Hydrology in a California oak woodland watershed:
a 17-year study
, M.J. Singer
*, R.A. Dahlgren
, K.W. Tate
Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA
Department of Agronomy and Range Science, University of California, Davis, CA 95616, USA
Received 13 April 2000; revised 4 September 2000; accepted 5 September 2000
The western foothills of the Sierra-Nevada are some of the most rapidly developing lands in California. Use of these lands
includes vineyards, retirement and family home construction, livestock grazing, and fuelwood harvesting. These many uses
require varying levels of woodland conversion and oak tree removal that alters the nutrient cycling, wildlife habitat and
hydrology of these watersheds. There is little long-term hydrologic data to help determine the impact of these land use changes
on water yield or quality. To ®ll this gap, precipitation and stream ¯ow data were collected for 17 years in a 103 ha California
oak woodland watershed, from which oaks were removed from 14% of the land area. These data were combined with measured
potential evapotranspiration (PET) to develop a simple water balance and to investigate changes in water yield from oak
removal. Hydrologic data included continuous stage height records from a three-foot Parshall ¯ume and a one-foot 908V-notch
weir. Rainfall measurements were made using a tipping bucket rain gage. Average annual rainfall, runoff, and estimated
evapotranspiration (ET) for the 17 years were 708, 344, and 364 mm, respectively. In this Mediterranean climate, ET is less
dependent upon rainfall than is runoff because the majority of precipitation coincides with the period of lowest PET. Mean
annual base¯ow depth was 24 mm ranging between 15 and 40 mm. Depth of base¯ow was more strongly associated with the
annual rainfall than with rainfall from previous years, indicating that changes in soil moisture storage approaches zero on an
annual time-scale. Effective depth for watershed soils was calculated to be 217 mm. Potential soil water storage between
bedrock and the top of the clay-rich subsoil (Bt Horizon) was 52 mm. This quantity accounts for summer ET and stream
base¯ow. A weakly signi®cant difference between the pre- and post-harvest mean monthly effective rainfall was observed,
indicating that oak removal, from 1984 to 1986, had little in¯uence on watershed hydrology. Peak monthly effective rainfall
corresponded to peak monthly runoff. The threshold of response to signi®cantly increase water yield from oak harvesting is
greater than 14% of a watershed area for the Sierra-Nevada foothills oak woodlands. q2000 Elsevier Science B.V. All rights
Keywords: Forest hydrology; Water balance; Oak woodlands; Wildland watersheds
California's oak woodlands represent an important
resource for fuelwood, rangeland, aquifer recharge,
and wildlife habitat (Standiford and Howitt, 1993;
Bolsinger, 1987). It is also an ecosystem being rapidly
developed for suburban dwellings and hillslope
vineyards. Management under this multiple-use
system includes varying levels of woodland vege-
tation conversion from complete removal of all oaks
to isolated harvest of individual trees. Although past
Journal of Hydrology 240 (2000) 106± 117
0022-1694/00/$ - see front matter q2000 Elsevier Science B.V. All rights reserved.
* Corresponding author. Fax: 11-530-752-1552.
E-mail address: firstname.lastname@example.org (M.J. Singer).
investigations included the effects of oak harvesting
on changes in forage quality and quantity (Kay, 1987;
Holland, 1980) and water yield (Lewis, 1968), there is
little published long-term hydrologic data for oak
woodland watersheds in California.
Motivated by water resource management efforts to
increase water yield, research on forest conversion to
grasslands has attempted to quantify the relationship
between the area of land converted and increased
annual water yield or runoff (Stednick, 1996; Bosch
and Hewlett, 1982; Pitt et al. 1978; Burgy and
Papaza®riou, 1971). Bosch and Hewlett (1982)
compiled data from 94 watershed studies and found
that every 10% of land area converted generated
40 mm of additional annual runoff in conifer and
eucalyptus forests, 25 mm in deciduous forests, and
10 mm in scrubland. They concluded that regions of
higher mean annual precipitation were more sensitive
and demonstrated stronger annual runoff responses to
vegetation conversion than regions of lower precipi-
tation. Their results also indicate a lack of research
conducted for small areas of vegetation conversion
and the need to identify thresholds of response.
Expanding on Bosch and Hewlett's review,
Stednick (1996) identi®ed percent area thresholds of
vegetation conversion at which point water yield
would begin to increase. Based on results from 95
paired watershed studies in the United States, he
concluded that a minimum of 20% area conversion
must take place to generate changes in annual runoff.
