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How to Simulate Soil Moisture Accounting and Variable Efficiency

StateMod has the ability to store in the soil reservoir and subsequently use soil moisture as a water supply. Additionally, StateMod has the ability to simulate under variable efficiency, whereby the model allows irrigation efficiency to vary from zero to a user-specified maximum value. These two functions are generally used together, and the soil moisture function requires the variable efficiency option be used.

The soil moisture option allows diverted water to be stored in the soil zone up to its defined capacity considering the diverting structures (direct diversion or well) efficiency. It uses an operating rule to specify an administration date that controls when water is available to be taken out of the soil zone to satisfy a consumptive demand. StateMod initializes the soil moisture reservoir contents to be 50% of the soil moisture capacity.

The variable efficiency option allows the model to vary the efficiency in which it meets a demand. For example, variable efficiency will operate at the maximum efficiency when a demand is water-short, but a lower efficiency would be used when a system is water-long. The following notes should be considered with variable efficiency:

  • Variable efficiency uses the Modified Direct Solution Algorithm and can be used with or without soil moisture storage.
  • When variable efficiency is used, the efficiency data provided in the diversion station (*.dds) file and well station (*.wes) file are ignored for structures with irrigation demands provided in the irrigation water requirement file (*.iwr).
  • Variable efficiency capability calculates the maximum system efficiency for a diversion to be the conveyance efficiency times the maximum flood efficiency provided in the annual time series file (*.ipy).
  • Variable efficiency capability calculates the maximum system efficiency for a well to be the maximum flood efficiency or maximum sprinkler efficiency provided in the annual time series file (*.ipy). The control file variable isprnk controls whether flood or sprinkler efficiency data are used. Sprinkler efficiency is used preferentially up to the acres served by sprinklers. Thereafter, any remaining acres served by wells are served by using the maximum flood efficiency.
  • Variable efficiency capability applies to all direct diversion, well pumping and carrier to diversion structure operations if irrigation demands provided in the irrigation water requirement file (*.iwr).

The following steps in sections 7.3.1 and 7.3.2 are recommended to implement soil moisture accounting and variable efficiency, respectively.