Continuous short-term rainfall time series are used for design and evaluation of sewer systems and other hydrological components in the urban environment. In a continuous simulation by a hydrologic/hydraulic model, not only the system response to single rainfall events can be assessed but also long-term volumes of e.g. treatment plant inflows and sewer overflows.
Generally an observed time series with a very high time resolution (1-10 minutes) from a single location is used in this type of modeling. As Regional Climate Model (RCM) results have a coarser resolution both in time (30 min - 1 hour) and space (25-50 km) they cannot be directly used in urban hydrological simulations, but downscaling is required.
Downscaling by Delta Change
A common downscaling method is Delta Change. In this method, the RCM results are used to calculate the expected future changes. These changes are then transferred to historical local rainfall observations. The generated data thus represent the local scale with modifications to take climate change into account. It should be emphasized that he method is based on the assumption that future changes at the RCM scale is valid also for the local scale, which is not certain but a necessary working hypothesis.
In the Common Services, users may upload local rainfall time series. Based on detailed analyses of high-resolution RCM results, the future changes of rainfall with different intensity in different seasons may be estimated (Figure 1). In the SUDPLAN tool, these changes will be projected onto the uploaded series, generating a new time series representing a selected future period.
Changes in frequency
Delta Change downscaling generally only takes into account changes in rainfall intensity and not frequency. This is a limitation, especially in a pan-European application as climate projections for some regions indicate pronounced changes in frequency. In SUDPLAN, the Delta Change procedure will be developed to allow for changes also in frequency.