Here is an email exchange I recently had with Kathy Mrowka at the Water Board. Enough folks have asked me that I had to ask, and Kathy has a short and sensible answer. Her answer will be frustrating to some, and a relief to others; either way, it represents the state of technology today as well as legal requirements.
From: Shawn Pike
To: John O’Hagan, Kathy Mrowka, and Jeffrey Yeazell
“Hi John, Kathy, and Jeff,
“Several intelligent ranchers have asked me, why doesn’t the Water Board just 
use satellite imagery to monitor flow and water storage? I did not have a good answer why not, since high-resolution satellite imagery has been available for more than 40 years, and very high resolution aerial photography is taken with exponentially increasing frequency.
“It is common knowledge that the Water Board has used aerial photography to find thousands of reservoirs and stockponds. The Department of Water Resources has used aerial photography and some ground-checking to map land and water use, including estimates of applied water, for decades.
“I’ll bet some inventive brains at your place are making progress on using photography to monitor and quantify water flow through diversions, as well as impounded water storage. Given that increasingly detailed data are derived from photography, farmers and ranchers are very interested in having DWR and/or the Water Board use the same technology to monitor diverted flows and storage. It would be a heck of a lot less expensive – a few percent of the thousands of dollars each diverter now has to spend on measuring devices, data loggers, and reporting their diversions.
“Do you have some good news to pass on to individuals, the Farm Bureau, Cattlemen’s Association, and others?
“Thank you,
Shawn”
Kathy’s answer:
“The Department of Water Resources and others, including the State Water Board, utilize satellite imagery for ground-checking information. The imagery can also be used with other models for estimates of applied water. An estimate of applied water unfortunately, does not provide the total amount diverted from the source. For instance, where delivery is by long irrigation canals, the water diverted to account for canal losses is greater than the amount applied for irrigation. Aerials also cannot distinguish between water supplies from groundwater wells, purchased supplies and diversions of water from a stream. A water right is issued for a specific diversion amount and season and measurement of the diversion is necessary to quantify the amount diverted under the specific water right and priority (which can include reasonable conveyance losses) from the amount consumed or estimated as applied water. We believe the measurement of diversion is important. Additionally, the diversion information combined with satellite imagery can help ensure water diversions are reasonable.
Kathy Mrowka”
