Seasonal river runoff calculated from a global atmospheric model

A global atmospheric general circulation model is used to examine seasonal river runoff for the world's largest rivers. The model's river runoff is not directly comparable to observed runoff at the mouth of a river basin because the model assumes that all runoff within a river's drainage basin reaches the river mouth instantaneously. It is useful, however, to examine the model's monthly runoff to determine the extent to which it does agree with runoff as understood by hydrologists, The runoff and precipitation for the river drainage basins are compared with observations. For most river basins the model produces too much precipitation and runoff. However, for the world's largest river, the Amazon, the mean annual runoff is less than half the observed. A common characteristic of high‐latitude rivers is a large runoff peak in the spring, primarily due to melting snow. The model reproduces these peaks but generally leads the observed runoff by one month. This arises because the model does not allow runoff to move between grid boxes but calculates only the total monthly runoff into the entire drainage basin. The runoff in dry river basins is poorly simulated by the model, in part because the precipitation is too large but also because the model's surface parameterizations do not accurately apportion the water lost between evapotranspiration and runoff. The present version of the atmospheric model contains two soil layers and specifies the grid box runoff at each time step as a linear function of the soil moisture storage and the precipitation. A more physically based model should improve the model's simulation of river runoff. A model's ability to simulate monthly river runoff is a good test of its hydrologic cycle and surface parameterizations in different climatic regions and provides a useful diagnostic for climate modelers.