In order to better understand the water, nutrient and treatment dynamics of aridland constructed treatment wetlands, we have developed datasets tracking primary productivity (aboveground and belowground), nutrient and water budget dynamics, soils, and aquatic metabolism at the Tres Rios wetlands, operated by the City of Phoenix Water Services Department, since July 2011. The 3-cell Tres Rios Wetlands were completed in 2010 and are associated with the 91st Avenue Wastewater Treatment Plant, the largest in Phoenix. This project is focused on the largest of the three wetlands treatment cells which was the first to be planted and became operational in Summer 2010. The wetland cells are bounded by roads (the "shoreline"), and the system we study is 42 ha in size, approximately half of which is open water and half of which is fringing vegetated marsh. Water depth is relatively consistent across the marsh (approximately 25cm) and effluent inflow to the cell varies seasonally from 95,000 to over 270,000 m3 d-1. Measurements are taken along two gradients representing the two hydraulic pathways of the system: The whole-system, from inflow to outflow, within the vegetated marsh itself.

Constructed treatment wetlands (CTW) provide cost effective and ecosystem-service based solutions to the problem of urban wastewater treatment. They are a particularly attractive option for water reuse in arid cities, where water resources are scarce, and understanding CTW function in these environments is critical to facilitating sustainable water use practices. Although CTW are well established and studied in mesic climates, how they function in and respond to hot, arid climates is comparatively not well understood. Specifically, large atmospheric water losses via evaporation and plant transpiration comprise a much larger component of the whole-system water budget than in mesic climates. Additionally, given the primary role that emergent macrophytes play in nitrogen removal, the effects of plant community composition and primary productivity patterns on system performance in the context of aridland constructed treatment wetlands have not been extensively studied. Our goal is to develop a model of how these "working wetlands" perform in arid climates by developing and comparing nutrient and water budgets. At the Tres Rios constructed treatment wetland in Phoenix, AZ, USA, we measure atmospheric water losses via plant transpiration and open water evaporation as well as inorganic nitrogen fluxes at the whole system and the vegetated marsh scales. Total water losses via evaporative pathways peaked at 300,000 m3 per mo-1 (714 L H2O m-2 mo-1) in the hot summer months and represented more than 70% of the whole-system water budget over a 27 month time period. These evaporative losses are nearly an order or magnitude higher than rates observed in mesic systems. Peaks in above-ground biomass ranged from 1586±179 to 2666±164 gdw m-2, with Typha spp. accounting for up to two-thirds of total biomass. Overall, the vegetated marsh removed almost all of the inorganic N supplied to it, and large transpirative water losses were observed to move large volumes of replacement water into the marsh via a plant-mediated "biological tide." This process providing additional opportunities for soil microbes and emergent macrophytes to process target solutes, and potentially enhancing the treatment efficacy of the aridland Tres Rios constructed treatment wetland relative to more humid and mesic systems.