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Determining the contributions of climate change and human activities to runoff variation is important for water resource management. An improved double mass curve method (IDMC), which can d...

The objectives of this study were to examine records from long-term rain gauges in Mākaha Valley for data homogeneity and to compare methods of estimating missing data. Double mass analysis was used to investigate data homogeneity. Results show that tree growth near one gauge has reduced rainfall catch by 21–25% since 1974. Four methods for estimating missing daily rainfall data were then tested using index gauges selected from a network of 21 active rain gauges. The number of index gauges and their order of selection were varied according to proximity and correlation. Selection by correlation significantly improved the performance of the station average and inverse distance methods for most cases, as well as the normal ratio method for the case when only one gauge is used. The normal ratio method produces the lowest error when two to five index gauges are used; the inverse distance method yields the lowest error when six or seven index gauges are used. Direct substitution produces better accuracy than th...

Hydrologists use double-mass analysis to verify the consistency of a precipitation or streamflow record. Precipitation of a station is plotted against the combined precipitation of the surrounding stations. The consistency of the record is verified if a single straight line fitted through the points is reasonable. The analysis is done on the computer by fitting a straight line and a fourth-degree polynomial through the points and comparing the two sums of squares of deviations of the points from the fitted curves. If a record is found to be inconsistent, it is adjusted by bringing the deviated points along a desired single straight line. The method should be of value to the hydrologists having access to computers.