Hydrology

Introduction

In order to manage the worlds increasingly scarce water resources we must have a sound understanding of how water moves around the planet and what influences water quality. Fundamentals of Hydrology provide an engaging and comprehensive introduction to this subject and provide real-life examples of water resource management in a changing world.

Hydrology

Hydrology is the study of the circulation of water between the earth and the atmosphere. This endless circulation of water is known as the hydrologic cycle.   The components of the hydrologic cycle can also be examined at the level of a wetland resource. In this case, we often use the terms “wetland hydrology”, “water budget”, and “hydrologic regime”.

The Hydrologic Cycle

Water is found in the earth’s atmosphere, on the earth’s surface (surface water), and within its subsurface (groundwater). The hydrologic cycle is the continuous process of water moving among these three general locations. Figure 1.1 depicts the basic components of the hydrologic cycle.

Water, which exists in the atmosphere as water vapor, reaches the earth’s surface by

Precipitation (rain, snow, hail, fog). Once on the surface, water moves by the force of gravity, and may follow a number of paths. As precipitation falls on the ground, some of the water remains on the surfaces of plants, process known as interception. Some of the water is stored in the irregularities and small depressions on the land surface, as depression storage. During winter months in temperate climates, some precipitation may be stored on the surface as snow, ice, and frost.

Some of the water that falls as precipitation enters the ground through soil pores, and is called infiltration. A portion of this infiltrated water may be stored in the soil, as soil moisture. Within the root zone, this soil moisture becomes available for plants. The remainder of the water entering deeper into the ground is referred to as recharge, and moves through the soil as interflow (unsaturated flow through the soil) or groundwater flow (saturated flow through the soil). The zone in which groundwater flow occurs is commonly referred to as the groundwater table.

 When precipitation exceeds the combined effects of interception, depression storage, and

Infiltration, the remaining water flows over the surface of the ground as direct runoff (also referred to as “excess rainfall”). This runoff flows over the surface to natural channels such as topographic swales, gullies, intermittent streams, perennial streams, and rivers. Some of this water is also stored on the earth’s surface in wetland systems, ponds, lakes, reservoirs, and the oceans.

Water beneath the ground surface may also flow to these various water resources through the processes of interflow and groundwater flow. The portion of flow in streams and other water bodies that originate from interflow and groundwater discharge is generally known as base flow.

Water eventually returns to the atmosphere by direct evaporation from the surfaces of the land and water bodies. Water also returns to the atmosphere from vegetation by evapotranspiration, which is the combined process of evaporation from plant surfaces and the uptake and release of water through the biological process known as transpiration. This guidance manual deals primarily with surface water flows. The behavior of subsurface flows is treated only to the extent of describing certain important relationships to surface water flows. Similarly, the behavior of water in the atmosphere is considered only to the extent of its effect on inputs to or losses from the surface water system.