Chemistry of stormwater runoff reflects a myriad of factors such as precipitation dynamics, antecedent activity on the landscape, and catchment features. Few studies of stormwater chemistry have focused on urban streams, and fewer still focus on aridland, urban streams. We studied stormwater runoff and solute export in Indian Bend Wash, an ephemeral stream that drains a large portion of Scottsdale, Arizona. This large watershed (~ 230 km2) encompasses a wide range of land cover types, including native Sonoran desert, commercial and recreational, but is predominantly residential. Indian Bend Wash is considered a model of a multi-purpose stormwater management infrastructure, was designed as a passive storm-control feature, and consists largely of golf courses, sports fields, artificial lakes and ponds, and paths. We sought to identify controls on stormwater runoff and solute export from this dynamic watershed by examining runoff from 90 storms over a 9-year period that were sufficiently large to produce flow at the watershed outlet. We used an automated water sampler to collect stormwater samples for chemical analyses during periods of flow, and obtained discharge from an adjacent USGS gauge. Only storms where samples were collected throughout the hydrograph (n = 21) were included in the analyses of solute export.
Results/Conclusions
Storm flow and precipitation were highly correlated, and events varied considerably in duration with continuous flow in the typically dry channel lasting between <2 to ~110 hours. Some of this variability may reflect the presence of retention structures throughout much of the watershed. The total export of solutes during storms was highly correlated with cumulative discharge during storms, but there was considerable variation within patterns of export (e.g., the timing of delivery) across both analytes and storms. Patterns of storm hysteresis were not consistent across analytes and storm size, but the pattern exhibited by conservative ions (e.g., chloride, sodium) often was opposite that of the nutrient (nitrate, ammonium, phosphate) species. The event mean concentration of most analytes was higher during flashier summer, monsoon storms than for frontal winter storms. While overall solute export was driven consistently by cumulative discharge, the export patterns (hysteresis, timing of delivery) within storms and across analytes suggest chemistry of stormwater runoff in aridland, urban catchments is complex and influenced by a dynamic of surface and storm features.