Spatial Overview - How it Works
The California Irrigation Management Information System (CIMIS) currently manages over 145 active weather stations throughout the state. Archived data is also available for 85 additional stations that have been disconnected from the network for various reasons. Most of the CIMIS stations produce estimates of reference evapotranspiration (ETo) for the station location and their immediate surroundings, often in agricultural areas. Because of California's diverse landmass and climate, many locations within the state lack a representative CIMIS station. Some counties, for example, do not have a CIMIS station and others have only one or two stations. As a result, there are significant spatial ETo data gaps, especially in urban areas. In an attempt to mitigate this problem, CIMIS initiated a study to investigate the possibility of coupling remotely sensed satellite data with point measurements from the CIMIS weather stations to generate spatially distributed ETo values (ETo maps).
A contract was awarded to the University of California Davis (UCD) remote sensing group, lead by Professor Susan Ustin, to conduct the study. The Department of Water Resources (DWR) formed an advisory committee comprised of individuals with expertise in remote sensing, GIS, modeling, and water management from DWR and UCD. The committee met, on an as needed basis to discuss new developments and plan future actions. After thorough research, the team decided to use combinations of data derived from satellites and interpolated from CIMIS station measurements to estimate ETo at a 2 kilometer (km) spatial resolution. The resulting product has been vigorously tested and has demonstrated a degree of accuracy that is acceptable for most irrigation applications. The CIMIS program will continue to evolve and expand to meet the future irrigation information needs of California. For a brief description of the methodology used to generate the ETo maps, see the Spatial Model discussion below.
Daily reference evapotranspiration (ETo) at a 2 km spatial resolution are calculated statewide using the American Society of Civil Engineers version of the Penman-Monteith equation (ASCE-PM). Required input parameters for the ASCE-PM ETo equation are solar radiation, air temperature, relative humidity, and wind speed at two meters height. These parameters are estimated for each 2 km pixel using various methods.
Daily solar radiation is generated from the visible band of the National Oceanic and Atmospheric Administration's (NOAA) Geostationary Operational Environmental Satellite (GOES) using the Heliosat-II model. This model is designed to convert images acquired by the Meteosat satellite into maps of global (direct plus diffused) irradiation received at ground level. The model has also been used with other geostationary satellites such as the GOES. For details on the Heliosat-II model and its accuracy, please refer to the HelioClim web page.
Air temperature, relative humidity, and wind speed values at each pixel were obtained by interpolating point measurements from CIMIS stations. Originally two interpolation methods, Spline and DayMet, were selected based on accuracy of results, code availability, and computational efficiency. Spline – the method currently used – is an interpolation method that fits a surface through or near known points using a function with continuous derivatives. Two- or three-dimensional Spline is used based on which weather parameter is to be interpolated.
The accuracy of both methods has been tested using cross-validation analysis, but DayMet is no longer used. DayMet is an interpolation method that was developed at the University of Montana to generate daily surfaces of temperature, precipitation, humidity, and radiation over large regions of complex terrain. It determines the weights associated with a given weather station for each point where weather parameters are to be determined depending on the distance and density of the stations.
The accuracy of ETo values estimated from these methods depends on many factors. For example, solar radiation remotely sensed through GOES is significantly affected by such factors as cloudiness and snow cover. Therefore, mountainous areas with snow cover and coastal areas with cloud and fog are more susceptible to errors. Also, interpolation accuracy is affected by the density of the weather stations and geographic features of the region. CIMIS stations are purposely placed in irrigated, open, flat areas – usually valleys – to provide the best reference data for adjacent farmlands and other irrigated areas. As a result, interpolation in valleys between CIMIS stations may not provide accurate data for mountainous terrain. Despite these potential problems, however, we believe the ETo estimates provided will be superior to only using data from a distant weather station with a different microclimate. For CIMIS station Siting criteria click the “Siting” tab.
For detailed descriptions of the methodology used to map daily ETo, refer to the RESOURCES navigation button https://cimis.water.ca.gov/Resources.aspx
If you would like to get ETo data created using the methods described here, you may do so by logging into your account, clicking on the SPATIAL navigation bar, then on the Spatial Report tab. If you do not have a CIMIS account, you can create one by registering with us. Please note that CIMIS data is provided free of charge and registration is required for statistical purposes only. https://cimis.water.ca.gov/Default.aspx
The California Irrigation Management Information System (CIMIS) is a program unit in the Water Use and Efficiency Branch, Division of Regional Assistance, California Department of Water Resources (DWR) that manages a network of over 145 automated weather stations in California. CIMIS was developed in 1982 by DWR and the University of California, Davis (UC Davis). It was designed to assist irrigators in managing their water resources more efficiently. Efficient use of water resources benefits Californians by saving water, energy, and money. https://cimis.water.ca.gov/
CIMIS reflects the energy that has arrived to drive water use by crops. It is the past. Now the National Weather Service has devised a way of forecasting water use, so that growers can be better prepared for irrigation. The question is how well does this new system work. Well, here are the results or a study that should lend confidence in the predictive value of the system.
