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3.1 - Crop Irrigation Water Requirement by CU Location

Monthly crop evapotranspiration (ET), or potential consumptive use (PCU), can be determined at a specific location (climate station, structure, farm, etc.) using the Soil Conservation Service (SCS) Blaney-Criddle method outlined in Irrigation Water Requirements Technical Release 21 (SCS TR-21, 1970), the FAO-24 original Blaney-Criddle method (FAO, 1977), or the Pochop Method for bluegrass outlined in Elevation – A Bias Error in SCS Blaney-Criddle ET Estimates (ASAE, 1984). Daily ET can be determined using the Penman-Monteith, ASCE Standardized Penman-Monteith, or Modified Hargreaves approach. The potential consumptive use for a CU location is determined based a unit acreage and corresponding crop type assigned to a climate station (Climate Station Scenario) or an actual acreage and crop types associated with a structure (Structure Scenario).

The potential crop consumptive use estimates can be reduced by an amount of monthly or daily precipitation considered effective at serving crop needs to determine the irrigation water requirement. In the case of a monthly consumptive use method, only monthly effective precipitation methods are available. With a daily consumptive use method, both monthly and daily effective precipitation methods are available.

3.1.1 - Monthly Potential Consumptive Use - SCS TR-21 Modified or Original Blaney-Criddle Method

StateCU allows either the SCS TR-21 modified Blaney-Criddle or the original Blaney-Criddle procedure to estimate monthly evapotranspiration (ET). The empirical equation relates ET with mean air temperature and mean percentage daytime hours. The SCS TR-21 method was modified from the original Blaney-Criddle method to reasonably estimate short-period consumptive use. The modifications include the use of (1) climatic coefficients that are directly related to the mean air temperature for each of the consecutive short periods which constitutes the growing season and (2) coefficients which reflect the influence of the crop growth rates on consumptive use rates (SCS TR-21).

The basic relationship assumes that ET varies directly with the products of mean monthly air temperature and monthly percentage of annual daylight hours for an actively growing crop with adequate soil moisture. This is expressed mathematically as:

Where \(U\) is the estimated ET in inches for the growing season; \(K\) is the empirical consumptive use coefficient for the growing season; \(F\) is the sum of monthly consumptive use factors for the growing season; \(k\) is the monthly consumptive use crop coefficient by month and crop; \(u\) is the monthly consumptive use of the crop in inches; and \(f = t p/100\), where \(t\) is the mean monthly air temperature and \(p\) is the mean monthly percentage of annual daytime hours.

The SCS TR-21 modification includes the use of a composite climatic crop coefficient:

where \(k_t = 0.0173t – 0.314\) with the modified Blaney-Criddle method and \(k_t = 1\) with the original method; and \(k_c\) is the coefficient reflecting the growth stage of the crop. The values are obtained from the crop growth stage coefficient curves prepared for each crop. Examples of crop curves for 25 crops are provided in SCS TR-21. - SCS TR-21 Modified or Original Blaney-Criddle Method with ET Elevation Adjustment

The ASCE Manuals and Reports on Engineering Practice No. 70, Evapotranspiration and Irrigation Water Requirements (1990), recommends elevation adjustments for both the SCS and the FAO-24 Blaney-Criddle methods of 10% adjustment upward for each 1,000 meters increase in elevation above sea level. The adjustment corrects for lower mean temperatures that occur at higher elevations at a given level of solar radiation (i.e. mean temperatures do not reflect crops’ reactions to warm daytime temperatures and cool nights). The adjustment is applied to the potential consumptive use estimate and can be applied to any crop type. Note, however, that if locally calibrated crop coefficients are used in lieu of standard SCS or FAO-24 crop coefficients, the mean temperature correction is represented, and an additional elevation adjustment should not be applied. - SCS TR-21 Modified or Original Blaney-Criddle Method with ET Elevation Adjustment

StateCU also allows the use of the Pochop Method for bluegrass (lawn grass). StateCU applies the recommended modifications to the SCS Blaney-Criddle formula to calculate bluegrass potential consumptive use with the Pochop Method. The original research by Pochop, also described a methodology for adjusting alfalfa. Based on discussions with Pochop, the Colorado Division of Water Resources no longer applies this methodology to adjust the calculated PCU for alfalfa and this method should only be used with bluegrass. The recommended crop naming convention to distinguish a crop with the Pochop Method is BLUEGRASS.POCHOP.

