Patterns and Controls of Monsoonal Urban Runoff Hydrologic and Hydrochemical Responses of Semi-arid Catchments

Abstract

Projections of population growth, urban expansion and decreasing water resources in arid and semi-arid regions have increased the importance of urban runoff as a potential renewable water resource that, through enhanced recharge, can augment groundwater supplies. However, it is unclear how urbanization alters hydrologic and hydrochemical responses of small catchments ( < 5 km²) in these regions. This body of work identified controls on the spatial and temporal variability of hydrologic and hydrochemical responses of urbanized catchments in the Tucson Metropolitan area of semi-arid southern Arizona. The temporal distribution and characteristics of rainfall did not control urban catchment hydrology where overall, there was no evidence of significant seasonal catchment wetting. Land use did not control hydrologic responses although runoff was more frequent and of longer duration in urbanized than in non-urban catchments. Runoff depth and runoff ratios were controlled by the combined effect of imperviousness, the characteristics of the stormwater drainage system and rainfall depth. Runoff hydrochemistry did not vary in response to land cover or imperviousness. Rapid increases in solute stores between rainfall-runoff vents resulted in invariant seasonal runoff solute concentrations. Four major factors controlling runoff hydrochemistry were identified: 1) landscape heterogeneity and catchment connectivity, 2) the spatial extent of pervious and biogeochemically active areas, 3) the efficiency of overland flow and runoff routing mobilization and 4) the extent of catchment wetting. The stormwater drainage system, and specifically the characteristics of the stream channel substrate, emerged as significant controls of runoff responses. Conservative transport of biogeochemically active solutes during runoff was observed in piped, concrete and gravel lined waterways, whereas solute sourcing and retention was more dynamic in grass lined reaches. Biogeochemical processing in the stream channel substrate between runoff events indicates that pervious waterways alter soil solute pools available for subsequent solute transport, and that stream channel biogeochemical processes are tightly linked to the characteristics of the channel substrate and cyclical channel wetting and drying. This body of work indicates that successful stormwater management strategies in the semi-arid Southwest should focus on the stormwater drainage network and the presence, density and characteristics of pervious channels.