IGNACIO RODRIGUEZ-ITURBE
Universidad Simdn Boh'var, Caracas, VenezuelaThis work recognizes rainfall as a multidimensional stochastic process. By using the knowledge of such processes and of multivariate estimation theory a procedure for designing an optimal network to obtain the areal mean precipitation of an event over a fixed area is developed. The methodology used in this problem allows consideration of the following aspects of network design: (1) spatial uncertainty and correlation of process, (2) errors in measurement techniques and their correlation, and (3) nonhomogeneous sampling costs. Optimal networks are given in terms of the number and location of stations together with the resulting cost and mean square error of rainfall estimation.
INTRODUCTIONThe problem of rainfall data collection network design has been divided into various levels [Rodda eta!., 1969]. Levels 1 and 2 can be classified as problems in regional estimation; i.e., there is no clearly defined final goal or use for the collected data. The problem of rainfall monitoring for estimating the areal total precipitation average for a storm event and the problem of finding the long-term (time) areal mean precipitation belong to these two levels. Level 3 networks are those designed to collect data for a specific clearly defined objective which implies known net benefits and utility of the data. The problem of rainfall monitoring for use together with a flood forecasting system theoretically fits within this framework. This work deals with the level 1 problem of designing a network to obtain the areal average of an event. Another article by Bras and Roddguez-lturbe [1975, 1976b] deals with a problem similar to the latter example on level 3 design. The areal mean of an event has innumerable uses in hydrology. It is a traditional parameter in runoff and water yield studies. Given a set of rain gages and data points, the hydrologists have devised numerous techniques to obtain estimates of the mean of the process over a given area. The well known methods of Thiessen polygon and isohyetal analyses are among the available estimators of the areal mean of an event. These data analysis techniques acknowledge uncertainty, due to spatial variability, of the rainfall process. Rainfall is a random function in time and space. Total rainfall depth of an event is a random function in space. With no clearly defined utility functions the objectives in the network design are then accuracy and minimum cost. Both accuracy and cost are functions of the density of the network (number of observation stations in the area of interest), location of the observation sites, and instruments used in the observations (accurate instruments are usually related to higher costs). The accuracy of the network is also a function of the stochasticity of rainfall itself in terms of its spatial and temporal variations. Most existing network design methods consider cost or accuracy or both as a function of only density of observations. Recently, Rodrt'guez-lturbe and Mej;a [1974] used mean...