Peak crop coefficient values for Shaanxi, North-west China

Lysimeter and soil water instruments were used in North-west China to determine peak crop coefficients and calibrate irrigation scheduling software developed for commercial use in Scotland. The (1963) Penman equation, previously used in Scotland, was compared with a particular formulation of the Penman-Monteith (PM) equation proposed by Allen et al. [Allen, R.G., Smith, M., Perrier, A., Pereira, L.S., 1994a. An update for the definition and calculation of reference evapotranspiration. ICID Bull. 43 (2), 1-34; Allen, R.G., Smith, M., Perrier, A., Pereira, L.S., 1994b. An update for the calculation of reference evapotranspiration. ICID Bull. 43 (2), 35-92]. Measures of soil water loss for winter wheat, maize and peaches were compared with scheduling model predictions. Predicted values of cumulative actual water use were comparable for the two ET equations when a soil water stress function was applied. However, the new PM equation provided slightly higher model efficiency, when used with a peak crop coefficient of 1.25 for maize. A peak value of 1.25 also appeared appropriate for wheat, though the evidence was less strong. Inclusion of the slow mobile water concept slightly improved agreement between measured and predicted soil water changes for the maize and peaches. For peaches, the revised PM equation provided minimum values of root mean square error (R.M.S.E.) and bias, when combined with a slow mobile water representation and using a peak crop coefficient of 0.75.