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Alaska AVHRR NDVI Derived Phenological Metrics

Seasonal properties of vegetation covering northern boreal and arctic landscape are considered as important input to a number of climate change studies. The data sets presented here contain multi-temporal phenological data of Alaskan vegetation for 1991 - 2000. These data were produced using seasonally truncated, composited Advanced Very High Resolution Radiometer (AVHRR) derived Normalized Difference Vegetation Index (NDVI) data. Phenological metrics were derived from each yearly NDVI data set and grouped into three general categories: 1) temporal metrics, 2) value metrics, and 3) derived metrics. In all, 10 different metrics were calculated.

Temporal metrics are a function of Julian date and include date of onset of greenness, date of senescence, date of maximum greenness and total days. They indicate the time when greenness begins and ends, when maximum greenness may occur, and length of greenness growing season. The first three metrics contain are individual dates and the fourth is a temporal quantity. Date of onset of greenness indicates when spring foliage became dense enough to be detected by the sensor over background conditions (tree and shrub stems and boles, previous years dead foliage, barren ground, snow cover). The only exception would be evergreen needleleaf trees, which retain green biomass year round, and may indicate greenness early on in the season before major photosynthetic activities begin. Date of senescence is recorded at the end of the growing season when leaves begin to loose chlorophyll. This date will vary depending on fall climatic conditions (cloud or early snow cover), leaf type and longevity, or, in the case of the 1994 data, termination of data acquisition because of sensor malfunction. Date of maximum greenness indicates that period of the growing season when vegetation is most lush or covers the greatest amount of surface area for any given location. Total days represents growing season length and is the number of days from initial vegetation green-up in the spring to senescence in the fall; it is based on the date of green-up and date of senescence metrics.

Greenness metrics contain the NDVI value recorded during a given time period. Four metrics were obtained for this study: maximum NDVI, mean NDVI, onset of greenness, and senescence of greenness. Maximum NDVI is the maximum measurable NDVI recorded during the year and is normally associated with the peak of green during the growing season. Mean NDVI is the maximum NDVI value obtained for each recording period during the growing season divided by the total number of periods. The last two greenness metrics, onset of greenness and senescence of greenness, are the recorded NDVI values that occurred during onset of greenness in the spring, and decline of greenness or leaf-fall in the autumn, respectively. These two metrics are based on greenness values that first equaled or exceeded 0.09 in the spring, or before greenness dropped below 0.09 in the fall. The 0.09 threshold has been shown to indicate when green vegetation is abundant enough to be recorded by the sensor, although different values have been shown more applicable in some situations.

All of the above metrics should be considered relative indicators of actual dates or times. Cloud cover may prevent the sensor from recording the event, or the compositing process may select a higher NDVI value during the two week composite time periods, thus bypassing the actual day in which the event occurred.

Rate metrics provide information about how quickly seasonal greenness activity occurs and are described by rate of green up and rate of green down (senescence). Rate of green-up (RGU) is calculated using maximum NDVI value recorded during the growing season divided by the number of days from date of onset to date of maximum greenness. Rate of senescence or green-down (RGD) is the reverse of rate of green-up in that it is the maximum NDVI value recorded during the growing season, divided by the number of days from date of maximum greenness to date of senescence. These two metrics are only relative and do not mimic actual rates of green-up and green-down.

These data are stored as raster image data and covers the State of Alaska (please see meta data files). The two figures below show examples of some of the data based on a random, 1000 point sample of vegetated areas in Alaska.

In general, these data indicate that there was a trend towards earlier higher maximum greenness from 1992 to 1998 and higher mean greenness from 1992 to 1997. The initial drop in greenness values after 1991 may have been due to atmospheric perturbations caused by the 1991 Mt Pinatubo eruption. Onset of greenness dates occurred earlier each year from 1992 to 1999. The last day of greenness in the autumn, however, was roughly the same from 1995 to 1999. Earlier green-up dates in the spring resulted in a lengthened growing season of up to 20 days in some areas of Alaska from 1994 to 1999. Climate data did not always collaborate with these findings, however.

Related Documentation

Markon, C.J. 1999, CHARACTERISTICS OF THE ALASKAN 1 KM ADVANCED VERY HIGH RESOLUTION RADIOMETER DATA SETS USED FOR ANALYSIS OF VEGETATION BIOPHYSICAL PROPERTIES. U.S. Geologial Survey Open-File Report 99-401, 90p.

Markon, C.J. 2001, SEVEN-YEAR PHENOLOGICAL RECORD OF ALASKAN ECOREGIONS DERIVED FROM ADVANCED VERY HIGH RESOLUTION RADIOMETER NORMALIZED DIFFERENCE VEGETATION INDEX DATA. U.S. Geological Survey Open-File Report 01-11, 58p.

