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Discharge-Area Relations from Selected Drainages
on the Colorado Plateau: a GIS Application GISWR Term Project By W. Scott Cragun
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Outline Problem to be addressed Part I Methods Results Conclusions
Part II
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Problem to be addressed:
A is often substituted for Q in fluvial models Example: Family of equations knows as Stream Power Laws Used to: model aggradation / incision within rivers model incision rates within rivers Basic Equations: Z = kQmSn OR Z = kAmSn k is proportionality constant Q is discharge S is slope m/n are exponents whose values are debated A is drainage area
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Commonly held that: Q = bAc ; A is the contributing drainage area
b is a proportionality constant c is a scaling exponent (1 is most common value) Most commonly a linear (or nearly linear) relation Perhaps true for alluvial rivers in humid climatic zones What about for mixed alluvial-bedrock rivers in semi-arid climatic zones?? From: Mitchell, D.K., 2000
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Part I - What I’ve Done: Six river basins selected Gunnison River
Upper Colorado River San Juan River Paria River Little Colorado River Fremont/Dirty Devil River Encompass the southern/arid basins and the northern/relatively wetter basins
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Part I - What I’ve Done: The raw DEM’s
DEM’s downloaded from USGS seamless data ( 30m DEM projected in UTM and interpolated at 100m cell size Run TauDEM tools in order to determine flow direction and flow accumulation Upper Colorado Fremont Gunnison Paria San Juan Little Colorado
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Part I - What I’ve Done: Obtain USGS gaging station data for each of the 6 drainages ( Mean annual flow Mean peak flow 35 total gaging stations Import gage location in GIS Manually extract contributing area at each gaging station compare GIS contributing area to USGS drainage area plot discharge against contributing drainage area Little Colorado River
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Part I - Results: Significantly more scatter in the data from the
Colorado Plateau Y = 0.08A0.95 R2 = 0.82
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Part I - Results: Individual basins show unique downstream discharge patterns
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Part I - Results: Hydrographs show peak runoff during - spring (snow melt) : northern drainages - fall (monsoon) : southern drainages northern drainages southern drainages
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Part I - Results: However… Individual basins show: 1) grouping of 3 northern basins 2) unique relations in southern 3 basins
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Part I - Results: Splitting drainage basins into two groups…
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Part I - Conclusions: 1) Close Q-A relation in northern drainages
But… scaling exponent (c) is not nearly 1 : value is 0.70 - suggests that streams in humid environments are capable of generating greater discharges per unit drainage area than in arid and semi-arid regions 2) Large scatter in Q-A relation in southern drainages (lower r2) Suggests that - geologic controls influence discharge generation - climatic influence = greater evaporation 3) Discharge generation per unit area varies between all 6 basins 4) Streams in ARID regions: appropriate Q-A relation should be carefully chosen even when significant water is available
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Part II - What I’ve Done:
Obtain precipitation data for western US from PRISM ( clipped out for each drainage resampled at 1000 m cell size Can RUNOFF be predicted from PRECIPITATION ?? runoff coefficients estimated for each basin From:
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Part II - Results: Initial runoff coefficients too high
Readjusted to better predict runoff Basin Initial Estimate K Refigured K Fremont 0.21 0.062 Gunnison 0.3 0.22 Little Colorado 0.14 0.0084 Paria 0.24 0.031 San Juan 0.26 0.17 Upper Colorado 0.42 0.30
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Part II - Results: First attempt to define precipitation-runoff relation through regression failed Try, try again… San Juan shaded relief map San Juan precipitation map
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Part II - Conclusions: Estimated runoff coefficients vary widely between 6 basins to 0.30 Suggests that - local climate conditions influence discharge monsoon vs. snow melt - local geologic characteristics influence discharge - local soil and vegetation distribution affects discharge 2) Runoff prediction from precipitation is not perfect oversimplified 3) Need a more robust coverage of drainage characteristics to establish representative discharge – precipitation relations in each basin
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