This threshold varies from 15% in the Rocky
Mountain Inland Intermountain region to 50% for
the Central Plains region. Potential reasons for this
variability include harvest location, harvest type,
pretreatment vegetation cover, or measurement
error. Because only two studies were available for
the Central Sierra Province region, the region in
which our study is located, a threshold could not be
established in Stednick's review. In addition to
threshold determinations, Stednick developed a
relationship for the 95 paired watershed studies
between increased runoff (y, mm) and percent area
of vegetation conversion (x) described by Eq. (1):
In the most recent, and arguably the only, study of
hydrologic response to vegetation conversion
conducted in the Sierra-Nevada Foothills, Lewis
(1968) compared runoff from a control watershed to
that in a watershed that had over 90% of land area
converted. Lewis' research represents climate,
geology, soils, and vegetation similar to those in this
study. His results indicate runoff was increased by
129 mm annually for the three years studied. Thus,
vegetation conversion on approximately 100% of
the area resulted in increased runoff in these oak
woodlands. What effects on runoff does vegetation
conversion of smaller areas have, as raised by Bosch
and Hewlett (1982)? Does the 14% area converted
within this study site generate an increase of 34 mm
in runoff as predicted by Eq. (1) and Stednick's (1996)
The scarcity of published data on oak woodland
watersheds combined with a growing demand for
their non-agricultural use highlights the need for
better understanding of the hydrology of oak wood-
lands and the role of oaks in nutrient and water
cycling. This study complements the work of Lewis
(1968), Bosch and Hewlett (1982) and Stednick
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117 107
Fig. 1. Location of the University of California Sierra Foothill
Research and Extension Center.
(1996) by presenting water yield data from 1981 to
1997 in a Northern California Sierra Nevada foothill
oak woodland watershed to meet two objectives. The
®rst is to quantitatively describe the annual rainfall,
runoff, and ET, as well as monthly average rainfall
and runoff for this oak woodland system. Second is to
investigate the in¯uence of selective oak harvesting
on water yield. Water quality results for the study are
presented in Lewis (1998).
2. Materials and methods
2.1. Site description
Located on the University of California's Sierra
Foothills Research and Extension Center (SFREC),
the 103 ha Schubert watershed is approximately
96 km northeast of Sacramento in Yuba County,
California (Fig. 1). Elevations within the watershed
range from 152 to 427 m with slopes ranging from 2 to
50% and average of 18 % (Fig. 2). The annual rainfall
distribution corresponds to California's Mediterra-
nean climate with maximum average monthly rainfall
in winter and minimum average monthly rainfall in
Dominated by blues oaks (Quercus douglasii) and
intermixed with interior live oaks (Q. wislizenii) and
foothills pine (Pinus sabiniana), the vegetation of the
site typi®es Sierra-foothills oak woodlands (Grif®n,
1977). The uneven distribution of trees creates a
mosaic of open grasslands, savanna, and woodlands
(Epifanio, 1989; Jansen, 1987). Annual grasses and
legumes dominate the ground cover, with differing
species diversity under the oak canopy compared to
open grasslands (Jackson et al., 1990; Jansen, 1987).
Soils in the Schubert watershed are Ruptic-Lithic
Xerochrepts on the steep side slopes and Mollic
Haploxeralfs on the more level areas (Lytle, 1998).
Soils are shallow to moderately deep, medium
textured, gravely, and rocky formed in basic meta-
volcanic (greenstone) bedrock (Beiersdorfer, 1979).
They are rich in Fe-oxides. Soil pit observations at
Schubert consistently identi®ed very stony clay
subsoils with 40% average clay content (Huang,
1997). The clay subsoil, starting at 35±40 cm depth,
and bedrock at the site limit deep percolation of water
(Dahlgren and Singer, 1994).
The watershed is used for light to moderate seasonal
beef cattle grazing primarily from January to March
and August to October. Total animal unit months,
AUM/ha, averaged 0.24 with a minimum of zero in
1986±1987 and a maximum of 0.56 in 1984±1985.