The three modifications included in StateCU, as recommended for the Pochop Method are as follows:

  1. Temperature Coefficient – The bluegrass temperature coefficient (\(k_t\)) in English units is calculated as: where \(t = \) temperature in degrees F.

  2. Crop Growth State Coefficients – Following are the calibrated bluegrass crop growth stage coefficients \((k_c)\):

    Month Coefficient \((kc)\)
    April 0.97
    May 1.00
    June 1.10
    July 1.06
    August 0.98
    September 0.97
    October 0.89
  3. Elevation Adjustment – The base elevation for the bluegrass crop growth stage coefficients shown above is 4,429 feet (1,350 meters). The Pochop Method recommends elevation adjustments to account for increased or decreased evapotranspiration at elevations above or below the ‘base’ elevation as follows:

    April, May, and September:

    2.9% per 1,000 feet (9.4% per 1,000 meters) above or below the base elevation

    June through August:

    2.3% per 1,000 feet (7.6% per 1,000 meters) above or below the base elevation

    The Pochop Method includes at elevation adjustment in the equation, therefore an additional elevation adjustment should not be applied.

3.1.2 - Monthly Effective Precipitation - Irrigation Season Precipitation

Effective precipitation (i.e. effective rainfall, \(R_e\)) is the component of precipitation that is available to meet consumptive use. It does not include precipitation lost through runoff and deep percolation below the root zone. StateCU estimates the monthly effective precipitation using one of two methods. The first method is based on the SCS methodology in which effective precipitation is dependent on the net depth of application and average monthly consumptive use. It is expressed mathematically as follows:

where: \(R_e\) is the average monthly effective precipitation; \(R_t\) is the total monthly precipitation; \(c_u\) is the average monthly consumptive use; \(F\) is a function of \(D\); and \(D\) is the net depth of application \((F = 0.531747 + 0.295164(D) – 0.057697(D^2) + 0.003804(D^3))\). It should be noted that \(R_e\) can not exceed average monthly precipitation or \(c_u\). When this happens, \(R_e\) is set equal to the lesser of the two.

The second method is based on the United State Bureau of Reclamation (USBR) methodology in which the effective precipitation (\(R_e\)) is linearly related to the monthly precipitation (\(R_t\)). Different linear relationships are used for different ranges of precipitation. They are expressed as follows:

Monthly effective precipitation methods can be used with both monthly and daily consumptive use analyses. When a monthly effective precipitation method is used in daily consumptive use analyses, StateCU requires a daily precipitation file that is summed on a monthly basis by the program and then applies either the SCS or USBR effective precipitation calculations depending on the user input. - Non-Irrigation Season (Winter) Precipitation

The StateCU soil moisture accounting system allows the user to select the option to add a percentage of total winter (non-growing season) precipitation to the soil moisture ‘reservoir’ so that it is available for the crops to use at the beginning of the growing season. Total winter precipitation volume is calculated as total nonirrigation season precipitation times that year’s irrigated acreage, reduced by the user-defined effective percentage. These volumes are further reduced, as necessary, so their total will fit into the available capacity of the soil moisture reservoir (total capacity less water stored during previous irrigation season).

In this context ‘winter’ is defined as the non-growing season for the entire collection of crops under the structure, i.e. when the calculated potential ET for the structure is zero. Since crop seasons vary and structures often have multiple crops, this assures that there is not winter carry-over in the soil reservoir under some crops in the same month that other crops might be using soil moisture. It also assures there is no double accounting of precipitation distribution – precipitation can not be effective in meeting crop demands under a ditch at the same time it is being stored in the soil zone as winter carry-over.

