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The uncertainty and subjectivity inherent in estimating the resistance coefficient is one of the main sources of error in the application of flow resistance equations to natural channels. Various studies have shown that it is possible to relate discharge successfully with variables related to the hydraulic flow geometry without the need for an independent estimate of the resistance coefficient. The aim of this paper is to calibrate and validate three models, previously proposed by other researchers, using an extensive empirical base, made up of 904 data from over 400 reaches of gravel-bed rivers and mountain streams from various regions of the world.Thanks to the cross validation procedure, the three models were calibrated using the full database, which allowed the fitted equations to be based on the maximum number of observations. There are no important differences between the three models calibrated with regard to their explanatory power. These models show that the exponent of the hydraulic radius is greater than 2/3 and that the exponent of the slope is closer to 1/4 than 1/2 in gravel-bed rivers and mountain streams. Validation confirmed the precision of the fitted equations, by reaching predictive power values comparable with those from calibration. The fitted equations can be successfully applied in reaches of gravel-bed rivers and mountain streams (for non-sinuous, un-vegetated and hydraulically-wide channels and for flow higher than 0.1 m 3 /s and lower or equal to bankfull discharge) for which there is no specific detailed information about flow resistance available. ª

Please, cite this article as: Puig-Bargués, J.; Arbat, G.; Elbana, M.; Duran-Ros, M.; Barragán, J.; Ramírez de Cartagena, F.; Lamm, F.R. AbstractFlushing is an important maintenance task that removes accumulated particles in microirrigation laterals that can help to reduce clogging problems. The effect of three dripline flushing frequency treatments (no flushing, one flushing at the end of each irrigation period, and a monthly flushing during the irrigation period) was studied in surface and subsurface drip irrigation systems that operated using a wastewater treatment plant effluent for three irrigation periods of 540 h each. The irrigation systems had two different emitters, one pressure compensating and the other not, both molded and welded onto the interior dripline wall, placed in laterals 87 meters long. Dripline flow of the pressure compensating emitter increased 8% over time, while in the nonpressure compensating emitter, dripline flow increased 25% in the surface driplines and decreased 3% in the subsurface driplines by the emitter clogging. Emitter clogging was affected primarily by the interactions between emitter location, emitter type, and flushing frequency treatment. The number of completely clogged emitters was affected by the interaction between irrigation system and emitter type. There was an average of 3.7% less totally clogged emitters in flushed surface driplines with the pressurecompensating emitter as compared to flushed subsurface laterals with the nonpressure compensating emitter.

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