watershed management what is a watershed? line of separation between two catchments (uk) [chambers...

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

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Page 1: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Watershed ManagementWatershed Management

What is a watershed?What is a watershed?

• Line of separation between two catchments Line of separation between two catchments (UK) [Chambers dictionary](UK) [Chambers dictionary]

• Watershed (USA) = Basin (USA) = Catchment Watershed (USA) = Basin (USA) = Catchment (UK) [US usage is most common overseas](UK) [US usage is most common overseas]

• Frequently (incorrectly) used to refer to upper Frequently (incorrectly) used to refer to upper or steep part of watershedor steep part of watershed

• often problems with boundary - underground often problems with boundary - underground watershed may not correspond to surface watershed may not correspond to surface topography.topography.

• surface boundary may be obscure in flat surface boundary may be obscure in flat areas.areas.

Page 2: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 3: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 4: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Reasons for adopting a watershed Reasons for adopting a watershed approachapproach

• Downstream activities / resource utilisation Downstream activities / resource utilisation

are often influenced by upstream are often influenced by upstream management policies, e.g.:management policies, e.g.:• dams (siltation leads to dam having a dams (siltation leads to dam having a

short life span)short life span)• hydro-electric schemes (damage may be hydro-electric schemes (damage may be

caused to equipment by abrasion, caused to equipment by abrasion, clogging, etc.)clogging, etc.)

• floods caused by increased runoff or floods caused by increased runoff or lowered infiltration rate (costly if of lowered infiltration rate (costly if of residential or agricultural land)residential or agricultural land)

Page 5: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 6: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 7: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

• fisheries (water quality affects production fisheries (water quality affects production especially if there is chemical pollution especially if there is chemical pollution (due perhaps to fertilisers being washed (due perhaps to fertilisers being washed out of soil, or silt)out of soil, or silt)

• irrigation may be affected by silt or salts irrigation may be affected by silt or salts in water (silt may be beneficial initially in water (silt may be beneficial initially though eventually, land height will though eventually, land height will change and so command will decrease)change and so command will decrease)

• navigation (silting up of waterways and navigation (silting up of waterways and lakes)lakes)

• effects on amenities may affect tourist effects on amenities may affect tourist income (lakes, reservoirs, rivers, etc.)income (lakes, reservoirs, rivers, etc.)

• danger of landslides and avalanches to danger of landslides and avalanches to downstream residentsdownstream residents

Page 8: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

• Watershed management approach offers Watershed management approach offers possibility of :possibility of :• minimising downstream problemsminimising downstream problems• simple way of monitoring effects of land simple way of monitoring effects of land

use changes and management use changes and management improvementsimprovements

Page 9: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Watershed monitoringWatershed monitoring

Rainfall Rainfall • use daily gauges semi -randomly distributed use daily gauges semi -randomly distributed

BUT take aspect, elevation & access into BUT take aspect, elevation & access into accountaccount

• if used in forest areas, place so that there is if used in forest areas, place so that there is an angle of >45° elevation with top of an angle of >45° elevation with top of nearest treesnearest trees

• minimum of 1 recording to 10 daily gaugesminimum of 1 recording to 10 daily gauges

Other climatic variablesOther climatic variables• Relative Humidity, minimum & maximum Relative Humidity, minimum & maximum

temperatures, solar radiation, wind speed temperatures, solar radiation, wind speed (for calculating evapotranspiration)(for calculating evapotranspiration)

Page 10: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Runoff Runoff • install flumes (e.g. Parshall trapezoidal install flumes (e.g. Parshall trapezoidal

flume) which force water into narrow flume) which force water into narrow channel; flow is a function of heightchannel; flow is a function of height

• river height (but need to establish stage-river height (but need to establish stage-discharge relationships using current meter) discharge relationships using current meter) is not very reliable as downstream flow can is not very reliable as downstream flow can affect upstream flowaffect upstream flow

• can use for predicting flood frequency and can use for predicting flood frequency and designing reservoirs as well as monitoring designing reservoirs as well as monitoring the affect of SWCthe affect of SWC

• should also monitor water quality should also monitor water quality (Suspended sediment, Total Dissolved Solids (Suspended sediment, Total Dissolved Solids (TDS), and Biological Oxygen Demand (TDS), and Biological Oxygen Demand (BOD))(BOD))

Page 11: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 12: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

SedimentationSedimentation

• sediment load (suspended in water, depends sediment load (suspended in water, depends on velocity), on velocity),

• saltation load (sediment which jumps along saltation load (sediment which jumps along the bed 1 to 10 metres at a time)the bed 1 to 10 metres at a time)

• bed load (rolls along the river bed, e.g. bed load (rolls along the river bed, e.g. boulders)boulders)

• use runoff plots to calculate sediment use runoff plots to calculate sediment delivery ratio (SDR)delivery ratio (SDR)

