Trends in Iowa Runoff
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Trends in Iowa Runoff
Kristie J. FranzDepartment of Geological & Atmospheric
SciencesIowa State University
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Runoff – water that runs over landInfluenced by:
Soil texture and structure Land surface characteristics (slope, vegetation, bare soil,
land use) Water input rate
http://www.savannahenvironmental.com/stormwater.htm
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Runoff = Water input - Infiltration
Infiltration – entry of water into soilInfluenced by:
Physical characteristics of the surface and soil (e.g. soil texture and structure, land cover, land use)
Soil moisture
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Number and size of holes tell us how much water soil can hold
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Connections, size and content of open spaces tell us how fast water can move into and through soil.
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Amount of runoff also depends on water application rate
Runoff = Water input - Infiltration
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Trends in Iowa Runoff (Streamflow)
Of 10 watersheds studied:
•8 had increasing average daily streamflow (50% increase)
•9 had increasing number of high flow days
•3 had increasing number of extreme flow days
Conrad, 2010(10-year running averages)
Number of high flow days per year
1950 2000
1950 2000
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Modeling Watersheds10 year running average mean daily
discharge
• Model results from the 3 calibrations were plotted with the observed
• We would expect each simulation to match best with the period it was calibrated to
• Increasing streamflow in response to climate for all scenarios
•But none follow observed perfectly• We are not considering land use
change over time
“tune” the model to different time periods
Conrad, 20101950 2000
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Impacts of Land Use on Infiltration
Source: Bharati et. al., 2002Source: Bharati et. al., 2002
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Changes in Iowa Farmland
1930 1970 20100
5,000
10,000
15,000
20,000
25,000
30,000
oatshaysoybeansCorn
Acr
es o
f Cro
p - t
hous
ands
More row cropsRow crops have higher runoff rates
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SummaryWe get more runoff when:
soils are wetterwe get more intense precipitation
We have wetter soils when:we get more precipitation on average (snow or
rain) we get more frequent precipitation (no time to
dry or drain)Climate does explain increasing runoff trends
in Iowa...but need to also consider land use change