R² despite its wide use in assessing model performance has several drawbacks. While taking into account the drawbacks, this paper introduces another metric (coefficient of model accuracy MCA) which is capable of assessing "goodness-of-fits". Stepwise derivation of CMA comprises an analogy to the R². Suitability of CMA for assessing model performance was demonstrated through comparison of simulations by hydrological models calibrated using CMA and other existing objective functions.

To investigate the possible change in catchment behavior, which may interfere with the flow-rainfall relationship, three rainfall-runoff models were applied to the main catchments of the Nile Basin in Africa based on inputs covering the period from 1940 to 2003. There was close agreement between the changes in the observed and simulated overland flow from all the models. Thus, change in catchment behavior due to anthropogenic influence in the Nile basin over the selected time period was minimal.

Variability of rainfall in the Nile Basin was found linked to the large-scale atmosphere-ocean interactions. This finding is vital for a number of water management and planning aspects. To give just one example, it may help in obtaining improved quantiles for flood or drought/water scarcity risk management. This is especially important under conditions of (1) questionable data quality, and (2) data scarcity. These conditions are typical of the Nile Basin and inevitably need to be addressed.