Catchment Hydrology - Drainage Basin PDF

Catchment hydrology: the drainage basin as a system The drainage basin system -​ The drainage basin is a part of the global hydrological cycle, and is defined as a catchment area forming part of the Earth's surface area which is drained by a particular stream or river. -​...

Catchment hydrology: the drainage basin as a system The drainage basin system -​ The drainage basin is a part of the global hydrological cycle, and is defined as a catchment area forming part of the Earth's surface area which is drained by a particular stream or river. -​ All land belongs in a drainage basin. The dividing line between adjacent basins is called a drainage divide, or watershed. For small local-scale rivers, slight variations in relief can help to divide the land surface into different drainage basins. -​ In the case of continental rivers, the drainage divide may correspond with larger-scale relief features, such as a range of mountains. -​ Unlike the global-scale water cycle, the drainage basin is an open system. The main inputs, stores, flows and outputs of the drainage basin system are portrayed in Figure 4 in two contrasting ways. Drainage basin inputs Drainage basin inputs consist of different types of precipitation, including rain, snow, sleet, hail and frost. Alongside the type and amount of precipitation, its duration and intensity affect how a drainage basin system responds. High intensity rainfall of between 50 and 100mm per hour is rare in the UK. When it does occur, however, the result can be flash flooding. For instance, the devastating Boscastle floods of 2004 were a result of 185 mm rainfall arriving in just five hours (equivalent to around two months of rainfall normally). This kind of occurrence is rare: the intense rain experienced by Boscastle was a one-in-400-year event. In December 2016, the worst flooding in 50 years hit Maesteg in south Wales. -​ Low intensity but relatively long duration rainfall is far more common in the UK. -​ Figure 5 shows the relationship between rainfall intensity and duration for a small tributary of the River Thames. As you can see, it is unheard of to have high intensity rainfall lasting for very long, whereas low intensity rainfall often lasts for many hours  ​ Drainage basin flows Precipitation inputs may be transferred through a drainage basin via a number of different flows. Rainwater dripping from leaves and branches towards the ground is called throughfall. Water that falls directly onto vegetation but flows to the ground via stems and trunks is termed stemflow.  -​ Infiltration is the movement of water from the ground surface into the soil. -​ The rate at which water can pass into the soil is known as its infiltration capacity, usually expressed as millimetres per hour. -​ Rainwater will be held on the ground surface if it falls at a rate that is greater than the infiltration capacity. -​ It is only a temporary form of storage: some water may begin to flow from surface puddles downhill over the land towards the river or ocean, or it may evaporate directly to the atmosphere. In the case of falling snow, all precipitation is stored temporarily on the surface until melting begins. -​ Throughflow is the movement of water laterally (sideways) through the soil, via a matrix of pore spaces, fissures and pipes (wide gaps in the soil created by roots and animal burrows). -​ With the exception of pipe flow, throughflow movements are relatively slow. Throughflow is most effective in the surface horizons of the soil because these are, in general, less compacted and have high permeability. -​ On farmed land, the surface horizon has often been ploughed and so the soil structure is open, with many large spaces and fissures which water can soak into. -​ In contrast, lower soil horizons are more compacted by the weight of overlying material, which reduces the soil's permeability. When this happens, water that is soaking downwards under gravity is deflected laterally down slope in the soil. -​ Percolation is the transfer of water from the soil into the underlying bedrock. -​ Groundwater flow is the vertical and lateral movement of water through a drainage basin's underlying rock as a result of gravity and pressure. All rocks show some porosity (the volume of voids as a percentage of the bulk volume of material) and resulting permeability. -​ However, porosity and permeability vary enormously according to rock type. Coarse-grained sedimentary rocks show high permeability and permit high levels of groundwater flow, whereas fine-grained igneous rocks are relatively impermeable. -​ Below a certain depth, the bedrock of a drainage basin may be permanently saturated. This level below which the ground is saturated is called the surface of the water table and it can vary in depth according to the season. The importance of overland flow Water can sometimes flow over the surface at very fast rates. Overland flow (also called surface runoff) is the movement of a sheet of water across the ground surface towards a river, lake or ocean, sometimes at a very fast speed. Overland flow can occur after either long duration or intense rainfall. -​ Saturation-excess overland flow happens if rainfall continues for a long time. -​ All soil layers become saturated and throughflow is deflected closer and closer to the surface. This is because over time during a rainfall event the upper and more permeable  soil horizons become saturated as the water level in the soil rises (exactly the same thing happens if you pour too much water into a plant pot: the level of standing water rises until it reaches the surface). -​ In time the entire soil becomes saturated right up to the surface. Thereafter, saturation-excess overland flow begins. -​ Infiltration-excess overland flow is defined as overland flow which occurs when rainfall intensity is so great that not all water can infiltrate, irrespective of how dry or wet the soil was prior to the rainfall event. This process, first described by R.E. Horton in 1945, is extremely common in some parts of the world, especially semi-arid areas where high intensity rainfall encounters hard-baked ground with a relatively low infiltration capacity. In these environments, infiltration-excess overland flow can lead to flash flooding and the growth of large, deep channels called wadis which fill with water when it rains. In contrast, the process is less common in parts of the world with a humid temperate climate, such as the UK. It happens when a rare infiltration-excess overland flow event following a torrential downpour of summer rain, which falls so fast that a sheet of water begins immediately to flow across