T. McLendon

The Inappropriate Use of Crop Transpiration Coefficients (Kc) to Estimate Evapotranspiration in Arid Ecosystems: A Review

ABSTRACT The transpiration coefficient (Kc) method estimates evapotranspiration as a proportion of the evapotranspiration of a reference crop growing in ideal conditions. This approach was designed for irrigated crops and assumes that plants are not subjected to resource limitations. Other assumptions are that plants have high leaf area index and little stomatal resistance to water loss. These conditions are not common for arid-land vegetation. However, mainly due to its simplicity, some studies have proposed the use of transpiration coefficients as a method of determining evapotranspiration in arid environments. In this article, the documented applications of the Kc method in arid environments and their accuracy are reviewed. We also critically discuss the physiological and agronomic concepts that support the Kc method as they relate to water-limited environments. The Kc method typically overestimates water use when plants encounter suboptimal conditions of soil water because it does not consider stomatal regulation and plant adaptations to drought. We conclude that, although the transpiration coefficient method is simple to implement and widely recognized, it is not suitable for determining evapotranspiration of vegetation adapted to arid conditions.

Patterns of Water Use by Great Basin Plant Species Under Summer Watering

We analyzed temporal and spatial patterns of water use by a functionally-diverse group of Great Basin plant species and determined their water use rates at the whole-plant and individual-leaf scales under variable summer watering. Species studied were the desert grasses Distichlis spicata and Sporobolus airoides, the desert shrubs Artemisia tridentata, Ericameria nauseosa, and Atriplex confertifolia; the wetland/riparian plants Juncus arcticus, Leymus triticoides, and Salix exigua; and the annual exotic Salsola tragus. Plant species were individually grown in 5.8 m2 plots in a common garden in eastern California. Three irrigation treatments in the form of monthly pulses were applied during the summer: low (1.3 cm), medium (2.6 cm), and high (3.9 cm), in addition to a nonirrigated control. Whole-plant water uptake characteristics were determined by soil water depletion at different soil depths, while leaf transpiration was determined by gas exchange. Whole-plant water extraction and leaf transpiration varied similarly among species. Desert shrubs had low water extraction (35 to 395 g m−2 day−1) and were not affected by irrigation. The desert grasses and riparian/wetland species had higher water extraction, increasing with irrigation levels. L. triticoides and J. arcticus had the highest water extraction overall (>2,000 g m−2 day−1). Desert shrubs relied 10 times more on deeper water sources than herbaceous species. The average T/ET was 31%, but varied by species. Summer available water in environments such as the Great Basin favors desert grasses and riparian/wetland species, but not desert shrubs. The observed species differences provide alternatives for water and vegetation management.

A Review of: “Response to “Comment on The Inappropriate Use of Crop Transpiration Coefficients (Kc) to Estimate Evapotranspiration in Arid Ecosystems” by Or et al. (this issue)”

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