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4 Sefficiency (Sustainable Efficiency)
First, the California Department of Water Resources (CDWR) defines urban water
use efficiency as “Methods or technologies resulting in the same beneficial residential, commercial, industrial, and institutional uses with less water or increased
beneficial uses from existing water quantities” (CDWR 2019). Carefully reading this
definition, we reach the conclusion that it is about an expression with its numerator
being beneficial water use (X b ), and its denominator, total water quantity applied
(VA), which represents Eq. (4.16). To understand this affirmation, let us enumerate
the possibilities that the definition sets forth to improve efficiency:
A. X b1 = X b2 with VA 2 < VA 1 (this is the 1st part of the CDWR definition of urban
efficiency)
B. X b1 < X b2 with VA 2 = VA 1 (this is the 2nd part of the definition, which is after
“or”)
C. A third possibility is not presented in the above CDWR definition of urban
efficiency with X b1 = X b2 and VA 2 = VA 1 . Under these conditions, CE improves
if X b1 * VA 2 < X b2 * VA 1 .
Fig. 4.14 Contour lines of C1-WC1-iSE trade-offs and patterns along four WR1 values