Suitability of stem diameter variations and water potential as indicators for irrigation scheduling of almond trees

Successful irrigation often depends upon the development of a precise schedule based on crop monitoring. Measurements of stem water potential (SWP) and of trunk diameter variations (TDV) were performed on almond trees during 1999 in a commercial orchard in the San Joaquin Valley of California. The objective was to determine the feasibility of using SWP and TDV as indicators for irrigation scheduling, for which reference values, representing behaviour of both indicators under non-limiting soil water levels, will be needed. The trees were irrigated in excess of their requirements with microsprinklers and subsurface drip to ensure their responses were affected only by the aerial environment. Regression analyses between three environmental variables (vapour pressure deficit, VPD; air temperature, Ta; and reference evapotranspiration, ETo) and the SWP and maximum daily shrinkage (MDS) gave 2 best results for the correlations of MDS with daily VPD (r(2) = 0.64) and Ta (r(2) = 0.59). Goodness of fit for the linear regressions of SWP against all environmental variables was lower than for MDS. (r(2) values of 0.48 and 0.37 for daily VPD and Ta, respectively). The relation between MDS and SWP was weak and showed significant hysteresis as the season progressed. Both indicators responded sharply to soil water deficits when another set of trees were allowed to dry before harvest, but the signal strength (stressed/fully irrigated) of MDS, which ranged from 0.08 to 0.35 mm, was greater than that of SWP, which declined from -1.0 to -1.8 MPa, during the drying cycle. It was concluded that while there may be uncertainties in establishing reference levels for both SWP and MDS, the use of MDS is a promising approach for the development of automated irrigation scheduling in almonds.