Using GIS and the PSIAC Using GIS and the PSIAC Method to Predict Method to Predict

Watershed Sediment Watershed Sediment Yield RatesYield Rates

Eric Eric BerntsenBerntsen

PSIAC MethodPSIAC Method

PSIAC stands for Pacific Southwest PSIAC stands for Pacific Southwest Interagency CommitteeInteragency Committee

predicts total annual sediment predicts total annual sediment yield (not just sheet and rill erosion yield (not just sheet and rill erosion like USLE/RUSLE)like USLE/RUSLE)

first developed in 1968, updated in first developed in 1968, updated in 19911991

PSIAC Method (cont’d)PSIAC Method (cont’d)

originally intended for watershed areas originally intended for watershed areas greater than 10 migreater than 10 mi22 but can be used on but can be used on smaller watershedssmaller watersheds

sediment yield represented by nine sediment yield represented by nine factors: surficial geology, soils, climate, factors: surficial geology, soils, climate, runoff, topography, effective ground runoff, topography, effective ground cover, land type and management, upland cover, land type and management, upland erosion, and channel erosion and erosion, and channel erosion and sediment transportsediment transport

PSIAC Method (cont’d)PSIAC Method (cont’d)

the sediment yield characteristic of the sediment yield characteristic of each factor is assigned a numerical each factor is assigned a numerical value representing its significant to value representing its significant to sediment yieldsediment yield

total sediment yield is found by total sediment yield is found by

summing up numerical valuessumming up numerical values

ErosionErosion

ErosionErosion is the is the process in which, by process in which, by the actions of wind the actions of wind

or water, soil or water, soil particles are particles are detached and detached and transported.transported.

ErosionErosion is the is the process in which, by process in which, by the actions of wind the actions of wind

or water, soil or water, soil particles are particles are detached and detached and transported.transported.

SedimenSedimentt

Sediment is eroded Sediment is eroded material suspended material suspended

in wind or water.in wind or water.

Sediment is eroded Sediment is eroded material suspended material suspended

in wind or water.in wind or water.

SedimentationSedimentation

Sedimentation is Sedimentation is the deposition of the deposition of eroded material.eroded material.

Sedimentation is Sedimentation is the deposition of the deposition of eroded material.eroded material.

Splash Splash ErosioErosio

nn

Raindrop Raindrop impact impact

energy is energy is enough to enough to dislodge dislodge surface surface

sedimentssediments..

Raindrop Raindrop impact impact

energy is energy is enough to enough to dislodge dislodge surface surface

sedimentssediments..

Sheet ErosionSheet Erosion

Sheet erosion is caused by shallow Sheet erosion is caused by shallow “sheets” of water flowing over the soil “sheets” of water flowing over the soil

surface .surface .

Sheet erosion is caused by shallow Sheet erosion is caused by shallow “sheets” of water flowing over the soil “sheets” of water flowing over the soil

surface .surface .

Rill ErosionRill Erosion

As flow As flow concentrates, concentrates, small channels small channels

begin to form in begin to form in the soil surface.the soil surface.

As flow As flow concentrates, concentrates, small channels small channels

begin to form in begin to form in the soil surface.the soil surface.

Rill ErosionRill Erosion

Gully Gully ErosionErosion

Gullies are Gullies are formed formed

when runoff when runoff cuts rills cuts rills

deeper and deeper and wider or wider or

when flows when flows from several from several

rills come rills come together together

and form a and form a large large

channelchannel..

Gullies are Gullies are formed formed

when runoff when runoff cuts rills cuts rills

deeper and deeper and wider or wider or

when flows when flows from several from several

rills come rills come together together

and form a and form a large large

channelchannel..

Gully ErosionGully Erosion

ChanneChannel l

ErosionErosion

Channels Channels may may

become become unstable unstable due to due to

increased increased flows or flows or changes changes

in in upstream upstream sediment sediment

load.load.

Channels Channels may may

become become unstable unstable due to due to

increased increased flows or flows or changes changes

in in upstream upstream sediment sediment

load.load.

ChanneChannel l

ErosionErosion

Sediment yield should Sediment yield should not be confused with not be confused with

erosion; erosion; the terms are not the terms are not interchangeableinterchangeable

Sediment YieldSediment Yield

……is the amount of eroded soil that is the amount of eroded soil that is delivered to a point in the is delivered to a point in the watershed that is remote from the watershed that is remote from the origin of the detached soil origin of the detached soil particles.particles.

Study AreaStudy Area

16.4 mi2 16.4 mi2

4% impervious4% impervious

86% of watershed high and very 86% of watershed high and very high fuelshigh fuels

County boundaries.shpAmerican river basin.shpWhiterock.shpSfar scenic roads.shp

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EW

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Site Map

MethodsMethods

GIS files from South Fork American River GIS files from South Fork American River Watershed Stewardship StrategyWatershed Stewardship Strategy

Aerial photos (2003 flight)Aerial photos (2003 flight) Geologic Map of Sacramento QuadrangleGeologic Map of Sacramento Quadrangle NRCS Runoff Curve Number MethodNRCS Runoff Curve Number Method Site reconnaissanceSite reconnaissance

Surficial Geology (0 to 5)Surficial Geology (0 to 5)

massive, hard formations (i.e. massive, hard formations (i.e. Sierra Nevada Batholith) assigned Sierra Nevada Batholith) assigned a value of 0a value of 0

marines shales and other fine-marines shales and other fine-grained rocks assigned a value of 5grained rocks assigned a value of 5