In the first month of each year that potential ET for the collection of crops under a structure is greater than zero, the accumulated winter precipitation soil moisture volumes from the previous months are available and are used to reduce the irrigation water requirement (IWR) of the crops under the structure. This is done before using other sources (surface water diversions, other soil moisture, or ground water pumping). At this point in the calculations the IWR has already been calculated as the crop potential ET minus the current month’s effective precipitation.

If the current month’s surface water diversions are in excess of what is needed to meet the remaining IWR, that water (at the same application efficiency) is used to fill empty space in the soil moisture reservoir. If the soil moisture reservoir is full and there is still winter precipitation carry-over in the soil, it will be replaced (i.e. ‘pushed out’) by the surface water diversions to the extent necessary. This common approach maximizes the use of diverted water to meet crop consumptive use.

3.1.3 - Daily Penman-Monteith Crop ET

The Penman-Monteith method for determining daily reference crop ET uses minimum and maximum temperature, vapor pressure, solar radiation and wind run data. Reference crop ET for alfalfa is the rate at which water, if available, is removed by the processes of evaporation and transpiration from soil and plant surfaces expressed as the depth of water used by a standard reference crop per unit time.

The Penman-Monteith equations used to determine daily ET are from the ASCE Manual No. 70 Evapotranspiration and Irrigation Water Requirements (ASCE-70) for alfalfa as follows:

where \(\lambda ET_r\) is the vapor flux density for the base reference in mega joules per day per area in meters squared \([MJm^{-2}d^{-1}]\) of alfalfa; \(R_n\) is the net radiation flux density to the plant canopy \([MJm^{-2}d^{-1}]\) ; \(G\) is the soil heat flux density \([MJm^{-2}d^{-1}]\) ; \(\Delta\) is the slope of the saturation vapor pressure curve \([kPa(^oC)^{-1}]\); \(\gamma\) is the psychometric constant \([kPa(^oC)^{-1}]\); \(e_a\) and \(e_d\) are the saturation vapor pressures at the current and dew-point air temperature, respectively \([kPa]\); \(\rho\) is the atmospheric density \([Kgm^{-3}]\); \(\lambda\) is the latent heat of vaporization \([MJKg^{-1}]\); and \(U\) is the wind speed \([ms^{-1}]\).

The daily ET of a particular crop is estimated from the expression:

where \(k_c\) is the experimentally derived crop coefficient. The crop coefficients are empirical ratios of specific crop ET to the ET of the reference crop. The \(k_c\) is derived from control experiments and its value depends on the crop’s growth stage, crop canopy characteristics, and surface soil moisture conditions. The same reference ET values should be used to estimate the ET as were used in developing the \(k_c\) values. It should be noted that the monthly crop coefficients developed for the SCS TR-21 Blaney-Criddle method should not be used for Equation 14.

3.1.4 - Daily ASCE Standardized Penman-Monteith Crop ET

The Penman-Monteith method was ranked as the number one (most accurate) method for potential consumptive use determinations in a detailed review presented by Jensen (1970). The primary required inputs to the 1965 implementation of the Penman-Monteith equation include temperatures (maximum and minimum air, and dewpoint), solar radiation, vapor pressure and wind speed. Jensen describes many secondary equations (i.e. air density, latent heat of vaporization, canopy resistance) used in the PenmanMonteith calculation. The result is a very technical and complicated calculation process to arrive at a Penman-Monteith consumptive use value.

