Re: “Is Drip Irrigation for You? Research at New Mexico State University shows how to determine crop coefficients for drip-irrigated Xeriscapes – saving time, labor and precious water resources” (November 2010).
It should be emphasized that what the authors of this article have determined are landscape coefficients (KL), not crop coefficients (Kc) as indicated in the article subtitle and in the heading for the sample list of species. Crop coefficients (Kc) are derived experimentally, to quote the authors “under standard conditions where the crop is grown in large monocultures that are disease-free, well-fertilized, grown under optimum soil water conditions, and which achieve full production under given climatic conditions.” The present study, as stated, did not include a monoculture, optimum soil water conditions or measurable production values. We can only assume that the studied plants were disease-free and well-fertilized.
The authors are correct in recognizing that landscapes are not crops, and that water use in most landscapes need not be applied in amounts that result in optimum plant growth. It is usually sufficient to attain only “acceptable growth and quality.” Therein, however, lies the rub. What is “acceptable growth,” and how does one measure “quality?” Deprived of “optimum” water, is a shorter, denser pine acceptable? If a species takes five years instead of three to attain full height, is that acceptable? Questions like these have no definitive answers unless everyone agrees on species-specific growth parameters.
Keeping estimated landscape coefficients (KL) and scientifically determined crop coefficients (Kc) separate and distinct is important, since these findings have a way of becoming codified in follow-up articles and landscape recommendations (read: mandates). A reference to the complete study would have been helpful.
Authors of plant-related subjects should also take note of the findings in this article. Without geographical attribution, authors have labeled at least five of the top water users “drought tolerant” in their articles. Should these plants have been planted to the truly drought tolerant “no-irrigation zone garden” or “low-irrigation zone garden” in this article, most likely they would be dead.
Jim Borland is correct in pointing out that the values recorded in the table and referred to in the title of the article are not crop coefficients (Kc) and should not be interpreted as such. In fact, they may not even be viewed as landscape coefficients (KL) either, since KL values (at least in the WUCOLS publication) are adjusted with other coefficients to account for density (Kd) and microclimate (Kmc) variability. So, what we’ve actually shown is the species coefficient (Ks) (or KL if Kd and Kmc are assumed to be 1.0). We must also agree with Mr. Borland that “quality” and/or “acceptability” is quite subjective and, hence, may be viewed differently by different people. Our ratings were based on feedback from master gardeners and other visitors to the garden, as well as our opinions and those of other research technicians. Our primary objective was to quantify the vague terms “low,” “medium” and “high,” which are so often used to describe plant water requirements in most Xeriscape gardening publications.