Climate Change Impacts on the Water Cycle

Emmanouil AnagnostouDepartment of Civil & Environmental Engineering

Environmental Engineering ProgramUCONN

manos@engr.uconn.edu

2014 NECPUC Annual Symposium, Stowe, Vermont

Global surface temperature anomalies relative to 1951-1980 average for(a) annual and 5-year running means through 2010, and (b) 60-month and 132-month running means through July 2012. Green bars are 2-σ error estimates.

(Credit: James Hansen, NASA GISS & Columbia University)

Global Surface Temperature

2014 NECPUC Annual Symposium, Stowe, Vermont

Impacts on Evaporation and Transpiration (ET)

ET increases globally by approximately 2-4%/˚C, as a result of

– Increased availability of net radiative energy at the surface

– Increased atmospheric demand for evaporation, due to the increased capacity of the atmosphere to hold moisture (by ~7%/˚C per the the Clausius-Clapeyron relationship)

2014 NECPUC Annual Symposium, Stowe, Vermont

Impacts on Precipitation Amount & Characteristics

• Global average increases: 2-4%/˚C

• Increase of flooding risk due to increase in rain volumes

2014 NECPUC Annual Symposium, Stowe, Vermont

Impact on Precipitation Characteristics (cont’d)

Decrease of rain frequency and/or duration– Increase in the number of consecutive dry days

droughts

2014 NECPUC Annual Symposium, Stowe, Vermont

The most important spatial pattern (top) of the monthly Palmer Drought Severity Index (PDSI) for 1900 to 2002.

The time series (below) accounts for most of the trend in PDSI.

Drought patterns are changing

Mainly decrease in rain over land in tropics and subtropics, but

enhanced by increased atmospheric demand for

evapotranpiration with warming

Most areas of the U.S.: relative increases in precipitation ranging from 9-31%; northeastern U.S.: precipitation increase ~67%Discharges are expected to increase, and with it the flood risk!- Collins et al. 2009: 25/28 New England gauges show upward trends in peak discharges- Wake & Markham 2005; Hayhoe et al. 2009 (using VIC) Greater winter discharge and earlier shift of peak flows

Hydrologic Extremes in the NE USA

2014 NECPUC Annual Symposium, Stowe, Vermont

Suitable for application in cold environments: seasonal and permanently frozen soils, snowpack on ground and canopy

Provided by Dr. Fischer from the NASA Jet Propulsion Laboratory

Regional Hydrologic Modeling

2014 NECPUC Annual Symposium, Stowe, Vermont

• Relative change from 1950 to 2011 in the amount of precipitation falling on days with precipitation exceeding the 99th percentile of daily precipitation, where the 99th percentile is estimated using 1950-1999 as the reference period. This change is based on the value for 1950 and 2011 derived from linear regression with time, instead of the actual value in these two years

• Basin Average change: ~240%• large values found in the 21st Century are highly influential• Fraction of total: increased by 0.146• Some things we may expect: larger runoff ratios (flooding), greater peak discharge, soil

moisture diminished, exacerbated drought conditions however, region specific

2014 NECPUC Annual Symposium, Stowe, Vermont

Runoff ratio: basin average increase of 0.0552– 10% relative increase

1950-2011 Trends - Basin Mean

Season PrecipitationEvapotran-spiration

RunoffSoil

Moisture

DJF-0.00977

mm/day/yr-0.000069 mm/day/yr

0.00153 mm/day/yr

3.67*10-5

vol. fraction/yr

MAM0.00539

mm/day/yr-0.000227 mm/day/yr

0.00363 mm/day/yr

5.36*10-5 vol.

fraction/yr

JJA0.01621

mm/day/yr0.000581

mm/day/yr0.01228

mm/day/yr

1.86*10-4 vol.

fraction/yr

SON0.01297

mm/day/yr-0.000188 mm/day/yr

0.01238 mm/day/yr

1.82*10-4 vol.

fraction/yr

Annual3.06529 mm/yr/yr

0.04297 mm/yr/yr

2.71428 mm/yr/yr

1.15*10-4 vol.

fraction/yr

2014 NECPUC Annual Symposium, Stowe, Vermont

Future Projections (2046-2065)

2014 NECPUC Annual Symposium, Stowe, Vermont

These signals of increased duration and 5 day maximum precipitation are indicators of increased flood severities (e.g. 100 yr return flood)

2014 NECPUC Annual Symposium, Stowe, Vermont

Greatest Change in the water

budget variables has been to ET

Potentially entering a new regime

characterized by increasing ET

Climate warming could increase

risk of both extreme floods and/or

drought

Evapotranspiration

2014 NECPUC Annual Symposium, Stowe, Vermont

• Globally we expect both drought and flood risk changes due to changes in the precipitation patterns.

• The NE USA has been experiencing an increasingly wet regime during the latter part of the 20th century through early 21st century– indications of change towards more extreme precipitation, increasing discharge, increasing

runoff ratio and negligible trend in ET

• Significant increases in precipitation extremes have been almost entirely accounted for by increases to runoff Flood Risk

How do these trends continue or change in the mid 21th Century?• Precipitation extreme indicators remain statistically similar to early 21th century• Flood Frequency has decreased, but the largest storms produce floods that

persist for longer durations major events (100 yr return floods) present a greater risk

• NE has gone from a slightly increasing ET trend to a large increase (~8% in the summer) possible acceleration of the hydrologic cycle

• It is possible the NE US will enter a new climate regime characterized by greater drought and flood risks

Conclusions

2014 NECPUC Annual Symposium, Stowe, Vermont