COMPARISON BETWEEN FORECASTS OF REFERENCE EVAPOTRANSPIRATION and ETo VALUES CALCULATED USING DATA FROM DIFFERENT CLIMATIC CONDITIONS.
G. Ben Hamouda, F. Ventura, D. Zaccaria, R.L. Snyder, K.M. Bali.
Evapotranspiration is the transfer of water from the earth's surface to the atmosphere. It comprises the sum of water losses to atmosphere due to the processes of evaporation of moisture from soil, water bodies and wet plant canopies, and the transpiration of water from plants. Forecasts of this crucial component of the hydrologic cycle can be very valuable for growers, farm managers, irrigation practitioners, water resource planners and managers, and reservoir operators for their planning, allocation, delivery and scheduling decisions, as well as to hydrologic scientists for research purposes. Verifying the reliability of models' forecasts is among the critical tasks for development and performance evaluation of physical models. In fact, the verification allows understanding the models' behavior under different climatic conditions, and evaluating their applicability and dependability.
The US National Weather Service (NWS) has released a product that provides forecasts of reference evapotranspiration (FRET) at 2.5-km grid resolution for the entire continental US. In this study, a comparison is made between ETo estimates from FRET and ETo values calculated by the California Irrigation Management Information System (CIMIS) for 68 days during summer 2019. Both the FRET forecasts and ETo values were obtained from NWS and CIMIS, respectively, on the basis of 15 CIMIS locations that are representative of different climatic conditions in California. In addition, air temperature, dew point temperature, relative humidity, wind speed, and vapor pressure deficit (VPD) data were also collected/calculated from the NWS and CIMIS websites to analyze the sensitivity of FRET forecasts to predictions of these parameters. All FRET forecasts were performed with timescales of 1, 3, 5 and 7 days. Statistical indices were calculated to assess the dependability of FRET values. They showed a good correlation of the FRET model outputs with CIMIS ETo data, with some differences depending on the climatic characteristics of selected weather stations' locations, suggesting that FRET data could be valuable for anticipating near-future water demand and improve irrigation management in California.
Look for a more detailed description of this study in a forthcoming edition of the Journal of Irrigation and Drainage Engineering – ‘Evaluation of Forecast Reference Evapotranspiration (FRET) for Different Microclimates Regions in California to Enable Prospective Irrigation Scheduling' - https://ascelibrary.org/journal/jidedh
FRET is available at:
Advances in Citrus Water Management Virtual Workshop
September 9-10, 2021
Virtual Workshop via Zoom. Zoom information will be sent to registrants closer to the event.
Objective: Join UC Irrigation Specialists Daniele Zaccaria and Khaled Bali, and other University of California experts in this 2-day on-line Workshop to learn about the latest research and advances in Citrus irrigation. Topics include: Citrus water relations, evapotranspiration and crop coefficients, soil-water budgeting, soil characterization and mapping, tree water status, remote sensing, micro-irrigation systems' evaluation, interpreting results of water analysis, and strategies for Citrus irrigation management under limited water supplies. A set of irrigation-related methods, technologies, and products to help schedule irrigation will also be presented during the Workshop. Registration includes participation to the two sessions and access to the recorded workshop presentations. You can view a current event agenda here.
Intended audience: citrus growers and farm managers, representatives from the citrus production industry, irrigation consultants and practitioners, CCAs, CPAs, water resource managers, water conservation districts' personnel, irrigation districts' managers, extension specialists and advisors, professional researchers, university students, and representatives from state agencies.
Two additional RAs were added to the NMP implementation schedule as part of the October 2020 Water Quality Management Plan update: RA 11 (McGrath Lake Coastal) and RA 16 (Boulder Creek). Both are shown in green with a deadline of the end of April 2021.
Note: This workshop is intended for growers, and their representatives, who are looking to learn more about nitrogen management planning and/or intend to self-certify their plans. If you are only looking to fulfill the annual 2-hour continuing education requirement, there is an event approved for education credit on April 16 (below) and additional events will be announced as they are scheduled throughout the year.
**6 Hours of VCAILG CEUs pending Regional Board approval
Please register for Nitrogen Management Plan Self-Certification Webinar on Tuesday and Wednesday, November 17/18, 2020 9:00 AM - 12:00 PM PST at:
This workshop is intended for growers, and their representatives, who are looking to learn more about nitrogen management planning and/or intend to self-certify their plans. It is also a good education on how nitrogen works in our environment and how it can be managed. The program is sponsored by CA Department of Food and Agriculture, University of CA Cooperative Extension and the Ventura County Irrigated Lands Group.
Attendees must participate in both sessions to receive education credit and qualify to take the online certification test after the final webinar session.
After registering you will receive a confirmation email containing information about joining the training.
This workshop will open a half hour early at 8:30am, to allow attendees to test their connection and access the GoTo Training webinar link.
Email organizer: firstname.lastname@example.org
Image of nitrogen deficient avocado leaf on left