Water balance of a Southern Moravian floodplain forest under natural and modified soil water regimes and its ecological consequences

Stand water balance was calculated in a floodplain forest in Southern Morav ia. A model was applied to actual and theoretical scenarios of climate and soil water supply. Rooted and root-free soil volumes were considered separa tely because root development was poor for these trees. Input data, i.e., m easured flows within the system including sap flow rate, characterize both the period of regular natural floods and period when floods were interrupte d for over 20 years because of canalization of rivers in the region. Under non-limiting underground water supply, net precipitation supplied only abou t 50 and 25% of water for actual evapotranspiration, E-T, under mild and dr y weather, respectively, and the other 30 and 60% came from underground sou rces. The model also characterizes the theoretical situation of no undergro und water supply, when E-T may decrease significantly. An important limit f or water supply to the trees may be the maximum hydraulic conductivity K, a llowing horizontal transport in heavy soils, because a small decrease in so il water content (2 to 4%(vol)) causes a large drop in K. K may become supp ly-limiting before soil water potential becomes a limiting factor. Trees wi th smaller or damaged root systems or lower root/shoot ratio were especiall y threatened by drought even on relatively moist heavy soils.