Long-term trends in climate and hydrology in an agricultural, headwater watershed of central Pennsylvania, USA

Study region: The WE-38 Experimental Watershed, which is a small (7.3 km²) basin in the Ridge and Valley physiographic region of east-central Pennsylvania. Study focus: We used non-parametric Mann-Kendall tests to examine long-term (1968 to 2012) hydroclimatic (precipitation, temperature, streamflow) trends in WE-38 in the context of recent climate change across northeastern US. New hydrological insights for the region: Annual mean temperatures in WE-38 increased 0.38 °C per decade, leading to an expansion of the growing season (+2.8 days per decade) and a contraction of frost days (-3.6 days per decade). Consistent with increased temperatures, annual actual evapotranspiration rose significantly (+37.1 mm per decade) over the study period. Precipitation also trended upward, with October experiencing the most significant increases in monthly total rainfall (+8.2 mm per decade). While augmented October precipitation led to increased October streamflow (+5.0 mm per decade), the trend in WE-38 streamflow was downward, with the most significant declines in July (-1.2 mm per decade) and February (-7.5 mm per decade). Declines in summertime streamflow also increased the duration of hydrological droughts (maximum consecutive days with streamflow < 10th percentile) by 1.9 days per decade. While our findings suggest some challenges for producers and water resource managers, most notably with increased fall rainfall and runoff, some changes such as enhanced growing seasons can be viewed positively, at least in the near term.