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Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary] Watershed (USA) = Basin (USA) = Catchment (UK) [US usage is most common overseas] Frequently (incorrectly) used to refer to upper or steep part of watershed

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Watershed Management What is a watershed?


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    1. Watershed Management • What is a watershed? • Line of separation between two catchments (UK) [Chambers dictionary] • Watershed (USA) = Basin (USA) = Catchment (UK) [US usage is most common overseas] • Frequently (incorrectly) used to refer to upper or steep part of watershed • often problems with boundary - underground watershed may not correspond to surface topography. • surface boundary may be obscure in flat areas.

    2. Reasons for adopting a watershed approach • Downstream activities / resource utilisation are often influenced by upstream management policies, e.g.: • dams (siltation leads to dam having a short life span) • hydro-electric schemes (damage may be caused to equipment by abrasion, clogging, etc.) • floods caused by increased runoff or lowered infiltration rate (costly if of residential or agricultural land)

    3. fisheries (water quality affects production especially if there is chemical pollution (due perhaps to fertilisers being washed out of soil, or silt) • irrigation may be affected by silt or salts in water (silt may be beneficial initially though eventually, land height will change and so command will decrease) • navigation (silting up of waterways and lakes) • effects on amenities may affect tourist income (lakes, reservoirs, rivers, etc.) • danger of landslides and avalanches to downstream residents

    4. Watershed management approach offers possibility of : • minimising downstream problems • simple way of monitoring effects of land use changes and management improvements

    5. Watershed monitoring • Rainfall • use daily gauges semi -randomly distributed BUT take aspect, elevation & access into account • if used in forest areas, place so that there is an angle of >45° elevation with top of nearest trees • minimum of 1 recording to 10 daily gauges • Other climatic variables • Relative Humidity, minimum & maximum temperatures, solar radiation, wind speed (for calculating evapotranspiration)

    6. Runoff • install flumes (e.g. Parshall trapezoidal flume) which force water into narrow channel; flow is a function of height • river height (but need to establish stage-discharge relationships using current meter) is not very reliable as downstream flow can affect upstream flow • can use for predicting flood frequency and designing reservoirs as well as monitoring the affect of SWC • should also monitor water quality (Suspended sediment, Total Dissolved Solids (TDS), and Biological Oxygen Demand (BOD))

    7. Sedimentation • sediment load (suspended in water, depends on velocity), • saltation load (sediment which jumps along the bed 1 to 10 metres at a time) • bed load (rolls along the river bed, e.g. boulders) • use runoff plots to calculate sediment delivery ratio (SDR) • some redistribution within catchments (use plots, pins, trees to monitor), so not all erosion reaches the river and not reaching top end of river reaches mouth

    8. Soil water • Measure using: • neutron probes • satellites (infra-red, thematic mapping) • manual sampling (weigh then oven dry, then reweigh) • gypsum blocks (absorb water until energy potential in equilibrium with soil, then electrical conductivity measured) • tensiometers • Time Domain Refractometry (TDR) based on electrical pulses

    9. Vegetation & crop yield changes • Monitor these as they influence sediment & runoff • rates and can be used to assess benefits of SWC • Water balance • Use above parameters to calculate water balance • calculate comparative water use (actual water use • / predicted ET)

    10. Sediment in waterways Bedload Rapid increase at about 500 cumecs

    11. Transport of suspended sediment Power law relationship but each catchment is different - needs instrumentation - see diagrams from Kenya

    12. Suspended sediment rating curve, Uaso Nyiro River, Archer’s Post, Kenya note power law relationship and logarithmic variation in flow and sediment

    13. Result of improved land management over a 20 year period -Tana River, Garissa, Kenya at 500 cumecs, sediment discharge has reduced by x 10 Important to monitor sediment to evaluate interventions

    14. Sediment delivery ratio • SDR depends on size, slope, relief and other characteristics of catchment - see diagrams

    15. Central and SE USA (Roehl, 1962)

    16. SDR v ratio of difference in height between top and bottom of catchment and the length of the catchment S USA (Maner, 1958; Roehl, 1962)

    17. Use USDA or Stewart relationships for SDR v. catchment size if no data.

    18. Sediment deposited in reservoirs • Sediment deposited (Sd) is calculated from average erosion rate (Eg) within catchment, sediment delivery ratio (SDR) and the sediment trapping efficiency of the reservoir (Etrap). • Brune (1953) gave a relationship for Etrap against the ratio of volume of water passing through reservoir to the capacity of the reservoir (see diagram).

    19. After Brune, 1953

    20. Example taken from actual case study in Guinea Dam name: Labe Touri Mean annual rainfall: 1670 mm Mean open water evaporation: 2200 mm Catchment area : 27 km2 Average catchment slope : 2.5% Main channel length : 11 m Geology: sandstone / schist / dolerite Soils: sandy/silty clays Vegetation: grass savannah SDR (from USDA curve) : 0.18

    21. Reservoir capacity = 400,000 m3 (from designers, according to water requirement of people, size of river and topography) Average annual catchment runoff, R = 18,400,000 m3 Sutcliffe & Piper (1986) made a map of Guinea showing net rainfall [Pn] (gross rainfall - evaporation - interception - infiltration) and proposed that: R = Pn -300 hence Net rain (from the map) = 980 mm Mean annual runoff = 980 - 300 = 680 mm

    22. Thus, volume of runoff = area x 680 mm = 27,000,000 x 0.68 [m3] = 18,360,000 m3 Capacity/Runoff = 0.022 Therefore (from Brune curve) trapping efficiency = 60% Plot level erosion rate = 4,000 t/km2/yr (from monitoring in catchment or modifications of USLE) Gross erosion = rate x area = 4000 x 27 = 108,000 t/yr

    23. Bulk density = 1.6 t/m3 Erosion = 108,000/1.6 m3/yr = 67,500 m3/yr Sediment deposited in reservoir = 67,000 x .18 x .6 = 7290 m3/yr Annual % loss of storage capacity = 7290/400,000 = 1.82% Time taken to half fill with sediment = 27 years

    24. Experimental techniques • use paired / replicated catchments (similar size, soils, slopes, vegetation, etc. • nesting (study sub-catchments in more detail, then extrapolate to larger catchment) • can use small artificial catchments or plots • for water year, use dry season to dry season

    25. Forestry & watershed behaviour Some evidence for reforestation leading to lower water tables (trees absorb water that would otherwise reach water table) Over 11 million ha of forest per year removed in tropical areas Evidence that clearing leads to increases in runoff: pines /eucalyptus 40 mm / 10% deciduous 25 mm / 10% bush 10 mm / 10% (Bosch & Hewlett, 1982, J. Hydrol. 55 3-23)

    26. Other potential important areas of study • final land use & proportions on erosion • road design on erosion • logging practices on erosion • clearing on micro-climate • clearing on soil nutrients • clearing on water quality • clearing on run-off rates

    27. Institutional aspects of watershed approach • There are often problems of co-operation between neighbouring administrative areas or neighbouring countries as watershed rarely coincides with administrative boundaries. • Often watersheds are run by a parastatal or quango e.g. • Tana River Devel. Auth. • Kerio Valley Devel. Auth. • Lake Basin Dev. Auth. • Usually units are made too large; 100 - 1000 sq. km. is feasible; but > 10,000 sq. km. is very difficult to manage

    28. Watershed management must be associated with improvements in: • communication systems • transport • health • education • government institutions / administrative structures