The catchment is a fundamental unit of study in hydrology. It is normally well defined

topographically, can be studied as a series of nested units (larger catchments are

made of many smaller sub-catchments), and is an open system for measuring inputs

and outputs of mass and energy. Catchments are usually delineated by land-surface

topography and are made of hillslopes and channels. The proportion of hillslope area

to channel density or total channel length may determine how efficiently water can be

removed from a catchment since water in channels tends to move much more quickly

than water across and through hillslopes. Thus the spatial layout of hillslopes and

channels is important. This article describes some basic principles of catchment

hydrology and illustrates how determining spatial factors involved is fundamental for

understanding how environmental change may impact on runoff production and resulting

river flow

Holden, J.

English

White Rose Research Online

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 eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/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 eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Geospatial Aspects of Catch Where Water Flows into Mapping The catchment is a fundamental unit of study in hydrology. It is normally well definedtopographically, can be studied as a series of nested units (larger catchments aremade of many smaller sub-catchments), and is an open system for measuring inputsand outputs of mass and energy. Catchments are usually delineated by land-surfacetopography and are made of hillslopes and channels. The proportion of hillslope areato channel density or total channel length may determine how efficiently water can beremoved from a catchment since water in channels tends to move much more quicklythan water across and through hillslopes. Thus the spatial layout of hillslopes andchannels is important. This article describes some basic principles of catchmenthydrology and illustrates how determining spatial factors involved is fundamental forunderstanding how environmental change may impact on runoff production and result-ing river flow.By Joseph HoldenHillslope Runoff Most water reaches rivers via hillslopes.Various hillslope flow pathways exist andthese delay flow to different extents.Hydrological processes on hillslopes rangefrom overland flow (OLF) through subsurfaceflow involving micropores, macropores andnatural soil pipes, to displacement flow andgroundwater discharge. The dominance ofthese processes varies with climate, topogra-phy, soil character, vegetation cover and landuse, but may vary at one location (e.g. sea-sonally) with antecedent soil moisture andwith storm intensity and duration. There aretwo possibilities for water once it reachesthe hillslope; either to infiltrate into the soilor to fill up any depressions on the surfaceand flow over the surface as OLF.Infiltration-excess overland flowIf surface water supply is greater than infil-tration then surface flow will commence; thisis called infiltration-excess OLF. In many tem-perate areas infiltration-excess OLF is a rareoccurrence except in urban locations becausethe infiltration capacity of many soils is toohigh (i.e. water can enter the soil at a fasterrate than it can rain). Infiltration-excess OLFis more likely in semi-arid areas on soilcrusts and where rainfall events can beintense. Often infiltration-excess OLF willoccur only on spatially localised parts of ahillslope (rather than the whole catchment)such as in tractor wheelings on arable land.This localised occurrence is known as thepartial contributing area concept.Saturation-excess overland flowWhen water infiltrates it will fill up availablepore spaces (gaps between solid soil parti-cles). When all pore spaces are full the soilis saturated and the water table is at thesurface. 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 lowerrainfall intensities than required to generateinfiltration-excess OLF and can occur evenwhen it is not raining. This might happen,for example, at the foot of a hillslope. Waterdraining through the soil is known asthroughflow (see below). Throughflow fromupslope can fill up soil pores at the slopefoot and so the soil becomes saturated. AnyOctober/November 2004Ar t ic le22Figure 1. A soil pipe outlet with coarse sediment delivered from its base.extra water is then forced out onto the sur-face to become OLF. This water is known as‘return flow’ and is a component of satura-tion-excess OLF (the other component isfresh rainwater). Therefore saturation-excessOLF is more likely to occur at the bottom ofa hillslope, or on shallow soils where thereis restricted pore space for water storage.The area of a catchment or hillslope thatproduces saturation-excess OLF variesthrough time. During wet seasons, more of acatchment or hillslope will be saturated andtherefore able to generate saturation-excessOLF than during dry seasons. If the catch-ment starts off relatively dry then duringrainfall only a small area will generate satu-ration-excess OLF, but as rainfall continuesmore of the catchment becomes saturated,especially in valley bottoms. Hence a largerarea of the catchment will produce satura-tion-excess OLF. This is known as the ‘vari-able source area concept’ and is the domi-nant concept in catchment hydrology. Atcertain times of year some parts of a catch-ment will provide OLF whereas at othertimes the same parts will not deliver OLF inresponse to the same-sized rainfall event.This is important for example, where surfacepollutants (such as microbes attached to cat-tle droppings) might be delivered from agiven part of a catchment or hillslope only atcertain times of the year. Thus understandingsuch spatial processes helps us inform deci-sion-making.ThroughflowIf water infiltrates into soil several things canhappen:- It can be taken up by plants and transpired(or be lost from the soil by evaporation).