Hi Elvis,

I think we need to close the budget and look at different seasons. Increased Runoff due to increased precipitation may have occurred due to more surface runoff and thus reaching rivers faster - if that's the case, it won't give enough time for water to infiltrate and replenish soil moisture to contribute to ETa. Additionally the gap between PET and ETa may be caused by increases in PET in response to warming while ETa may have remained at its prior level. Thus, one would need to look at each term to see if the trends are really conflicting.

Seasonal variation may also reveal things better. Runoff may have increased in a season (possibly spring) while PET would have increased in another (summer) and thus creating the disconnect.

Finally, the uncertainty in determining any of the water balance terms can be large. If trends are based on the mean of several methods or observational data sets, then you may see contractions that do not exist.

Mohamed

Thanks for the insightful discourse @Mohamed. I agree that the water deficit is often quantified in terms of the PET which is itself very sensitive to global warming. It is also true that seasonality can play a major role towards understanding this apparent contradiction. One can also deduce from your exaplanation that the increases in runoff are the result of substantial increases in precipitation during the cool NH months (October-March, say) when PET (warming dependent) increases are relatively small, and the largest PET increases often occur during the warm months (April-September, say).

Right, observational and methodological uncertainties are ubiquitous, but one may also theorize that the contradiction occurs because water deficit (PET dependent) and runoff represent different components of the water and energy budgets?. In most cases, hydroclimatic water deficits are approximated as PET – ETa, while runoff is often computed as Precip – ETa. I doubt if the contradicion is evident if we derive runoff simply with the Budyko water balance formulation given precip and PET?

Thanks a lot for further insights. It is important to understand some of these details before making a general conclusion of trends in water defict and runoff in the NH for the historical epoch, then the future projections.

Elvis

Dear Asong,

You will find the correct answer by reading the attached article properly.

With my best regards

Prof. Bachir ACHOUR