investigators 

concluded, therefore, that there was no meaningful 

change in sedimentation as a result of the chemical 

clearing treatment.

Hansen (1966) defined the concentration index 

as a parameter to be an approximation of the average suspended sediment concentrations originating 

from (arbitrarily defined) 1-square-mile watersheds 

in the pinyon-juniper woodlands of northern Arizona. 

He reported that the concentration index was related 

significantly to precipitation characteristics, elevation 

of the watersheds, and litter accumulations on the 

soil surface of the watersheds. Hansen summarized 

the suspended sediment concentrations approximated 

by the concentration index by elevational strata to be 

45 to 128,000 parts per million (ppm) on watersheds 

less than 6,000 ft in elevation and 18 to 1,720 ppm on 

watersheds 6,000 to 6,500 ft in elevation. The lower 

concentrations of suspended sediments on watersheds 

at lower elevations were attributed largely to the lower 

volumes of streamflow from the lower watersheds to 

transport sediments.

Sediment rating curves relating suspended sediment concentrations to streamflow discharge (Brooks 

and others 2003) provide another approach in determining the effects of vegetative management practices 

on sedimentation processes. Sediment rating curves 

developed for watersheds in the pinyon-juniper woodlands on Beaver Creek indicated that the mechanical 

clearing of tree overstories by chaining resulted in 

increased suspended sediments at selected rates of 

streamflow discharges because of the soil disturbances caused by uprooting of the trees (Lopes and others 

1996, 1999). The highest suspended sediment concentrations (approaching 80 ppm) were associated with 

the high intensity, short duration rainfall events occurring in the summer, while the lowest concentrations 

(less than 5 ppm) were attributed to the lower volumes 

of streamflows generated mostly by the late-winter and 

early-spring snowmelt-runoff events. There were no 

changes in the sediment rating curve on the watershed 

where the herbicidal conversion treatment was applied 

in relation to the sediment rating curve before the treatment. The researchers reported that the soil surface on 

the watershed was not disturbed significantly by this 

treatment since most of the chemical was applied by 

helicopter.

The primary mover of the sediments that accumulate in stream channels is the infrequent large 

rainstorm events that produce the intermittent streamflows of sufficient energy to transport sediments that 

occur throughout the pinyon-juniper woodlands of the 

western region. The movement of sediments in stream 

channels, therefore, is episodic in nature. However, 

the accumulation, storage, and transport of sediments 

through a stream channel are (collectively) complex 

processes (Heede 1985, Shen and Julien 1993). Loss of 

tree overstories by clearing, excessive livestock grazing, or the occurrence of wildfire can result in “large” 

overland flows of water that become sufficiently concentrated in the stream channels to move sediment 

accumulations. Regardless of the mechanism, sediments entrained in streamflows are transported further 

downstream than the larger bedload materials in these 

events.

Quality of Streamflow Water—Samples of 

streamflow water collected on the Beaver Creek watersheds before and after clearing of tree overstories 

by mechanical methods and the application of herbicides provide insight to the changes in the quality of 

streamflow water that can occur as a result of these 

conversion practices. Chemical constituents analyzed 

in these samples included pH, electrical conductivity, 

total dissolved solids, hardness, silica as SiO2, calcium, 

magnesium, sodium, potassium, biocarbonate, carbonate, sulfate, chloride, and fluoride. Iron, manganese, 

boron, nitrate, and phosphate were also included in the 

analysis of some of these samples.

Samples obtained from all of the watersheds cleared 

of tree overstories on Beaver Creek generally exceeded 

the minimum quality standards—at the time that the 

samples were obtained—for agricultural irrigation, 

public water supplies, and aquatic life (Clary and others 1974, Ffolliott and Thorud 1975). However, the iron 

content in a few of the samples exceeded recommended quality standard for drinking water and aquatic life. 

All of the samples collected on the watersheds had low 

sodium absorption ratios. Electrical conductivity values in the streamflow water from the watershed where 

the hand-clearing treatment was applied were low. 

While some of the samples obtained on the watershed 

where the chaining treatment was applied showed a 

“slight deterioration” of the quality standards specified 

for irrigation purposes, these samples still exceeded 

the quality standards for drinking water and aquatic 

life. Streamflow water from the chained watershed was 

“moderately hard” while it was “soft” from watershed 

that was hand- cleared.

The rate of picloram applied on the Beaver Creek 

watershed treated with herbicides to kill the trees was 

known to be “excessive” at the time of its application. 
USDA Forest Service RMRS-GTR-271. 2012. 15

However, the rates were applied nevertheless to aid 

researchers in planned studies of the post-treatment 

picloram residue in streamflow water. The highest concentration of picloram (0.32 ppm) was detected in the 

initial samples obtained following the treatment (Clary 

and others 1974), while the picloram concentrations 

were less than 0.01 ppm by the spring following its 

application. It was determined that only 1.3 percent of 

the applied picloram left the watershed in streamflow 

water, with 90 percent of this concentration leaving the 

watershed in the first seven months after its application. No picloram was detected in streamflow water 

after 3 years.

