Conservation of Mass

Now that you are familiar with the concepts of unit area and the SPA system, the principle of conservation of mass can be applied to this system for determination of the root zone water balance.

• When analyzing water fluxes, the time step or duration of the fluxes must be specified.
• In irrigation management, a daily (24-hour) time step is convenient to use since weather variables such as precipitation are routinely recorded on a daily basis.  Thus, the amount of stored water in the soil root (d_w) on any given day can be estimated using the following water accounting equation for the soil root zone:

Equation 1

Click on each “i” symbol to find out more about each part of the equation.

Equation 1 Notes

• Note that d_w represents the total amount of water stored in the root zone and that all water fluxes in equation 1 are for the current day (i.e., most recent 24-hour period). The calculation of d_w will be discussed in Lesson 103.
• Run on and lateral flow that are shown in Fig. 1-2 are disregarded as they are usually negligible when the unit area is level. This would not be the case for landscapes with significant slope.

Equation 1 applies conservation of mass (water) to the soil root zone and says that the total amount of water in the SPA system on the current day (d_wc) can be determined by knowing the initial stored water (d_wp) during the period, adding to it all the fluxes that increase stored water (i.e., P, I_rr, C), and subtracting all the fluxes that decrease stored water (i.e., ET_c, SRO, DP). Later in this course series, we will use a simplified version of equation 1 to schedule irrigations.