Data for 1991-2000

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Data for 1991

Metadata

Download Data

Date of onset of greenness (minday1991.gz)

Date of senescence (lastjd1991.gz)

Date of maximum greenness (maxjd1991.gz)

Total days (totdays1991.gz)

Maximum NDVI (max1991.gz)

Mean NDVI (meannd1991.gz)

Onset NDVI of greenness (onsetnd1991.gz)

Senescence NDVI of greenness (lastnd1991.gz)

Rate of green-up (rguo1991.gz)

Rate of green-down (rgus1991.gz)

Data for 1992

Metadata

Download Data

Date of onset of greenness (minday1992.gz)

Date of senescence (lastjd1992.gz)

Date of maximum greenness (maxjd1992.gz)

Total days (totdays1992.gz)

Maximum NDVI (max1992.gz)

Mean NDVI (meannd1992.gz)

Onset NDVI of greenness (onsetnd1992.gz)

Senescence NDVI of greenness (lastnd1992.gz)

Rate of green-up (rguo1992.gz)

Rate of green-down (rgus1992.gz)

Data for 1993

Metadata

Download Data

Date of onset of greenness (minday1993.gz)

Date of senescence (lastjd1993.gz)

Date of maximum greenness (maxjd1993.gz)

Total days (totdays1993.gz)

Maximum NDVI (max1993.gz)

Mean NDVI (meannd1993.gz)

Onset NDVI of greenness (onsetnd1993.gz)

Senescence NDVI of greenness (lastnd1993.gz)

Rate of green-up (rguo1993.gz)

Rate of green-down (rgus1993.gz)

Data for 1994

Metadata

Download Data

Date of onset of greenness (minday1994.gz)

Date of senescence (lastjd1994.gz)

Date of maximum greenness (maxjd1994.gz)

Total days (totdays1994.gz)

Maximum NDVI (max1994.gz)

Mean NDVI (meannd1994.gz)

Onset NDVI of greenness (onsetnd1994.gz)

Senescence NDVI of greenness (lastnd1994.gz)

Rate of green-up (rguo1994.gz)

Rate of green-down (rgus1994.gz)

Data for 1995

Metadata

Download Data

Date of onset of greenness (minday1995.gz)

Date of senescence (lastjd1995.gz)

Date of maximum greenness (maxjd1995.gz)

Total days (totdays1995.gz)

Maximum NDVI (max1995.gz)

Mean NDVI (meannd1995.gz)

Onset NDVI of greenness (onsetnd1995.gz)

Senescence NDVI of greenness (lastnd1995.gz)

Rate of green-up (rguo1995.gz)

Rate of green-down (rgus1995.gz)

Data for 1996

Metadata

Download Data

Date of onset of greenness (minday1996.gz)

Date of senescence (lastjd1996.gz)

Date of maximum greenness (maxjd1996.gz)

Total days (totdays1996.gz)

Maximum NDVI (max1996.gz)

Mean NDVI (meannd1996.gz)

Onset NDVI of greenness (onsetnd1996.gz)

Senescence NDVI of greenness (lastnd1996.gz)

Rate of green-up (rguo1996.gz)

Rate of green-down (rgus1996.gz)

Data for 1997

Metadata

Download Data

Date of onset of greenness (minday1997.gz)

Date of senescence (lastjd1997.gz)

Date of maximum greenness (maxjd1997.gz)

Total days (totdays1997.gz)

Maximum NDVI (max1997.gz)

Mean NDVI (meannd1997.gz)

Onset NDVI of greenness (onsetnd1997.gz)

Senescence NDVI of greenness (lastnd1997.gz)

Rate of green-up (rguo1997.gz)

Rate of green-down (rgus1997.gz)

Data for 1998

Metadata

Download Data

Date of onset of greenness (minday1998.gz)

Date of senescence (lastjd1998.gz)

Date of maximum greenness (maxjd1998.gz)

Total days (totdays1998.gz)

Maximum NDVI (max1998.gz)

Mean NDVI (meannd1998.gz)

Onset NDVI of greenness (onsetnd1998.gz)

Senescence NDVI of greenness (lastnd1998.gz)

Rate of green-up (rguo1998.gz)

Rate of green-down (rgus1998.gz)

Data for 1999

Metadata

Download Data

Date of onset of greenness (minday1999.gz)

Date of senescence (lastjd1999.gz)

Date of maximum greenness (maxjd1999.gz)

Total days (totdays1999.gz)

Maximum NDVI (max1999.gz)

Mean NDVI (meannd1999.gz)

Onset NDVI of greenness (onsetnd1999.gz)

Senescence NDVI of greenness (lastnd1999.gz)

Rate of green-up (rguo1999.gz)

Rate of green-down (rgus1999.gz)

Data for 2000

Metadata

Download Data

Date of onset of greenness (minday2000.gz)

Date of senescence (lastjd2000.gz)

Date of maximum greenness (maxjd2000.gz )

Total days (totdays2000.gz)

Maximum NDVI (max2000.gz)

Mean NDVI (meannd2000.gz)

Onset NDVI of greenness (onsetnd2000.gz)

Senescence NDVI of greenness (lastnd2000.g z)

Rate of green-up (rguo2000.gz)

Rate of green-down (rgus2000.gz)


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