INTRODUCTION

A logical point of departure for analysis of water management and natural environment would be a description of the natural state of waters worldwide before human alterations of their quantity and quality began to take effect. For a variety of reasons, this is possible only to a very small extent. Any presentation of the natural condition of waters of the globe necessarily must also take account of the magnitude and diversity of ways in which alterations have been taking place independent of human influences, and in which for more than five thousand years humans have changed inputs and outputs. It is difficult to estimate all of those changes accurately. Only a few rough estimates can be made of the truly natural state of the hydrologic cycle worldwide and in the United States, taking account of major components of precipitation, streamflow, groundwater and transpiration, and their chemical and biological quality.

HYDROLOGIC CYCLE

In simplest terms, the natural waters of the globe may be viewed at any one time as atmospheric moisture plus precipitation plus soil and underground storage plus glaciers and ice fields plus oceans plus moisture stored in or transpired from plants and animals. The concept of a hydrologic cycle has evolved from very simple to very complex (Figure 4.1). There is rich literature in hydrology and hydraulic engineering that defines and examines ways of measuring the various elements in the hydrologic cycle (Chow, 1964; Grigg, 1996; McCuen, 1989).