In 1999, the Irrigation Association requested a technical committee of the American Society of Civil Engineers (ASCE) that is involved with evapotranspiration to propose one standardized equation and set of procedures for calculating evapotranspiration. The goal was a consistent model that would have wide acceptance. A subcommittee (task force), chaired by Ivan Walter of Colorado, initiated meetings and debates to arrive at a standardized equation. The result of this process was that two equations (one for a short crop named \(ET_o\) and one for a taller crop named \(ET_r\)) were developed. These equations were based on framework suggested in a FAO56 (Allen, 1998) document that assumed a constant for the psychometric constant, simplified the air density term and simplified the vapor aerodynamic resistance term. For each type of reference crop (short or tall), constants were assigned in the ASCE Standardized Penman-Monteith equation for the vegetation height, latent heat of vaporization, and surface resistance. The resulting equation for ET (for both short and taller crops) is:

Where \(ET_{sz}\) is the standardized reference crop evapotranspiration for a short or tall reference crop, \(\Delta\) is the slope of saturated vapor pressure curve, \(R_n\) is the net radiation flux, \(G\) is the sensible heat flux into the soil, \(\gamma\) is the psychometric constant, \(C_n\) is the numerator constant for the reference crop type and time step (see table below), \(T\) is the temperature (usually daily mean air temperature), \(U_2\) is the wind speed, \({e_s}^o\) is the mean saturated vapor pressure, \(e_a\) is the mean daily ambient vapor pressure, \(C_d\) is the denominator constant for the reference crop type and time step (see table below).

Values of \(C_n\) and \(C_d\):

Calculation Time Step Short Reference Crop \((ET_{os})\) Tall Reference Crop \((ET_{rs})\)
\(C_n\) \(C_d\) \(C_n\) \(C_d\)
Daily 900 .34 1600 .38
Hourly, daytime 37 .24 66 .25
Hourly, nighttime 37 .96 66 1.7

The units of the variables in the ASCE Standardized Penman-Monteith Equation and more detail on the calculation process can be found in the manual and associated appendices for the ASCE Standardized Penman-Monteith equation available from

While the ASCE Standardized Penman-Monteith equation remains complicated, considerable simplification and standardization has been performed compared to previous versions of the Penman-Monteith equation. Real life performance tests reported at an April 2004 seminar (Colorado Bar, 2004), demonstrate good agreement between results using the ASCE Standardized Penman-Monteith equation and measured evapotranspiration. This equation appears to have the support of most of the experts in crop consumptive use calculations. StateCU supports the ASCE Standardized Penman-Monteith methodology for tall (alfalfabased) crops.

3.1.5 - Daily Modified Hargreaves Crop ET

StateCU uses a Modified Hargreaves radiation method to estimate ET that was developed by Agro Engineering, Inc. This method was specifically developed for the San Luis Valley and should be used with caution in other areas. The original Hargreaves method used temperature and radiation data to estimate ET for an Alta fescue grass crop. The Modified Hargreaves approach includes a wind function to recognize the advective transfer of water away from plant stomatal openings under windy conditions as follows:

where \(R_s\) is the incoming short wave solar radiation in langleys \([cal/cm^2/day]\) and \(T_{avg}\) is the average daily temperature in degrees Fahrenheit. The \(1498.6\) term represents the latent heat of vaporization at 55 degrees Fahrenheit multiplied by the density of water. The latent heat of vaporization term converts the solar radiation from langleys to inches of water per day. The following wind function, \(F\), is used in the model. Note that this function was developed for the San Luis Valley, and may not be appropriate for other areas.

where \(U_2\) represents the wind run at a two meter height in miles per day. The wind function has units of \([^oF^{-1}]\).

A crop coefficient is used to convert the reference ET into the actual ET used by the crop. The crop coefficients are a function of crop variety, canopy development, and stage of growth. The actual ET is calculated as follows:

where \(K_c\) is the crop coefficient for a crop growing under conditions of optimum fertility and soil moisture and achieving full production and water use potential. The crop coefficient is calculated as follows:

where \(D\) is the current day of the year, \(D_{10\%}\) is the date of 10 percent cover, \(D_{cover}\) is the date of effective full cover, \(D_{mature}\) is the date of the start of maturity, and \(D_{harvest}\) is the date of harvest. Note that for alfalfa and pasture grass, \(D\) can exceed the harvest date. \(K1\), \(K2\), and \(K3\) are the values of the crop coefficient at 10 percent cover, effective full cover, and harvest respectively.