• some redistribution within catchments (use some redistribution within catchments (use plots, pins, trees to monitor), so not all plots, pins, trees to monitor), so not all erosion reaches the river and not reaching erosion reaches the river and not reaching top end of river reaches mouthtop end of river reaches mouth

Page 13: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 14: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Soil waterSoil water

Measure using:Measure using:• neutron probesneutron probes• satellites (infra-red, thematic mapping)satellites (infra-red, thematic mapping)• manual sampling (weigh then oven dry, then manual sampling (weigh then oven dry, then

reweigh)reweigh)• gypsum blocks (absorb water until energy gypsum blocks (absorb water until energy

potential in equilibrium with soil, then potential in equilibrium with soil, then electrical conductivity measured)electrical conductivity measured)

• tensiometerstensiometers• Time Domain Refractometry (TDR) based on Time Domain Refractometry (TDR) based on

electrical pulseselectrical pulses

Page 15: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 16: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Vegetation & crop yield changesVegetation & crop yield changes

Monitor these as they influence sediment & Monitor these as they influence sediment & runoffrunoff

rates and can be used to assess benefits of rates and can be used to assess benefits of SWCSWC

Water balanceWater balance

Use above parameters to calculate water Use above parameters to calculate water balancebalance

calculate comparative water use (actual water calculate comparative water use (actual water useuse

/ predicted ET)/ predicted ET)

Page 17: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Sediment in waterwaysSediment in waterways

BedloadBedloadRapid increase at Rapid increase at about 500 cumecsabout 500 cumecs

Page 18: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 19: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Transport of suspended sedimentTransport of suspended sediment

Power law relationship but each catchment is Power law relationship but each catchment is different - needs instrumentation - see different - needs instrumentation - see diagrams from Kenyadiagrams from Kenya

Page 20: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Suspended Suspended sedimentsedimentrating curve, rating curve, Uaso Nyiro River, Uaso Nyiro River, Archer’s Post, Archer’s Post, Kenya Kenya

note power law note power law relationship and relationship and logarithmic logarithmic variation in flow variation in flow and sedimentand sediment

Page 21: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Result of improved Result of improved land management land management over a 20 year over a 20 year period -Tana River, period -Tana River, Garissa, KenyaGarissa, Kenya

at 500 cumecs, at 500 cumecs, sediment sediment discharge has discharge has reduced by x 10 reduced by x 10

Important to Important to monitor sediment monitor sediment to evaluate to evaluate interventionsinterventions

Page 22: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Sediment delivery ratioSediment delivery ratio

• SDR depends on size, slope, relief and other SDR depends on size, slope, relief and other characteristics of catchment - see diagramscharacteristics of catchment - see diagrams

Page 23: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Central and SE USA (Roehl, 1962)Central and SE USA (Roehl, 1962)

Page 24: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

SDR vSDR vratio of ratio of difference in difference in height height between top between top and bottomand bottomof of catchment catchment and the and the length of the length of the catchmentcatchmentS USAS USA(Maner, (Maner, 1958; Roehl, 1958; Roehl, 1962)1962)

Page 25: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

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

Page 26: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Sediment deposited in reservoirsSediment deposited in reservoirs

• Sediment deposited (SSediment deposited (Sdd) is calculated from ) is calculated from average erosion rate (Eaverage erosion rate (Egg) within catchment, ) within catchment, sediment delivery ratio (SDR) and the sediment delivery ratio (SDR) and the sediment trapping efficiency of the reservoir sediment trapping efficiency of the reservoir (E(Etraptrap).).

• Brune (1953) gave a relationship for EBrune (1953) gave a relationship for Etraptrap

against the ratio of volume of water passing against the ratio of volume of water passing

through reservoir to the capacity of the through reservoir to the capacity of the

reservoir (see diagram).reservoir (see diagram).

Page 27: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

After Brune, 1953After Brune, 1953

Page 28: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Example taken from actual case study in GuineaExample taken from actual case study in Guinea

Dam name: Dam name: Labe TouriLabe TouriMean annual rainfall: Mean annual rainfall: 1670 mm1670 mmMean open water evaporation: Mean open water evaporation: 2200 mm2200 mmCatchment area : Catchment area : 27 km27 km22

Average catchment slope : Average catchment slope : 2.5%2.5%Main channel length :Main channel length : 11 m11 mGeology: Geology: sandstone / sandstone / schist / schist / doleritedoleriteSoils: Soils: sandy/silty clayssandy/silty claysVegetation: Vegetation: grass savannahgrass savannahSDR (from USDA curve) :SDR (from USDA curve) : 0.180.18

Page 29: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Reservoir capacity Reservoir capacity = = 400,000 m400,000 m33 (from designers, according to water (from designers, according to water requirement of people, size of river and requirement of people, size of river and topography)topography)Average annual catchment runoff, R Average annual catchment runoff, R

