There is a close relationship between available soil water and water consumption as well as plant growth if we assume adequate soil aeration. Transpiration and plant growth increase with an increasing amount of available water, which can be provided either from the soil or from the groundwater, i.e., capillary rise. Interaction between growth and transpiration is based on the stomata regulation of the plant which is involved in both water consumption and photosynthesis. Plants transpire when their stomata are open and at the same time CO2 diffuses into the plant from the atmosphere. These processes are interrupted when stomatas are closed. Using a calibrated simulation model for cropland, grassland, and pine forest, the actual evapotranspiration Eact was determined for various soils and groundwater depth levels. Eact increases in the sequence cropland < grassland < coniferous forest. Results show a strong correlation between the ratio of transpiration (Eact) and water vapour pressure gradient (De) of the air and field measured production of dry matter.