Comparison of calculations for crop transpiration

It is significant to research crop evapotranspiration, including plant transpiration and soil evaporation, for field water cycle. There are some techniques to obtain transpiration index. In this paper, comparisons of three techniques for calculating evapotranspiration of grape under alternate partial root zone drip irrigation were curried. The results showed that cumulative transpiration can be calculated by sap flow and Penman-Monteith. Compare to water balance, the relative error was not beyond 10%. Considered the relationship of delay lag between the variety of sap flow of grape vine and the change of the transpiration, the modified coefficient K related with (R_n - G)was introduced. The results showed there was a good correlation between the modified temporal sap flow and the transpiration rate. The correlation coefficient is 0.8344, and the slop of correlation equation is 0.9038.