Determining water consumption in olive orchards using the water balance approach

21 pages, 9 figures, 3 tables, 39 references.

Efficient irrigation regimes are becoming increasingly important in commercial orchards. Accurate measurements of the components of the water balance equation in olive orchards are required for optimising water management and for validating models related to the water balance in orchards and to crop water consumption. The aim of this work was to determine the components of the water balance in an olive orchard with mature `Manzanilla' olive trees under three water treatments: treatment I, trees irrigated daily to supply crop water demand; treatment D, trees irrigated three times during the dry season, receiving a total of about 30% of the irrigation amount in treatment I; and treatment R, rainfed trees. The relationships between soil water content and soil hydraulic conductivity and between soil water content and soil matric potential were determined at different depths in situ at different locations in the orchard in order to estimate the rate of water lost by drainage. The average size and shape of the wet bulb under the dripper was simulated using the Philip's theory. The results were validated for a 3 l h-1 dripper in the orchard. The water amounts supplied to the I trees during the irrigation seasons of 1997 and 1998 were calculated based on the actual rainfall, the potential evapotranspiration in the area and the reduction coefficients determined previously for the particular orchard conditions. The calculated irrigation needs were 418 mm in 1997 and 389 mm in 1998. With these water supplies, the values of soil water content in the wet bulbs remained constant during the two dry seasons. The water losses by drainage estimated for the irrigation periods of 1997 and 1998 were 61 and 51 mm, respectively. These low values of water loss indicate that the irrigation amounts applied were adequate. For the hydrological year 1997-1998, the crop evapotranspiration was 653 mm in treatment I, 405 mm in treatment D and 378 mm in treatment R. Water losses by drainage were 119 mm in treatment I, 81 mm in treatment D and 4 mm in treatment R. The estimated water runoff was 345 mm in treatments I and R, and 348 mm in treatment D. These high values were due to heavy rainfall recorded in winter. The total rainfall during the hydrological year was 730 mm, about 1.4 times the average in the area. The simulated dimensions of the wet bulb given by the model based on the Philip's theory showed a good agreement with the values measured. In a period in which the reference evapotranspiration was 7.9 mm per day, estimations of tree transpiration from sap flow measurements, and of evaporation from the soil surface from a relationship obtained for the orchard conditions, yielded an average daily evapotranspiration of 70 l for one I tree, and 48 l for one R tree.

This study was supported with funds of the Spanish CICYT, project HID96-1342-CO4-01.

Peer reviewed