In the Project Options property sheet, DRAINS allows users to apply a multiplier to all design rainfalls.

In Book 1 of ARR 2019, climate change issues are covered in Chapters 2, 4 and 6. Australia is divided into eleven ‘clusters’ that are shown in Figure 1.6.1. Tables 1.6.2 to 1.6.9 set out the expected increases in temperatures for each cluster over dates from 2030 to 2090.  It is necessary to go through a process described in Section 6.3 of Book 1 of ARR 2019 to arrive at an appropriate temperature increase T_m (^oC) for the task being undertaken. Once this is obtained, rainfalls can be adjusted by the simple formula set out as Equation 1.6.1 in ARR 2019:

                I_p = I_ARR × 1.05 ^Tm

Where  I_p is the projected rainfall intensity or equivalent depth (mm/h or mm), 

              I_ARR is the rainfall depth or intensity for current conditions (mm/h or mm), and 

              T_m is the expected temperature increase at a selected time in the future (^oC).

The multiplier used in DRAINS is 1.05 ^Tm.  For each 1^oC rise in temperature, rainfalls are increased 5%.  More specific climate change rainfall multipliers are available from the ARR Data Hub, covering the period from 2040 to 2090.

To illustrate the process, suppose that we are planning an addition to the existing stormwater drainage system at in suburban Sydney. The steps in defining a climate change adjustment are shown below:

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This adjustment is quite simple, as it applies to all rainfall durations and AEPs.  It is presumably the best general method to apply at present.  

Applying a climate change rainfall multiplier will obviously increase rainfalls, but this must be set against (a) positive or negative changes to design rainfall intensities at the location of interest, and (b) application of the median design procedure with the rainfall ensembles.  Use of the IL-CL model in place of Horton (ILSAX) or ERM will also have impacts, especially since the equivalent impervious area is less than the paved or impervious area assumed in other models.  Drainage authorities and designers will need to perform their own tests to assess the effects of all these changes.