2-COMPONENT LOG-NORMAL DISTRIBUTION OF IRRIGATION-AFFECTED LOW FLOWS

A new approach is used for the determination of the theoretical distri bution of low flows in a highly developed basin. As estimates of water diversions and return flows from irrigation may have high errors, dif ficulties arise in transforming an observed series of river flows to a natural series, which is a prior requirement in a statistical analysi s of this type. A priori reasoning suggests that the net diversions ma y be represented by linear, non-linear or truncated functions of river flows. A two-component log-normal distribution is proposed, in which the first component represents the natural low flows in the basin, una ffected by irrigation, and the second component models the effects of diversions, return flows and other regulations. The adopted diversion rule provides the link between the components. The maximum likelihood method is applied for parameter estimation, but simplified procedures based on the method of moments are investigated, and one which results in a sufficiently good fit is obtained. The feasibility of relating t he constants of the net diversion rules to physical parameters such as catchment area is investigated. The method is applied to the Po basin , the largest in Italy.