EXPERIMENTAL STUDIES OF EVAPOTRANSPIRATION IN SMALL STAND WETLANDS

Evapotranspiration (ET) is driven primarily by solar radiation in large wetlands (> 1 ha), where water loss approximately equals open-water evaporation. In wetlands smaller than 1 ha, water loss from ET exceeds open-water evaporation due to advective transport of dry regional air (i.e. wind) through the plant canopy. The difference in ET between large and small wetlands may lead to inaccurate water budget and net recharge calculations if not taken into account. Also, wetland plant transpiration may play a role in generating a vertical hydraulic gradient needed to transport harmful contaminants to the hydrosoil where they can be treated and sequestered through biogeochemical processes. In this study, a flowing wetland lysimeter constructed using 265-L storage containers filled with sand and broadleaf cattails (Typha latifolia) was used to record ET during summer 2011. Measured ET was compared with reference ET calculated from field collected data (temperature, dew point, wind, and solar radiation) to determine crop coefficients required for predicting ET from small stand wetlands. Results indicate that ET from the wetland lysimeter was 2.5 times greater than the calculated reference ET (R² = 0.96), and the calculated crop coefficient was 2.2-2.5 times greater than crop coefficients reported for large wetlands containing cattails. Peak-hourly ET in the lysimeter was greater than 2 mm h^-1. Vertical tracer tests were performed on bench-scale wetlands with either trimmed or untrimmed cattails to measure transport time of tracer solution from the water surface to a depth of 5 cm. Mean tracer arrival time differed significantly (p = 1.2 x 10^-8) between the untrimmed and trimmed bench-scale wetlands (104 minutes versus 450 minutes, respectively) demonstrating that plant transpiration significantly contributes to vertical flow. The difference in crop coefficients between values determined from the lysimeter tests in this study and values reported previously for large wetlands confirms that wetland size is an important factor in estimating ET. In addition, the results of the tracer test indicate that cattail transpiration can aid in transporting contaminants to the hydrosoil where they may be treated and sequestered.