Browsing by Author "Fernandez, B"

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    Relating autocorrelations and crossing rates of continuous- and discrete-valued hydrologic processes
    (ASCE-AMER SOC CIVIL ENGINEERS, 2001) Salas, JD; Chung, CH; Fernandez, B
    The return period and risk of extreme droughts can be derived from hydrologic series of wet and dry years. If Z(t) denotes a continuous-valued hydrologic series such as annual streamflows, a series of wet and dry years, X-t, can be obtained by clipping Z(t) by z(0) such that X-t = 1 if Z(t) greater than or equal to z(0), and X-t = 0 if Z(t) < z(0). A method is presented for relating the autocorrelation functions (k)(Z) and rho (k)(X). In addition, the relationships between the crossing rate gamma and rho (j)(Z) and rho (j)(X) are derived. The method assumes that the underlying hydrologic series is stationary and normally distributed. The applicability of the methods and derived relationships has been examined and tested by using annual streamflow series at several sites and by simulation experiments based on low-order ARMA and DARMA models. The analysis of 23 series of annual flows reveals that the derived relationship between rho (k)(X) and rho (k)(Z) are applicable and reliable. The same conclusion is reached when simulated samples from the ARMA model are utilized. In addition, it has been shown that the autocorrelation function,(X) obtained (by using the derived relationship) from rho (k)(Z) of a low-order ARMA model, can be fitted by a low-order DARMA model. The significance of the relationships between the referred autocorrelation functions has been documented in terms of estimating certain drought properties. It has been shown that significant differences can be obtained for estimating the return periods and risks of certain drought events if the sample autocorrelations rho (k)(X) are used instead of the derived autocorrelations <()over tilde>X-k). Furthermore, it has been shown that the derived relationships between gamma and rho (t)(Z) and gamma and rho (i)(X) apply quite well for annual streamflows.
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    Risk of scarcity of monthly precipitation and streamflows in semiarid regions
    (1998) Fernandez, B; Vergara, A
    In semiarid regions, periods of one to several consecutive months of scarcity of precipitation or low flow rates in rivers and streams frequently occur. In these regions, scarce precipitation considerably affects grazing activities od rangelands, irrigation agriculture, and flora and fauna of surface stream environments. This paper is concerned with estimating probabilities of occurrence for such events and associated risks by using statistical information provided by monthly precipitation and streamflow series. Conventional return period and risk concepts as customarily used in hydrology are applied, and events of interest such as meteorological droughts are considered by using monthly precipitation as a time-related series of supply, thereafter addressing hydrological droughts in terms of streamflow supply. For these two kinds of events, periodic situations of time-independent phenomena are considered for the case of precipitation, with time-dependent phenomena considered for streamflows. The method is applied to the semiarid portion of Chile to calculate probabilities of occurrence of specific numbers of consecutive months with no significant precipitation, or now rates insufficient to meet stream ecological needs.
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    Statistical estimation of runoff characteristics of watersheds in central Chile
    (IAHS PRESS, INST HYDROLOGY, 1996) Fernandez, B; Pizarro, G
    Estimation of monthly runoff statistical properties, such as monthly means and variances, is usually needed to design and evaluate water resource systems. If no local recorded data are available, a transfer of information through different alternative procedures can be used. In this paper, the use of linear Transfer Function (TF) models with precipitation series as inputs is proposed to estimate statistical properties of the resulting runoff series. Empirical relationships based on data from watersheds in the mountainous zone of central Chile are suggested to estimate parameters of low-order TF models and some of their properties.