Trends in grazing intensity as a function of time were
not detected for the duration of this study r20:004:
In addition to grazing, oaks were harvested on 14%
of the site from 1984 to 1986 (Epifanio, 1989). A total
of 1352 trees were removed from 15 ha, or 14% of the
watershed, during three periods of cutting. From 7/22/
84 to 8/16/84, 880 trees were removed, followed by
removal of 416 trees between 4/1/85 and 9/30/85 and
56 trees between 4/26/86 and 5/26/86. Woodcutters
were prohibited from cutting in obvious waterways
whether or not water was present. They were also
prohibited from cutting on slopes over 30%. Cutting
was allowed in winter but wood removal was
restricted to summer months. Converted areas remain
covered by annual grasses since oak harvesting.
2.2. Watershed instrumentation
In 1978, the watershed was instrumented with a
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117108
Fig. 2. Locations of oak removal and instruments in the Schubert
watershed. Contours are in meters.
three-foot Parshall ¯ume for storm ¯ow measure-
ments and a one-foot 908V-notch weir for base ¯ow
measurements (Fig. 2). Stage height was continuously
recorded with stage height recorders inside stilling
wells connected to the ¯ume and weir. Rainfall
amount and intensity were recorded with tipping
bucket rain gages and recorders at three different
elevations within the watershed to provide redun-
dancy and account for rainfall variability resulting
from orographic in¯uences. Comparisons of annual
and storm-event rainfall indicate that the variability
between these three gages is negligible (Huang,
1997). Rainfall reported here is for one gage at the
highest elevation. Because of instrument incon-
sistencies and settling of disturbed soil around the
¯ume and weir, data from 1978, 1979, and 1980
were not included in the long-term record.
2.3. Data analysis
Annual rainfall and runoff were calculated as the
respective sums of every rainfall event and daily
discharge measured during each water year (October
1 to September 30). Monthly average rainfall and
runoff were calculated as the mean of each month's
rainfall and runoff from 1981 to 1997. In addition,
rainfall± runoff relations were investigated by calcu-
lating annual and mean effective rainfall according to
Haan et al. (1994) and Pilgrim and Cordery (1992).
Evapotranspiration (ET) on an annual basis was
estimated as the difference between rainfall and runoff
using a simple water balance method (Haan et al.,
1994; Peters, 1994; Hewlett, 1982). Annual potential
evapotranspiration (PET) values were obtained from
the SFREC's weather station and evaporation pan
Mean annual base¯ow, effective rainfall, and mean
annual rainfall that initiated observable stream runoff
increase from storms were determined. These were
compared to effective depth calculations made from
soil bulk density data presented by Dahlgren et al.
(1997). Base¯ow separation was made using annual
runoff hydrographs and hydrograph separation
methods (Haan et al., 1994; Pilgrim and Cordery,
1992). Storm runoff was identi®ed as the runoff
between the points on the annual hydrograph where
tangents to the recession curve touched. This runoff
depth was excluded from the summation of inter
storm-events and summer base¯ow.
Water yield analysis was conducted using effective
rainfall (Haan et al., 1994) on annual and monthly
time scales. For purposes of this paper, effective
rainfall represents runoff as a percent of precipitation
at annual and monthly time steps (Black, 1990).
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117 109
Fig. 3. Schubert watershed mean monthly rainfall and runoff. Error bars represent one standard deviation from the mean rainfall.
Monotonic trends and step trends were developed for
effective rainfall using statistical methods recom-
mended by Helsel and Hirsch (1995) as well as Hirsch
et al. (1991, 1982). These included linear regression
and t-test of means on data transformed to natural
logarithmic values. Normal distribution of data was
tested using the probability plot correlation coef®cient
test (Looney and Gulledge, 1985). This test indicated
that monthly effective rainfall was not normally
distributed but natural logarithmic transformed data
was. Use of step trends is in keeping with the structure
of a before and after treatment study (Spooner et al.,
1985), and facilitates the comparison of pre- and post-
oak harvest effective rainfall. Time series analysis of
pre- and post-cutting runoff was made using the SAS
statistical program autoreg procedure after data
3. Results and discussion
3.1. Monthly rainfall and runoff
Mean monthly rainfall and runoff for the 17-year
period are typical of the annual pattern found in Cali-
fornia's Mediterranean climate, with 88% of rain fall-
ing from October to March (Fig. 3). Mean monthly
rainfall and the resulting monthly runoff are highly
variable, with the greatest variability in the winter
and the least variability in the summer. Peak mean
monthly rainfall and runoff occurred in January
followed by a second maximum in March (Fig. 4).
