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Glossary 489evapotranspiration – the combination of evaporation of water from the earth’s watersurfaces and soil and the transpiration of water by plants. Evapotranspiration is frequentlymeasured in two ways: (1) the amount that occurred; and (2) the potentialamount that would have occurred if enough water had been present to meet all evaporationand transpiration needs. In arid areas the actual amount is frequently less than thepotential amount.extreme events – (from IPCC AR3 WGI): “an event that is rare at a particular place andtime of year. Definitions of rare vary, but an extreme weather event would normally beas rare as or rarer than the lowest or highest 10 percent of all weather events that haveever been observed in a location or region. By definition, the characteristics of what iscalled extreme weather may vary from place to place in an absolute sense. Single extremeevents cannot be simply and directly attributed to anthropogenic climate change,as there is always a finite chance the event in question might have occurred naturally.When a pattern of extreme weather persists for some time, such as a season, it may beclassed as an extreme climate event, especially if it yields an average or total that is itselfextreme (e.g., drought or heavy rainfall over a season).”exurban land – land on the outer ring of suburbs, here defined as having a housing densityof one unit per 2.5 to 40 acresfeedback, natural system feedback – Feedbacks are interactions in which outputs froma process have an effect on the inputs to that same process. Sometimes feedbacks canoffset or inhibit a change (negative feedback), and sometimes they can amplify a change(positive feedback). An example of a positive feedback is when the atmosphere heatsup it melts ice. Ice reflects a lot of incoming energy from the sun, so when it melts andis replaced by heat-absorbing water, soil or vegetation, the land surface warms morequickly and warms the atmosphere, making it easier for more snow and ice to melt in asustained manner that can eventually cause extensive snowmelt. An example of a negativefeedback is when carbon dioxide increases, it causes plants to grow faster, whichallows them to absorb more carbon dioxide, which eventually can lead to a large reductionof carbon dioxide.forb – a herbaceous flowering plant that is not a grassforcing – (n.) From IPCC AR4 WGI: “The climate system can be driven, or “forced” byfactors within and external to the system. Processes within the system include thoserelated to the atmosphere, the cryosphere (ice-covered parts of the Earth), the hydrosphere,the land surface, and the biosphere. Volcanic eruptions, solar variations andhuman-caused changes in the composition of the atmosphere and land use change areexternal forcings.”forward contract – a bilateral agreement to buy and sell an asset at a specified time andprice in the future, with both terms agreed upon todayfreeze-free season – the period between the last frost of spring and first autumn frost;the length of the season equals the number of consecutive days during which minimumdaily temperatures are above freezing. An increase in the length of the season may beconsidered a proxy for potential increased evaporative or heat stress on plants in thearid Southwest.