Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 1

1 Basic Concepts and Management Issues of Rivers
Abstract
Basic concepts are introduced in this chapter to help the readers understand the contents of the other
chapters in this topic. The water cycle and modes of stream network development, Horton's laws, sediment
and sediment load, and various river patterns are defined and presented. Concepts of the water environment
and stream ecology are briefly introduced. The major river management issues, such as water resources
management, flood defense, reservoir management, river bed incision and geological disasters, erosion
control, and river uses are also discussed.
Key words
Water cycle, Stream order, Sediment load, River patterns, Ecology, Management issues
1.1 Basic Concepts
1.1.1 Hydrological Cycle
Precipitation is the water falling over the land from the atmosphere primarily in the form of rain and
snow. Precipitation can return to the atmosphere; move into the soil; or run off the earth's surface into a
stream, lake, wetland, or other water body. More than half of the precipitation falling over the land of
China evaporates to the atmosphere rather than being discharged as stream flow to the oceans. This
“short-circuiting” of the hydrologic cycle occurs because of the two processes, interception and
transpiration. A portion of precipitation never reaches the ground because it is intercepted by vegetation
and other natural and constructed surfaces. The amount of water intercepted in this manner is determined
by the amount of interception storage available on the above-ground surfaces.
Transpiration is the diffusion of water vapor from plant leaves to the atmosphere. Unlike intercepted
water, which originates from precipitation, transpired water originates from water taken in by the roots of
plants. Evaporation of soil moisture is, however, a much slower process due to the capillary and osmotic
forces that keep the moisture in the soil and the fact that vapor must diffuse upward through soil pores to
reach surface air at a lower vapor pressure. When calculating the hydrologic budget of a watershed the
transpiration from vegetation and the evaporation from the soil typically are considered together as
evapotranspiration.
Infiltration —Close examination of the soil surface reveals millions of particles of sand, silt, and clay
separated by channels of different sizes. These macropores include cracks, “pipes” left by decayed roots
and wormholes, and pore spaces between lumps and particles of soil. Water is drawn into the pores by
gravity and capillary action. Gravity is the dominant force for water moving into the largest openings,
such as worm or root holes. Capillary action is the dominant force for water moving into soils with very
fine pores. Infiltration is the term used to describe the movement of water into soil pores. The infiltration
rate is the amount of water that soaks into soil over a given length of time. The maximum rate at which
water infiltrates into a soil is known as the soil's infiltration capacity .
Ground water —The size and quantity of pore openings also determines the movement of water within
the soil profile. Gravity causes water to move vertically downward. This movement occurs easily through
larger pores. As pores reduce in size capillary forces eventually take over and cause water to move in any
direction. Water will continue to move downward until it reaches an area completely saturated with water,
the phreatic zone or zone of saturation. The top of the phreatic zone defines the ground water table or
phreatic surface . In mountainous area the channels are incised very deep and lower than the phreatic