RELATING UNIT AND SUB-UNIT IRRIGATION PERFORMANCE
The relationship between on-farm and district-wide irrigation efficiency is not well understood. No analytical 
method exists to describe the relationship between performance parameter values for different geographic scales. Often 
on-farm efficiencies have different numerical values than district-wide or region-wide efficiencies. The ambiguous 
relationship between irrigation performance at differing scales is a frequent source of confusion, leading potentially to 
erroneous assumptions or policy decisions. A major item influencing irrigation performance over different geographic 
scales is the fact that tailwater (runoff) or drainage from one field may be reused on another, at least to the extent that the 
water quality of these flows is not limiting. Such reuse complicates the quantification of consumptive or beneficial uses of 
water. This article presents analytical expressions relating irrigation performance values for a unit and its constituent 
sub-units, when tailwater or drainage from one sub-unit may be reused on another. A simple numerical approximation to 
the exact relationship is also offered. Two irrigation performance parameters are considered: Irrigation Consumptive Use 
Coefficient (ICUC) and Irrigation Efficiency (IE). Expressions are presented for two cases: when there is a terminal subunit 
from which no further reuse is possible, and when non-consumed water from any sub-unit may be reused on some 
other sub-unit. Application of the theory is discussed.