Interannual climatic variability is also high. Rain-
fall and runoff were below average in water years
1987 to 1992. This time span corresponds to a recog-
nized regional drought and contrasts with the time
periods of above average rainfall and runoff that
preceded and followed it. This variability was not
limited to time spans of several years but was
observed between sequential years, as demonstrated
in the annual hydrographs for the 1994 and 1995 water
years (Fig. 5a and b). Water years 1994 and 1995
represent the minimum and maximum runoff
measured over the 17-year period. Similar juxtaposi-
tion of high and low rainfall and runoff years occurred
in 1981 and 1982, as well as 1985 and 1986.
Consistent with the mean annual rainfall pattern
presented in Fig. 3, 85 and 86% of all rainfall fell
between October and March for 1994 and 1995,
respectively. In response to this rainfall pattern, the
runoff for both years was elevated during this same
period. Initial rainfall events produced little or no
stream ¯ow response until soil water holding capacity
was recharged. The initiation of increased runoff
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117110
Fig. 4. Schubert watershed annual rainfall and runoff from 1981 to 1997.
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117 111
Fig. 5. (a) Schubert watershed annual hydrograph for October 1 1993 through September 30 1994. (b) Schubert watershed annual hydrograph
for October 1 1994 through September 30 1995.
occurred in early December for both years, indicating
a similar soil recharge response. Once in the excess
moisture phase, subsequent rainfall events were
recorded as increased stream ¯ow. Peak runoff for
1995 was more than four times that of 1994 because
of greater precipitation and closer timing of storm-
events during 1995. During 1995, 21 observable
storm runoff events occurred, as indicated by the
rise and fall of the annual hydrograph. In 1994 only
ten such events occurred. In addition, base¯ow
between storm runoff events was greater in 1995
than in 1994.
3.2. Annual rainfall, runoff and ET
Mean annual rainfall, runoff, and ET over the
17-year record were 708 (SD 259), 344 (SD
241), and 368 mm (SD 89), respectively (Table
1). The 17-year average rainfall is comparable to
749 mm (SD 276) recorded at the nearby SFREC
weather station for the same time period (SFREC,
1997). Annual total runoff and rainfall were signi®-
cantly correlated r20:90 p0:001while annual
ET and rainfall were not r20:14 p0:139(Fig.
6). These relationships were similar to those for an
oak woodland watershed in Northeastern Spain
receiving 857 mm annual precipitation (Avila and
Roda, 1990). Furthermore, the site is responding as
a humid watershed with relatively constant ET in the
face of highly variable annual precipitation (Likens et
al., 1995). Lastly, this response is consistent with the
precipitation threshold identi®ed by Bosch and
Hewlett (1982) for the behavior of mean annual ET.
ET is constant and not correlated with rainfall where
precipitation is greater than 600 mm and increasingly
variable and correlated with rainfall below this
Although the water balance method is not effective
for short-time-scale water budget calculations and
lacks a control for climatic factors, it does provide
an ef®cient way to calculate annual ET and is often
used to check the accuracy of other methods. A rele-
vant example is work conducted in a California
coastal oak woodland watershed by Fisher et al.
Assumptions made in using this method include
integration of surface and lateral ¯ow in stream
discharge or runoff, changes in soil water storage
approaching zero when analyzing a long record, and
minimal water ¯ux to deep seepage as a result of site
soils and geology. Previous ET calculations for the
watershed showed that site conditions met the ®rst
two of these assumptions (Huang, 1997 and Dahlgren
and Singer, 1994). Fisher et al. (1996) also con®rmed
that changes in soil water storage approached zero at
the annual time scale for the California oak woodland
watershed in their study. Dahlgren and Singer (1994)
used the ®eld capacity method of Maule and
Chanaskye (1987) for ET calculation and estimated
that deep seepage at our site was less than or equal to
5% of precipitation for the 1992±1993 water year. In
contrast, Huang (1997) and Parton and Jackson (1989)
estimated it to be as much as 20% of precipitation.
Thus, site conditions might be met for the third con-
dition. A lack of deep percolation is further supported
by the observed high subsoil clay content and shallow
depth to bedrock that restrict downward ¯ow and
promote lateral ¯ow at the site (Dahlgren et al.,
1997; Epifanio, 1989).
