where the soil is locally saturated and by capillarity pull to drier parts of the soil. Most plant water needs are supplied from the suction caused by evaporation from plant leaves (transpiration) and a lower fraction is supplied by suction created by osmotic pressure differences between the plant interior and the soil solution. Plant roots must seek out water and grow preferentially in moister soil microsites, but some parts of the root system are also able to remoisten dry parts of the soil. Insufficient water will damage the yield of a crop. Most of the available water is used in transpiration to pull nutrients into the plant. Soil water is also important for climate modeling and numerical weather prediction. The Global Climate Observing System specified soil water as one of the 50 Essential Climate Variables (ECVs). Soil water can be measured in situ with soil moisture sensors or can be estimated at various scales and resolution: from local or wifi measures via sensors in the soil to satellite imagery that combines data capture and hydrological models. Each method exhibits pros and cons, and hence, the integration of different techniques may decrease the drawbacks of a single given method. Moisture level concepts Field capacity A flooded field will drain the gravitational water under the influence of gravity until water's adhesive and cohesive forces resist further drainage at which point it is said to have reached field capacity. At that point, plants must apply suction to draw water from a soil. By convention it is defined at 0.33 bar suction. Available water and unavailable water The water that plants may draw from the soil is called the available water. Once the available water is used up the remaining moisture is called unavailable water as the plant cannot produce sufficient suction to draw that water in. Wilting point The wilting point is the minimum amount of water plants need to not wilt and approximates the boundary between available and unavailable water. By convention it is defined as 15 bar suction. At this point, seeds will not germinate, plants begin to wilt and then die unless they are able to recover after water replenishment thanks to species-specific adaptations. Water retention Water is retained in a soil when the adhesive force of attraction that water's hydrogen atoms have for the oxygen of soil particles is stronger than the cohesive forces that water's hydrogen feels for