Present hydroclimate observations and archived data in the United States use the English system of units. Almost all other nations and the scientific literature use Systeme Internationale (SI) units for these variables. Unit conversions from one system to another are widely available and are not presented here. Only SI units are used in this book.
Symbols for representing hydroclimatic data are another issue. The quantities to be represented far exceed the capacity of the Greek and Roman alphabets. Also, some symbols commonly used in climatology are used for a different purpose in hydrology. One example is the Greek letter delta (A). The symbol A in climatology represents the slope of the saturation vapor pressure curve at the mean wet-bulb temperature of the air. In hydrology, the symbol A is used to indicate "a change in'' a quantity. The symbols used in this book are based on common usage in both disciplines. Consequently, there are a few instances where a symbol has more than one meaning. However, the context in which the symbol appears clearly identifies the intended nature of the symbol. This approach is preferred to introducing a symbol set unique to this book.
Climate data are viewed traditionally as point data. Instruments for collecting climate data are designed to measure precipitation, temperature, radiation, wind speed, evaporation, pressure, and humidity for a specific location. The point sensors are located on the ground, a tower, or attached to balloons and are intended to record data for a given location. Streamflow is a volume measurement integrating processes operating over an area. Consequently, observed climatic and hydrologic data are not immediately comparable due to the contrasting nature of point and areal data. Computational methods available for estimating areal data from point data and remote sensing technology provide improved spatial data that alleviate point observational biases in estimating spatial variability.
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