Understanding spatial and temporal patterns of effluent discharge for dryland riparian ecosystems

A new hydro-dynamic is emerging for riparian ecosystems in the southwestern United States as water consumption for urban development is increasing effluent generation. We summarize changing spatial patterns of effluent discharge in Arizona since the mid-twentieth century based on aerial photography and discharge data and discuss potential future changes.  We also address effects on riparian ecosystem recovery and development.  Preliminary data indicate the existence of 60 point sources of municipal effluent production discharging at 90 outfalls. The fate of the effluent varies widely.  Some is used intentionally to create riparian/wetland habitat, some is released into surface water bodies or aquifers where it may or may not sustain riparian habitat, and some is re-used. Of the portion released into river channels, approximately three-quarters flows into dry (ephemeral) channels.  Effluent-sustained waterways are fundamentally different from both dry and intermittent streams and naturally perennial rivers because the discharged wastewater is nutrient rich and steadily released, fluxing diurnally with urban consumption patterns and often remaining disconnected from the floodplain aquifer.  The consequences of effluent discharge for riparian ecosystems vary depending on hydrogeomorphic traits of the receiving stream.  For example, preliminary data for the effluent-sustained lower Santa Cruz River, characterized by a wide, dynamic floodplain and deep water table, indicate that the pioneer forests are younger, less abundant, and more willow-dominated than along a perennial sister river (San Pedro).  Wetland species diversity is comparatively low, perhaps due to narrow width of the wetted zone and to greater isolation from other perennial reaches. Effluent discharge is altering patterns at larger landscape scales, by creating many short, perennial stream reaches within a watershed. Over coming decades, both the number of municipal discharge points and total effluent discharge are expected to increase substantially, intensifying the urgency for understanding ecological outcomes of effluent discharge.