3.1.6 - Daily Effective Precipitation

Daily effective precipitation (i.e. effective rainfall, \(R_e\)) may be estimated in StateCU using three methods that can be selected by the user. Daily effective precipitation methods can only be used with daily consumptive use analyses. The first method uses a user-specified maximum effective precipitation in inches per day. In this method the effective precipitation (\(R_e\)) is equal to the total daily precipitation (\(R_t\)) if \(R_t\) is less than or equal to the user-specified maximum effective rainfall. If \(R_t\) is greater than the user-specified maximum, the \(R_e\) is assumed to be equal to the user-specified maximum effective rainfall.

The second method estimates a fixed percentage of total rainfall as effective as follows:

where \(R_e\) is the daily effective precipitation; \(F\) is a user-specified factor with the value ranging from 0 to 1, and \(R_t\) is the daily total rainfall.

The third method is based on the SCS NEH4 method for estimating direct runoff from storm rainfall (SCS NEH, 1964). Effective precipitation is calculated from the difference of the total daily precipitation and the estimated runoff. The direct runoff is estimated from the following expression:

where \(R_o\) is the runoff; \(P\) is the potential maximum runoff; and \(S\) is the potential maximum abstraction. The potential maximum runoff can be assumed equal to the total precipitation of the day. The \(S\) value can be calculated from the expression:

where the parameter \(CN\) is the runoff curve number or hydrologic soil cover complex number. The three required \(CN\) values, provided to StateCU by the user, represent \(CN\) values for antecedent moisture conditions I, II, and III estimated based on the soil type, and the land use and treatment classes. StateCU determines which antecedent moisture condition applies based on the total precipitation in the 5-day period preceding the storm.

3.1.7 - Climate Station Weighting and Orographic Adjustments

Climate station data can be combined and ‘weighted’ to better represent the location of irrigated lands. StateCU allows the user to assign up to five climate stations and corresponding weights factors. If the location of irrigated lands is midway between two climate stations, the user can select both climate stations with a weight of 50 percent each. For each month, the temperature used in the ET calculation will be 0.50 times the first climate station plus 0.50 times the second climate station. The precipitation data can also be weighted for the effective precipitation calculation.

An orographic adjustment can be used to adjust climate station data to the location of the irrigated lands. This type of adjustment is commonly applied when irrigated lands are at a location that varies significantly from nearby climate stations. Data from multiple climate stations can also be weighted then used in the adjustment, for instance in areas with better climate station coverage.

StateCU currently allows an orographic adjustment only with the monthly consumptive use methods (Modified Blaney-Criddle, Original Blaney-Criddle, or Pochop). There is a separate adjustment factor for temperature and precipitation data. The temperature adjustment is based on a user-specified degree Fahrenheit per 1,000 foot rise in elevation between the structure elevation and the climate station elevation. If the climate station is located at a lower elevation than the structure, then the temperature values are adjusted downward. If the climate station is located at a higher elevation than the structure, then the temperature values are adjusted upward. The precipitation adjustment is based on a user-specified ratio that is used to adjust the total precipitation data at the climate station. Commonly used adjustments are as follows:

Temperature Adjustment - Adjust the temperature down by 3.6 degrees per 1,000 feet rise in elevation (based on the standard meteorological Environmental Lapse Rate).

Precipitation Adjustment - Using annual precipitation maps, compute the ratio of the annual precipitation at the location of the irrigated acreage divided by the annual precipitation at the climate station, then multiply the monthly values at the climate station by the ratio to estimate monthly values at the irrigated lands.

When the orographic adjustment is initially selected through the StateCU GUI, the GUI displays the default temperature adjustment of 3.6 degrees Fahrenheit per 1,000 feet and the default precipitation adjustment of 1.0 (no adjustment). These default adjustments can be changed and saved through the GUI.

Note that orographic adjustment is independent of the ET elevation adjustment and may be applied to temperature data that is then used in the Modified or Original Blaney-Criddle method with elevation adjustment. A separate elevation adjustment is already built into the Pochop method.

