Using Remote Sensing Based Metrics to Quantify the Hydrological Response in a City

We propose a remote-sensing based metric approach to evaluate the hydrological response of highly urbanized areas and apply it to the city of Brussels. The model is set-up using 2 m resolution hyperspectral data. Next, it is upscaled to the city level, using multi-spectral Sentinel-2 data with 20 m resolution. We identify the total impervious area, the vegetation cover and the leaf area index as important metrics to derive a timeseries of spatially distributed net rainfall, runoff and infiltration from rainfall data. For the estimation of the actual evapotranspiration we use the potential evapotranspiration and the available water storage based on the interception, the depression storage and the infiltration. Additionally, we route the runoff to the outlet of selected sub-catchments. An important metric for the routing is the timing to the outlet which is approximated using the total impervious area and the hydrological distance to the outlet. We compare our approach to WetSpa model simulations and reach R 2 values of 98% for net rainfall, 95% for surface runoff, 99% for infiltration and 97% for cumulative evapotranspiration. The routing in the Watermaelbeek catchment is evaluated with discharge observations and reaches NSE values of 0.89 at a 2 m resolution and 0.88 at a 20 m resolution using an hourly timestep. At the timestep of 10 min and a 20 m resolution the NSE is reduced to 0.76. For the Roodebeek catchment we reach an NSE of 0.73 at a spatial resolution of 20 m and an hourly timestep. The results presented in this paper are optimistic for using spatial and temporal metrics retrieved from remote sensing data to quantify the water balance of urban catchments.

An alternative method for estimating total impervious area in catchments using high-resolution colour aerial photography

Estimation of total impervious area (TIA) is a pre-requisite for ecohydrological research to allow for a direct prediction on stream ecosystem health within catchments. This paper presents an alternative to using multi-spectral imagery for estimating TIA at a catchment scale, by using high-resolution colour aerial photography. The method was applied to a number of catchments in South East Queensland, Australia, some of which were gauged and some of which were part of an Ecosystem Health Monitoring Program (EHMP). The results from this method were compared to TIA estimates, for some of the same catchments, that were derived through three other techniques, i.e. manual digitization of geo-referenced aerial photos, Brisbane City Council data derived from image analysis using Landsat TM imagery and rainfall runoff depth relationship. The high-resolution colour aerial photography method compared favourably to the other techniques with standard deviations of TIA (%) ranging between 0.8% and 8%. The major constraints were shading effects, particularly on roads and grassed areas, and from the similarity in colours between some surface types, some of which can be reduced by appropriate selection of signature colours and multiple iterations of a supervised classification. It was concluded that while infra-red spectral wave bands could help considerably, the high-resolution colour photography could be applied with confidence to derive catchment-scale TIA estimates.

A GIS-based methodology for selecting stormwater disconnection opportunities

The purpose of this paper is to introduce a geographic information system (GIS)-based decision support tool that assists the user to select not only areas where (retrofit) sustainable drainage systems (SuDS) could be implemented within a large catchment (>100 ha), but also to allow discrimination between suitable SuDS techniques based on their likely feasibility and effectiveness. The tool is applied to a case study catchment within London, UK, with the aim of increasing receiving water quality by reducing combined sewer overflow (CSO) spill frequency and volume. The key benefit of the tool presented is to allow rapid assessment of the retrofit SuDS potential of large catchments. It is not intended to replace detailed site investigations, but may help to direct attention to sites that have the greatest potential for retrofit SuDS implementation. Preliminary InfoWorks CS modelling of ‘global disconnections’ within the case study catchment, e.g. the removal of 50% of the total impervious area, showed that CSO spill volume could be reduced by 55 to 78% during a typical year. Using the disconnection hierarchy developed by the authors, the feasibility of retrofit SuDS deployment within the case study catchment is assessed, and the implications discussed.