To What Extent Is An Understanding Of Hydrological Pathways Important In Predicting Flooding?

Flooding occurs when peak river discharge is sufficient to cause overtopping of banks and inundation of surrounding land (Eisenbies et al., 2007). The hydrological pathways that transport precipitation to waterways can be an essential component in understanding the likelihood of floods occurring in certain areas and in certain time scales. Using the equation P (precipitation) = Q (discharge) + E (evapotranspiration) + I (interception) + MGS (soil water, ground water and storage), all the different pathways of precipitation can be recognised, and can be used alongside precipitation forecasts to interpret flood hydrographs and locally predict flooding (Holden, 2008). This essay will discuss how knowledge of hydrological pathways is gained, and how this can be used in relation to small scale localities to predict the probability of a flood. However, the essay will proceed to argue that an understanding of hydrological pathways is insufficient to predict flooding accurately on large scales. Furthermore, knowledge of pathways can only provide highly simplified models of flooding, and reliable, useful prediction methods require greater understanding of other factors in order to not only calculate when and where a flood may occur, but also predict the hazards it may involve. One aspect of hydrological pathways which needs to be understood in order to predict lag times in a flood hydrograph is the relationship between infiltration and overland flow. The Hortonian model of overland flow relies on the notion of overland flow only occurring when the infiltration capacity of the localised area of soil has reached its maximum. Infiltration is affected by the porosity of soil, involving size, shape and occurrence of pore spaces within the top layer, and in this model, overland flow only occurs on the rare occasions when rainfall intensity is high enough to entirely saturate the soil (Holden, 2008). Hewlett and Hibbert (1967) however, explore alternative ways overland flow can transpire. These include overland flow from high intensities of rain that cause the water table to rise above the normal level, and also, the intersection of water tables with hillslopes, affecting the gradient of the water flow and causing overland flow to occur before maximum infiltration. The occurrence of overland flow depends on factors such as land management, topography and soil porosity, and because precipitation transported by this route produces lag times15-20 times shorter than those produced by subsurface flows such as pipe flow (Jones, 1997), flash flooding will be more likely in catchments dominated by overland flow. This shows that an understanding of which pathways produce the shortest lag times could aid in assessing area vulnerability. Although overland flow is useful is predicting the location of a flood, in order to predict when a flood may occur, understanding subsurface flows is essential. One way in which water can travel once it has infiltrated is through macropores. Beven and Germann (1982) describe these pores (usually formed by the growth of plant roots and the funnelling of soil fauna) as 1mm-50mm in diameter, contributing up to 35% of space in forest soils, and providing a large surface area for not only transportation of water but also lateral infiltration through the walls of the pores. Jones (1997), states that pipeflow and macropore contributions to lag time in a storm hydrograph are 30-40% shorter than normal throughflow due to the large surface area of the channels, depicted clearly in Figure 1 (Beven and Germann, 1981cited in Beven and Germann, 1982 pp.1319); however, because slowing infiltration processes are involved, the pathways produce longer lag times than overland flows. Transportation of water can also occur through subsurface hillslope flows. In a study in the Cascade Mountains, Oregon in 2002-2003, soil moisture levels were measured during several different storm periods, and the levels were shown to peak at similar times along the path from hillslope to stream, displaying a high connectivity through the hillslope topsoil (McGuire and McDonnell, 2010). These pathways of subsurface flow demonstrate a need for hydrological connectivity within the soil in order to maximise the efficiency of transportation of water, suggesting that catchments with connected soil pore networks will create flashier flood hydrographs than catchments dominated by storage, percolation, and saturated throughflows. Soil infiltration, subsurface flows, overland flows and other hydrological pathways can all be affected by land use, meaning an understanding of pathways is essential in predicting how a change in land use may affect flood magnitude and frequency. Vegetation is one of the most influential factors in determining precipitation pathways, and is often used to manipulate land in a manner that will lessen the risk of flooding. Osterkamp and Hupp (2010) state that vegetation can trap and build up any sediment that is carried over it by flood waters. This causes an accretion of sediment which could eventually form a well vegetated flood plain, protecting the surrounding areas by decreasing the competence of the river flow and attenuating any possible floods. In a forest environment, over 90% of precipitation reacts with the soil instead