atmosphere Article Streamflow Predictions in a Small Urban–Rural Watershed: The Effects of Radar Rainfall Resolution and Urban Rainfall–Runoff Dynamics Lauren E. Grimley 1,* , Felipe Quintero 2 and Witold F. Krajewski 2 1 Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27514, USA 2 IIHR—Hydroscience and Engineering, University of Iowa, Iowa City, IA 52242, USA; [email protected] (F.Q.); [email protected] (W.F.K.) * Correspondence: [email protected] Received: 27 May 2020; Accepted: 21 July 2020; Published: 23 July 2020 Abstract: The authors predicted streamflow in an urban–rural watershed using a nested regional–local modeling approach for the community of Manchester, Iowa, which is downstream of a largely rural watershed. The nested model coupled the hillslope-link model (HLM), used to simulate the upstream rural basins, and XPSWMM, which was used to simulate the more complex rainfall–runoff dynamics and surface and subsurface drainage in the urban areas, making it capable of producing flood maps at the street level. By integrating these models built for different purposes, we enabled fast and accurate simulation of hydrological processes in the rural basins while also modeling the flows in an urban environment. Using the model, we investigated how the spatial and temporal resolution of radar rainfall inputs can affect the modeled streamflow. We used a combination of three radar rainfall products to capture the uncertainty of rainfall estimation in the model results. Our nested model was able to simulate the hydrographs and timing and duration above the threshold known to result in nuisance flooding in Manchester. The spatiotemporal resolution the radar rainfall input to the model impacted the streamflow outputs of the regional, local, and nested models differently depending on the storm event. Keywords: hydrologic modeling; radar-rainfall; streamflow predictions; urban hydrology 1. Introduction In Iowa, the dominant contributor to streamflow is the runoff generated from regional watersheds, which are characterized by more natural terrain, but the urban areas primarily experience the negative effects of fluvial and pluvial flooding [1,2]. Hydrological and hydraulic models are commonly used as tools to understand past, present, and future flood risk profiles at various scales. [1,3–5]. Integrating regional- and local-scale hydrological models is difficult because of the varying heterogenous spatial and temporal scales and dimensionality within the watershed [6–8]. These kinds of challenges are not unique to Iowa. In this study, the authors simulated historical peak streamflow and urban flood extents using a nested regional–local model with three radar rainfall products as inputs. In the context of this paper, “nested” refers to one-way coupling of a coarse resolution model to a model with a finer resolution. We use “local” to refer to an urban area of interest that is within a “regional” watershed where rural landscapes and natural terrain are dominant. We used a nested model to study flooding in a small town, Manchester, IA in the regional Maquoketa River basin (Figure1). This area was suitable for a case study because we were able to use radar rainfall products, streamflow measurements, rain gauge data, and the statewide forecasting model managed by the Iowa Flood Center (IFC). Our objective Atmosphere 2020, 11, 774; doi:10.3390/atmos11080774 www.mdpi.com/journal/atmosphere Atmosphere 2020,, 11,, x 774 FOR PEER REVIEW 2 2of of 22 21 was to gain an improved understanding of the impacts of the spatial and temporal resolution of radar was to gain an improved understanding of the impacts of the spatial and temporal resolution of radar rainfall at the regional and local scales for extreme rainfall events, and to identify the benefits of using rainfall at the regional and local scales for extreme rainfall events, and to identify the benefits of using a nested modeling approach for streamflow prediction and local flood modeling. a nested modeling approach for streamflow prediction and local flood modeling. (a) (b) FigureFigure 1. 1. (a()a )The The extent extent of ofthe the regional regional model model was was 725 725km2km of rural2 of rural land landin the in upper the upper Maquoketa Maquoketa River watershed,River watershed, located located in northeast in northeast Iowa. Iowa. The Themajo majorr tributaries tributaries are areCoffins Coffi nsCreek Creek and and Honey Honey Creek, Creek, which drain into thethe MaquoketaMaquoketa RiverRiver at at the the north north end end of of the the city city limits limits of of Manchester. Manchester.(b )(b The) The extent extentof ofthe the local local model model included included Manchester Manchester city limits,city limit whichs, which are divided are divided by the by Maquoketa the Maquoketa River. TheRiver. Eastern The EasternTributary Tributary drains into drains the Maquoketainto the Maquoketa River near River the United near the State United Geological State Ge Surveyological USGS Survey streamflow USGS streamflowgauge and NWSgauge Coop and NWS rain gauge Coop locatedrain gauge at the located model at outlet. the model outlet. AA hydrological hydrological and and hydraulic hydraulic model model developed at the the regional regional scale is is typically typically used used to to generate generate streamflowstreamflow estimates estimates along along a a major major stream stream network, network, and and the the dominant dominant rainfall–runoff rainfall–runoff andand routing processes can can be be described described at at a coarser a coarser scale scale (i.e., (i.e., datasets datasets at a at resolution a resolution of 50 of m 50 or mgreater) or greater) and with and simplificationswith simplifications (such (such as channel as channel geometry) geometry) [1,9–11]. [1,9– Local11]. Local hydrodynamic hydrodynamic models models are aredeveloped developed on anon “as an “asneeded” needed” basis basis for a for specific a specific purpose, purpose, with witha desired a desired set of set output of output variables variables and accuracy and accuracy such assuch floodplain as floodplain mapping mapping or evaluating or evaluating changes changes in the water in the level water from level a proposed from a proposed engineering engineering project [12,13].project [In12 these,13]. In cases, these a cases,fine-resolution a fine-resolution model is model often is necessary often necessary for simulating for simulating pluvial pluvial flooding flooding in an urbanin an urban landscape, landscape, because because the dominant the dominant hydrologic hydrologic processes processes vary vary spatially spatially and and temporally temporally due due to to increasinglyincreasingly complexcomplex drainagedrainage networks networks with with subsurface subsurface (e.g., (e.g., storm storm sewers) sewers) and surface and surface (e.g., streets, (e.g., streets,buildings) buildings) routing. routing. 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AnAn important important source source of of uncertainty uncertainty in in hydrological hydrological models models is isthe the spatiotemporal spatiotemporal resolution resolution of theof therainfall rainfall input input (see Villarini (see Villarini and Krajewski and Krajewski [19] for [a19 review] for aof review radar rainfall of radar uncertainty rainfall uncertaintymodeling), becausemodeling), it heavily because influences it heavily influencesthe rainfall–runof the rainfall–runof processesff processes simulated simulated in the model in the [20–22]. model [ 20Many–22]. researchersMany researchers seek to accurately seek to accurately capture the capture non-linear, the non-linear,spatiotemporal spatiotemporal patterns of rainfall patterns and of how rainfall this variabilityand how this will variability change the will accura changecy of the flood accuracy models of and flood their models predictions and their [20,22–28]. predictions Furthermore, [20,22–28]. radarFurthermore, and satellite radar rainfall and satellite datasets rainfall can be datasetsresolved canand beused resolved as input and to usedlocal scale as input or urban to local models scale [20,22,25,27,29].or urban models By [20 using,22,25 ,27multiple,29]. By rainfall using multipleinputs, we rainfall produced inputs, an we ensemble produced of an hydrological ensemble of predictions that implicitly accounted for the uncertainty in the equations and parameters that were Atmosphere 2020, 11, 774 3 of 21 Atmosphere 2020, 11, x FOR PEER REVIEW 3 of 22 hydrologicalused to describe predictions the physical that implicitly system accounted[30]. In this for paper, the uncertainty the authors in thedescribe equations the andstudy parameters area and thathistorical were usedflood to events,