Soils (0 to 10) Soils (0 to 10)

soils with appreciable amounts of soils with appreciable amounts of aggregated clays, high percentage aggregated clays, high percentage of rock fragments, or high organic of rock fragments, or high organic matter assigned a value of 0matter assigned a value of 0

fine textured, easily erodible soils fine textured, easily erodible soils

assigned a value of 10assigned a value of 10

Climate (0 to 10)Climate (0 to 10)

watersheds with humid or arid climate watersheds with humid or arid climate and low intensity storms, arid climate and low intensity storms, arid climate with rare convective storms, or snow with rare convective storms, or snow assigned a value of 0 assigned a value of 0

watersheds with frequent intense, watersheds with frequent intense, convective storms, freeze-thaw, or convective storms, freeze-thaw, or storms that last several days assigned storms that last several days assigned a value of 10a value of 10

Runoff (0 to 10)Runoff (0 to 10)

low peak flows and volume of runoff low peak flows and volume of runoff per unit area or rare runoff events per unit area or rare runoff events assigned value of 0assigned value of 0

high peak flows and volume of runoff high peak flows and volume of runoff per unit area assigned value of 10per unit area assigned value of 10

runoff curve numbers calculated runoff curve numbers calculated from union of hydrologic soil group from union of hydrologic soil group and coverand cover

Wrockhsg.shpABCD

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EW

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Hydrologic Soil Group

Wrccover.shpAGRBARCONHDWHEBMIXSHBURBWAT

N

EW

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Cover Type

Union1.shpAGRBARCONHDWHEBMIXSHBURBWAT

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EW

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Union of HSG and Cover

Topography (0 to 20)Topography (0 to 20)

gentle upland slopes (< 5%) gentle upland slopes (< 5%) assigned value of 0assigned value of 0

steep upland slopes (> 30%) steep upland slopes (> 30%) assigned value of 20assigned value of 20

Wrpsiacslope.shp000011223455677899101011

N

EW

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Topography

Effective Ground Cover (-10 to Effective Ground Cover (-10 to 10)10)

effective ground cover = effective ground cover = vegetation % + litter % + rock %vegetation % + litter % + rock %

vegetation % < 20% little to no vegetation % < 20% little to no litter, no rock assigned value of 10litter, no rock assigned value of 10

an area completely protected by an area completely protected by vegetation, litter, and rock vegetation, litter, and rock assigned value of -10assigned value of -10

no recent logging, or good grazing no recent logging, or good grazing management assigned value of - management assigned value of - 1010

overgrazed, burned, or roads in overgrazed, burned, or roads in need of repair assigned value of 10need of repair assigned value of 10

Land Type and Management QualityLand Type and Management Quality (-10 to 10)(-10 to 10)

Upland Erosion (0 to 25)Upland Erosion (0 to 25)

no apparent signs of erosion (i.e. rills no apparent signs of erosion (i.e. rills and gullies), assigned a value of 0and gullies), assigned a value of 0

if more than 50% of area if more than 50% of area characterized by concentrated characterized by concentrated erosion (i.e. increasing gully erosion (i.e. increasing gully development), assigned a value of development), assigned a value of 25 25

Channel Erosion and Sediment Channel Erosion and Sediment Transport (0 to 25)Transport (0 to 25)

wide, shallow channels with flat gradients wide, shallow channels with flat gradients or channels in massive rock, large or channels in massive rock, large boulders, or well vegetated assigned boulders, or well vegetated assigned value of 0value of 0

eroding banks, active headcuts and eroding banks, active headcuts and degradation in tributary channels degradation in tributary channels assigned value of 25assigned value of 25

Results and DiscussionResults and Discussion

PSIAC FactorPSIAC FactorGIS-based PSIACGIS-based PSIAC

AverageAverage RangeRange

GeologyGeology 33 2 to 42 to 4

SoilsSoils 33 3 to 73 to 7

ClimateClimate 44 44

RunoffRunoff 44 2 to 82 to 8

TopographyTopography 44 0 to 110 to 11

Ground CoverGround Cover -3-3 -5 to 5-5 to 5

Land UseLand Use -6-6 -8 to -3-8 to -3

Upland ErosionUpland Erosion 33 1 to 81 to 8

Channel ErosionChannel Erosion 33 1 to 81 to 8

TOTALTOTAL 1515

Results and DiscussionResults and Discussion

according to PSIAC rating sheet, 15 according to PSIAC rating sheet, 15 equals 0.14 ac-ft/miequals 0.14 ac-ft/mi22/yr/yr

sedimentation in Chili Bar Reservoir sedimentation in Chili Bar Reservoir (when adjusted for trap efficiency) is 0.11 (when adjusted for trap efficiency) is 0.11 ac-ft/miac-ft/mi22/yr./yr.

approx. 21% difference is consistent with approx. 21% difference is consistent with other studies (e.g. Woida and Clark 2001)other studies (e.g. Woida and Clark 2001)

ConclusionsConclusions

GIS proved to be a powerful tool, GIS proved to be a powerful tool, especially for runoff and topographyespecially for runoff and topography

GIS-enhanced PSIAC appears to be a GIS-enhanced PSIAC appears to be a good screening tool; more site specific good screening tool; more site specific methods (i.e. RUSLE or volumetric methods (i.e. RUSLE or volumetric approaches for gully and channel approaches for gully and channel erosion) could be used to refine erosion) could be used to refine estimatesestimates

AcknowledgementsAcknowledgements

USDA Natural Resources Conservation USDA Natural Resources Conservation Service – Placerville Field OfficeService – Placerville Field Office

El Dorado County and Georgetown Divide El Dorado County and Georgetown Divide Resource Conservation DistrictsResource Conservation Districts

Sierra Ecosystem AssociatesSierra Ecosystem Associates