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Introduction Water planners in arid regions are faced with the problem of allocating scarce water resources to the multiple demands of fish and wildlife, recreation, agriculture, municipalities, and industry. An important factor they must consider is "conveyance losses," natural pathways by which water in streamflow is diverted into temporary storage or permanently lost from fluvial systems. Examples include water surface and soil evaporation, percolation to deep aquifers, streambank water table storage, and riparian vegetation transpiration.
Consumption of water by riparian plants is a two-edged sword. On the one hand, the water they transpire is removed from the riparian ecosystem and can no longer contribute to streamflow. This has led to attempts to increase streamflows by riparian vegetation removal (e.g., Culler 1970). On the other hand, riparian vegetation stabilizes streambanks, reducing erosion and building river terraces where water fluxes into and out of water tables help reduce peak flows in spring and increase late-summer flows (Mizell and Skinner 1986). Riparian vegetation also benefits fish, wildlife, water quality, and recreation (Johnson and Haight 1984).
A knowledge of riparian plant transpiration is useful in a number of contexts, including predicting water delivery from mountain snowpacks to the lowlands, partitioning conveyance losses into various pathways (Hasfurther and Pahl 1986), and weighing the water "cost" of increased transpiration vs. the flood control, water quality, wildlife, and recreation benefits of degraded riparian zone restoration (Platts and Nelson 1985, Skinner et al. 1986).
Attempts to measure riparian zone evapotranspiration (the combination of leaf transpiration and soil evaporation) have occurred for many years. The most widespread method, used extensively in the American Southwest, has been direct measurement of water losses from non-weighing lysimeters (Gatewood et al. 1950, McDonald and Hughes 1968, Robinson 1970, van Hylekama 1974, Borrelli and Burman 1982, Davenport et al. 1982). These instruments provide long-term records of water use within specific stands of vegetation, but they have several disadvantages. First, they are difficult to install and have long start-up times, making replication expensive. Second, their accuracy is dependent on careful measurement of the specific yield of the soil inside them, an inherently inaccurate procedure. Finally, their temporal resolution is about one month. Thus, non-weighing lysimeters cannot be used to predict how evapotranspiration changes in response to short-term changes in weather and canopy leaf area, necessary components of any predictive model for riparian evapotranspiration.
Use of direct-weighing lysimeters, with no need to measure specific yield, has provided daily and even hourly accounting of evapotranspiration from crops and trees (e.g., Ritchie and Barnett 1968, Fritschen et al. 1973, Rodrigue et al. 1983, Klocke et al. 1985). However, these instruments are costly and difficult to maintain, so that replication is seldom feasible.
Instantaneous water loss from individual leaves may be measured with diffusion porometers. This method focuses on transpiration separate from evaporation and has a temporal resolution of minutes or seconds. Provided adequate sampling occurs and total leaf areas are measured, water use rates of entire stands of vegetation can be estimated (e.g., Dolan 1988). Smith et al. (1987) used porometry to estimate seasonal water use by several riparian vegetation types in southeast Wyoming. The major drawback is the problems associated with extrapolation from leaves to whole canopies, e.g., spatial variability in stomatal apertures (Leverenz et al. 1982). Nevertheless, the low cost and high portability of porometry holds great promise for its widespread use to estimate short- and long-term water use by various riparian vegetation types, stand ages, and species. However, confidence in the results of porometry requires calibration against independent methods known to be of high accuracy.
Purpose of This Research
In 1984, the Department of Agricultural Engineering at the University of Wyoming installed a sensitive weighing lysimeter for measurement of evapotranspiration by shortgrass prairie in the Laramie Basin (Sayler et al. 1985). Initial tests of the instrument indicated a resolution of 0.2 mm of water, or a daytime temporal resolution of less than one hour at high rates of evapotranspiration (> 5 mm d-1). Thus, this instrument promised sensitivity and resolution comparable to porometry, making it well-suited as a rigorous method, accounting for all water inputs and outputs, against which porometry could be calibrated.This research involved a comparison of transpiration of a particular riparian species independently and simultaneously estimated by the weighing lysimeter and with a diffusion porometer.
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