Mean annual ET was 19% of mean annual PET
(Table 1). This is expected in Mediterranean climates
where ET is a larger component of the annual water
budget than runoff (Peters, 1994; Swift et al., 1988)
and maximum PET occurs during summer when little
or no precipitation occurs (Fig. 3). For example,
monthly average PET for December and January
were 41 and 42 mm compared to 303 and 281 mm
for July and August (SFREC, 1997). The lack of
synchronous PET and ET with precipitation results
in the large disparity between PET and ET over the
17-year record. Further indication of this rainfall,
runoff, and ET relationship is the signi®cant p,
0:001positive correlation, r20:90;between
rainfall and effective rainfall on an annual basis.
The inference is that additional rainfall primarily
contributes to runoff but does not increase ET.
With the differences in runoff volume in response to
rainfall during the winter, it is interesting to note a
consistent decline and endpoint value of base¯ow
during the summer months in 1994 and 1995. The
minimum daily runoff for 1994 and 1995 was 0.05
and 0.04 mm, respectively, and total summer
base¯ow for 1994 and 1995 was 15 and 9 mm,
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117112
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117 113
Schubert watershed annual rainfall, runoff, potential (PET) and estimated (ET) evapotranspiration, effective rainfall, cumulative rainfall to annual peak runoff, peak runoff, annual
cumulative rainfall to sustained rise in runoff, runoff at sustained rise in runoff, base¯ow, and base¯ow as a percentage of rainfall for water years 1981±1997
to annual peak
to sustained rise
in runoff (mm)
Runoff at sustained
rise in runoff (mm)
1981 545 106 2207 439 19 467 9 278 3 36 7
1982 1184 668 1865 516 56 725 38 156 1 16 1
1983 1014 766 1811 247 76 266 28 91 1 17 2
1984 754 369 2178 386 49 429 30 94 4 23 3
1985 469 189 2232 281 40 333 36 113 1 19 4
1986 840 507 2062 333 60 617 62 113 1 21 3
1987 366 107 2264 259 29 252 8 152 1 40 11
1988 461 150 2166 311 33 350 8 167 1 27 6
1989 623 307 1859 317 49 508 35 154 3 18 3
1990 589 139 1745 450 24 284 12 179 1 28 5
1991 570 139 1722 431 24 291 19 175 3 30 5
1992 547 220 1768 327 40 398 39 175 2 32 6
1993 962 426 1684 536 44 693 21 181 3 15 2
1994 411 87 1702 324 21 241 5 141 2 33 8
1995 1205 848 1670 357 70 559 44 161 5 16 1
1996 694 354 1892 340 51 381 27 170 1 21 3
1997 807 470 1738 337 58 711 57 163 2 19 2
Mean 708 344 1916 368 44 441 28 157 2 24 4
SD 259 241 217 89 17 167 17 43 1 8 3
Potential evapotranspiration reported in SFREC (1997) annual report.
Evapotranspiration estimated as the difference between rainfall and runoff.
respectively. 1994 was preceded by a year of above
average rainfall and runoff. If soil water storage at an
annual time step did not approach zero the juxta-
position of the divergent winter runoff values and
similar summer runoff values would not be expected.
Base¯ow during the summer without precipitation
raises questions about subsurface water sources.
Lewis and Burgy (1964) indicated that deep cracks
contributed to the water budget in a California coastal
oak woodlands watershed. As an alternative to deep
drainage, subsurface ¯ow from saturated soils has
been observed as the principal source of base¯ow in
forested watersheds (Mosley, 1979; Hewlett, 1961).
Given the uncertainty in the source of base¯ow and
leaks either into or out of the watershed, the simple
water budget approach applied in this study is only a
Application of this approximation to the Schubert
watershed is informed by discussion of effective depth
and annual base¯ow at the site as they relate to runoff
and rainfall. Using bulk density results presented by
Dahlgren et al. (1997), the estimated effective water
depth stored in the soil pro®le at saturation is 217 mm.
Above the Bt horizon we estimated potential storage
of 165 mm. Lewis (1968) indicated that between 152
and 203 mm of rainfall was required to prime that
study's watersheds and generate runoff. The average
priming precipitation depth for sustained rise in runoff
from summer base¯ow to fall and winter storm ¯ow in
the Schubert watershed was 157 mm (SD 43)
(Table 1). The similarity between the 157 mm
required to initiate stream response and the calculated
effective depth supports the contention that water
¯owing through the soil above the Bt horizon is
responsible for initial stream response to rainfall.
