Accuracy and caveats

The figures provided are estimates, and real water use by individual farmers may differ.

One reason is that the main input to determine a field’s consumptive use fraction depends on identifying the specific crop grown on each field. We did not obtain records from the Imperial Irrigation District showing precisely which crop was grown where and when, so we relied on a U.S. Department of Agriculture’s Cropland Data Layer to identify crops grown in 2022. That data identifies which crop is most likely to have been grown on each 30-meter square of land based on analysis of satellite imagery, and it assigns a score for how confident it is that the crop is correctly identified. When that confidence score was low, we used the California Department of Water Resources Statewide Crop Mapping database (2021 provisional). Additionally, the irrigation district reported what farmers planned to grow on some fields in 2022 through its On-farm Efficiency Conservation Program. That data was used to correct the identified crop when it disagreed with the district’s data.

Incorrect crop classification would cause a field’s estimated water use to vary according to the consumptive use fraction. For example, a field with evapotranspiration of 10 acre-feet in a given month would yield an estimate of 27 acre-feet applied to the field if it was classified as lettuce, but only 15 acre-feet if classified as sudan grass, a common summer cover crop. For that reason, fields classified as growing vegetables and leafy greens may be overestimated for some months if satellite sensing and records don’t accurately reflect crops switching during the year. Most of the acreage in question, however, does not see multiple plantings of distinct crops over the course of a year.

Figures may also vary depending on how accurately all of the inputs described above portray a farmer’s actual practices on a particular field, given that the estimates are based on averages for certain conditions. In cases when the crop identified disagreed with common practices in the region, such as a leafy green growing in the summer, the district’s average consumptive use fraction was used for that field instead.

Taken all together, the Propublica and Desert Sun estimates closely match the total water deliveries to agricultural users reported by the Imperial Irrigation District. Our estimates add up to about 2,372,000 acre-feet delivered by the district to fields in 2022. The district reported 2,369,000 acre-feet delivered in 2022 to agricultural users, about a 3,000 acre-foot, or 0.1 percent, difference across the entire valley.

Several experts, including Jay Famiglietti, who uses NASA satellite data to track global water depletion, told us our methodology was sound.

“(The analysis is) very solid, and it’s very physically based. It’s probably the best you’re going to do without direct access to data from IID,” Famiglietti said. “I don’t think that I could do a better job.”