TMJC 2025 JC1H2 C2 Lecture 8 Drainage Basin Hydrology

1 Higher 2 (9173) Cluster 2: Tropical Environments TMJC C2 Lecture 8 Drainage Basin Hydrological Cycle 2025 Hydrologic Cycle and Drainage Basin A Catchment is also known as a drainage basin and both define an area of land in which water flowing across the surface drains into a particular stream or river. A catchment is a convenient unit because it is normally well defined topographically and it is an open system for which inputs and outputs of mass and energy can be defined and measured. Large drainage basins, like the area that drains into the Mississippi River contain thousands of smaller drainage basins. One important concept to remember is that the Drainage Basin Hydrological Cycle would consist of Inputs, Outputs, Pathways (flows) and Stores. Figure 1: Each stream, no matter how small, has its own drainage basin, the area from which the stream and its tributaries receive water. This basin displays a pattern reminiscent of a tree leaf and its veins. The basin hydrological cycle depicts the movement of water among storage places for discrete areas of the earth, for example ecosystems or most often drainage basins. Drainage basins are by definition closed to inputs of surface water thus the number of inputs is minimized or essentially reduced to one, precipitation, although inter-basin transfer of groundwater may occur, but is difficult to quantify and generally assumed to be insignificant. Outputs from drainage basins are by evapotranspiration, flow at the mouth of the main stream which ultimately accounts for all runoff, and possibly output (seepage) of groundwater.

2 Figure 2. Drainage Basin Hydrological Cycle Characteristics of the drainage basin: 1) inputs – in the form of precipitation 2) outputs – in the form of water lost from the system through river flow into the sea or through evapo-transpiration 3) stores – in lakes and / or in soil 4) pathways – in a series of transfers or flows: infiltration, percolation, throughflow, surface flows, etc… 1. Explain the Various Inputs and Outputs of Water into the Hydrological Basin 1.1 Inputs into the drainage basin usually come in the form of precipitation. Precipitation is usually expressed in units of length such as mm or cm. This is because it is assumed that precipitation has fallen uniformly over a given area and so the volume of water is divided by the surface area of the catchment to give a depth of water. It is the deposition of atmospheric moisture at the surface of the earth in the form of rain, dew, mist, hail, sleet, snow. Water evaporates from water bodies like streams, seas, rivers, ponds and from land (evaporation) and vegetation (transpiration) in the form of water vapour. Amount of precipitation received influences the overall scale of evaporation and runoff losses. Variation of Precipitation: - short term variations o variations in precipitation intensity during a rainfall event - climatic regions o de