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Luo Y.,Hohai University | Li S.,Hohai University | Peng S.,Hohai University | Wang W.,Hohai University | And 3 more authors.
Paiguan Jixie Gongcheng Xuebao/Journal of Drainage and Irrigation Machinery Engineering | Year: 2013

To explore a method for forecasting short-term reference crop evapotranspiration (ET0) precisely, a proposal that is based on temperature forecast and the Hargreaves-Samani (HS) equation was raised. Thus, the observed daily meteorological data from 2001 to 2011 in Nanjing Station and the daily weather forecast data for 4 day forecast horizon in 2011 were collected. Subsequently, daily ET0 was calculated by using the FAO-56 Penman-Monteith (PM) equation, the parameters in the HS equation were corrected with 2001-2010 PM ET0 values. Then the calibrated HS equation and tempe-rature forecast were used to forecast the 4 day ET0. The forecasted and estimated ET0, forecasted and calculated temperatures were compared to evaluate the accuracy in ET0 forecast and identify the causes for the errors. The results showed that the average accuracy in forecast for the minimum temperature was 81.9%, and for the maximum temperature it was 80.1%. After calibration, the accuracy of the HS equation was relatively high. The mean accuracy, MAE, RMSE and correlation coefficient were 85.7%, 1.01 mm/d, 1.42 mm/d and 0.74, respectively, for ET0. Besides, the errors were enlarged with increasing forecast horizon. The main reason for the errors is that the mean wind speed and relative humidity are not taken into account in the HS equation. In general, the approach for predicting ET0 based on temperature forecast and corrected HS equation has a better accuracy and can provide relatively accurate ET0 data for irrigation forecast and decision making.

Yang S.,Hohai University | Han J.,Yellow River Institute of Hydraulic Research | Peng S.,Hohai University | Gao X.,Hohai University | And 2 more authors.
Paiguan Jixie Gongcheng Xuebao/Journal of Drainage and Irrigation Machinery Engineering | Year: 2012

To find a guide to water management in field irrigation, the vertical distribution of streamwise velocity in a U-shaped open channel was clarified by means of indoor and field experiments. In addition, a two-power law for such a distribution was established by using the law of velocity in asymmetric closed channel. The methods for defining the parameters in the two-power law were given, and then the law was validated by the experimental data. The results showed that the flow velocity was lowered with increasing water depth, but the peak velocity appeared under the free surface. The predicted velocity distribution by the two-power law agreed with the experimental data in the 0.10-0.95 dimensionless depth so well that the error was as small as 5%, suggesting the law can involve the effect of free surface and channel wall. Compared with the traditional law of velocity distribution, the two-power law is more accurate, and may be used in an open channel with a cross-section of other shape.

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