Landscape, hydrological and social factors affecting water quantity and quality in urban headwater streams of central Iowa
Urban landscapes are complex and dynamic systems, and in areas of strong human population growth urban areas are undergoing rapid expansion. Increases in impervious surface area (e.g. roads, buildings, and parking lots) are typically associated with urban expansion, and usually represent irreversible change on the landscape. These impervious surface areas provide services to urban residents, but also have environmental consequences, including alteration of urban stream hydrology and stream water quality. This dissertation documents research conducted to quantify urban expansion, and to explore the complex relationships among terrestrial landscape features, urban stream hydrology, social system factors, and water quantity and quality variables in 20 urban stream watersheds located in five central Iowa cities. First, urban land cover change was quantified for four cities using planimetric data applied to aerial photo images from 1940 to 2011. Impervious surfaces accounted for about 1.5% of the area in four study cities in 1940, and increased to 16.8% in 2011, a 1,020 % increase. Much of the expansion of impervious surface was contributed by buildings and parking lots. High, medium, and low percent impervious surface zones were dominated by parking lots, buildings, and roads, respectively. Second, both climate change and land cover change were predicted and simulations were performed using the Storm Water Management Model to determine their potential effects on stream hydrology (with unit-area peak discharge, flashiness index, and runoff ratio as response variables) in five selected study watersheds. Simulation of climate change predicted effects on unit-area peak discharge, while simulation of land cover change predicted changes in all three response variables. Different distributions of additional impervious surface simulated within a single watershed had a greater effect on timing of delivery than on total amount of discharge. Third, the direct, indirect and total effects of human system, terrestrial landscape features, and stream hydrology variables on water quality outcome variables were assessed using path analysis. Stream water conductivity, total nitrogen concentration, and total phosphorus concentration were strongly influenced by road density, percent of crop land, and percent of residents with college-level education, respectively. Finally, a semi-quantitative analysis of the implementation of the National Pollutant Discharge Elimination System (NPDES) permitting program was linked to water quality outcomes in 20 urban streams in five cities to examine the effectiveness of these regulatory requirements. Cities with higher ratings for completeness of responses to the NPDES stormwater program also had better stream water quality. Based on multiple regression analysis, a strong relationship was detected between the quality of municipal responses to the NPDES program and stream water concentrations of phosphate, total phosphorus, total suspended solids and turbidity. Evidence presented in this dissertation indicates that urban stream hydrology and water quality are related to characteristics of urban populations, landscapes, and that impacts of changes in these variables on urban stream water quality and quantity in central Iowa are likely to be more pronounced over time. The information provided by this research should be helpful for civic officials in these and other cities to mitigate against potential negative impacts of these changes to urban streams.