3.1.8 - Crop Coefficients

StateCU allows user-specified crop coefficients to be entered and applied with any of the PCU methods. The recommended crop naming convention to distinguish between crop coefficients is CROP_NAME.XXXX where the ‘XXXX’ extension describes the type of coefficients. StateCU automatically recognizes the following crop coefficient identifiers associated with crop coefficients from the original ET methods or calibrated crop coefficients developed and documented under CDSS efforts and other planning efforts:

Source Identifier CU Method Description
SCS TR-21 CROP_NAME.TR21 Modified B-C SCS TR-21 crop coefficients
SPDSS Calibrated Coefficient\(^1\) CROP_NAME.CCUP CROP_NAME.CCLP Modified B-C Upper South Platte and Lower South Platte coefficients developed by calibrating modified Blaney-Criddle to the ASCE Penman-Monteith method
RGDSS Calibrated Coefficient\(^2\) CROP_NAME.CCRG Modified B-C developed by calibrating modified Blaney-Criddle to the Daily Modified Hargreaves method
Denver Water South Park Study Calibrated Coefficients\(^3\) CROP_NAME.DWHA Original B-C high altitude grass pasture coefficients developed for Denver Water by calibrating lysimeter data to the original Blaney-Criddle method
Calibrated Coefficients for the Upper Gunnison Subordination Report CROP_NAME.UGHA Original B-C high altitude grass pasture coefficients developed and used by the UGRWCD for the 'Subordination of the Wayne N. Aspinall Unit Water Rights within the Upper Gunnison Basin' annual report
Pochop Method CROP_NAME.POCHOP Pochop bluegrass crop coefficients developed for use with the Pochop Method
ASCE Standardized Penman-Monteith CROP_NAME.ASCEPM ASCE Std P-M ASCE Standardized Penman-Monteith crop coefficients
Agro Engineering Inc. CROP_NAME.MHG Modified Hargreaves Daily Modified Hargreaves crop coefficients

\(^1\)The Upper South Platte calibrated coefficients were developed for use in Water District 1, 2, and the lower portions of Water Districts 3, 4, 5, 6, 7, 8, and 9 (below 6,500 feet). The Lower South Platte calibrated coefficients were developed for use in Water District 64. See SPDSS Task 59.1 technical memorandum for more information.

\(^2\)See RGDSS Historic Crop Consumptive Use Analysis report.

\(^3\)The high altitude portion of the SPDSS study area is defined as areas west of the foothills (above 6,500 feet) including Water Districts 23, 47, 48, 76, and 80 and the upper portions of Water Districts 3, 4, 5, 6, 7, 8, 9. See SPDSS Task 59.1 technical memorandum for more information.

StateCU also recognizes other crop coefficient identifiers, although the user must appropriately indicate the consumptive use method using the flag in the control file (flag1 in the *.ccu file) and in the case of a monthly consumptive use analysis, the flag in the crop coefficient file (ktsw in the *.kbc file).

There are two common approaches to developing ‘locally calibrated coefficients’. The first approach involves developing calibrated crop coefficients using data from local studies. This method has been predominantly applied to high altitude irrigated grasses based on lysimeter studies but could be developed for any crop type. Locally calibrated crop coefficients should be applied with the PCU equation under which they were originally developed.

The second approach involves calibrating coefficients based on a more accurate but data-intensive method and then applying the calibrated, less precise method back in time. This approach can be valuable at locations where the data required for the more precise method is limited while the data for the less precise method is more complete. For example, locally calibrated coefficients can be developed using a daily PCU method (i.e., ASCE Standardized Penman-Monteith) then adjusting the SCS Blaney-Criddle coefficients. The daily method is applied with daily data for the same period as the SCS Blaney-Criddle method with monthly data. The ratio of the average monthly PCU from the daily method to the average monthly PCU from the SCS Blaney-Criddle method is used to adjust the SCS Blaney-Criddle coefficients until the average monthly PCU estimates match. The calibrated SCS Blaney-Criddle coefficients are then applied with the SCS Blaney-Criddle method back in time. This method can be applied to any crop type.