Intra‐annual variability of urban effects on streamflow

Abstract

While considerable research has established the impacts of urbanization on streamflow, there has been little emphasis on how intra-annual variations in streamflow can deepen the understanding of hydrological processes in urban watersheds. This study fills this critical research gap by examining, at the monthly scale, correlations between land-cover and streamflow, differences in streamflow metrics between urban and rural watersheds, and the potential for the inflow and infiltration (I&I) of extraneous water into sewers to reduce streamflow. We use data from 90 watersheds in the Atlanta, GA region over the 2013-2019 period to accomplish our objectives. Similar to other urban areas in temperate climates, Atlanta has a soilwater surplus in winter and a soil-water deficit in summer. Our results show urban watersheds have less streamflow seasonality than do rural watersheds. Compared to rural watersheds, urban watersheds have a much larger frequency of high-flow days during July-October. This is caused by increased impervious cover decreasing the importance of antecedent soil moisture in producing runoff. Urban watersheds have lower baseflows than rural watersheds during December-April but have baseflows equal to or larger than baseflows in rural watersheds during July-October. Intraannual variations in effluent data from wastewater treatment plants provide evidence that I&I is a major cause of the relatively low baseflows during December-April. The relatively high baseflows in urban watersheds during July-October are likely caused by reduced evapotranspiration and the inflow of municipal water. The above seasonal aspects of urban effects on streamflow should be applicable to most urban watersheds with temperate climates.

The Role of Inflow and Infiltration (I/I) in Urban Water Balances and Streamflow Regimes: A Hydrograph Analysis Along the Sewershed‐Watershed Continuum

“…However, we did not detect any correlations with low flows in this study. This result therefore differed from existing studies which detected low flow reduction by I/I, for example, in Long Island, NY (Plunhowski & Spinello, 1978;Simmons & Reynolds, 1982), Baltimore, MD , and Atlanta, GA (Diem et al, 2021;Pangle et al, 2022). The lack of an effect on baseflow could be further evidence that I/I in our study area originated from other non-stream sources.…”

Evapotranspiration From Developed Land and Urban Watersheds in a Humid Subtropical Climate

“…Compared to rural areas, urban areas have lower ET in summer and tend to have higher ET in winter, thereby resulting in reduced intra‐annual variability in ET in urban areas. This reduced intra‐annual variability also has been found for multiple streamflow variables in urban watersheds (Bhaskar & Welty, 2012; Diem et al., 2021). Urban areas have relatively high ET totals in winter resulting from evaporation from a relatively large coverage of impervious surfaces and also from the anthropogenic heat flux enhancing ET from vegetated land cover.…”

Contextualizing Inflow and Infiltration Within the Streamflow Regime of Urban Watersheds

“…The second component of total I&I is groundwater infiltration (GI) that occurs during and between precipitation events and across seasons. Time series of sanitary‐sewage flow often show persistently higher values during the winter and spring seasons, which gradually decline throughout the summer toward minimum values that typically occur in late summer or early autumn (e.g., Bares et al., 2009; Diem et al., 2021; Dirckx et al., 2009). This pattern is attributed to enhanced GI occurring during cold seasons when groundwater storage and the water‐table elevation are relatively high and relatively lower rates of GI during warm seasons when the water‐table elevation has decreased (Figure S6 in the Supporting Information ).…”