SPDSS_Task59-1_Develop Locally Calibrated Blaney-Criddle Crop Coefficients

Home Browse Search

Table 2 Average Annual (1950 - 2003) PCU for Pasture Grass using Lysimeter-Derived Crop Coefficients vs. Standardized Coefficients with Elevation Adjustment Climate Station Lysimeter-Derived Coefficients Modified Blaney- Criddle w/Elev Adj Ratio (Elev Adj/Lysimeter) Antero 2.17 feet 1.33 feet 61% Bailey 2.52 feet 1.66 feet 66% As presented above, even with an elevation adjustment, use of the modified Blaney-Criddle method and standard TR-21 coefficients significantly understates (by over 30%) the estimated potential consumptive use of grass pasture when compared to lysimeter data. This is also shown in Figure 7.49 of Manual No. 70 where South Park lysimeter results are compared to several temperature-based ET methods (ASCE, 1990). It is recommended that lysimeter-derived coefficients from the South Park study by Walter et al. (Table 1 and Table 8 below) be used with the original Blaney-Criddle method for high altitude areas of the SPDSS, using the growing season trigger of 42 degrees mean temperature. 4.2 Plains Portion of the SPDSS Study Area The following approach was used to develop locally calibrated modified Blaney-Criddle crop coefficients for the plains area. • Identify and obtain available climate data. • Identify irrigated crop types throughout the SPDSS study area. • Compare the ASCE Standardized method and modified Blaney-Criddle method PCU estimates. • Adjust the modified Blaney-Criddle crop coefficients to represent the PCU results predicted using the ASCE Standardized method. Identify and Obtain Available Climate Data The average monthly temperature data and frost dates required for the modified Blaney-Criddle consumptive use method are readily available for long study periods from the National Climate Data Center through the National Oceanic and Atmospheric Administration (NOAA), who is responsible for operation of the stations, and are included in the State's central database, HydroBase. Daily maximum temperature, minimum temperature, wind speed, solar radiation, and vapor pressure data required for the ASCE Standardized calculations are primarily available from two sources throughout the SPDSS study area, the Northern Colorado Water Conservancy District (NCWCD) and the Colorado Agricultural Meteorological Network (CoAgMet). Longer periods of record are available for CoAgMet climate data. Three CoAgMet climate stations and one NCWCD climate station were selected for use in the calibration of modified Blaney-Criddle crop coefficients (Figure 4 and Table 3). Task59_l.doc (updated January 11, 2008) 8 of 32