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 dependent on frequency of rain Precipitation intensity is determined by raindrop size and the speed of the rain falling on the soil. The bigger the drops of rain and the higher the speed, the more intense the rain. Precipitation also comes in other forms such as hail, snow or sleet and the time taken for them to be absorbed into the water cycle is also different. At its core, precipitation intensity is determined by its raindrop size and the speed of the rain falling on the soil. The bigger the drops of rain and the higher the speed, the more intense the rain.

3 1.2 Outputs from the drainage basin hydrological cycle usually comes in the form of evaporation and transpiration (Evapo-transpiration). It is the loss of moisture at the earth’s surface by direct evaporation from water bodies and soil AND transpiration from growing plants. As it is rather difficult to separate the 2, the term evapo -transpiration is used to describe the combined effect of both. Evaporation – is the direct loss or diffusion of water to the atmosphere from surfaces of water bodies (lakes, rivers, wet roads, moist soil). Relatively less evaporation takes place from large water bodies than small ones because more incoming solar radiation is used in heating the water to depth and not just the surface. High salinity water has lower evaporation rates than fresh water as impuri ties in the water reduces rate of evaporation. Evaporation from soil surfaces is limited by the availability of water or evaporation opportunity. Soil texture also determines the rate of evaporation. Clays have a lower rate of evaporation than sand and gravel. Transpiration – is a biological process involving the evaporation of water from leaves of plants (through the stomata). It occurs when vapour pressure in the leaf cell is greater than atmospheric vapour pressure. Transpiration is controlled by atmospheric factors like temperature and by plant factors like leaf area, amount of soil moisture, etc. River Discharge – is also an important form of output as the water is leaving the drainage basin into the sea. Factors affecting Evaporation (E) and Transpiration (T) (from water surface): - Temperature o E is highest when air temperature is at its maximum (e.g. summer) and lowest at night and in winter. Solar radiation is the most important factor involved in evaporation as it affects rate of water loss. As air temperature increases, potential for holding water vapour increases and hence greater E. o However, there must be temperature differences between the water body and the surrounding air for E, compared to just air temperature alone. This is because temperature differences control the vital vapour pressure deficit. Water vapour molecules flow from high to low vapour pressure areas. o Solar radiation also enhances photosynthesis, thus EVPT are greatest in the noon and in summer. - Atmospheric Humidity o When relative humidity (RH) is high, EVPT is low, and vice-versa o When the air is saturated (RH=100%), there is little room for EVPT. - Wind speed o EVPT is higher when winds speed increases , surrounding water vapor is blown away, reducing relative humidity which allows for more water vapor to evaporate. - Vegetation type and density o Different types of plants have different rates of EVPT. o Deep-rooted vegetation transpires more water during droughts than shallow-rooted plants o Xerophytic desert plants transpires less as there are fewer stomata o Light colored plants with a higher albedo reflects solar radiation, thus have lower rates of EVPT.

4 - Water Quality o E decreases by 1% for every 1% increases in salinity. - Water body o Shallow waters have higher E in summer and lower in winter (faster heating up of water body) o Deep waters have higher E in winter than in summer (slower cooling down of water body) o The larger the lake, the greater the total volume of water evaporated. Factors affecting EVPT (from soil surface): - Soil moisture content o E is higher when soil moisture content (on the surface) is higher - Soil color o Darker the soil, the higher the E, because darker soils have a lower albedo (ability to reflect the sunlight), thus gets heated up faster. - Vegetation o Vegetation will reduce soil evaporation by shading the soil and thus lowering surface temperature, reducing wind speed and increasing RH in the lower layers of air.

5 2 & 3. Explain the different flows/pathways and water stores Another important aspect that we need to investigate about the drainage basin hydrological cycle is how water flows (pathways) and how it is stored throughout the cycle when it arrives as inputs and leave as outputs. This section will explain various pathways and stores. Try to classify them in terms of surface/sub-surface and also by their direction. Interception/interception storage Precipitation can be held up on the canopy or b