|Thank you for submitting the revised version of your manuscript. All my minor comments were well addressed.|
However, some clarifications regarding my comments referring to the methodological limitations / uncertainties are still needed.
The authors explain that they perform an area-weighted sum of the variables from each GCM over the grid cells in the basin and thus the previous assumption “Our modified SPEI calculation assumes that both precipitation and glacial runoff are distributed evenly across the drainage basin...” does not hold anymore. Can you clarify this? Does an area-weighted sum mean that you weighted the precipitation output of each GCM grid cell in the respective basin by the area of the corresponding grid cell? And the sum gives the mean precipitation of a basin? However, in Equation 1 a fraction of the total precipitation in the basin is replaced by the glacier runoff from Huss and Hock (2018). If I understood your explanation correctly, my first comment remains: There is the possibility of an increased moisture source term in the modified SPEI that does not stem from glacier runoff but from the methodological limitation. This might be the case if glaciers are located in a rather wet part of the basin, e.g. Amu Darya basin (see the toy example in the previous comment). If you would want to circumvent this limitation, you would have to subtract the glacier area fraction from the precipitation amounts of the actual grid cells where glaciers are located and not from the basin-averaged precipitation.
I do not expect you to change this methodology, as this might only very slightly impact the results, but I expect some discussion of this potential limitation similar to the discussion of the precipitation correction factor. An example quantification of whether this methodological limitation affects the main results of your study would be helpful to understand it. This quantification could be combined with the following comment as both methodological limitations likely lead to an increased moisture source term in the modified SPEI which does not relate to glacier runoff itself and could potentially affect the results regarding buffering of droughts by glacier runoff.
Precipitation Correction Factor
The added paragraph to the discussion is well written and explains concisely and honestly the potential limitations. Thanks a lot! It also shows for one example basin how much the precipitation would increase assuming a default precipitation factor.
What would still be needed to understand the implications of this limitation is the connection between the increase in precipitation and the changes in SPEI. The effect of an increase in precipitation on SPEI is not straightforward. The higher the potential evaporation, the larger the effect of increased precipitation on Di values (Eq. B1). Therefore, the effect will be larger in summer and if the basin is located in lower latitudes/altitudes, I assume. Then, to calculate the SPEI, the values are compared to the standardization set of SPEI. It is not clear to me how much the SPEI and your results would change if there would be a 10% increase in precipitation in the Cooper basin.
I agree that it makes no sense to implement a precipitation correction for all basins in the manuscript. However, it is important to know that the methodological limitations have no significant effect on the results. Therefore, I suggest the authors follow their calculations of the results for two basins, given no glacier melt but an increase in precipitation. One good example would be the basin with the largest glaciation as suggested by the authors already, where the increase in precipitation would be largest. Another example could be a basin with high potential evapotranspiration, e.g. Indus basin (see Fig. 2 in Laghari et al, 2012) where a change in the moisture source term leads to larger Di values.
It would be valuable to see if this increase in precipitation changes the SPEI and the amount of droughts / frequency of droughts or whether the results are robust to these methodological limitations.
Laghari, A. N., Vanham, D., and Rauch, W.: The Indus basin in the framework of current and future water resources management, Hydrol. Earth Syst. Sci., 16, 1063–1083, https://doi.org/10.5194/hess-16-1063-2012, 2012.