Quantifying relationships between urban land use and flow frequency of small Missouri streams.

Flow frequency is an important hydrologic statistic to consider in environmental flows assessment. However, there is a paucity of focused interdisciplinary hydrologic assessments that quantify human development influence on flow frequency of small streams (drainage area < 282 km). Relationships between urban land use and land cover (LULC) and flow frequency were assessed for general trends at current gauged watersheds (n = 32) of Missouri, USA. Urban land use - flow frequency relationships changed from linear in developed areas with <50% total impervious surfaces (i.e. low density urban areas), to non-linear in developed areas with >50% total impervious surfaces (i.e. high density urban areas). Urban land use influence on flow frequency was not detected in events below median flow (0.02510 < R > 0.03356; n = 32). Conversely, urban land use - flow frequency relationships were relatively strong above median flow (0.55500 < R > 0.78703; n = 32). Further, explained variance generally increased to meso-scale flows (0.58350 < R > 0.82470; n = 32), and then, decreased during high flows (0.34912 < R > 0.61805; n = 32). More specifically, the greatest observed influence of urban land use on flow frequency increased from a 0.2 to 1 year return period in low density urban areas, to a 1 to 2 year return period in high density urban areas in small Missouri streams. Thus, results indicate that management efforts should focus on reducing the frequency of 1 year events in low density urban land use areas, and 2 year events in high density urban areas to secure environmental services of small urban streams in Missouri, USA. These results hold important implications for other regions globally, where urban land use has increased the frequency of streamflow response.