# 4.28 - Monthly Drain File (*.dra)

The monthly drain file (*.dra) contains supplemental surface water, such as tailwater, drain flows, or off-channel reservoir supplies, that are not delivered through the river headgate, therefore not included in the historical diversion data file. This additional supply can be used to meet irrigation water requirements and excess can be stored in the soil moisture reservoir. The drain file can be used with any supply-limited consumptive use analysis. The drain file does not incur conveyance losses but the maximum application efficiencies read in the irrigation parameter yearly data file (*ipy) are used to estimate the portion of tailwater and/or drain flows available to the crop. The last column in the file contains the structure ID of the source of the drain water, if applicable; however this column is not used by StateCU. The file can include only structures with supplemental surface water sources. The file must contain data for the years defined in the model control file by nyr1 and nyr2 (see Section 4.4).

The drain file can also be used to offset non-irrigation diversions that are included in the historical water supply (*.ddh) file, for instance diversions to on-ditch reservoirs or recharge sites. Including the negative of these non-irrigation diversions (with or without conveyance loss) in the drain file is one way to make sure they are not available to meet crop consumptive use directly or to be stored in the soil zone and available to meet crop consumptive use in subsequent months. If negative values supplied in the drain file result in a farm delivery less than zero, farm delivery is set to zero internally and reflected in the output reports. Missing data is not allowed in the drain file. Using -999 in the ‘negative’ drain file (idrain= 2) will indicate an actual negative quantity of 999 ac-ft and will offset diversions accordingly. Values of -999 in the ‘positive’ drain file (idrain= 1) will be converted to zeros during the analysis simulation.

The drain file is only read if idrain= 1 or 2 in the model control file (1 ignores negative values while 2 allows negative values). An example is provided in Appendix A.

Row-Data Program Variable Description
Row 1 Format (6x, i4, 11x, i4, 7x, a3)
1-1 jyr1 Beginning year of diversion data
1-2 jyr2 Ending year of diversion data
1-3 idum3 CYR for calendar year or WYR for water year
Row 2 Format (i4, 1x, a12, 12(f8.2))
2-1 tyr Year
2-2 tid Structure ID
2-3 tail(1,tyr,1-12) Drain water for months 1-12(acre-feet)
2-4 Structure ID of the source of the drain water (not read by StateCU)

Repeat for the number of structures that receive drain water.

Repeat for each year jyr1 through jyr2.