Demand Considerations

StateMod allows demands to be set at the headgate or well structure as an irrigation water requirement at the irrigated land by month or by year (12 values repeated each year). The following are noted.

1. When a total demand is provided for a direct diversion structure the variable idvcom of the direct diversion station file (*.dds) should be set to 1 for monthly data and 2 for annual data. Similarly for a well structure the variable idvcomw of the well station file (*.wes) should be set to 1 for monthly data and 2 for annual data (annual data option for wells is reserved but not yet active). By providing total demand data, StateMod recognizes that a structures demand includes inefficiencies associated with conveyance and on-farm irrigation practices. The fate of inefficient water is controlled by the return flow data provided. This standard approach is recommended when wells are not part of an analysis.
2. When an irrigation water requirement is provided for a direct diversion structure the variable idvcom of the direct diversion station file (*.dds) should be set to 3 for monthly data and 4 for annual data. Similarly for a well structure the variable idvcomw of the well station file (*.wes) should be set to 3 for monthly data and 4 for annual data (annual data option for wells is reserved but not yet active). By providing an irrigation water requirement as demand data, StateMod recognizes that a structure's demand does not includes losses associated with conveyance and on farm irrigation practices. Therefore these adjustments are done within StateMod using the efficiency data provided in the direct diversion station file (*.dds) and the well station file (*.wes). The fate of inefficient water is controlled by the return flow data provided. This approach is recommended when wells are part of an analysis since the system efficiency associated with surface water and ground water are often significantly different.
3. When co-mingled supplies (surface and ground water sources) are used to meet a common demand, the control variable icondem of the control file (*.ctl) controls how demand data are provided to and treated by StateMod.
   • Historical Demand Approach. Set icondem = 1 to indicate surface water demands are provided in the diversion demand file (*.ddm), well demands are provided in the well demand file (*.wem) and no addition to determine a total structure demand occurs. This means that any surface water shortages cannot be supplied by ground water and vice versa. Also, the diversion and well station demand type variables (idvcom and idvcomw) are typically set to 1 or 3 which means monthly total demands (1) or monthly Irrigation Water Requirement demands (3) will be provided. Note this option is typically used during a historical model calibration when historical diversions and pumping are known or estimated from a StateCU scenario.
   • Historical Sum Demand Approach. Set icondem = 2 to indicate surface water demands are provided in the diversion demand file (*.ddm), well demands are provided in the well demand file (*.wem) and they are added together to determine a total structure demand. This means that any surface water shortages for a structure can be supplied by ground water and vice versa. The priority of the surface and ground water rights (limited by water right, capacity, etc.) dictates which source (surface water or ground water) will try to supply water. Also, the diversion and well station demand type variables (idvcom and idvcomw) are typically set to 1 or 3 which means monthly total demands (1) or monthly irrigation water requirement demands (3) will be provided in the monthly well demand file (*.wem). This option is typically used during calibration to quantify the impact of what occurs when priorities dictate the water supply source.
   • Structure Demand Approach. Set icondem = 3 to indicate one demand is provided for structures served by both surface and ground water in the direct diversion demand file (*.ddm). For well only lands demand is provided in the well demand file (*.wem). Similar to when icondem = 2, this means that any surface water shortages for a structure can be supplied by ground water and vice versa. The priority of the surface and ground water rights (limited by water right, capacity, etc.) dictates which source (surface water or ground water) will try to supply water. Also, the well station demand type variable (idvcomw) is typically set to 6 indicating demands will be provided in the direct diversion demand file (*.ddm) and no demand data are expected in the monthly well demand file for co-mingled structures. The diversion station demand type variable (idivcom) dictates if the data provided in the monthly demand file (*.ddm) is total demand or irrigation water requirement. This option is typically used during calibration and a baseline run when a structure's total demand is known but the mixture of surface water and ground water supplies is not.
   • Supply Demand Approach. Set icondem = 4 to indicate data are provided in the same way as when icondem = 3 (e.g. for co-mingled structures one demand is provided in the direct diversion demand file(s) (*.ddm) and for well only lands demand is provided in the well demand file (*.wem)). This method requires the variable efficiency method be operational (control variable ieffmax = 1). It allows surface water and ground water demands to operate somewhat independently. Like all demand options surface and ground water use under the Supply Demand Approach are dictated by the priority of each source and when diversion or pumping occurs the structures CIR is reduced as a function of the efficiency of the supply source. The Supply Demand Approach allows surface water to be diverted up to the user-supplied demand even if there is no CIR. Ground water is only allowed to pump when a CIR exists. This option is typically used during a calculated model calibration and a baseline run to better match historic operations. Its net effect is to 1. Divert surface water up to the user specified demand when available and in priority regardless of how efficient or inefficient that surface water will be used, and 2. Pump ground water only when there is a CIR. Note it operates most effectively in conjunction with the sprinkler option which allows a structure to pump preferentially on lands with sprinklers but still divert surface water to meet both CIR and recharge demands.
   • Decreed Demand Approach. Set icondem = 5 to indicate data are provided in the same way as when icondem = 3 or 4 (e.g. for structures with both a surface and ground water supply one demand is provided in the direct diversion demand file(s) (*.ddm) and for well only lands demand is provided in the well demand file (*.wem). This method requires the variable efficiency method be operational (control variable ieffmax = 1) and operates surface and ground water supplies exactly the same as when icondem = 4. In addition, the Decreed Demand Approach overrides demand data provided for structures with both surface and ground water supplies to equal the total of their surface water decrees if there is a CIR in that time step. Like the Supply Demand Approach, the Decreed Demand Approach 1. Allows surface water to be diverted up to the user-supplied demand (water rights) even if there is no CIR and 2. Allows ground water to be pumped only when a CIR exists. This option is typically used during a calculated model calibration and a baseline run to better match historic operations. Note it operates most effectively in conjunction with the sprinkler option which allows a structure to pump preferentially on lands with sprinklers but still divert surface water to meet both CIR and recharge demands.

Note that the Supply Demand Approach (icondem = 4) and Decreed Demand Approach (icondem = 5) could be extended to assist in determining when to use reservoir supplies (i.e. only make a reservoir release if a CIR exists).