An Overview of Possible Changes in Regional Climate and Hydrology

On average, New York State receives about 40 in of precipitation per year. About 50% of this evaporates away, leaving another 50% to enter streams and rivers or to replenish withdrawn groundwater. Rainfall is relatively even over most of the year with the exception of lower amounts in the winter months. However, most evaporation and transpiration occurs between May and October when plants are active, making streamflows lower and soils dryer during summer and early fall (see Figure 1 for an illustration of typical monthly rainfall amounts and streamflows). In addition to the lack of plant transpiration, spring streamflows may be further elevated by the contribution of melting snow. This section reviews both current trends and future projections for these different hydrologic processes.

rainfall per month

Figure 1. Average monthly streamflow discharge and precipitation for the Fall Creek watershed in Central NY.

Historic Trends

A number of studies have examined climate and hydrological trends in New York State. A study specific to the Catskill Mountain region (Burns et al. 2007) noted upward trends in annual temperature, precipitation, potential evapotranspiration, and runoff over a 50-year period starting in the 1950's. The most notable changes were the shift in peak snowmelt from early April at the beginning of the historic record to late March by the end of the record and an increase in annual warm-season (June-October) runoff. A study in Monroe County, NY (Coon 2005) assessing trends from 1965 to 2005 noted an increase in temperature, precipitation, and 7-day low-flows in rural streams. Trends within New York are consistent with those observed elsewhere in the U.S.. For example, Hodgkins et al. (2003) documented that peak river flows are occurring several weeks earlier throughout the Northeast. McCabe and Wolock (2002) assessed streamflow at 400 sites in the conterminous U.S. from 1941-1999 and documented an increase in annual minimum and median daily stream flow beginning around 1970, particularly in the East. Notably, peak streamflow (i.e. floods) did not show a consistent increase in the studied streams.

In regards to precipitation, it is not just the total annual amount that has been changing but also distribution and intensity. Recent work by DeGaetano (2009) assessed trends in extreme rainfall occurrence across the conterminous U.S. Approximately two-thirds of the trends in rainfall amounts for storms with 2- 5- and 10-year return periods were positive, and an even higher percentage were positive in the Northeast. A positive trend in the number of events greater than one inch (within 24 hrs) from 1961-2000 in NYS is shown in Figure 2 (red line).

rainfall events over one inch

Figure 2. The number of rainfall events over one inch from 1960-2100 averaged over four stations in NYS. Downscaled United Kingdom Meteorological Office Hadley Centre Climate Model version 3 (HADCM3) predictions are shown in blue and observations are shown in red.

Future Projections

In terms of precipitation, climate projections suggest that increases in the frequency of large rainfall events will continue. Applying a model (the Statistical Downscaling Model, (SDSM) Version 4.2) that relates large-scale circulation patterns and atmospheric moisture to local weather conditions, DeGaetano (2009) simulated daily rainfalls from 1961-2100. These simulations suggest a 0.2 in. increase in the 100-yr storm by the end of the century. Given that the model is underestimating current trends, this is also likely to be an underestimate of the expected change.

It is important to note that only recently have researchers started to investigate changes in the intensity of sub-daily precipitation events (Lenderink and Van Meijgarrd 2008). The intensity of sub-daily rainfall (particularly at less than an hour) is of particular relevance since it is usually these short events that generate rainfall intensities that exceed a soil's ability to infiltrate water, resulting in surface runoff which can cause flooding and the transport of pollutants. There is evidence from historical data and regional climate modeling to suggest that the intensity of sub-daily rainfall events will increase in a warming climate. For example, Lenderink and Van Meijgarrd (2008) found that 1-hr rainfall amounts increased 7% per degree Fahrenheit of air temperature warming in the Netherlands. Similar analyses of sub-daily rainfall intensities have not yet been carried out for New York State.

Beyond predicting precipitation, several efforts have attempted to predict future streamflows in the Northeastern U.S. The basic approach in these studies is the same: 1) GCMs project future temperature and precipitation amounts for large-scale regions of the globe, 2) a downscaling procedure is used to adjust these projections for the climate conditions of the areas of interest, 3) the downscaled climate data are incorporated into a hydrologic model that predicts streamflows and groundwater levels. Within each study, several scenarios comprising different emission levels, GCMs, downscaling techniques, and model parameterizations may be chosen, resulting in an average and range of possible outcomes. However, there is a growing recognition that a large number (on the order of thousands) of equally plausible scenarios could be used. For a study in the UK, New et al. (2007) demonstrate that streamflow projections are most dependent on the choice of GCM, but that each GCM and hydrologic model can also be parameterized slightly differently to result in additional variation in possible outcomes. In brief, no one outcome based on a single scenario should be granted much weight.

In general, nearly all studies focused on the Northeastern US have estimated that on average annual streamflow should change little (Hayhoe et al. 2007, Neff et al. 2000, Frei 2002).Additionally, these studies project increased late winter and spring flows and a shift in the timing of spring snowmelt. This means that even if there is more annual streamflow, it may be distributed unevenly over the year, with lower flows in the late summer and autumn, and higher flows in the late winter and spring. This temporal shift in flow magnitudes has already been observed in stream records, as noted above.

References:

Burns, DA., J. Klaus, and M.R. McHale. Recent climate trends and implications for water resources in the Catskill Mountain Region, New York, USA. Journal of Hydrology, 336: 155-170.

Coon, W. 2005. Hydrologic Evidence of Climate Change in Monroe County, NYS. US Geological Survey. OFR 2008-1199.

DeGaetano, A.T. 2009. Time-dependent changes in extreme precipitation return-period amounts in the continental U.S. Journal of Applied Meteorology and Climatology, 48: 2086-2099.

Frei, A., R.L. Armstrong, M.P. Clark, and M.C. Serreze. 2002. Catskill Mountain water resources: Vulnerability, hydroclimatology, and climate-change sensitivity, Annals of the Association of American Geographers, 92: 203-224.

Hayhoe, K., C.P. Wake, T.G. Huntington, L. Luo, M.D. Schwartz, J. Sheffield, E. Wood, B. Anderson, J. Bradbury, A. DeGaetano, T.J. Troy, and D. Wolfe. 2007. Past and Future Changes in Climate and Hydrological Indicators in the US Northeast. Climate Dynamics, 28: 381-407.

Hodgkins, G.A., R.W. Dudley, T.G. Huntington,. 2003. Changes in the timing of high river flows in New England over the 20th century. J. Hydrology 278: 244-252.

Lenderink, G. and E. Van Meijgarrd. 2008. Increase in hourly precipitation extremes beyond expectations from temperature changes. Nature Geosciences, 1: 511-514.

McCabe, G.J. and D.M. Wolock. 2002. A step increase in streamflow in the conterminous United States. Geophysical Research Letters, 29: 2185.

Neff, R., H. Chang, C.G. Knight, R.G. Najjar, B. Yarnal, and H.A. Walker. 2000. Impact of climate variation and change on Mid-Atlantic Region hydrology and water resources. Climate Research, 14:207-218.

New, M. , A. Lopez, S. Dessai, and R. Wilby. 2007. Challenges in using probabilistic climate change information for impact assessments: an example from the water sector. Phil. Trans. R. Soc. A, 365: 2117-2131.

Learn More

Climate Change Links

Intergovernmental Panel on Climate Change (IPCC) (link)

Northeast Climate Choices (UCS Reports) (link)

Climate Change and Northeast Agriculture (link)

Climate Change and Water Resources (NCAR) (link)

USDA Global Change Program Office (GCPO) (link)