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Spatial Model
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Daily reference evapotranspiration (ETo) at 2 km spatial resolution was 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 (Rs), air temperature (Ta), relative humidity (RH), and wind speed at two meters height (U2). These parameters are estimated at each pixel (2 km square grid) using various methods.
Daily Rs 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. Heliosat-II is a European model 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:
(http://www.helioclim.org/heliosat/heliosat2_soft_descr.pdf).
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Air temperature, relative humidity, and wind speed values at each pixel were obtained by interpolating between point measurements from the CIMIS stations. Two interpolation methods, DayMet and Spline, were selected based on accuracy of results, code availability, and computational efficiency. 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 http://www.daymet.org. 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.
Spline is an interpolation method that fits a surface through or near known points using a function with continuous derivatives. Two- and three-dimensional Spline is used based on which weather parameter is to be interpolated. The accuracies of the results obtained from both methods have been tested using cross-validation analysis.
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The accuracy of ETo values estimated from these methods depends on many factors. One such factor is the accuracy of the remotely sensed data, which 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. Another factor is the accuracy of the interpolation methods used. Interpolation methods are affected by the density of the weather stations and geographic features of the region. Since most of the CIMIS stations are concentrated in lowland agricultural and urban areas, the mountains are again more susceptible to errors resulting from data interpolation due to the low density of weather stations. Despite these potential problems, however, we believe the ETo estimates provided using this method will be superior to only using data from a distant weather station with a different microclimate.
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For detailed descriptions of the methodology used to map daily ETo, refer to a pdf file at (http://wwwcimis.water.ca.gov/cimis/pdf/EToMapping.pdf). If you would like to get ETo data created using the methods described here, you may do so by clicking on the Map Reports link on the left sidebar.
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We would like to inform CIMIS data users that this is a newly emerging technique and is in the process of being refined. Although the data quality presented here is acceptable for most applications, we do not guarantee its accuracy. Therefore, neither the CIMIS program, the Department of Water Resources (DWR), UC Davis, nor any other party who participated in the development of this product shall be responsible for errors in this data, and/or for any resulting consequences from using this data.
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