Wallach, R., Seagram Center for Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, Hebrew University of Jerusalem, P.O. Box 12, Rehovot, Israel
Raviv, M., Dept. of Ornamental Horticulture, Agricultural Research Organization, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel

An alternative definition of water availability is suggested and verified in the current study. According to this definition, the term water availability expresses the balance between the atmospheric water demand (Vapour pressure deficit, VPD) and the capability of the growing medium to supply this demand at the compatible rate. As such, water availability is a relative property that does not depend only on the levels of water content or tension in the growing medium. Water can be fully available at a certain VPD and partially available at a higher VPD, and vice versa. A controlled greenhouse study was conducted to verify the actual water availability under "wet" and "dry" irrigation treatments. The containers weight, water tension (tensiometers) and moisture content (TDR) at two heights within the container, and drainage from the container bottom were measured at 5 minutes interval. Water in the medium, at a certain tension, can be fully available at a particular VPD and partially available at a higher VPD, and vice versa. Water in the medium can be considered available, although not fully, if the pattern of water uptake rate follows the VPD daily pattern, namely, it does not decrease when VPD increases. A temporary high level of water availability is obtained during and soon after irrigation during high VPD hours which caused to a temporary increase in water uptake rate. Both, water availability and rate of water uptake decreased afterwards to the "smooth" pattern of uptake rate that is typical to the frequent irrigation treatment. This smooth pattern is higher than the water uptake rate pattern associated with the drier irrigation treatment. The relationship between the simultaneously-measured moisture content and water tension in the container (denoted as containerized-media retention curve) are different than the one measured in the laboratory when equilibrium in moisture content has been reached for each measured tension. The containerized-media retention curve is not unique; it depends on irrigation frequency and the location of the tensiometers and TDR probes within the container.