Water is one of our most valuable natural resources and water conservation continues to be a major national priority [Vickers, 2001; Texas Water Development Board (TWDB), 2007]. As a result of population growth, current potable water supplies will be insufficient by the year 2050 in Texas (TWDB, 2003). Currently, 7.8 billion gallons, or ≈30% of all potable water, is used outdoors (U.S. Geologic Survey, 2006) primarily for landscape irrigation (Kjelgren et al., 2000; Vickers, 2001; White et al., 2004).
Landscape plants provide an aesthetic appeal to urban landscapes, prevent erosion of the soil that impairs surface water supplies, sequester carbon, add oxygen to the atmosphere, and improve recharge of groundwater (Beard and Green, 1994). Irrigated areas within the built landscape can also increase property values. Yet, end-user lack of understanding of best management practices for landscape water management will routinely contribute to excess water use. In a study of 800 home consumers in College Station, TX, it was estimated that more than 24 to 34 million gallons of excess water, that is water in excess of an irrigation coefficient of 1.0 of the yearly reference evapotranspiration, were used annually for landscape irrigation during 2001 through 2003 (White et al., 2004). Appropriate landscape design and planning has been heralded for decades as a step toward water conservation (Welsh et al., 2000), yet water consumer irrigation practices have not changed with landscapes designed for water conservation (Peterson et al., 1999).
Evapotranspiration (ET) is the amount of water lost through evaporation from the soil and plant surface plus that lost through plant transpiration. Reference evapotranspiration (ETo) water loss rate is based on environmental demands for a cool-season turf completely covering the ground. Landscape irrigation based on ETo is an emerging area of water conservation that links plant water use to irrigation water replacement rates and schedules. There is evidence that ETo weather station data can be used in irrigating landscape plants (Shaw and Pittenger, 2004; White et al., 2004), yet there is a lack of information on the fundamental seasonal relationships between ETo and actual evapotranspiration of turfgrasses, native grass species, tree species, and mixed species landscapes under different climates. An understanding of this relationship is critical to providing accurate recommendations for landscape irrigation based on ETo.
A variety of state-of-the-art technologies are available for reducing irrigation water use in amenity landscapes. One of these “smart irrigation” technologies is an ET-based controller. McCready et al. (2009) compared the effectiveness of an ET-based controller technology treatment with a time-based system with 2 d of irrigation per week without any type of sensing mechanism. Compared with the time-based system, the ET-based treatments used 25% to 62% less water without compromising turf quality. This demonstrates the benefit of using ET-based controllers in landscape irrigation, but there is a gap in the knowledge of what fraction (e.g., 0.7, 0.8) of the ETO is needed in the mixed-species landscape. Coupling ET-based plant water use with ET-based irrigation controllers can provide a means of accurately applying water to the landscape.
It is well documented that sodic and saline soil conditions can alter soil water use and transpiration in landscape plant materials (Eom et al., 2007; Munn, 2002; Wang and Nii, 2000). Dean et al. (1996) demonstrated a differential response in bermudagrass and tall fescue growth in arid climates where excess salt and water-induced stress were factors. The Dean et al. study also demonstrated that both grasses could be grown with moderately saline water if irrigation water volume was above a species-specific threshold value. Carrow and Duncan (1998) documented how excess soil sodium (Na) levels can lead to soil structural deterioration and to specific ion toxicity in shoot and root tissue. Sodic soil conditions may develop in amenity landscapes if high Na irrigation water is used. Therefore, the actual ET/ ETO relationship of plants may vary between sodic landscape sites and non-sodic landscape sites. As sources of potable fresh water are depleted, lower-quality water increasingly becomes used for irrigation of turf and woody plants.
The objectives of this study were to 1) compare landscape crop coefficients (KL; actual ET to ETo) by landscape plant treatment; and 2) determine if seasonal differences in KL occur within sites.
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