# 3.2 - Water Supply Limited Crop Consumptive Use by Structure

If water supply information is available, the user can choose to determine water supply limited consumption on a monthly basis. The water budget routine is not available on a daily basis. If the scenario is based on a daily consumptive use method, then the daily potential consumptive use or irrigation water requirement is summed on a monthly basis and used to estimate monthly water supply limited consumption.

With a water supply limited consumptive use analysis, the user can use the model to account for storage in the soil moisture reservoir. The model calculation process for water supply limited consumptive use is the same with and without soil moisture accounting; if soil moisture is not considered, the capacity of the soil moisture reservoir is simply set to zero. The starting soil moisture can be initialized by the user or can be determined by running a presimulation analysis and setting initial soil reservoir contents based on ending presimulation contents.

The user can choose to account for winter carry-over precipitation in the water supply limited analysis. A user-supplied percentage of winter precipitation is stored in the soil reservoir, if capacity is available, and used directly to reduce irrigation water requirements during the irrigation season. Similar to effective precipitation during the irrigation season, it is not accounted for under the crop consumptive use category, as it is not an irrigation supply.

StateCU estimates monthly historical water supply available to the crops, based on historical diversion records, conveyance efficiency, and maximum irrigation application efficiency input for each structure. Maximum application efficiency is determined for each structure based on the acreage split between sprinkler and flood irrigation methods, defined by the user and described in Section 4.15. The water supply for the crop is compared on a monthly basis to the irrigation water requirement. When the water supply exceeds the potential crop needs, the crop requirements become the actual consumptive use, the excess supply is used to fill the soil moisture reservoir up to its capacity, and any remaining water is not used. When the water supply is less than the irrigation water requirement, the water supply plus additional available water taken from the soil moisture zone, becomes available for consumptive use. The soil moisture reservoir capacity is determined based on a user-supplied soil moisture capacity for each structure, in inches per inch, and the associated root depth of the crop types.

If lands are irrigated or supplemented with tailwater, drain flows, or other off-river supplies that are not included in historical diversion records, the user can include those supplies in the analysis by including a separate monthly drain flow file (*.dra). Similar to historical diversions, this supply can be used to meet irrigation water requirements directly and excess can be stored in the soil moisture reservoir. Crop supply from the drain file is limited by maximum application efficiency, but is not reduced by conveyance loss.

The drain file also can be used to ‘remove’ diversions that are included in the historical river supply but are not available for irrigation, such as diversions to on-ditch reservoirs or to recharge sites. Negative values in the drain file will reduce the amount of water available to the crop at the farm level.

Missing water supply is indicated by a –999 value in the historic water supply data file. If water supply is missing for a given month, then water supply limited calculations are not made for the year with missing water supply data. The program has several options to fill ‘on-the-fly’ missing diversion data or estimate water supply limited consumptive use through the proration concept. To determine ‘prorated’ water supply limited consumptive use, the ‘calculated’ shortages of consumptive use are summarized by Water District and by month and applied to the irrigation water requirements for structures with missing data. The historic supply estimated for a structure with missing diversion records is based on the ‘prorated’ water supply limited consumptive use divided by the use efficiency. Detailed water budget output produced by the program indicates whether water budget terms were ‘Calculated’ or ‘Prorated’. If there are no other structures in the scenario for that water district, StateCU sets missing surface water diversions to meet the full supply.

The user has options to fill ‘on-the-fly’ missing diversion data for use during analysis without modifying the historical water supply file or original input data files. The user can fill missing diversion records with the average monthly diversions based on data in the historic water supply file. Note that if there are no diversion data for a particular month through the entire analysis period, then a historic average can not be calculated and the missing data can not be filled. Also, note that there is not a minimum sample size for creating a historical average (i.e. a historical average could be calculated based on one data point and used to fill the entire record). The user can also fill missing diversion records with zeros.

The structure water balance accounts for the use of water supplies to satisfy irrigation water requirement ($$CU_i$$). The amount of water stored and used from the soil moisture reservoir and irrigation diversions are tracked on a monthly basis as follows:

When $$SW \geq CU_i$$:

When $$SW < CU_i$$:

where $$SW$$ is surface water available to the crop (river diversion * conveyance efficiency * maximum application efficiency); $$Cu_w$$ is water supply limited consumptive use; $$SS_m$$ is the maximum soil moisture reservoir storage; $$SS_i$$ is the initial soil moisture reservoir storage, $$SS_f$$ is the final soil moisture reservoir storage, $$SR$$ is the return from surface water not consumed by crops.