6.3 - Consumptive Use Component

The consumptive use component of the StateCU model performs the calculations required for estimating monthly potential consumptive use by the SCS TR-21 Blaney-Criddle method, the Original Blaney-Criddle method, and the Pochop method for bluegrass. Daily potential consumptive use can be calculated using the daily Penman-Monteith, ASCE Standardized Penman-Monteith, or the Modified Hargreaves methods. The program was written in FORTRAN for use on desktop computers. Analysis options and input files can be viewed and changed through the StateCU GUI. The user can create simulations to estimate consumptive use for individual structures or a combination of structures. Parameters used in the consumptive use analysis can be tied to climate stations, individual diversion structures, water districts, counties, or other defined locations.

Each simulation location (structure) is identifiable by acreage, crop types, available soil moisture capacity, and other crop characteristics. A maximum of 800 structures can be used in a simulation. A structure can have crop acreage that varies by year and the simulation period can be up to 100 years.

6.3.1 - General Options

The program provides six options for determining crop ET: SCS Modified Blaney-Criddle monthly estimation, Original Blaney-Criddle monthly estimation, Pochop monthly estimation, Penman-Monteith daily estimation, ASCE Standardized Penman-Monteith daily estimation, and Modified Hargreaves daily estimation. Other options provided by the program are for the effective rainfall method, incorporation of water supply information, and detail of output. When water supply information is incorporated, soil moisture accounting, supply accounting by priority, ground water use, and river depletions considering return flow parameters can also be activated. The length of growing season can be based on daily temperature minimum (28 or 32 degrees F), monthly mean temperature, or set by the user.

Several levels of output detail are available. ASCII matrices of potential consumptive use and water supply limited consumptive use, plus structure water budgets are available output from the StateCU FORTRAN program. Graphical output is also available through the GUI.

6.3.2 - Growing Season

The growing season of the crop potentially extends from frost to frost or from the last killing frost in the spring to the end of a definite period of time thereafter (USDA 1970). Actual planting and harvesting dates can be used to signify the beginning and ending of the growing season. The user-specified dates are evaluated in terms of the earliest and latest moisture use based on the spring frost temperature and killing frost in the fall, respectively. For example, if the planting date is earlier than the allowable date for moisture use, the allowable date is assumed to be the beginning of the growing season. If the harvesting date is after the latest date corresponding to the killing frost in the fall, the killing frost date is assumed to be the ending of the growing season. These are particularly important for annual crops. For perennial crops, the growing season can be completely determined by the frost dates. Therefore, the planting and harvesting dates for perennial crops should be specified as the first day (January 1) and the last day (December 31) of the year respectively. Frost dates are based on historical records and can be estimated using minimum daily temperature values. Note that SCS TR-21 recommends the use of actual frost dates (either 32 degrees Fahrenheit or 28 degrees Fahrenheit) of the use of mean monthly temperature to estimate frost dates, depending on crop.

6.3.3 - Crop Coefficients

The crop coefficient curves are converted to tabular values to make them readable by the program. For the Blaney-Criddle monthly method, crop coefficient values are in terms of percentage days of the growing season for annual crops; and day of the year for perennial crops. The table of crop coefficients for each crop should include a constant number of discrete pairs (day, \(k_c\)) of data points: 25 for perennial crops and 21 for annual crops. Normally, a crop coefficient value is assigned for every 5 percent of the growing season for annual crops; and at every 15 days for the year (starting from January 1) for perennial crops.

Crop coefficients for Winter Wheat are based on two separate curves – one that represents the fall portion of the growing season (WHEAT_FALL) and one that represents the spring portion of the growing season (WHEAT_SPRING). Currently, to represent both portions of Winter Wheat’s growing season, the user must include acreage for both crop types WHEAT_FALL and WHEAT_SPRING. For example, a farm with 100 acres of Winter Wheat should be modeled with 100 acres of WHEAT_FALL and 100 acres of WHEAT_SPRING. The user needs to be aware that even though the *.cds file shows 200 acres for the farm, the actual total acreage is 100. The resulting total consumptive use, however, accurately reflects that 100 acres planted with Winter Wheat, although the AF/ac column in both the *.cir and *.wsl should not be used directly. To accurately represent the consumptive use, a starting date (refer to the Colorado Irrigation Guide) must be specified for the fall portion in the crop characteristic file (*.cch), and an ending date must be specified for the spring portion.

For the Penman-Monteith and ASCE Standardized Penman-Monteith based ET estimation procedures, alfalfa-based mean crop coefficients are used. The Penman-Monteith crop coefficient curves are converted to tabular values. The \(k_c\) values are in terms of the percentage of the growing season. For alfalfa, different sets of crop coefficients can be assigned to the growing periods between the planting and the first cut; first cut to second cut; and between the second and third cut. For pasture, a set of crop coefficients is specified for the planting to harvest.

For the Modified Hargreaves ET estimation procedure, crop coefficients are in terms of the percentage of the growing season. The crop coefficient is a segmented linear coefficient.

6.3.4 - Climate Data

The Blaney-Criddle and Pochop based ET estimation procedures requires monthly mean temperature, monthly percentage daylight hours, and monthly total rainfall. The temperature and rainfall data are generally read from files generated by TSTool, a DMI that accesses the CDSS database, HydroBase.

The Penman-Monteith and ASCE Standardized Penman-Monteith based ET estimation procedures require daily climate data that includes minimum temperature, maximum temperature, vapor pressure, solar radiation, wind speed, and rainfall. The daily climate data files are be generated by the TSTool DMI.

The Modified Hargreaves ET estimation procedure requires daily climate data that includes average temperature, solar radiation, wind speed, and rainfall.

Individual structures may use different climate stations. Since climatic data are located at weather stations, each structure is assigned a set of weights associated with representative weather stations. A different set of weights can be assigned to both temperature and precipitation data.

ET can not be estimated for a year if temperature values are missing. Likewise, irrigation water requirement can not be estimated if precipitation values are missing. StateCU will report a missing value for every month (-999). Climate station data can be filled using several techniques through TSTool or filled with month averages during simulation, by setting the imiss flag in the control file (Section 4.4).

Climate data can be adjusted to represent a field location with a different elevation or in an area that receives a different amount of annual precipitation than the climate station location through the use of an orographic adjustment. For more information, see Section 3.1.7.