== 18,400,000 m18,400,000 m33

Sutcliffe & Piper (1986) made a map of Guinea Sutcliffe & Piper (1986) made a map of Guinea showing net rainfall [Pshowing net rainfall [Pnn] (gross rainfall - ] (gross rainfall - evaporation - interception - infiltration) and evaporation - interception - infiltration) and proposed that:proposed that:

R = PR = Pnn -300 -300 hencehenceNet rain (from the map) Net rain (from the map) == 980 mm980 mmMean annual runoff Mean annual runoff = = 980 - 300 980 - 300

= = 680 mm680 mm

Page 30: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Thus, volume of runoffThus, volume of runoff = = area x 680 mm area x 680 mm = = 27,000,000 x 0.68 27,000,000 x 0.68

[m [m33]]= = 18,360,000 m18,360,000 m33

Capacity/Runoff Capacity/Runoff = = 0.0220.022Therefore (from Brune curve)Therefore (from Brune curve) trapping efficiency trapping efficiency = 60%= 60%Plot level erosion rate Plot level erosion rate = 4,000 t/km= 4,000 t/km22/yr /yr (from monitoring in catchment or modifications of USLE)(from monitoring in catchment or modifications of USLE)Gross erosion Gross erosion == rate x area rate x area

= = 4000 x 274000 x 27 == 108,000 t/yr108,000 t/yr

Page 31: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Bulk density Bulk density = 1.6 t/m= 1.6 t/m33

Erosion Erosion = 108,000/1.6 m= 108,000/1.6 m33/yr/yr = 67,500 m= 67,500 m33/yr/yrSediment deposited in reservoir Sediment deposited in reservoir

= 67,000 x .18 x .6= 67,000 x .18 x .6 = 7290 m= 7290 m33/yr/yrAnnual % loss of storage capacity Annual % loss of storage capacity

= 7290/400,000= 7290/400,000 = 1.82%= 1.82%

Time taken to half fill with sediment = 27 years Time taken to half fill with sediment = 27 years

Page 32: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Experimental techniquesExperimental techniques

• use paired / replicated catchments (similar use paired / replicated catchments (similar size, soils, slopes, vegetation, etc.size, soils, slopes, vegetation, etc.

• nesting (study sub-catchments in more nesting (study sub-catchments in more detail, then extrapolate to larger detail, then extrapolate to larger catchment)catchment)

• can use small artificial catchments or plotscan use small artificial catchments or plots• for water year, use dry season to dry seasonfor water year, use dry season to dry season

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Page 34: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments
Page 35: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Forestry & watershed behaviourForestry & watershed behaviour

Some evidence for reforestation leading to Some evidence for reforestation leading to lower water tables (trees absorb water that lower water tables (trees absorb water that would otherwise reach water table) would otherwise reach water table)

Over 11 million ha of forest per year removed Over 11 million ha of forest per year removed in tropical areasin tropical areasEvidence that clearing leads to increases in Evidence that clearing leads to increases in runoff:runoff:

pines /eucalyptus pines /eucalyptus 40 mm / 10%40 mm / 10%deciduousdeciduous 25 mm / 10%25 mm / 10%bushbush 10 mm / 10% 10 mm / 10%

(Bosch & Hewlett, 1982, J. Hydrol. 55 3-23)(Bosch & Hewlett, 1982, J. Hydrol. 55 3-23)

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Page 37: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Other potential important areas of studyOther potential important areas of study

• final land use & proportions on erosionfinal land use & proportions on erosion• road design on erosionroad design on erosion• logging practices on erosionlogging practices on erosion• clearing on micro-climateclearing on micro-climate• clearing on soil nutrientsclearing on soil nutrients• clearing on water qualityclearing on water quality• clearing on run-off ratesclearing on run-off rates

Page 38: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

Institutional aspects of watershed Institutional aspects of watershed approachapproach

• There are often problems of co-operation There are often problems of co-operation between neighbouring administrative areas between neighbouring administrative areas or neighbouring countries as watershed or neighbouring countries as watershed rarely coincides with administrative rarely coincides with administrative boundaries.boundaries.

• Often watersheds are run by a parastatal or Often watersheds are run by a parastatal or quango e.g.quango e.g.

• Tana River Devel. Auth.Tana River Devel. Auth.• Kerio Valley Devel. Auth.Kerio Valley Devel. Auth.• Lake Basin Dev. Auth.Lake Basin Dev. Auth.• Usually units are made too large; 100 - Usually units are made too large; 100 -

1000 sq. km. is feasible; but 1000 sq. km. is feasible; but > 10,000 sq. km. is very difficult to manage> 10,000 sq. km. is very difficult to manage

Page 39: Watershed Management What is a watershed? Line of separation between two catchments (UK) [Chambers dictionary]Line of separation between two catchments

• Watershed management must be associated Watershed management must be associated with improvements in:with improvements in:• communication systemscommunication systems• transporttransport• healthhealth• educationeducation• government institutions / administrative government institutions / administrative

structuresstructures