About the Data

California Water Watch brings together existing available data from various sources. Most of this data is not physically collected by DWR although in some cases we are a compiler of data provided to us by others. The data here thus vary in format, frequency of updates, and available statistical analyses. Specifics of individual datasets are described below.

As available from the original source of the data, statistics such as percent of average or percentile are used to characterize the current data. Since our different data sources have used different rankings to characterize their data, percentile categories vary among the datasets. The U.S. Geological Survey (USGS), for example, shows the 10th and 90th percentiles but then uses categories other than tenths for the values in between those points. In contrast the reservoir page uniformly breaks down percentiles by tenths. A percentile is a value below which a given percentage of the observations fall. For example, if an observation is in the 90th percentile it is higher than 90 percent of the other observations.

Precipitation and Temperature

DWR has acquired gridded precipitation and temperature data via a licensing agreement with Oregon State University for its Parameter-elevation Regressions on Independent Slopes Model (PRISM) dataset. Using gridded data (spatial data that can be analyzed in a Geographic Information System) allows incorporation of many observations from multiple climate networks and permits calculation of statistics over broad areas such as watersheds.

Entering an address or clicking on the map will bring up precipitation and temperature information for the corresponding HUC-8 watershed. Point values of precipitation and temperature, such as observations collected at an individual weather station, can vary significantly over very short distances because of factors such as the immediate surroundings of an individual station (e.g., the urban heat island effect), changes in ground surface elevation, or very localized thunderstorm activity. Smoothing out these localized variations over the scale of a watershed gives a more representative picture of hydrologic conditions. From the watershed page you can also select the option to see precipitation and temperature statistics at the larger regional and statewide scale.

Reservoir Conditions

Reservoir storage and elevation data come from DWR’s California Data Exchange Center (CDEC) website. CDEC contains information on 150-plus of the state’s largest reservoirs and compiles the reservoir conditions data provided to us by reservoir owners. Not all the reservoirs on CDEC have daily data; California Water Watch only serves information for the reservoirs with daily data. Clicking on a reservoir on the map will open a text box with information on reservoir storage. A small subset of the reservoirs has additional information as indicated by the “more details” link in the pop-up, allowing viewers to see historical graphs of reservoir elevation.


Clicking on a dot on the map will open a pop-up window for a USGS stream gauge. There are more than 400 stations in California that provide daily data and more than 500 stations in total available on California Water Watch. We provide USGS’ information for the larger number of stations, but the pop-up box will have streamflow data and statistics only for those serving daily data. The information provided in the pop-up box is exactly as USGS provides it. Links in the menu box on the left side of the screen direct the viewer to other sources of information, including a link to water temperature data where available.


Groundwater level data come from DWR’s Sustainable Groundwater Management Act (SGMA) Data Viewer. Most of the data on this site are provided by local agencies in response to requirements stemming from that legislation. Some of the data comes from measurements performed by DWR, primarily in northern California, and some is provided by USGS. The SGMA data are only required to be submitted for designated groundwater basins where most of the state’s groundwater use occurs. Almost no water level data are available for wells drilled in fractured bedrock areas such as the Sierra Nevada foothills because these areas are not covered by the SGMA data reporting requirement. Water levels in wells relying on fractured rock sources are highly variable over very short distances, making it impractical to support monitoring programs in these areas. Most of the groundwater level data is only available twice a year (spring and fall, times commonly taken to represent the annual maximum and minimum water levels). Simplified information on statewide water level conditions is available at DWR’s California Groundwater Live website which summarizes information for more than 6,000 wells.

Clicking on a dot on the California Water Watch website’s groundwater map will open a pop-up with available data for that well. Available data for individual wells varies widely. Some will have very few water level measurements while others may have sustained long-term measurements.


Snowpack water content is provided from snow sensors having daily data reported through DWR’s Cooperative Snow Surveys program and served on CDEC. Clicking on a sensor on the map will open a pop-up window providing the data on that sensor and comparing it to historical values. There are about 130 active sensors on CDEC. Owners of these sensors provide the data to DWR so that it can be widely distributed to people wishing to estimate the snowpack runoff that will contribute to local water supplies. Snowpack data are only available during the months when there is snow on the ground. Other sources of snowpack information come from periodic manual measurements at snow courses and from periodic aircraft-based and satellite-based remote sensing data collection and processing. Viewers interested in these other sources are encouraged to check the snow pages on CDEC.

Soil Moisture and Vegetation Conditions

Soil moisture and vegetation conditions are very different from the other types of data shown on California Water Watch in that they are indirectly estimated from other types of measurements. Although there are localized networks that directly measure soil moisture or some aspect of vegetation conditions, these sources are far too minimal to assess conditions over a state as large and climatically diverse as California. Instead, researchers have developed datasets through satellite-based measurements of factors that can be correlated to soil moisture or vegetation conditions. These datasets are mostly used for academic research and highly technical applications. Most California Water Watch viewers will want to use these maps only to gain a general understanding of relative conditions across the state.

The soil moisture data are from the National Aeronautics and Space Administration (NASA) and are an estimation of moisture in the top 100 centimeters of the soil. NASA updates the data every six hours. Vegetation conditions are represented by a NASA daily dataset of the evaporative stress index, a value that shows relative differences in the evapotranspiration of water by plants. The units of this index are the statistical measure of standard deviations from the mean.