WWRC 86-12
Transpirational Water Loss from Streamside Vegetation in Wyoming: High Elevation Source Streams and Alluvial/Irrigation Channels of Valleys
Abstract
Transpirational water use (TWU) by riparian vegetation in the
Little Laramie River drainage of southeast Wyoming was estimated
from field measurements of leaf areas, stomatal conductances, and
microclimate at five representative study sites. These sites
were large and small natural streams and large and small
irrigation ditches at low elevation (below 8,000 ft), and an
alpine source basin at high elevation (10,600 ft). Woody
phreatophytes - cottonwood and willow species - were the major
consumers of water, per unit of leaf area and for entire riparian
zones, at the low-elevation sites. In the alpine source basin,
dwarf willows, broadleaved herbs, and sedges were the major
consumers of water. Cottonwood used more water per unit leaf,
area (185 lbs ft-2) than did the willows (103 to 119 lbs ft-2) at
low elevation, but annual willow TWU was larger than cottonwood
because willow thickets occupied a greater area than did
cottonwood groves. At high elevatiol), marsh marigold used more
water per unit leaf area (40 lbs ft-2) than did willows or sedges
(30 to 33 lbs ft-2), but its annual TWU was less than the other
species because of its lesser total leaf area.
Riparian zone TWU for the growing season of 1986 was 5 inches
in the alpine source basin and ranged from 19 to 47 inches in the
low-elevation riparian zones. TWU/streamflow ratios increased as
streamflows decreased because the ratio of transpiring leaf area
to streamflow was higher along smaller streams. The combination
of low streamflows, maximum riparian leaf areas, and maximum
daily water use rates in late summer resulted in greater
TWU/streamflow ratios in August.
Annual TWU/streamflow ratios in 1986 were less than one
percent for the large and small natural streams and the large
irrigation ditch, two percent for the small irrigation ditch, and
four percent for the alpine source basin. Mid- to late summer
TWU/streamflow ratios exceeded ten percent for both the small
natural stream and the small irrigation ditch. When TWU was
extrapolated to the entire Little Laramie drainage, the annual
TWU/streamflow ratio was about three percent. The proportion of
total drainage TWU contributed by each major class of riparian
zone decreased from low-elevation natural streams (62 percent) to
low-elevation irrigation ditches (34 percent) to alpine source
basins (4 percent).
Riparian zone TWU is a relatively minor quantity compared to
total streamflow along most stream channels in southeast Wyoming.
Small streamflow increases might be achieved in mid- to late
summer by substituting willows for cottonwoods along irrigation
ditches. TWU of alpine riparian vegetation needs to be accounted
for in models of water delivery from mountains to valleys. When
growing season precipitation and water storage in reservoirs and
water tables is taken into account, riparian TWU is a
quantitatively unimportant component of river drainage water
budgets in southeast Wyoming.
Water Resources Publications List
Water Resources Data System Library |
Water Resources Data System Homepage