These results further indicate that runoff, primarily
as lateral ¯ow (Dahlgren and Singer, 1994), increases
prior to saturation of the entire soil pro®le, thus
limiting deep water seepage.
Mean annual base¯ow and base¯ow as a percentage
of rainfall in the Schubert watershed were 24 mm
SD 8and 4% SD 3;respectively (Table 1).
There is suf®cient subsoil soil water storage to
generate this base¯ow. We are not con®dent that
subsoil stored water is the only source of summer
base¯ow because other similar watersheds in the
area have seasonal, not perennial, ¯ow. Other sources
are the perennial springs, unique to this watershed,
that contribute to base¯ow with stored soil water
that reaches the stream as ¯ow from these springs.
If, however, it is assumed that base¯ow is generated
from another source, that source is only a minor
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117114
Fig. 6. Schubert watershed annual runoff and actual ET as functions of rainfall. ET was estimated as the difference between rainfall and runoff.
contributor to runoff missed by the water budget
approach. Further analysis using end-member mixing
methods may help to determine the source of summer
3.4. Effect of tree removal
Removal of trees from14% of the area produced no
trend over time for annual r20:008;p0:912or
D. Lewis et al. / Journal of Hydrology 240 (2000) 106±117 115
Fig. 7. (a) Schubert watershed effective rainfall and total monthly runoff over the 17-year record. Months with zero rainfall are not included.
Each water year begins in October of one year and ends in September of the next year. (b) Schubert watershed pre- (open circles) and post-
harvest (closed circles) monthly effective rainfall as a function of monthly rainfall.
monthly r20:002;p0:645effective rainfall.
There was also no signi®cant difference between the
pre- and post-harvest mean monthly effective rainfall
means ln transformed 0.054). The weak-
signi®cance of these results runs the risk of a Type
II error, but the null hypothesis could not be rejected
using the signi®cance level of p0:05:Admittedly,
the power of these tests is limited by the small
pretreatment sample size.
Further analysis of the runoff data using time series
analysis showed a weakly signi®cant difference
between pre- and post-harvest runoff Pr .utu
0:041:Before time-series analysis, a log±log trans-
formation of the monthly runoff data was made to
correct for non-normal distribution. The Shapiro-
Wilk statistic W0:960indicated that the trans-
formation did not fully normalize the distribution; it
remained thick-tailed. A one month lag and a one year
lag were signi®cant Pr .utu,0:0001:As expected,
the previous month's runoff was strongly correlated to
the runoff in the present month. The seasonality of the
watershed's behavior is shown by the strong relation-
ship among the same months each year.
Peak monthly effective rainfall corresponded to
peak monthly runoff (Fig. 7a). There was no signi®-
cant correlation between effective rainfall and
monthly rainfall either pre- r20:085;p0:089
or post-cutting r20:020;p0:151(Fig. 7b).
Monthly effective rainfall greater than 100% in Fig.
7a and b represent late winter and early spring months
with low precipitation but continued high runoff. This
high runoff results from the continued discharge of the
previous month's rainfall that was held within the soil.
For example, February 1995 had 6 mm of precipi-
tation and 33 mm runoff resulting in 534% for
effective rainfall. Precipitation for January 1995 was
330 mm, however, which contributed to the February
Rainfall was strongly correlated with runoff but not
ET. With precipitation falling in winter when PET is
lowest, there is little opportunity for abstraction of
water from the watershed other than through runoff.
Runoff and effective rainfall increase as rainfall
increases, while ET remains relatively constant. This
response is more consistent with cool±humid water-
sheds than more xeric conditions. Winters in northern
California are cool and wet mimicking cool±humid
There is a weak indication that removal of 14% of
the oak cover increased monthly effective rainfall and
runoff. The threshold for signi®cantly increased water
yield from oak removal is greater than 14% of the land
area. This threshold is expected to be approximately
20% of the watershed area (Stednick, 1996). In
addition, the staggering of tree removal into three
sequential harvesting events and the directives to
woodcutters to abstain from cutting in waterways
and timber removal in winter are practices that
prevented signi®cant watershed disturbance.
Partial support for this work came from the Uni-
versity of California Jastro-Shields fund and Inter-
national Agricultural Development Graduate Group.
Thanks to Mike Conner, Dave Labadie and the Sierra
Foothills Research and Extension Center staff for
continued support in watershed instrumentation and
measurement. Dr Mitchell Watnik of the UC Davis
Statistical Laboratory provided statistical support.
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