The contribution of evapotranspiration to the annual water budget of an aridland urban wastewater treatment wetland

One of the most important aspects of systems-level analysis of wetlands is the water budget. Quantifying how evaporation and evapotranspiration contribute to water residence time is crucial to understanding the cycling of biogeochemically active and non-active solutes through the water column, plants and soils—particularly in arid climates. Since June 2011, we have collected bi-monthly measurements of evapotranspiration, evaporation, and conductivity in the Tres Rios constructed treatment wetland. Our primary objectives were: 1) to determine species-specific transpiration rates using a handheld infrared gas analyzer (IRGA); 2) quantify aboveground biomass and species composition of the plant community, and; 3) calculate a whole-system annual water budget using these rates plus inflow and outflow data. We hypothesized that; 1) leaf-specific transpiration rates are controlled by photosynthetically-active radiation, relative humidity, and air temperature, but: 2) annual evapotranspirative water losses will be driven by seasonality in macrophyte biomass and community composition. We are working with the City of Phoenix on an adaptive management plan for the Tres Rios treatment wetlands that will maximize the ecosystem services provided by the wetland, and this water budget is an important component of the management plan.

We combined our bi-monthly IRGA measurements with data from a nearby meteorological station and plant biomass sampling to scale our leaf-specific rates to the entire system. Daily evapotranspirative losses ranged from near 0 in the winter to 6.5 cm day^-1 during the summer. These values are 5 – 10 times higher than published results from mesic wetlands. These losses represented up to 6 – 8% of the total effluent inflow to the system during the summer. We observed that seasonal variations in plant biomass affected total evapotranspirative losses. On the average, Typha latifolia and domingensis and Schoenoplectus acutus transpired 4-5 times more water than S. americanus and S. tabernaemontani. These interspecies differences were driven by differences in plant-specific biomass, plant-specific transpiration rates, and relative proportion of each species to whole-system biomass. Bi-monthly measurements of conductivity showed increases of up to 25% along transects within the wetlands from open water to the shoreline, suggesting that the evapoconcentration of nutrients and solutes is occurring. This poster and research project is part of a broader effort to understand the ability of wetlands in arid climates to deliver ecosystem services related to wastewater treatment.