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For proper growth and function plants require water. Water is used as building material (Figure 3.1 in §3.1), nutrient solvent, transport- and cooling agent. Water, moreover, plays an important role in photosynthesis. For most crops water is taken up by the roots. There are three main forces that drive water movement trough the plant: root pressure, capillary action and transpiration. Simply put, root pressure is an osmotic pressure in the root system that causes water to travel from outside the root inside the vascular tissue (xylem). This uptake causes a pressure in the xylem, which pushes the water upwards trough the xylem. However, when transpiration is high, xylem sap is usually under tension due to transpirational pull and not under pressure generated by the roots. Plant scientist refer to evaporation if water passively vaporizes from liquids on the outside surface of plant organs (e.g. leafs and stems) into a gaseous phase.  The term transpiration refers to the water that evaporates from within their tissue. If it is hard to discriminate between evaporation and transpiration, then the term evapotranspiration is used. Evapotranspiration is the cumulative amount of water that vaporizes from the substrate, the plant inside and the plant surface.