Guesses the pressure at the last (most remote) emitter, then steps the Energy Grade Line back toward the supply, reach by reach, adding friction and minor losses along the way. Elevation and velocity head are subtracted at each node to report the actual pressure there. The guessed far-end pressure is adjusted (bisection) until the computed required supply pressure matches the entered supply pressure — the same closed-loop problem addressed by the pipe-flow solver on the Manning Pipe Flow calculator, extended to a branching network.
Main vs. Lateral Reaches
Each row is one reach along the single hydraulically worst path (the test path) from the supply to the last emitter. A Main reach only passes flow to laterals not on the test path, so its withdrawal is a simple multiplication (design flow × the reach’s total emitter count) — no local pressure sensitivity. The main is a shared trunk pipe, so the reach of the main line that ends at the test lateral line must include not just laterals between its own endpoints but also any laterals still further down the main beyond that point, or sharing the same junction (e.g. an opposite-side lateral) — their flow travels through that same reach before splitting off, whether or not they appear anywhere else in this table. A Lateral reach is a segment of the test lateral itself: emitter discharge is computed from the actual local pressure via q = k·Hx, and friction loss is reduced by Christiansen’s F(n) factor to account for flow decreasing as each emitter in the reach withdraws water.
Limitations
Models one fixed supply pressure (no pump curve), one test path only (not the full field), and a 2-parameter emitter curve (set the exponent near 0 to approximate a pressure-compensating emitter). Two different uniformity ratios are reported, deliberately kept separate: qlast/qavg,field is an approximation of the standard low-quarter Distribution Uniformity (low-group average ÷ population mean); but this is from a small modeled sample and a user-estimated correction instead of the standard full-field statistical sample. Also, the test lateral is deliberately the presumed worst case, so its raw, uncorrected average would understate the true field average and make uniformity look better than it is; the Δpressure input exists specifically to counter that bias. Values for uniformity at or above 1 are still possible: they only mean that the last emitter’s pressure is at or above the estimated field average, so some other emitter is the point of lowest pressure. This could be because the last emitter is on low ground or because the Δpressure estimate is too small. qlast/qdesign is a different, non-uniformity check against the manufacturer’s rated flow — useful for detecting an over- or under-pressured system overall, but it is a separate check to read alongside the uniformity number, since the design/rated flow is independent of the system’s actual mean operating pressure.
Reference
Christiansen, J.E. (1942). “Irrigation by sprinkling.” California Agricultural Experiment Station Bulletin 670. ASAE/ASABE standards for microirrigation design use the same multi-outlet friction-loss approach.
Application Design
Application rate and system/zone flow use the estimated field-average emitter flow (qavg,field — the test lateral’s own average, corrected by the entered Δpressure estimate), not a guessed rate: PR = qavg,field / Ae, fed by the corrected modeled value. Spacing and system-wide lateral/emitter counts are separate inputs here because the test path only models one worst-case branch, not every lateral in the field.