| Climate change is not only increasing the Earth’s temperature but is also altering the hydrological cycle. This study aims to identify indicators of long term climate change in relation to hydrological cycle – changes in precipitation, pan evaporation, aridity index and to identify footprints of climate change in agricultural ecosystem– changes in thawing period, changes in freezing period of soil, changes in growing season, and air temperature. The data for precipitation, pan evaporation, and precipitation from metrological stations from 10 states (Alaska, Minnesota, Ohio, Tennessee, Alabama, Hawaii, Florida, Arizona, New Mexico, and Texas) across a latitudinal gradient in the U.S. were analyzed for 20-80 years. Selection of the sites was based on the type of biome it represents, accuracy of data, length of the observation period, data availability online, and on the least missing data since the historical climate records have been archived for these stations. Precipitation was analyzed annually and for the warm season. Precipitation trends are increasing in some regions across U.S., such as Minnesota, Tennessee, Ohio, Oklahoma and Alabama while decreasing in others such as, for example Florida and Hawaii. In New Mexico, Arizona and Texas there is an increasing trend in long-term precipitation. However, in the last 20 years precipitation has been decreasing. The pan evaporation trends were analyzed for the summer season (May to September). Based on the trends, it is apparent that the pan evaporation has increased overtime in some regions across U.S. (i.e. Tennessee, Ohio, Florida, Hawaii and Alabama) but has also decreased in others (i.e. Alaska, Arizona, New Mexico, Minnesota, Texas), assuming that there are no systematic error in the weather stations’ observations. The sites which showed an increase in mean annual temperature (MAT) are the ones in Alaska, Texas, Florida, New Mexico, Arizona and Hawaii. In contrast, sites which showed a decreasing trend in average temperature are in Minnesota, Alabama and Tennessee. The increase in minimum temperature has been much higher and prominent than the increase in maximum temperature. Fairhope (Alabama) is the only site in this study which showed a significant decreasing trend in minimum temperature. At all the other sites, the minimum temperature increased in the last few decades. The thermal or climatological growing season is the frost-free period between the last frost in spring to the first frost in autumn. The number of consecutive days with minimum temperature above 0°C can be assumed to be conducive for growth. The length of climatological growing season was studied for 5 sites in Ohio and one site each in Alaska and Minnesota. All five sites in the Midwestern state Ohio (Wooster, Coshocton, Circleville, Bellefontaine and Bowling Green) as well as the two sites in the northern states of Minnesota (Waseca) and Alaska (University Experiment Station) show an increasing trend in the length of growing season. The rate of increase varies amongst different sites, but they are all statistically significant. This means the frost-free period is getting longer, advancement of spring season and delay of autumn frost. |
