Unsaturated Zone Water Storage

In the irrigated cells of the Lower East Coast, the volume of water in the unsaturated zone is modeled as illustrated in Figure 12. There are several methods for unsaturated zone accounting, the SFWMM model uses the following method in the Lower East Coast Developed Area. The water balance equation for the unsaturated zone is defined as

S = INFILT + NIRRSUPTOT - ETIU - ETNU - PERC,

where INFILT is the infiltration from ponding to the unsaturated zone, NIRRSUPTOT is the net irrigation from the public water supply and all irrigation use types, ETIU is the evapotranspiration from the irrigated unsaturated zone, ETNU is the evapotranspiration from the non-irrigated unsaturated zone, and PERC is the percolation from the unsaturated zone to the saturated zone (water table aquifer). During water restrictions, the unsaturated zone water balance equation is adjusted by the amount of the cutback, ETIUADJ.

S = INFILT + NIRRSUPTOT - ETIUADJ - ETNU - PERC.

The ET/Recharge Model or the SFWMM computes all the terms except for the PERC and S. It is important that an accurate estimate of the PERC value be made because percolation affects the recharge of the water table aquifer. The SFWMM uses an iterative definition for accounting of the unsaturated zone storage based on the fact that percolation occurs when the soil moisture exceeds the upper limit of the soil's field water storage capacity. This upper limit, SWMAX, is available from the Agricultural Field Scale Irrigation Requirements Simulation (AFSIRS) Model. The SFWMM method, similar to that used by AFSIRS, is based on an iterative definition for the unsaturated zone storage capacity. The iterative solution is required because SWMAX = fn(water table) and in the SFWMM the size of the unsaturated zone is a fn(water table). The SFWMM method:

St = St-1 + NIRRSUPTOT - ETIUADJ - ETNU

if (St > SWMAX) then

PERC = St - SWMAX

St = SWMAX

else

PERC = 0

endif.

AFSIRS uses the following formulas to calculate storage capacity (STO) and delta storage (STO):

STO = WHC * ERZD,

where WHC is water holding capacity and ERZD is the effective root zone depth.

STO = RAIN + NIRR - ET - DRAIN - RUNOFF,

where NIRR is the net irrigation requirements, DRAIN is drainage below the root zone and RUNOFF is the lateral flow.

The portion of the inefficient irrigation that evaporates, IRREVP(UT) is typically small (~5%) so the term is set to zero to simplify the algorithm.

IRREVP(UT) = 0

and the portion of the inefficient irrigation that returns to the water table aquifer, IRRETURN(UT), is quantified by:

IRRETURN(UT) = GIRRSUP(UT) - NIRRSUP(UT),

where GIRRSUP(UT) is the gross irrigation supplies for use type UT and NIRRSUP(UT) is the net irrigation supply for use type UT.

Figure 12. Hydrologic processes affecting water storage in the unsaturated zone in the Lower East Coast Developed Area.

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