Water Rights Restored to the Gila River
The impacts of irrigation on vegetation health in the Gila River Valley
In 2004, the Akimel O’otham and Tohono O’odham tribes won a water rights settlement in the US Supreme Court. Using satellite imagery, we can see the effects of irrigation water on the local vegetation.
- Open raster or image data using code
- Combine raster data and vector data to crop images to an area of interest
- Summarize raster values with stastics
- Analyze a time-series of raster images
Reclaiming Water Rights on the Gila River
The Gila River Reservation south of Phoenix, AZ is the ancestral home of the Akimel O’otham and Tohono O’odham tribes. The Gila River area was known for its agriculture, with miles of canals providing irrigation. However, in the 1800s, European colonizers upstream installed dams which cut off water supply. This resulted in the collapse of Gila River agriculture, along with sky-rocketing rates of diabetes and heart disease in the community as they were force to subsist only on US government surplus rations.
In 2004, the Gila River community won back much of its water rights in court. The settlement granted senior water rights nearly matching pre-colonial water use. Work has begun to rebuild the agriculture in the Gila River Reservation. According to the Akimel O’otham and Tohono O’odham tribes, “It will take years to complete but in the end the community members will once again hear the sweet music of rushing water.”
Observing vegetation health from space
We will look at vegetation health using NDVI (Normalized Difference Vegetation Index). How does it work? First, we need to learn about spectral reflectance signatures.
Every object reflects some wavelengths of light more or less than others. We can see this with our eyes, since, for example, plants reflect a lot of green in the summer, and then as that green diminishes in the fall they look more yellow or orange. The image below shows spectral signatures for water, soil, and vegetation:
> Image source: SEOS Project
Healthy vegetation reflects a lot of Near-InfraRed (NIR) radiation. Less healthy vegetation reflects a similar amounts of the visible light spectra, but less NIR radiation. We don’t see a huge drop in Green radiation until the plant is very stressed or dead. That means that NIR allows us to get ahead of what we can see with our eyes.
> Image source: Spectral signature literature review by px39n
Different species of plants reflect different spectral signatures, but the pattern of the signatures across species and sitations is similar. NDVI compares the amount of NIR reflectance to the amount of Red reflectance, thus accounting for many of the species differences and isolating the health of the plant. The formula for calculating NDVI is:
\[NDVI = \frac{(NIR - Red)}{(NIR + Red)}\]
Read more about NDVI and other vegetation indices:
Your turn! Repeat this workflow in a different time and place.
It’s not only irrigation that affects NDVI! You could look at:
- Recovery after a national disaster, like a wildfire or hurricane
- Drought
- Deforestation
- Irrigation
- Beaver reintroduction