Water is always a contentious issue in California. With 38 million thirsty people, a $43 billion agriculture industry, often erratic rainfall, and a bewildering tangle of policies, it's no wonder. The creators of a new California water atlas are hoping to make data on the state's water resources more accessible and comprehensible through a series of interactive maps.

"We're hoping we can inspire dialogue around some of these policy issues," says Laci Videmsky, a designer and one of the directors of the project, which is funded by Patagonia and the Resource Renewal Institute, a non-profit organization based in Mill Valley, California. Videmsky and his co-director Chacha Sikes, a programmer, hope the new digital atlas will help journalists, activists, and ordinary citizens learn more about how water is allocated and used in the state.

The duo met last year at Code for Oakland, a hackathon for civic-minded software developers, and started brainstorming projects that could have state-wide impact.

Chacha Sikes (left) and Laci Videmsky. Photo: Alex Washburn/Wired

A major inspiration for the new project was a 1979 water atlas commissioned by California governor Jerry Brown (yes, he was governor back then too). The book became a cult classic among cartography geeks for its beautiful maps and infographics, Videmsky says, but it didn't reach many people and didn't have much impact on water policy.

Videmsky and Sikes hope to change that with an open and interactive digital atlas that is free for anyone to use online.

The first map they created for the new atlas illustrates water rights claims in the state. Water rights granted by the state give individuals, companies, and agencies permission to withdraw water from rivers, streams, and groundwater sources for what the state considers reasonable and beneficial uses.

Launched in April, the water rights atlas displays about 50,000 claims on file with the State Water Resources Control Board. Each dot represents a claim, and the color indicates what the water is used for: green is for irrigation, brown is for livestock, dark blue is for hydroelectric power, and so on. Users can hover over a dot to see who owns the claim, how much water it entitles them to, and other information.

The finished product helps tell the story of how water is allocated in California, and it illustrates a key element in the state's water use conundrum, Videmsky and Sikes say. The claims total 250 million acre feet per year (one acre foot is roughly the amount of water used in a year by a typical family of four). But only 200 million acre feet fall on the state each year as precipitation, and most of that evaporates, leaving only 71 million acre feet of new water. That discrepancy doesn't mean that California uses 3.5 times as much water as it receives – various policies give claim holders an incentive to claim more water than they actually use – but it does illustrate the potential for confusion and conflict in the current system.

The map took about 4 months to make. First, Sikes wrote a scraper program that downloaded water claims data from the state's database. That took about 2 months, due to the amount and complexity of the data. To begin building a prototype, she and Videmsky used Google Refine (now called Open Refine) to clean up the data, writing simple scripts to take care of annoying details like getting the longitude and latitude figures formatted correctly for their purposes. They used mongoDB, an open source database system, to test the data and make it searchable. They made the final version of the water rights markers in TileMill, a free program for designing interactive maps, and Sikes wrote additional code to customize the interactive features.The dataset and documentation on the code built into the atlas are available on GitHub.

The water rights map is just a beginning. Later this summer, Videmsky and Sikes hope to release a map of the state's groundwater aquifers, another large piece of the state's water use picture. Other plans include a map of the virgin waterscape of California, maps of water pricing and water quality, and real-time integration with flow sensors in streams and rivers around the state.