“
“The notions attached with hydrological drought generally refer to shortfalls in river flows, water levels in lakes,

ponds, wetlands, ground water reservoirs, etc. By and large river flows have been used in the analysis of hydrologic droughts and therefore the term streamflow drought has also been used. One index that has become popular in recent years for identifying meteorological droughts is the standardized precipitation index (SPI), which is a seasonally (monthly, weekly, etc.) standardized-and-normalized value of the precipitation time series (McKee et al., 1993). Sharma and Panu (2010) have suggested the standardized hydrological index (SHI) as a measure for defining and modeling the hydrological droughts, which is conceptually Osimertinib mw analogous to SPI except that SHI represents a standardized value (mean, μ = 0 and standard deviation, σ = 1 of SHI sequence) which is not normalized. The distinguishing feature JAK inhibitor between a standardized-and-normalized (also called standard normal) and standardized variable is that the former is obtained by subtracting mean from the original variable, xi and division

by the standard deviation of the variable ei = (xi − μ)/σ; ei is the standardized variable and transforming it into normal distribution (ei → zi becomes a normalized variable) while in the latter case the

transformation into the normal distribution is not conducted. For example, Branched chain aminotransferase when a standardized sequence, ei is derived from a Gamma distributed variable xi; it can be transformed into a standard normal distribution, zi using Wilson–Hilferty transformation ( Viessman and Lewis, 2003). In the case of SPI, the above transformation is conducted prior to analyzing the drought parameters whereas in the case of SHI, the above transformation is not conducted. This paper describes the analysis for drought parameters using SHI as a platform. In the case of annual flow series, which is generally regarded as a case of weak stationarity, the computations for creating SHI sequences is trivial as there is only one mean and one standard deviation. In the case of monthly and weekly flow series, the creation of SHI sequences is somewhat involved because it requires stationarising the seasonal (monthly or weekly) flow series. The process of stationarising means standardization of the flow series using month by month μ’s and σ’s, that catapults into a weak stationary series with constant μ equal to zero and σ equal to one. The SHI sequence so obtained inherits the non-normal character of the seasonal flow series as no attempt is exercised to normalize it. The non-normalization offers an advantage in that the flow values are not distorted.