- It can continue to percolate down into thebedrock- It can travel laterally downslope throughthe soil or rock – this is called throughflow.In fact most water reaches rivers bythroughflow, through the soil layers orthrough bedrock. Throughflow can bothmaintain low flows (baseflow) in rivers byslow subsurface drainage and can also con-tribute to peak flows (stormflow) through itsrole in generating saturation-excess OLF.There are different ways water can movethrough soil and this effects the timing ofwater delivery to the river. Soils are not uni-ronments. Often pipes can erode to formlarge gullies. Pipes may transmit a large pro-portion of water to the stream in somecatchments (10 - 40 % of discharge in peatcatchments). Sometimes it is difficult todetect soil pipes where they have not col-lapsed and are several metres below the sur-face. However, the author has used geophys-ical techniques such as ground penetratingradar as shown in (see Figure 2 ) to allowmapping of subsurface pipes and theirhydrological connectivity. The pipe networkscan be complex and provide rapid connectiv-ity of water, sediment and solutes couplingdistant parts of hillslopes with stream chan-nels.Groundwater flowGroundwater is water held below the watertable (in soils and rock) and has, to someextent, already been discussed in the sec-tions above. Groundwater may be a largestore of water, but in order for it to be avail-able to supply river flow the holding material(rock or soil) needs to be not just porousbut permeable (i.e with connecting pores).Layers of rock sufficiently porous to storewater and permeable enough to allow waterto flow through them in economic quantitiesare called aquifers. Sometimes aquifers canbe confined between impermeable rock lay-ers (aquitards) and are only open forrecharge and discharge at certain locations. Impact of Runoff Processes and LandManagement on StreamflowKnowledge of runoff mechanisms and theircontrols is important for determining catch-ment response to a precipitation event. Theamount of precipitation that reaches the riverchannel can vary from almost 100 % (insome urban areas) to less than 5 %.Changes in catchment management (or landuse) such as urbanization, ploughing,afforestation, deforestation and artificial soildrainage can alter hydrological flow paths,affecting the timing, volume and quality ofwater reaching the river channel. There isalways a lag between precipitation and peakriver discharge. This lag time is affected byhillslope runoff processes. Catchments domi-nated by infiltration-excess OLF have theshortest lag times and the highest peakform deposits as they have cracks and fis-sures within them. Water can move throughthe very fine pores of soil as matrix flow, orit can move through larger pores calledmacropores (macropore flow), or even largercavities called soil pipes (pipe flow). Matrix flowFlow through the soil matrix is controlled bysoil type (e.g. texture) and saturation. Sandysoils allow faster water movements throughthem than clay soils. Even after a longdrought most soils contain some waterwhich suggests that the forces involved mustbe very strong. A combined attraction of thewater molecules to each other and the waterto the soil particles means that water is heldin soil against the force of gravity. This isknown as capillary water and it will movewithin the soil from wet to dry areas. Thus itis possible to map the likely movement ofsoil water across the hillslope if there is asufficient spatial array of instruments.Macropore flowMacropores are pores larger than 0.1 mm indiameter and can promote rapid, preferentialtransport of water and chemicals through thesoil, not only due to their size but alsobecause they are connected and continuousover sufficient distances to bypass agricultur-ally and environmentally important soil lay-ers. Therefore if a field has many macrop-ores, surface fertiliser applications may getwashed through the macropore channels andmay not enter the main part of the soil.Macropores can be formed by soil fauna,plant roots, and cracking resulting from adried soil. They may not take up much spacebut can still play a large role in throughflow;studies in upland peat catchments by theauthor have indicated that 30 % of through-flow moves through macropores.Pipe flowNatural soil pipes are cavities greater than 1mm diameter that are continuous in lengthsuch that they can transmit water, sedimentand solutes through the soil and bypass thesoil matrix. Soil pipes are created by a rangeof processes including animal burrows andturbulent flow through macropores. Pipescan be up to several metres in diameter (seeFigure 1) and several hundred metres inlength and occur in a broad range of envi-October/November 2004Latest News? Visit www.geoinformatics.comAr t ic le23 ment Hydrologyflows. This is why urbanisation can lead toincreased flood risk downstream. If through-flow in the matrix dominates on hillslopesthen the lag time will be long and peak flowlow. There may even be two river