WaterWare On-line Manuals: watershed erosion

WaterWare
Reference &
User Manual

WaterWare On-line Manuals

Release Level

6.2

Release Date

2010 05

Revision Level

1.0

Last modified on: Saturday, 12-Jun-10 12:29 CEST

Rainfall-Runoff Model: Watershed erosion

The RRM rainfall-runoff model is a dynamic, spatially lumped but vertically semi-distributed basin scale water budget model that operates at a daily timestep.

Basin (subcatchment or watershed) parameters, precipitation input data and the runoff data generated by the model can also be used to estimate watershed erosion (specific and totasl sediment yield) and thus average turbidity for a scenario. For some background, and the ranges of observed turbidity data and specific sediment yield for a number of major rivers world wide, see the WaterWare erosion modeling pages.

The following data and concepts are used:

Basin Characteristics and Geometry:
Elevation difference m simply the difference between minimum and maximum elevation specified for that basin
Average slope m/km Elevation difference divided by basin length (RRM input)
Average slope length km watershed area divided by drainage density (length of all perennial channels)
Land use distribution in % summarised in a pie chart for forests, pastures, agriculture, and the residual
Elevation distribution km²/100m summarised in a hypsographic curve
Precipitation Input:
Total precipitation mm based on elevation corrected precipitation
Maximum precipitation mm derived from the daily precipitation time series input data (elevation corrected)
Precipitation > 2mm N Number of days with potentially erosive rain: derived from the daily precipitation input time series
Precipitation > 10mm N Number of days with heavy rain: derived from the daily precipitation input time series
Runoff (RRM Model output):
Total/average runoff mm, m³/s
Maximum runoff mm, m³/s
Total/average interflow mm, m³/s
Total/average surface runoff mm, m³/s
Days with surface runoff N
User input:
Soil erodibility symbolic low - medium - high
Erosion control symbolic none - medium - advanced
EROSION MODEL OUTPUT:
Annual sediment load tons, tons/ha
Average turbidity g/m³ calculated for total flow including basflow !

Model implementation

The erosion model is basically empirical, logically closely related to the Universal Soil Loss Equation (USLE, Wischmeier and Smith, 1965, 1978), with a numbner of modifications to fully exploit the structure of the data available and the spatioally lumped, vertically semi-distributed structure of the model.

The relationship is a simple multiplicative one as used in USLE:


EROSION(tons/ha) = scale * PRECIPITATION 
                   * SLOPELENGTH * SOIL * VEGETATION 
                   * TRANSPORT * MANAGEMENT

The following basic elements are being used:

EROSION depends on:

Precipitation (erosive energy);
Slope (directly proportional) and slope length (indirectly proportional), combined into a single factor, with slope dominating;
Soil (erodibility, depending on type and organic content);
Vegetation cover (land use class);
Sediment transport (driven by overland and interflow)
Erosion Control, including agricultural practices and the drainage system.

PRECIPITATION describes the erosive energy from precipitation input: SUM(Prec[day]-2) excluding snow accumulation
SLOPELENGTH combines the effects of average slop and average length of slopes:
SLOPE / SQRT(LENGTH) slope in m/km, slope length in km
SOIL soil factor representing erodibility, values are:
low(0.75), medium (1.0), and high (1.5)
VEGETATION land use/cover from RRM input, area weighted average:
FOREST(0.01); PASTURES(0.05); AGRICULTURE(1.0); RESIDUAL(0.20)
TRANSPORT sediment transport through the basin:
INTERFLOW + SURFACEFLOW², all in mm
MANAGEMENT erosion control measures, symbolic:
none (1.0), medium (0.80), advanced (0.50)

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