The conceptual framework and supporting plot 

studies in the pinyon-juniper woodlands near the Los 

Alamos National Laboratory reported by Wilcox and 

Breshears (1995) offer a means for predicting the longterm movement of water, sediments, and unspecified 

contaminants and nutrients. With respect only to contaminants, low-level contamination covering “many 

square miles” and localized high-level contamination 

such as found in landfills were the two primary sources 

of contaminants considered within this research effort. 

It was concluded by Wilcox and Breshears that the 

concentrations of contaminates would be “sufficiently 

low” not to warrant remedial action in some of the areas 

studied. At the same time, however, erosional processes 

could cause depositions of contaminated soil particles 

in canyon bottoms. The need to improve predictions 

of the movement of contaminants from pinyon-juniper 

woodlands on mesa-tops to canyon bottoms led to the 

development of this conceptual framework.

Conclusions

Estimates of the nature and magnitudes of hydrologic processes in the pinyon-juniper woodlands obtained 

from a literature review of plot studies and watershedlevel investigations of these processes are the focus of 

this report. These estimates should not be considered 

“all inclusive” for the full range of hydrologic conditions encountered in the woodlands, however, but only 

for the sites studied. Nevertheless, the estimates should 

be useful to hydrologists and watershed managers having responsibilities for sustaining the hydrologic health 

of the pinyon-juniper woodlands.

The studies and investigations that were reviewed 

centered ultimately on identifying the potentials for 

increasing the flows of high-quality streamflows originating on watersheds in the pinyon-juniper woodlands. 

However, it has been concluded from these plot studies 

and watershed-level investigations that the potential 

for increasing streamflow volumes by converting tree 

overstories to a herbaceous cover is poor (fig. 9). 

While this conclusion somewhat contradicts the earlier 

Figure 9—The potential 

for increasing 

streamflow volumes 

by converting 

tree overstories to 

herbaceous covers 

in the pinyonjuniper woodlands 

is poor. This 2009 

photograph shows 

tree recovery on 

a site in the Tonto 

National Forest 

near Payson, 

Arizona, that was 

mechanically 

treated in 1958.
16 USDA Forest Service RMRS-GTR-271. 2012.

recommendations of Barr (1956) and others, it should 

not necessarily be surprising.

Streamflow improvement in any vegetative type is 

based largely on the premise that streamflow volumes 

and, eventually, groundwater aquifers are increased by 

an amount that is equal approximately to the net reduction in evapotranspiration losses attributed to the 

conversion treatment implemented on the watershed 

(Hibbert 1979). However, there is little opportunity to 

reduce evapotranspiration losses on a watershed-basis 

where annual precipitation is less than a threshold value of 18 inches (Hibbert 1983) and its total is exceeded 

by the potential evapotranspiration (Thornthwaite and 

Mather 1957). This low amount of annual precipitation is not likely to penetrate far enough into the soil to 

influence the storage of moisture in the soil body that 

in turn impacts on overland flows of water and, eventually, streamflow volumes. This situation is found 

throughout most of the pinyon-juniper woodlands in 

the western region.

A similar conclusion that streamflow volumes 

are not increased significantly in the pinyon-juniper 

woodlands by converting tree overstories to herbaceous understories was also reached by Ffolliott and 

Brooks (1988) in their summary of the opportunities for enhancing streamflow volumes by vegetation 

management in the Mountain West; by Roundy and 

Vernon (1999) in their assessment of watershed values 

(including streamflow regimes) in the pinyon-juniper 

woodlands of the Interior West; and by Baker and 

Ffolliott (2000) in their analysis of the opportunities 

for increasing streamflow volumes through vegetative management practices within the Colorado River 

Basin.

Acknowledgments

The authors acknowledge the reviews by Roy 

L. Jemison, Regional Hydrologist, and Wayne A. 

Robbie, Supervisory Soil Scientist-Regional Inventory 

Coordinator, both with the Southwestern Region 

of the U.S. Forest Service of an earlier paper on the 

general content of the general technichal report titled 

“Hydrology and Hydrologic Processes of PinyonJuniper Woodlands: A Historical Review”. This earlier 

paper was presented at the 2010 Annual Meeting of 

the Society of American Foresters in Albuquerque, 

New Mexico. The authors also appreciate the reviews by Norman Ambos, Tonto National Forest, 

Phoenix, Arizona; Donald Decker, Natural Resource 

Conservation Service, Douglas, Arizona; and Douglas 

Page, Bureau of Land Management, Cedar City, Utah, 

of this more comprehensive review of hydrologic processes in the pinyon-juniper woodlands presented in 

this report.

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