This glossary provides definitions of important terms used in the descriptions of ISciences' capabilities and products.
Anomaly Levels are specified in five levels, each of which corresponds to a range of return periods as follows: abnormal=3-5 years, moderate=5-10 years, severe=10-20 years, extreme 20-40 years, and exceptional= greater than 40 years.
Composite water anomaly index combines multiple parameters to best capture water surpluses and deficits. Deficits are based on the most extreme of three water deficit indicators: soil moisture deficit, evapotranspiration deficit, and total blue water deficits. Surpluses are based on the most extreme of two water surplus indicators: runoff surpluses and total blue water surpluses. Maps of the composite water anomaly index show deficits in warm colors and surpluses in cool colors. Because surpluses and deficits are estimated using different parameters, it is possible for a single half-degree cell to indicate both surpluses and deficits. These cases are shown in shades of purple, with the return period based on the more extreme of the surplus or deficit indicators.
Deficits are declared when surface water levels fall below historical averages. [see return period]. Sustained deficits may become droughts, which are defined as persistent, abnormal deficits that have an adverse impact on vegetation, animals, or people [National Drought Policy Commission Report, May 2000). Deficits are specified in five levels, each of which corresponds to a range of return periods (see Anomaly Levels).
Evapotranspiration deficit is the difference between the amount of water that would be transpired or evaporated if water supply was unlimited (potential evapotranspiration), and the amount of water that is actually transpired or evaporated (estimated from temperature, precipitation, change in soil moisture, and soil characteristics). Large values relative to the norm indicate likely vegetative stress unless the difference is made up using irrigation.
Integration Period indicates the number of months being compared to historical data. For example, a composite index map for June through August (3-month integration period) compares those three months to the same three month period in each year from 1950 through 2009. Longer integration periods show longer-term trends, and tend to be less noisy than shorter periods.
Return period is expected number of years between events of a particular magnitude. A 30-year high temperature, for example, is a temperature that would be expected to occur once every 30 years, based on data distributions for the period 1950 through 2009. Return periods are computed locally, i.e., for each half-degree cell, based on historical data for that locale. Five levels of stress are defined, each of which corresponds to a range of return periods as follows: Notable=3-5 years, moderate=5-10 years, severe=10-20 years, extreme 20-40 years, and exceptional= greater than 40 years.
Runoff is precipitation or snow-melt which is not evaporated, absorbed by soil, or used by vegetation. Large values relative to the norm suggest potential for crop or pasture damage by flooding.
Soil Moisture is the amount of water held in the soil column. Small values relative to the norm indicate stress to rainfed agriculture and pasture lands depending on the seasonality of the crops and grassland production.
Surpluses are declared when surface water levels are higher than historical averages. [see return period] Monthly surpluses may result in flooding, depending on soil saturation, timing of precipitation within the month, and other factors. Surpluses are specified in five levels, each of which corresponds to a range of return periods (see Anomaly Levels).
Total blue water is uninhibited flow accumulated runoff. Low values relative to the norm suggest periods of water scarcity and increased competition amongst large point source users such as irrigation systems, urban water distribution systems, and electric power generation. Large values can indicate wide area flooding.
WSIM (Water Security Indicators Model) is ISciences' reduced form hydrological model that produces a suite of physical quantities for soil moisture, runoff, evapotranspiration, evapotranspiration deficit, snow water equivalent and total blue water (flow accumulated runoff), and is used to monitor and forecast water anomalies on a monthly, near-global basis. For more detail go to the WSIM page.
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