dischargepeaks caused by one rainfall event. Thismight occur where the first peak is satura-tion-excess OLF dominated (with some pre-cipitation directly in the channel). The sec-ond peak may be much longer and largerand caused by subsurface throughflow accu-mulating at the bottom of hillslopes and val-ley bottoms before entering the stream chan-nel. Throughflow may also contribute directlyto storm hydrographs by a mechanism calledpiston or displacement flow. This is wheresoil water at the bottom of a slope (oldwater) is rapidly pushed out of the soil bynew fresh infiltrating water entering at thetop of a slope. Climate change and landmanagement may alter infiltration, soil satu-ration, soil structure and vegetation and thedominance of particular runoff processes. Insome locations flooding has increased dueto changing precipitation regimes and landmanagement. Many solutions have involvedbuilding taller levees or embankments, orstraightening and clearing river channels(rather than tackling the problem on the hill-slopes). However, the flood waters have togo somewhere and these practices oftenlead to worse flooding; sending water quick-ly through the river system reduces thedownstream lag time and increases the floodpeak. Floodplains function as a temporarywater store reducing the flood peaks down-stream. However, demand for building/farm-ing on floodplains means that less area isreduced. Before ditches had been dug, pre-cipitation that landed on a hilltop wouldhave to move across the entire length of theslope (e.g. ~200 m) to its base before reach-ing the stream. After drainage the same pre-cipitation now only needs to travel a shortdistance (e.g. ~10 m) from the hilltop to aditch before it is then rapidly channelledaway. There are two conflicting processes;the first reduces the hillslope flood peak andincreases the lag time and the second hasthe opposite effect. However, the first pro-cess may also lead to increases in floodpeaks. This is because of spatial processes.Where a hillslope that once produced a floodpeak 30 minutes after rainfall, followingdrainage it now produces a flood peak 60minutes after rainfall due to the extra stor-age capacity available in the drained soil.While the flood peak from this hillslope islower than it was before drainage, its timingnow corresponds to when the main riverchannel has its flood peak. Such synchronici-ty leads to an overall increase in the mainchannel flood. Clearly the effects will dependon where in the catchment land manage-ment change takes place as this determinesthe impact of changing the timing of waterdelivery to the main channel. In addition, theeffects of ditching can result in small-scaleirreversible changes to hillslope processes.Lowered water tables in organic soils suchas those in the Yorkshire Dales can result insoil shrinkage and cracking. These cracksprovide new routes for water (as macroporeflow) which then enlarge into soil pipes. Thisleads to an interesting problem. If we blockditches in an attempt to ameliorate flooding,this may lead to more water (from blockedditches full of water) travelling to the streamvia newly created soil pipes on ditch walls.Subsurface pipe erosion (and resulting gullydevelopment) might ensue. Thus change tohillslope processes brought about by landuse change may not be simply reversible justby reverting back to the original land use.We must therefore think very carefully aboutimpacts of land use change within a catch-ment before those changes are implemented.Dr. Joseph Holden (j.holden@leeds.ac.uk), is Professorat the School of Geography, University of Leeds in theUK. ■available for water storage. Thus flooding inlowland areas is an increasing problem aswater is brought downstream more quicklyand in greater quantities than ever before. Case Study – Upland Drainage in the Yorkshire DalesThe idea that ‘upstream matters’ is importantbecause it makes us think about spatial pro-cesses. Even within an upland headwater,the impact of a management activity may bevery different depending on the location. Thefollowing case study provides an example.Floods have been more common on theRiver Ouse, at York, UK since the 1940s.Analysis of the rainfall record showed mini-mal change. Hence land management changeis a likely causative factor. Many parts of theupland Yorkshire Dales were drained byditches in the 1940s in order to improvegrazing and game bird habitat by loweringwater tables. Two main hillslope hydrologicalchanges are likely:There is increased water storage capacitywithin the soil (as a drained soil can storemore fresh rainfall). Therefore peak flowswould be reduced and lag times increased.This, on first impression, should result in adecrease in flooding. The ditches now pro-vide new channels for fast moving water toreach the stream. Instead of water moving asOLF through vegetated terrain or as through-flow in the soil matrix the water is able tomove much more quickly as OLF withindrainage ditches directly linked to streams.Thus the flowpaths for hillslope water will beOctober/November 2004Latest News? Visit www.geoinformatics.comAr t ic le25Figure 2. Mapping soil pipes using ground-penetrating radar. The instrument is transported or wheeled across thesurface and can detect soil pipes that are several meters below the surface.

Geospatial aspects of catchment hydrology

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Geospatial aspects of catchment hydrology

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