Basin

I

Introduction

Basin or Drainage Basin, area of the Earth’s surface which drains into a single river system. The boundaries of a basin are formed by watersheds that separate it from adjacent areas draining into other rivers. The size and shape of a basin are usually determined by the underlying geology. The pattern and density of streams and rivers draining the basin depend not only on geological structure, but also on land surface relief, climate, soil types, vegetation, and, increasingly, human impacts on the basin environment.

II

General Concepts

Basins can be treated as open systems within which hydrological processes can be studied; an open system is a set of interrelated elements and processes that exchanges energy with its surroundings, and through which materials flow. Their drainage characteristics can be measured and analysed quantitatively, a process called drainage basin morphometry. Hence, the basin represents the basic unit used in hydrology, the science concerned with the study of the different forms of water in the natural environment. Basins are major features of the landscape and over most of the world’s continents, landscape-forming processes are dominated by fluvial erosion, transport, and deposition. For these reasons, the basin also constitutes the fundamental unit of physical geography.

Flood hazards and water resources are also best assessed and managed using a basin-wide approach, because the input, storage, and output of water within a basin can be measured, budgeted, and modelled analytically. In addition, piecemeal approaches have been found to solve one problem while inadvertently generating others elsewhere in the drainage system. Consequently, integrated basin management is recognized as the best practice in water resource development and river regulation.

III

Basin Formation

Basins exist over a vast range of scales, from the ocean basins that define the largest drainage units on Earth to field-sized areas feeding small streams. Many basins are formed by geological processes involving deformation of the Earth’s crust through extension, downwarping, faulting, folding, or volcanic activity. Others are the result of erosion of the land surface by wind, water, or ice. The structure of the underlying rocks influences the distribution of erosion, with low and high areas developing on erodible and erosion-resistant rocks, respectively. Because rocks stretched upwards in an anticline are weaker than those compressed downwards in a syncline, erosion often leads to inverted relief, with high areas becoming basins and formerly low areas forming watersheds.

Where the rocks underlying a basin are permeable it is possible for water moving through the ground, or groundwater, to “leak” from one basin to another. Hence, the boundaries of a groundwater basin do not always coincide with the watersheds of the drainage basin above.

IV

Basin Hydrology

Water in the basin arrives in the form of precipitation as part of the water cycle (hydrological cycle). Some precipitation returns to the atmosphere, having been intercepted by vegetation and evaporated from the surfaces of leaves and branches. More is lost to evaporation from the ground surface and transpiration by plants. In arid and semi-arid climates all of the precipitation may be consumed in this way for most of the time; basin run-off occurs only occasionally, following intense storms. Where precipitation exceeds losses to evapotranspiration, the excess water makes its way through the drainage system. Its rate of progress is not uniform, however; water may be stored in lakes, soils, and as groundwater for considerable periods before it eventually arrives at the outlet, or basin channel, as basin run-off.

The major elements of basin hydrology are illustrated in the diagram “The Hydrology of a Drainage Basin”, which shows water following different pathways to reach the basin channel as run-off. Water which infiltrates to the permanently saturated groundwater, or phreatic, zone below the water table moves as baseflow; in the partially saturated aerated, or vadose, zone above, it moves as interflow and throughflow. Water that is unable to infiltrate the soil becomes overland flow. The proportion of run-off following the different pathways depends on a variety of factors, some of which are fixed properties of the basin (geology, structure, and relief). Other factors can vary with time and in response to human activities (climate, soils, vegetation), and some depend on the recent weather experienced by the basin (antecedent conditions). Subsurface drainage by interflow and groundwater seepage occurs much more slowly than surface drainage by overland flow, a characteristic that is important in maintaining baseflow in the river system between precipitation inputs.

Antecedent conditions are particularly important in determining the amount of overland flow. Where the soil is saturated rainfall cannot infiltrate. The resulting overland flow drains rapidly to the channel network. Following a series of closely spaced storms, or a prolonged period of precipitation, the area of saturated soil expands, increasing the amount of overland flow. This results in the rapid delivery of a large volume of water to the channel system, which may overwhelm its capacity and cause a flood. In basins which receive substantial precipitation in the form of snow, large volumes of water may be stored on the surface during the winter months. Basin run-off is often dominated by high flows during spring melting. Flooding is a risk if high temperatures or heavy rainfall induce rapid melting.