Trends in Iowa Water Run-off Rick Cruse, Greg Brenneman, and Matt

HelmersIowa Water Center and Iowa State University Extension

For a Given Water Application Rate - Runoff Caused By

Surface of sponge restricting water infiltration – paper towel covering sponge

Sponge full of water – nowhere for water to go

For a Given Sponge Condition – Runoff Caused byIncreased water application rate

InfiltrationInfiltration – entry of water into the soil.

Influenced by: Physical characteristics of the soil (e.g. soil texture and soil structure) Soil moisture Cover on the soil and management of the soil Rainfall intensity

Impacts of Land Use on Infiltration

Source: Bharati et. al., 2002Source: Bharati et. al., 2002

Landuse Example Pochohantas Co. Watershed

2002 Landuse

Grassland/herbaceous (6%)Pasture/hay (3%)Cultivated crops (78%)Other (13%)

1940 Landuse

Grassland/herbaceous (25%)Pasture/hay (26%)Corn (29%)Small grains (20%)

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

Land Cover TrendsMore row cropsRow crops have lower infiltration rates

Tile DrainageRemoves water from saturated ‘sponge’Creates storage space for subsequent rainfallReduces runoff for repetitive storms when

soil profile is full or nearly full

Cum

ulat

ive

surfa

ce ru

noff

(in a

cre-1

)

0

1

2

3

4

5

6

100% Row-crop10% Perennial Cover at Toe Slope10% Perennial Cover in Contour Strips20% Perennial Cover in Contour Strips

Date

Cum

ulat

ive

sedi

men

t exp

ort (

lb a

cre-1

)

0

5000

10000

15000

20000

100% Row-crop10% Perennial Cover at Toe Slope10% Perennial Cover in Contour Strips20% Perennial Cover in Contour Strips

Trends in Precipitation

Groisman, Pavel Ya., Richard W. Knight, David R. Easterling, Thomas R. Karl,Gabriele C. Hegerl, and Vyacheslav N. Razuvaev. 2005. Trends in intense precipitation in the climate record. J. of Climate. 18:1326-1350.

Cedar River Annual Flow Cedar Rapids

1880 1900 1920 1940 1960 1980 2000 20200

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

Ann

ual F

low

- C

FS

Cedar River Peak Flow Rates

Iowa River Peak Flow Rates

Iowa River Peak Flows - Marengo

Concluding CommentsLoss of continuous living cover, increased soil

disturbance, and limited use of conservation practices can increase runoffImpacts are greater for smaller events and dryer soilsMuch smaller impacts for larger events or wet soils

Removal of water from soil profile (evapotransporation, tile drainage) reduces runoff

Water runs off concrete, asphalt, and building roofsExtreme rainfall event frequencies have increasedHigh production and increased water infiltration

(10% perennial cover) may coexist