Seasonal and Species Variation in Baseline Functions for Determining Crop Water Stress Indices in Turfgrass
- G. L. Horst ,
- J. C. O'Toole and
- K. L. Faver
Empirically based relationships between canopy minus air temperature (TcTa) regressed on vapor pressure deficit (VPD) have been described as measures of crop water stress indices (CWSI) and indicators for irrigation scheduling. This study was conducted to determine seasonal and turfgrass species variation in empirical-baseline functions. Empirical and energy-balance CWSI functions also were compared to determine which was the most accurate estimate of CWSI over the range of turfgrass species and conditions studied. Field experiments were conducted to compare CWSI relationships derived during different climatic seasons and four different turfgrass species: buffalograss, Buchloë dactyloides (Nutt.) Engelm. cv. Texoka; common bermudagrass, Cynodon dactylon (L.) Pers. cv. Arizona common; St. Augustinegrass Stenotaphrum secundatum (Walter) Kuntze cv. Raleigh; and, tall fescue Festuca arundinacea Schreber cv. Falcon. Data were collected in midsummer of 1986 and late summer of 1987 from plots irrigated with a linear-gradient irrigation system. The CWSI relationships were calculated from the two lowest canopy temperatures in each plot during 7 July to 1 Aug. 1986 and 30 Aug. to 11 Sept. 1987. Differences between CWSI baseline functions from 1986 and 1987 for common bermudagrass, buffalograss, and tall fescue were highly significant (P < 0.01). Mean values of net radiation, VPD, and wind speed also were significantly different (P < 0.01) for the seasons. Vapor pressure deficit usually accounted for more than 50% of the variability in TcTa across seasons and turfgrass species. Using the energy-balance method to calculate CWSI and comparing these values with empirical calculated CWSI values reduced the portion of index differences that were greater than 0.1.
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