Abstract:
Evaporation was studied along the Middle Fork of the John Day River in northeastern Oregon. Evaporation pans and various meteorological sensors recorded data at nine sites during the summer of 1998; measurements were made at each site over a 3-4 day period. A Dalton-type evaporation model was produced using water loss measurements in conjunction with the following meteorological information: pan water temperature, air
temperature, relative humidity, and windspeed. This model was highly significant (r² = 0.88, F₂,₃₆₀ = 5572, p > F = 0.0001, n = 363). Regression coefficients composing the
evaporation relationship were found to vary as a result of unstable atmospheric conditions (overcast periods and/or variable daytime wind direction) and windspeed measurement height. At each of the nine sites, stream channel characteristics were analyzed to determine potential effects on the streams' capacity to cool by evaporation. The alignment of the stream channel with respect to wind direction, the degree of incision of the stream channel, and the gradient of stream bank side slopes in the direction of wind were found to potentially affect wind movement at the stream surface, thereby, restricting evaporative cooling.
Water movement was found to increase rates of evaporation relative to non-moving water during periods of low vapor pressure deficit and no air movement. At higher vapor pressure deficits with no wind and during periods of light wind movement with a low vapor pressure deficit, differences in evaporation rates between moving water and non-moving water were less important. Much of the field data used to produce the evaporation relationship presented in this study were recorded during periods of atmospheric conditions similar to those that demonstrated minimal differences in evaporation rates between moving water and non-moving water. Nonetheless, because water loss was measured in stagnant water evaporation pans, it can only be safely
assumed that the evaporation relationship presented in this document will tend to
underestimate rates of evaporation from streams with moving water. The evaporation relationship provided in this document is applicable to aridland environments. Application to other climatic regimes is probable but has not yet been validated by additional scientific research.
