Peak Daily ET & Net System Capacity Calculator
Estimates how big a system needs to be — the question behind every pump and pipe-sizing decision. Walks from your soil's allowable depletion, to a general-estimate peak daily crop water use (Table 5.3), to the gross system capacity a pump actually needs to deliver.
Step 1 — Allowable Depletion (optional helper)
AD = AWC (in/ft) × root depth (ft) × allowable fraction depleted
Step 2 — Peak Daily ET (Table 5.3)
Table 5.3 (USDA-SCS, 1970) gives a general planning-level estimate of peak daily crop ET, achieved about 70% of the time, based on allowable depletion and the crop's maximum monthly ET. This is a different, simpler method than the textbook's Example 5.7, which reads a probabilistic simulation result off a location/crop-specific graph (Figure 5.11, for Eastern Colorado corn) — the two methods will not give the same numeric answer for the same soil. Use local or crop-simulation data instead of Table 5.3 whenever it's available.
Auto-filled from Step 1 above, or enter directly. Table 5.3 covers 1.0-6.0 in.
Table 5.3 covers 5-12 in/mo.
Step 3 — Gross System Capacity
Cg = Cn / [(ELQ/100%)(1−Dt/100%)]
Auto-filled from Step 2's peak daily ET.
About this calculator
Net system capacity (Cn) is the rate of water supply needed to prevent crop water stress during peak demand. Gross system capacity (Cg) scales that up to account for application inefficiency and planned system downtime.
A useful conversion: 18.86 gpm/ac ≈ 1 in/d (derived here from the same precise acre-inch and gallon factors used throughout this site, rather than the rounded constant).