Defining a Topographic Index Threshold to Delineate Hydrologically Sensitive Areas for Water Resources Planning and Mana
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Defining a Topographic Index Threshold to Delineate Hydrologically Sensitive Areas for Water Resources Planning and Management Zeyuan Qiu 1
2
& Steve W. Lyon & Ellen Creveling
3
Received: 21 November 2019 / Accepted: 29 July 2020/ # Springer Nature B.V. 2020
Abstract
Hydrologically sensitive areas (HSAs) are runoff-generating areas often targeted for effective water resources planning and management actions. Commonly, HSAs can be mapped as areas in a landscape with a topographic index (TI) greater than a threshold level. This study explored the impact of a gradient of different TI threshold values for delineating HSAs using two popular TIs: a topographic wetness index (TWI) and a soil topographic index (STI). The resultant HSAs for each TI were compared to the Federal Emergency Management Agency (FEMA) 100-year floodplain map in New Jersey and its five water regions. Spatial comparison indicators were used to assess the spatial similarity between the HSAs delineated and the FEMA floodplain map. Such comparisons identified the threshold that delineated HSAs whose spatial distributions were most consistent with the FEMA floodplain at each spatial scale for each TI. For example, the identified threshold for using a TWI to delineate HSAs was 10.5 at the state level; however, this threshold varied by the water region. The HSAs delineated approximate the spatial extent of runoff-contributing areas to the 100-year flood relevant for water resources planning and management actions for flood hazard mitigation. Keywords Hydrologically sensitive areas . Topographic index . Thresholds . Kappa value . Floodplain . Water resources planning and management
* Zeyuan Qiu [email protected]
1
Department of Chemistry and Environmental Science, College of Science and Liberal Arts, New Jersey Institute of Technology, Newark, NJ, USA
2
School of Environment and Natural Resources, Ohio State University, Columbus, OH, USA
3
The Nature Conservancy, Southern New Jersey Office, Delmont, NJ, USA
Qiu Z. et al.
1 Introduction Hydrologically sensitive areas (HSAs) are the parts of a landscape that produce disproportionate amounts of surface runoff. HSAs can be targeted to effectively implement planning and management measures to control nonpoint source pollution (Amin et al. 2017; Qiu et al. 2019) and mitigate negative impacts of stormwater runoff (Qiu et al. 2014, Martin-Mikle et al. 2015; Kaykhosravi et al. 2019). Topographic indices (TIs) have long been used to parsimoniously delineate HSAs based on the concept of variable source area (VSA) hydrology (Walter et al. 2000; Heathwaite et al. 2005; Qiu 2009). Specifically, the TI values mimic the sequential distribution of water storage capacity of a landscape allowing identification of runoffgenerating areas (Dahlke et al. 2009). As such, the locations in a landscape with TI values greater than a given threshold delineate HSAs. There are several mechanistic ways to define such a threshold for delineating HSAs in landscapes, including the controlling specific storm event method (L
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