Finding the Greenest Place in Arizona

 I love plants, and I love places where plants are happy.  But how can I find the greenest places, especially when I live in Arizona?

In the Southwest deserts, plant phenology and growth are dependent on intermittent rains.  But the rains can be variable between nearby areas. 

Rainfall totals and percent of normal can be viewed using https://water.weather.gov/precip/ This uses observed (radar) precipitation. 

Displaying Last 90-Day Percent of Normal Precipitation from Sep. 20.

However, more important than total rainfall over the growing season is timing of rainfall.  A single deluge that brought 3 inches last week to an area that hasn't seen rain in 6 months, is not as effective in stimulating plant growth as regular weekly or biweekly 0.5 inch storms.

A better approach to locating areas of high plant growth is using NDVI (Normalized Difference Vegetation Index), or observed "greenness" from satellites.  Data updates every week or couple of weeks, depending on the satellite.

But some areas, like forests, will always look greener than other areas, like deserts.  What we really want to locate are areas of anomalous greenness.

This UA webpage calculates departures from last week, last year, and from the average year.

VIIRS NDVI difference from average from Sep 6.

Other resources include the USGS Vegetation Drought Model, VegDRI.  The Vegetation Drought Response Index (VegDRI) shows seasonal drought impacts on vegetation. The weekly index is produced using a model trained on the self-calibrated Palmer Drought Severity Index using MODIS satellite-derived greenness anomaly integrated with precipitation, land cover, soils, and other biophysical data sets. Updated every Monday by 10 AM. 

VIIRS NDVI as shown by the USGS VegDRI model.

Clicking on the map generates a time series, which can be useful to compare seasonal and yearly changes at a location.

Graph of VegDRI drought index for a location in Yavapai County over the last 4 years.

The United Nation's FAO also maintains an NDVI anomaly drought mapper that uses the European satellite AVHRR.  However, the data is only provided at the scale of entire nations.  

So where is the greenest place in Arizona?  As you may have noticed from the screenshots above, that depends on who you're asking!  I hope to "ground-truth" some of these models to learn which are most reliable for greenness-hunting.