of flowing directly into the waterway via surface runoff. This is because, as well as vegetation accreting sediment, it also stabilises the soil beneath it, and forms channels with its roots so ensure rainfall will infiltrate efficiently (Eisenbies et al., 2007). Vegetation can also intercept precipitation before it reaches ground level, allowing evapotranspiration to occur, so the volume of water that reaches the stream or river is significantly reduced. This understanding needs to be coupled with knowledge of the relationships formed with river capacity, hydrological pathways and flooding. Pinter and Heine (2005) describe the ways in which changes made to a section of the 4000km long Missouri River, USA, affect flooding. The river has been channelled and straightened for navigation purposes, causing higher velocities along the channel due to a reduction in friction along the river bank, and resulting in lower flood capacity. This shows how river management and engineering can interact with hydrological pathways to produce more frequent, damaging floods. These examples show that an understanding of hydrological pathways is essential in predicting flooding in certain areas based on their land use and management, and also in predicting how a change in land use may affect flooding in the future. Although these examples show that a basic understanding of hydrological pathways in catchments is necessary in order to interpret flood hydrographs; the usefulness is considerably limited due to the simplified flooding models, and the small, local scales that the understandings are gained from. Reffsgard et al., (2007) suggests that the environmental modelling used in the prediction of flooding needs to be assessed for uncertainty in order to acknowledge the possibility of error. When understanding of hydrological pathways is used to predict flooding, the context of the location is unclear, and the boundaries are not always static or defined, meaning a prediction could be misinterpreted. Also, the models of flood risk and the prediction of when and where flooding will occur are based on simplifications of complex hydrological processes that could behave in unexpected ways that are not entirely understood, especially when under particularly heavy or intense rainfall. O'Connell et al., (2004) states that a small scale understanding of hydrological processes and pathways is not enough to form reliable predictions of flooding, as catchments of varying scales may interact differently to high intensities of rainfall. The majority of investigations into hydrological pathways are conducted in areas of less than 10km2; meaning predictions of floods are limited to this area. Also, as urbanisation is increasing and intense large scale agriculture is becoming more prevalent, the different types of land uses are diversifying, which could cause unanticipated changes to the ways in which pathways react to rainfall (O'Connell et al., 2004). This shows that the level of uncertainty and lack of large scale understanding of hydrological pathways severely limits the usefulness of it as a method of flood prediction. Understanding hydrological pathways is useful when predicting the location and time scale of a possible flood; however, in order to predict the other many aspects of flooding, a greater understanding of the area is needed assessing more than temporal and spatial factors. In the US alone in the 1990's, floods caused 90 billion dollars' worth of damage (Eisenbies et al., 2007), showing the importance of predicting flooding accurately in order to reduce damage caused. In Pickering, Ryedale in North Yorkshire, Lane et al., (2001) follows a flood risk management and prediction scheme that involves wider understandings beyond those of hydrological pathways. The scheme is based on hydrological analysis, hydraulic analysis using GIS to map the likely spatial patterns of a flood, assessment of economic losses, and human interaction. The project implemented used the grounding of understanding hydrological pathways, however, the local knowledge of how the land functioned, including social, economic, political and environmental factors, provided a successful flood prediction and management scheme, for example, knowing which areas are most valuable socially and economically, and therefore most important to investigate in greatest detail in order to predict flooding more reliably (Lane et al., 2011). This method of flood prediction on a local scale not only predicts the likelihood of a flood, but also predicts the associated risks and hazards; utilising basic hydrological understanding in order to make flood predictions more reliable and useful. In conclusion, although hydrological pathways can provide important basic understanding when predicting flooding, its usefulness is greatly limited. Knowledge of the general pathways of precipitation is useful to some extent, however, on larger scales, land uses are diverse, and the ways in which water transportation occurs over spatial and temporal scales varies, meaning flood predictions are relatively simplistic. Understanding of hydrological pathways needs to act as foundation knowledge, used with broader understandings of the causes, effects and large scale patterns of flooding to provide more detailed, reliable and accurate flood predictions.

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