Geospatial Aspects of Catchment Hydrology

This is a repository copy of Geospatial aspects of catchment hydrology. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/1610/ Article: Holden, J. (2004) Geospatial aspects of catchment hydrology. GEOinformatics, 7. pp. 22-25. ISSN 1387-0858 Reuse See Attached Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ Article Geospatial Aspects of Catch Where Water Flows into Mapping The catchment is a fundamental unit of study in hydrology. It is normally well defined Hillslope Runoff topographically, can be studied as a series of nested units (larger catchments are Most water reaches rivers via hillslopes. Various hillslope flow pathways exist and made of many smaller sub-catchments), and is an open system for measuring inputs these delay flow to different extents. Hydrological processes on hillslopes range and outputs of mass and energy. Catchments are usually delineated by land-surface from overland flow (OLF) through subsurface flow involving micropores, macropores and topography and are made of hillslopes and channels. The proportion of hillslope area natural soil pipes, to displacement flow and groundwater discharge. The dominance of to channel density or total channel length may determine how efficiently water can be these processes varies with climate, topography, soil character, vegetation cover and land removed from a catchment since water in channels tends to move much more quickly use, but may vary at one location (e.g. sea- sonally) with antecedent soil moisture and than water across and through hillslopes. Thus the spatial layout of hillslopes and with storm intensity and duration. There are two possibilities for water once it reaches channels is important. This article describes some basic principles of catchment the hillslope; either to infiltrate into the soil or to fill up any depressions on the surface hydrology and illustrates how determining spatial factors involved is fundamental for and flow over the surface as OLF. understanding how environmental change may impact on runoff production and result- Infiltration-excess overland flow If surface water supply is greater than infil- ing river flow. tration then surface flow will commence; this is called infiltration-excess OLF. In many tem- By Joseph Holden perate areas infiltration-excess OLF is a rare occurrence except in urban locations because the infiltration capacity of many soils is too high (i.e. water can enter the soil at a faster rate than it can rain). Infiltration-excess OLF is more likely in semi-arid areas on soil crusts and where rainfall events can be intense. Often infiltration-excess OLF will occur only on spatially localised parts of a hillslope (rather than the whole catchment) such as in tractor wheelings on arable land. This localised occurrence is known as the partial contributing area concept. Saturation-excess overland flow When water infiltrates it will fill up available pore spaces (gaps between solid soil particles). When all pore spaces are full the soil is saturated and the water table is at the surface. Therefore any extra water has diffi- culty entering the soil. Hence OLF will occur. This type of OLF is known as saturation- excess OLF. This can happen at much lower rainfall intensities than required to generate infiltration-excess OLF and can occur even when it is not raining. This might happen, for example, at the foot of a hillslope. Water draining through the soil is known as throughflow (see below). Throughflow from upslope can fill up soil pores at the slope Figure 1. A soil pipe outlet with coarse sediment delivered from its base. foot and so the soil becomes saturated. Any 22 October/November 2004 Article ment Hydrology extra water is then forced out onto the sur- form deposits as they have cracks and fis- ronments. Often pipes can erode to form face to become OLF. This water is known as sures within them. Water can move through large gullies. Pipes may transmit a large pro- ‘return flow’ and is a component of satura- the very fine pores of soil as matrix flow, or portion of water to the stream in some tion-excess OLF (the other component is it can move through larger pores called catchments (10 - 40 % of discharge in peat fresh rainwater). Therefore saturation-excess macropores (macropore flow), or even larger catchments). Sometimes it is difficult to OLF is more likely to occur at the bottom of cavities called soil pipes (pipe flow). detect soil pipes where they have not col- a hillslope, or on shallow soils where there lapsed and are several metres below the sur- is restricted pore space for water storage. Matrix flow face. However, the author has used geophys- The area of a catchment or hillslope that Flow through the soil matrix is controlled by ical techniques such as ground penetrating produces saturation-excess OLF varies soil type (e.g. texture) and saturation. Sandy radar as shown in (see Figure 2 ) to allow through time. During wet seasons, more of a soils allow faster water movements through mapping of subsurface pipes and their catchment or hillslope will be saturated and them than clay soils. Even after a long hydrological connectivity. The pipe networks therefore able to generate saturation-excess drought most soils contain some water can be complex and provide rapid connectiv- OLF than during dry seasons. If the catch- which suggests that the forces involved must ity of water, sediment and solutes coupling ment starts off relatively dry then during be very strong. A combined attraction of the distant parts of hillslopes with stream chan- rainfall only a small area will generate satu- water molecules to each other and the water nels. ration-excess OLF, but as rainfall continues to the soil particles means that water is held more of the catchment becomes saturated, in soil against the force of gravity. This is Groundwater flow especially in valley bottoms. Hence a larger known as capillary water and it will move Groundwater is water held below the water area of the catchment will produce satura- within the soil from wet to dry areas. Thus it table (in soils and rock) and has, to some tion-excess OLF. This is known as the ‘vari- is possible to map the likely movement of extent, already been discussed in the sec- able source area concept’ and is the domi- soil water across the hillslope if there is a tions above. Groundwater may be a large nant concept in catchment hydrology. At sufficient spatial array of instruments. store of water, but in order for it to be avail- certain times of year some parts of a catch- able to supply river flow the holding material ment will provide OLF whereas at other Macropore flow (rock or soil) needs to be not just porous times the same parts will not deliver OLF in Macropores are pores larger than 0.1 mm in but permeable (i.e with connecting pores). response to the same-sized rainfall event. diameter and can promote rapid, preferential Layers of rock sufficiently porous to store This is important for example, where surface transport of water and chemicals through the water and permeable enough to allow water pollutants (such as microbes attached to cat- soil, not only due to their size but also to flow through them in economic quantities tle droppings) might be delivered from a because they are connected and continuous are called aquifers. Sometimes aquifers can given part of a catchment or hillslope only at over sufficient distances to bypass agricultur- be confined between impermeable rock lay- certain times of the year. Thus understanding ally and environmentally important soil layers (aquitards) and are only open for such spatial processes helps us inform deci- ers. Therefore if a field has many macrop- recharge and discharge at certain locations. sion-making. ores, surface fertiliser applications may get washed through the macropore channels and may not enter the main part of the soil. Impact of Runoff Processes and Land Throughflow Macropores can be formed by soil fauna, Management on Streamflow If water infiltrates into soil several things can plant roots, and cracking resulting from a happen: dried soil. They may not take up much space Knowledge of runoff mechanisms and their - It can be taken up by plants and transpired but can still play a large role in throughflow; controls is important for determining catch- (or be lost from the soil by evaporation). studies in upland peat catchments by the ment response to a precipitation event. The - It can continue to percolate down into the author have indicated that 30 % of through- amount of precipitation that reaches the river bedrock flow moves through macropores. channel can vary from almost 100 % (in - It can travel laterally downslope through some urban areas) to less than 5 %. the soil or rock – this is called throughflow. Pipe flow Changes in catchment management (or land In fact most water reaches rivers by Natural soil pipes are cavities greater than 1 use) such as urbanization, ploughing, throughflow, through the soil layers or mm diameter that are continuous in length afforestation, deforestation and artificial soil through bedrock. Throughflow can both such that they can transmit water, sediment drainage can alter hydrological flow paths, maintain low flows (baseflow) in rivers by and solutes through the soil and bypass the affecting the timing, volume and quality of slow subsurface drainage and can also con- soil matrix. Soil pipes are created by a range water reaching the river channel.

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