Daily precipitation records for 25 years at more than 100 stations in the conterminous United States have been analyzed. The order of the Markov chain representing the conditional dependence of precipitation occurrences at each station has been studied. This is done by using a decision procedure based on an extension of the principle of maximum likelihood. A loss function composed of a log-likelihood ratio term and a degree-of-freedom term is used as the decision criterion. Among several competing Markov orders, the one that minimizes this loss function is selected. The results show that the order of conditional dependence of daily precipitation occurrences depends on the season and geographical location. There exists a prevalence of first-order conditional dependence in summer and higher-order conditional dependence in winter. An explanation based on meteorology is proposed. In both seasons, there are areas where the Markov order deviates from the prevalent patterns. If the record length is too short (n << 1000 days), there is a tendency for a low-order chain to be misrepresented as the proper model. A specific example in which a third-order model is required to depict the precipitation occurrence in winter is also given in some detail. Therefore the proper Markov order describing the daily precipitation occurrence process has to be determined and cannot be assumed a priori. The common practice of assuming that the Markov order is always l is unjustified.

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Acre-foot (acre-ft). The volume of water required to cover 1 acre to a depth of 1 ft. It equals 43,560 ft3 (cubic feet), 325,851 gal (gallons), or 1,233 m3 (cubic meters). Contents. The volume of water in a reservoir or lake. Content is computed on the basis of a level pool or reservoir backwater profile and does not include bank storage. Convection cloud. A cloud which owes its vertical development, and possibly its origin, to convection. Convective cell. An organized, convective, fluid motion characterized by the presence of distinct upward motion in the center of the cell and sinking or downward flow in the outer regions. Cubic foot per second (ft3/s). A rate of discharge. One cubic foot per second is equal to the discharge of a stream of rectangular cross section 1 ft wide and 1 ft deep, flowing at an average velocity of 1 ft/s. It equals 28.32 L/s (liters per second) or 0.02832 m3/s (cubic meter per second). Cubic foot per second per square mile, (ft3/s)/mi2. The average number of cubic feet of water flowing per second from each square mile of area drained by a stream, assuming that the runoff is distributed uniformly in time and area. One ft3/s per square mile is equivalent to 0.0733 m3/s per square kilometer. Dew point (or dew-point temperature). The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content for saturation to occur. Drainage area of a stream at a specific location. An area, measured on a horizontal plane, bounded by topographic divides. Drainage area is given in square miles. One m2 is equivalent to 2.590 km2 (square kilometers). Exceedance probability. The probability, expressed as a percentage, that a given magnitude of flood discharge will be exceeded during any given year. The reciprocal of exceedance probability is recurrence interval. Thus a flood magnitude with an exceedance probability of 1 percent has a recurrence interval of 100 years. Flood. Any streamflow that overtops natural or artificial banks of a stream and inundates land not usually underwater. Front. The interface or transition zone between two airmasses of different densities. Gaging station. A particular site on a stream, canal, lake, or reservoir where systematic observations of gage height or discharge are made. K Index. A measure of the airmass moisture content and static stability given by: where T is temperature and TA is dewpoint, in degrees Celsius, and the subscripts denote pressure level in millibars. The larger the K index of the airmass, the more unstable it is. Low. Center of low barometric pressure. National Geodetic Vertical Datum of 1929 (NGVD of 1929). A geodetic datum derived from a general adjustment of the firstorder level nets of both the United States and Canada, formerly called "mean sea level." Millibar (mb). A unit of pressure equal to 1,000 dynes per square centimeter. Peak discharge. The maximum instantaneous discharge attained during a flood. Peak stage. The highest instantaneous stage attained during a flood. Precipitable water. The total ...

The U.S. Geological Survey and the National Weather Service have a long history of cooperation in monitoring and describing the Nation's water cycle the movement of water as atmospheric moisture, as precipitation, as runoff, as streamflow, and as ground water, and finally, through evaporation, its return to the atmosphere to begin the cycle over again. The cooperative effort has been a natural blending of technical talent and responsibility. The National Weather Service is the Federal agency responsible for monitoring and predicting atmospheric moisture and precipitation, for forecasting riverflow, and for issuing warnings of destructive weather events. The U.S. Geological Survey is the primary agency for monitoring the quantity and quality of the earthbound water resources, including both ground water and surface water.This report represents another step in the growth of our cooperative efforts. The working arrangement has been accelerated by many major flood disasters that have struck the Nation in the last few years, including hurricane Agnes in 1972, which has been called the worst natural disaster in the United States. Hundreds of lives have been lost, thousands of people have been made homeless, millions of acres of land have been inundated, and several billions of dollars in property damage in urban and industrial areas have been caused by floods.A tidal storm surge along the coast of Maine, February 2, 1976, caused by hurricane-force winds, resulted in a water-surface elevation more than 10 feet higher than the predicted astronomical tide at Bangor, Maine. The business section of Bangor was severely damaged. Roads, docks, and beaches along the coast between Eastport and Brunswick were also heavily damaged.These disasters emphasize the need for increased knowledge and respect of the force and flow of floodwater. The documentation of the flood in Bangor, Maine, in February 1976 should aid the understanding of such flood disasters and will help improve human preparedness for coping with future floods of similar catastrophic magnitudes.

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