The Inappropriate Use of Crop Transpiration Coefficients (K c ) to Estimate Evapotranspiration in Arid Ecosystems: A Review Ricardo Mata-Gonz alez Terry McLendon MWH Americas, Inc., Fort Collins, Colorado, USA David W. Martin Los Angeles Department of Water and Power, Bishop, California, USA The transpiration coefficient (K c ) 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 K c method in arid environments and their accuracy are reviewed. We also critically discuss the physiologi- cal and agronomic concepts that support the K c method as they relate to water-limited environments. The K c 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 coef- ficient method is simple to implement and widely recognized, it is not suitable for deter- mining evapotranspiration of vegetation adapted to arid conditions. Keywords arid-land vegetation, desert plants, stomatal control, rangeland, water use Water issues in arid areas of the world are subject to increasing debate due to the scarcity of fresh water and the growing demand for urban and agricultural uses. Evaluations of evapotranspiration in desert ecosystems are important because they provide valuable information for watershed management. These evaluations have been conducted with the objectives of (1) predicting rangeland production and assessing grazing effects (Wight & Hanks, 1981; Floret et al., 1982; Frank, 2003), (2) providing basis for groundwater management (Or & Groeneveld, 1994; Xu et al., 1998; Steinwand et al., 2001), and (3) achieving a better understanding of hydrological processes, which are essential in ecological modeling and land manage- ment of arid areas (Wight et al., 1986; Stannard, 1993; Malek et al., 1997; Liu & Kotoda, 1998; Goodrich et al., 2000). Received 14 September 2004; accepted 7 February 2005. Address correspondence to R. Mata-Gonz alez, MWH Americas, Inc., 760 Whalers Way, Suite A-100, Fort Collins, CO 80525, USA. E-mail: ricardo.mata-gonzalez@mwhglobal.com Arid Land Research and Management, 19:285–295, 2005 Copyright # Taylor & Francis Inc. ISSN: 1532-4982 print/1532-4990 online DOI: 10.1080/15324980590951469 285