Application Efficiency of the Low Quarter (ELQ) Calculator

Answers "how much of the water I paid to pump actually ended up usable by the crop?" — combining uniformity (DU), soil water deficit, and runoff into one efficiency number, the way an irrigation auditor or NRCS/extension evaluator would after a field uniformity test.

Calculator

Runoff recovery

Most sprinkler and microirrigation systems have no runoff recovery. This mostly matters for surface (furrow/border) systems with tailwater return.

Gross depth applied from the source (d_g)

Runoff depth (d_r)

Return ratio (R_t) — fraction of runoff that's recovered and reused

Mean depth infiltrated (d_z)

Enter the low-quarter depth (d_LQ) directly instead of DU

Distribution uniformity (DU)

From a catch-can test — see the Catch-Can Uniformity Calculator.

Low-quarter depth (d_LQ)

Soil water deficit (SWD)

About this calculator

Deep percolation happens for two different reasons: the field isn't watered perfectly uniformly (nonuniformity), and/or more water is applied on average than the root zone can hold (excessive application). The low-quarter depth (d_LQ) — the average depth received by the driest 25% of the field — is used as the practical "management depth": irrigating to d_LQ results in about 90% of the field being adequately watered.

d_ev = d_g − d_z − d_r — d_LQ = DU × d_z — d_e = min(d_LQ, SWD) — E_LQ = 100% × d_e / d_a

If d_LQ < SWD, the field isn't even fully refilled at the low quarter — the effective depth is limited by nonuniformity, not over-application. If d_LQ > SWD, the low quarter already exceeds what the root zone can hold — the effective depth is capped by SWD, and the excess is deep percolation from over-application. Either way, E_LQ tells you what fraction of the water pulled from the source is doing useful work for the crop.

Textbook Example 5.2: d_z=2.0 in, DU=0.75, d_a=2.2 in, no runoff, SWD=1.6 in → d_LQ=1.5 in (< SWD), d_e=1.5 in, E_LQ=68%.
Example 5.3 repeats this with SWD=1.2 in instead → now d_LQ > SWD, so d_e=1.2 in and E_LQ=55%.