The contribution of evapotranspiration and evaporation to the water budget of a treatment wetland in Phoenix, AZ, USA

Background/Question/Methods

One of the most important aspects of any wetland 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. We measured evapotranspiration and evaporation rates in a constructed treatment wetland in Phoenix during the summer, when both rates were at annual maxima and wastewater inflows were at an annual minimum. Our primary objectives were: 1) to measure the rates of wetland evaporation and evapotranspiration bi-weekly using a handheld infrared gas analyzer, and; 2) calculate a whole-system summer water budget using these rates plus inflow and outflow data. We hypothesized that; 1) the summer water balance will lead to seasonal evapoconcentration of bioactive solutes and salts, and; 2) this will put substantial stress on the ability of wetland plants and soil microbes to perform the desired ecosystem services of nutrient uptake and transformation. These water flux data and summer water budget will contribute to our overall goal of quantifying the hydrology budget for the Tres Rios treatment wetland, and will improve our general knowledge of wetland water treatment capacity in dryland areas.

Results/Conclusions

In arid climates, such as Phoenix, treatment wetlands face the challenge of the evapoconcentration of nutrients and dissolved salts during the summer, when rates of atmospheric water loss are high and effluent inflows are low. Before we can understand how evapoconcentration affects biogeochemical dynamics, we first had to quantify this process itself.  Our preliminary water budget indicated summer effluent inflow rates that are less than 50% of winter rates (during the peak “snowbird” tourist season in Phoenix).  Summer evapotranspiration and evaporation rates from arid wetlands often produce atmospheric water losses in excess of 2 cm day^-1.  The atmospheric water flux measurements and summer water budget estimates we present here were generated in June and July 2011 by a Research Experience for Undergraduates (REU) student (the first author) who came from the Florida Coastal Everglades LTER Program to the Central Arizona-Phoenix LTER Program during Summer 2011. 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 water treatment.