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.
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