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The first stop of the Field Day on Beaver Creek was at Watershed 14.
Dr. Malchus Baker and Dr. Dan Neary, USFS, explained the purpose of the
Beaver Creek Experimental Watershed: to determine the potential for increasing
water yield by manipulating ponderosa pine and pinyon-juniper vegetation
types and to determine effects of treatments on the other watershed resources.
- What is a watershed? All land enclosed by a continuous hydrologic
drainage divide and lying up slope from a specific point on a stream.
- What is the precipitation regime? Information on precipitation
regimes is based on 20 or more years of data from watershed studies
in the major vegetation types in Arizona (mixed conifer, ponderosa pine,
pinyon juniper, grasslands, chaparral). The most significant characteristic
of precipitation, besides being bimodal--summer and winter--, is its
variability. Average precipitation in the pinyon-juniper zone is 20
inches; in ponderosa pine zone average precipitation is 25 inches. Every
other year average or above precipitation occurs in the pinyon-juniper
type.
- What happens to the precipitation? Runoff (R) or streamflow
is a function of Precipitation (P) and evapotranspiration (ET) --water
loss by evaporation or transpired from plants (R = P - Et). To have
Perennial flow (streamflow all year long) precipitation must exceed
ET. If precipitation only exceeds ET part of the time them you will
have ephemeral flow (lasting for a short period of time). Over 20 years,
our average annual streamflow is 5 inches in the pine type vegetation
and 1 to 3 inches in the pinyon juniper type vegetation.
- What environmental factors affect runoff? Runoff regime is
highly dependent on soil type. Basalt-derived soils have high clay content
and are relatively resistant to erosion. Runoff is predominately an
overland flow regime on Beaver Creek. The saturated soil or surface
horizons reduce infiltration and produce overland flow conditions.
- Does the season affect runoff? Eighty to 90 % of the runoff
f rom ponderosa pine and pinyon-juniper vegetation types comes during
the winter season, from low intensity storms with relatively little
energy available for erosion. Runoff can be highly variable because
of differences in seasonal distribution and timing of snowmelt. Certain
storms can produce significant amounts of runoff. The Labor Day Storm
of 1970, with 4 to 7 inches of rain in 19 hours, produced flow floods
from treated and untreated watersheds. Significant runoff events can
occur, when tropical, fall, storms in the Pacific pump moisture into
Arizona.
- Do you think we got an increase in water yield by treating the
vegetation? There were no significant increases in runoff through
vegetation management in regions with 18 inches or less average precipitation.
This is because the existing vegetation can effectively utilize precipitation.
In areas with more than 18 inches of precipitation, we can expect to
get some increases in runoff following vegetation treatments. For example,
in areas where ponderosa pine trees were removed, there was a 2 inch
increase; however, this increase in runoff was lost in 7 years due to
sprouting and growth of gambel oak.
- Why is watershed condition important? Adequate vegetation
and litter cover lead to healthy watersheds. Healthy watersheds have
the capacity to absorb storm energies, provide regulation of storm flows
through the soil mantle, and bring stability to the entire basin. In
contrast, a watershed that has been abused often develops a more extensive
channel system throughout its basin, including an ephemeral gully network.
The expanded gully network is the hydrologic response to increased surface
flows which results from rapid, concentrated, runoff that produce head
cutting and gully formation. These expanded gully networks increase
peak flows and may produce large amounts of sediment.
- How does erosion affect the watershed? Erosion is episodic
in the Southwest due to the highly variable precipitation regime and
to the distribution and size of storm events. The predominant, small
storm events gradually move soil particles down slope and eventually
into the channel system. Large storm events, such as the Labor Day storm,
effectively flush out the channel system to produce relatively large
amounts of sediment in flow flows. Additionally, roads are major sources
of sediment.
- How do we monitor the watershed? Major factors that influence
monitoring frequency include precipitation distribution and variability.
Sediment in Verde River is only relatively high for a small amount of
the time when flow is over a give discharge amount (normally after an
intense rainstorm). For 90% of the time, flow is so low that water quality,
from sediment, is not a problem. Scientists must monitor often enough
to determine when and what amount of (IE. sediment) becomes a problem.
For more vocabulary: Water
Science Glossary by U.S. Geological Survey
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