the surface of the land: there is simply no time for the water to soak into the ground or run into drains. -​ An important aspect of overland flow dynamics is the way that additional parts of a drainage basin begin to contribute to saturation-excess overland flow during a long-lasting storm event. -​ As we have seen, under normal (i.e. non-intense) rainfall conditions, downslope overland flow will only occur once the ground below is completely saturated and water can no longer soak in. -​ Areas close to the bottom of a slope tend to become saturated first during a storm because they are receiving throughflow from higher up the slope in addition to directly infiltrating rainfall. As time continues, this saturated area begins to extend further up the slope towards the crest of the hill (Figure 7). -​ The saturated zone that develops is called the saturated wedge because it has a triangular or wedge shape upslope. Drainage basin stores Several drainage basin stores are also shown in Figure 4. They include the: -​ interception store (leaf and plant surfaces) -​ vegetation store (water held in the biomass itself, also called green water') -​ surface store (water collected on the surface of the ground in depressions and hollows, and also snow cover) -​ soil moisture store (water held in pore spaces in the soil matrix) -​ channel store (water held in the river channel itself at any moment in time) -​ groundwater store (water stored in solid rock and in any superficial deposits, e.g. gravels below the soil) interception may prevent almost all of the precipitation falling on an area from ever reaching the ground surface. Interception varies with the duration of precipitation and with the character of the vegetation. If a storm lasts a short time then a considerable proportion of the rainfall remains caught on the leaves and branches. All of this water may be evaporated eventually back to the atmosphere. However, during a longer storm these myriad small reservoirs will overflow and the rainfall drips to the surface as throughfall, or flows along vegetation surfaces to the ground as stemflow. As the duration of the storm increases so too does the proportion of rainfall reaching the surface beneath the plants. Of course, vegetation and land use may vary considerably from place to place in a drainage basin. As a result, interception storage is often uneven. In addition to any spatial variations, there can be marked seasonal differences in interception for drainage basins in climate zones with deciduous vegetation that sheds its leaves in autumn. Deciduous woodland in England and Wales has a greatly reduced ability to store water during winter months. Looking further afield, the capacity for interception storage of different global biomes varies enormously. -​ Tropical rainforest trees are well adapted to a hot humid equatorial climate by possessing drip tip' leaves: these encourage a more rapid flow of rainwater through the tree canopy and towards the ground via throughfall and stemflow. -​ High-latitude coniferous trees have sloping branches: snow slides towards the ground instead of building up on the branches, which might cause them to break. -​ As a result of this plant adaptation, snow is transmitted from the vegetation store to the drainage basin's surface store. -​ The effectiveness of surface or depression storage depends upon local relief factors. If the drainage basin contains relatively few flat areas or depressions and hollows, then there is little potential for depression storage. -​ However, if the land is relatively flat and contains hollows where water can collect, then a significant amount of water can be stored on the surface, especially if the ground is already saturated or has a naturally low infiltration capacity. -​ The collection of large amounts of surface water can result in a condition known as pluvial flooding for urban areas. The soil moisture store There are actually three different types of soil water: -​ Water adhering in thin films by molecular attraction to the surface of soil particles is called hygroscopic water (this form of water is not available for plants). Overall, this is a relatively insignificant form of water storage. -​ Water forming thicker films occupying smaller pore spaces in soil termed capillary water. It is held against the force of gravity by surface tension, and is available for plants to use. This is the water that remains in the soil when excess water has drained away after a storm event. It is vitally important for plant health. -​ The excess water that occupies all large and usually free-draining spaces in the soil is called gravitational water. This transitory water drains away soon after rain stops falling. If it did remain, the soil would become permanently waterlogged and unable to support much vegetation. Figure 8 shows different states of soil moisture storage, including field capacity (the total amount of water remaining in a freely drained soil after all gravity water has been drained away following the end of rainfall) and wilting point (when there is insufficient soil water to compensate for plant water losses from transpiration). Drainage basin outputs There are three principal catchment outputs -​ Evaporation is the change in state of water from a liquid to a gas. For this to happen, heat energy is required. Evaporation can occur from the surface of any water store including the ocean, surface water on the land and water intercepted temporarily on plant leaves. Many meteorological factors influence the rate of evaporation, including temperature, humidity and wind speed. -​ Transpiration is the diffusion of water from vegetation into the atmosphere, involving a change from liquid to gas. Water is lost through the stomata (pores) of leaves, and different types of vegetation vary greatly in terms of the rates of transpiration that are allowed. Tropical trees have large leaves, which maximises their rates of transpiration, while in contrast coniferous trees have needle leaves which minimise transpiration. The term evapotranspiration is used to describe a combination of evaporation and transpiration. -​ Channel discharge is the volume of water leaving a drainage basin via its main stream or river during a specified unit of time. A river's discharge is generally measured in cumecs (cubic metres per second). Temporal and spatial variations can be studied and observed in the relative importance of the three system outputs. Marked seasonal changes can be observed in many local and regional contexts, while different types of vegetation are adapted to the environment in ways that may maximise or minimise their rates of transpiration.