Using GIS and the PSIAC Method to Predict Watershed Sediment Yield Rates Eric Berntsen

PSIAC Method PSIAC stands for Pacific Southwest Interagency Committee predicts total annual sediment yield (not just sheet and rill erosion like USLE/RUSLE) first developed in 1968, updated in 1991

PSIAC Method (cont’d) originally intended for watershed areas greater than 10 mi2 but can be used on smaller watersheds sediment yield represented by nine factors: surficial geology, soils, climate, runoff, topography, effective ground cover, land type and management, upland erosion, and channel erosion and sediment transport

PSIAC Method (cont’d) the sediment yield characteristic of each factor is assigned a numerical value representing its significant to sediment yield total sediment yield is found by summing up numerical values

Erosion Erosion is the process in which, by the actions of wind or water, soil particles are detached and transported. Instructor Comments: Let’s start with some basic definitions so we are all on the same page. Erosion is…

Sediment is eroded material suspended in wind or water. Instructors Comments: Sediment is… Sediment is eroded material suspended in wind or water.

Sedimentation is the deposition of eroded material. Instructor Comments: Sedimentation is… Sedimentation is the deposition of eroded material.

Raindrop impact energy is enough to dislodge surface sediments. Splash Erosion Raindrop impact energy is enough to dislodge surface sediments. Instructor Notes: Slides 13 – 23 illustrate the various types of erosion.

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

Rill Erosion As flow concentrates, small channels begin to form in the soil surface.

Rill Erosion

Gully Erosion Gullies are formed when runoff cuts rills deeper and wider or when flows from several rills come together and form a large channel.

Gully Erosion

Channel Erosion Channels may become unstable due to increased flows or changes in upstream sediment load.

Channel Erosion

Sediment yield should not be confused with erosion; the terms are not interchangeable

Sediment Yield …is the amount of eroded soil that is delivered to a point in the watershed that is remote from the origin of the detached soil particles. Instructor Comments: Sediment yield includes eroded materials from all forms of erosion: sheet, rill, gully, etc.

Study Area 16.4 mi2 4% impervious 86% of watershed high and very high fuels

Methods GIS files from South Fork American River Watershed Stewardship Strategy Aerial photos (2003 flight) Geologic Map of Sacramento Quadrangle NRCS Runoff Curve Number Method Site reconnaissance

Surficial Geology (0 to 5) massive, hard formations (i.e. Sierra Nevada Batholith) assigned a value of 0 marines shales and other fine-grained rocks assigned a value of 5

Soils (0 to 10) soils with appreciable amounts of aggregated clays, high percentage of rock fragments, or high organic matter assigned a value of 0 fine textured, easily erodible soils assigned a value of 10

Climate (0 to 10) watersheds with humid or arid climate and low intensity storms, arid climate with rare convective storms, or snow assigned a value of 0 watersheds with frequent intense, convective storms, freeze-thaw, or storms that last several days assigned a value of 10

Runoff (0 to 10) low peak flows and volume of runoff per unit area or rare runoff events assigned value of 0 high peak flows and volume of runoff per unit area assigned value of 10 runoff curve numbers calculated from union of hydrologic soil group and cover

Topography (0 to 20) gentle upland slopes (< 5%) assigned value of 0 steep upland slopes (> 30%) assigned value of 20

Effective Ground Cover (-10 to 10) effective ground cover = vegetation % + litter % + rock % vegetation % < 20% little to no litter, no rock assigned value of 10 an area completely protected by vegetation, litter, and rock assigned value of -10

Land Type and Management Quality (-10 to 10) no recent logging, or good grazing management assigned value of - 10 overgrazed, burned, or roads in need of repair assigned value of 10

Upland Erosion (0 to 25) no apparent signs of erosion (i.e. rills and gullies), assigned a value of 0 if more than 50% of area characterized by concentrated erosion (i.e. increasing gully development), assigned a value of 25

Channel Erosion and Sediment Transport (0 to 25) wide, shallow channels with flat gradients or channels in massive rock, large boulders, or well vegetated assigned value of 0 eroding banks, active headcuts and degradation in tributary channels assigned value of 25

Results and Discussion

PSIAC Factor GIS-based PSIAC Average Range Geology 3 2 to 4 Soils 3 to 7 Climate 4 Runoff 2 to 8 Topography 0 to 11 Ground Cover -3 -5 to 5 Land Use -6 -8 to -3 Upland Erosion 1 to 8 Channel Erosion TOTAL 15

Results and Discussion according to PSIAC rating sheet, 15 equals 0.14 ac-ft/mi2/yr sedimentation in Chili Bar Reservoir (when adjusted for trap efficiency) is 0.11 ac-ft/mi2/yr. approx. 21% difference is consistent with other studies (e.g. Woida and Clark 2001)

Conclusions GIS proved to be a powerful tool, especially for runoff and topography GIS-enhanced PSIAC appears to be a good screening tool; more site specific methods (i.e. RUSLE or volumetric approaches for gully and channel erosion) could be used to refine estimates

Acknowledgements USDA Natural Resources Conservation Service – Placerville Field Office El Dorado County and Georgetown Divide Resource Conservation Districts Sierra Ecosystem Associates