Comparison of methods for estimating actual evapotranspiration in a row-cropped vineyard

The water consumption of grapevines is one of the main factors influencing wine quality. The general characteristics of the vines (deep and sparse rooting system, heterogeneity of canopy size) and of vineyards (sloping ground, stony soils, small fields) make it difficult to estimate the actual evapotranspiration of the vineyard using the soil water balance or energy balance methods. In this paper these two methods are compared to a method relying on separate measurements of soil evaporation and plant transpiration. An experiment was conducted for two months in 1994 in a one-hectare vineyard of Southern France in which five monitoring plots had been installed, In each plot, soil moisture was monitored down to 260 cm under the row and between the rows to take into account the heterogeneity of the rooting system. Water flux at 260 cm depth was computed from measurements of soil water potential and hydraulic properties. Evapotranspiration was then obtained from the soil water balance. Transpiration was monitored with two sap flow gauges per plot and evaporation with three mini-lysimeters installed at different distances from the row. The energy balance was determined with micrometeorological measurements taken in the field. The reliability of the measurements of evaporation from the soil and of plant transpiration was satisfactory. First, soil evaporation computed from mini-lysimeter measurements compared well with that calculated from the water balance of the soil surface. Second, the sap flow followed the diurnal pattern of net radiation at a time scale of less than one hour. The three methods for estimating actual evapotranspiration gave comparable mean field values and showed a similar evolution through the summer. Statistical tests showed that the observed differences between the estimated values were not significant, whatever the length of the period considered (3-31 days) and the rate of evapotranspiration (1.8-3.5 mm d(-1)). The experiment shows the inherent limits and advantages of the three methods. The soil water balance method must be used over periods of longer than one week to provide acceptable precision in estimating field evapotranspiration. The energy balance method gives small time scale information but requires constant technical follow-up. Measuring evaporation and transpiration separately to estimate vineyard evapotranspiration is easily adaptable to a large range of vine-growing conditions. Moreover, it proved to be more precise than the other two methods. (C) 1998 Elsevier Science B.V. All rights reserved.