A method for calculating the duration and intensity of salt intrusions: the Yangtze River estuary

Abstract. Studies of intrusions of salt water into estuaries are typically constrained by both the short duration of discharge records and the paucity of observations of discharge and salinity. Thus studies of intrusions of salt water into estuaries typically seek to identify the conditions under which intrusions occur, using detailed observations for periods of 20–60 days. This paper demonstrates a method by which to identify the conditions under which intense intrusions of long duration occur and applies that method to the Yangtze River estuary. The paper constructs a model of the relationship between salinity and discharge and then employs Monte Carlo simulation methods to reconstruct the probability of observing intrusions of differing intensities and durations in relation to discharge. The model predicts that the duration of intrusions with chlorinity ≥250 mg L−1 increases as the number of consecutive days with discharge ≤12 000 m3 s−1 increases; consecutive days of discharges ≤8000 m3 s−1 predict the duration of intrusions with chlorinity ≥400 or 500 mg L−1. In 26 of the 64 years analysed, the probability of an intrusion of at least 60 days at ≥250 mg L−1 is greater than 1 in 1000; in 17 years is greater than 1 in 100; and in ten years is greater than 1 in 10