Search for Physically Based Runoff Model—A Hydrologic El Dorado?

The search for physically based distributed runoff models has been underway for more than half a century. A great deal of progress has been made and we have a much better understanding of hydrological processes today than engineers and scientists did in the 1930s. Physically based models are being used more frequently in engineering practice, but questions are being raised regarding their "physical basis" and accuracy, and it has been suggested that simpler models are superior. Selected papers presenting tests of physically based models or comparisons with simpler models are reviewed. It is shown that, on a relatively small scale, these papers are overly pessimistic due to problems of hydrologic measurement and interpretation. However, there are great difficulties involved in scaling up to larger watersheds and we must realize that significant uncertainties are involved in predicting surface runoff.