ANALYSIS OF THE IMPORTANCE OF WEATHER AND CLIMATE TO DISCHARGE IN SMALL WATERSHEDS

According to the Northeast Climate Impact Assessment (Frumhoff et al. 2007), the northeastern United States annual temperatures have been warming at a rate of 0.5°F per decade. Since stream discharge represents the integrated basin response to various climatic inputs, especially precipitation and temperature (Hodgkins et al. 2003), changing temperatures and precipitation patterns will potentially affect the hydrology of the region, both annually and seasonally, with future trends towards more periods of summer drought. The impact on stream discharge may be seen first in small watersheds which are common along the lake plain of the Lake Ontario coastline.

In this study, we have examined past climate data, from 1926 through 2012, to evaluate trends for the area around Rochester, NY, and instrumented three watersheds to collect detailed current year data. Data analysis using 10-year moving averages of monthly average temperatures and cumulative precipitation show no significant change in minimum temperatures. Maximum temperatures were found to be increasing for February and March, which would impact the timing of peak spring runoff. Precipitation was found to be increasing for the months of June, August and September which is contrary to suggestions of increased drought. Analyses of Palmer Drought Indices (PDI) for the same period suggest a confirmation of this trend. One of the three watershed used in this study has USGS gauge records from 1989 to present. A comparison of the PDI with monthly average discharge values shows no significant correlation. This may suggest that the PDI is not valid for use in predicting the response of stream discharge for small watersheds. Here we evaluate the Reconnaissance Drought Index (RDI) with modifications to evaluate the response of three small watersheds to individual storm events and shorter time periods. Preliminary results suggest that these watersheds tend to be very responsive, flashy, to large storm events. Given the third wettest spring on record, we also see an extended response of base flow to change from a wet spring to, at present, a more typical summer season based on the most recent 10 year precipitation record. Using the RDI, as modified, we see the importance of large storm events to total discharge.