科研之友
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摘要
The FAO Irrigation and Drainage Paper No. 56 (FAO-56) is expected to provide a universally consistent methodology for obtaining reliable estimates of crop evapotranspiration (ETc) from standard weather data and crop coefficient (K-c) information. Yet, for many areas, including the southwestern desert region of the U.S., evaluation of the FAO-56 crop coefficient methods for local conditions has been limited. In this study, measurements of daily alfalfa ETc were used to evaluate the dual crop coefficient approach of FAO-56, which separates the single K-c into two coefficients, a basal coefficient, K-cb (primarily transpiration), and a wet soil evaporation coefficient, K-e. A second objective was to calibrate alfalfa K-cb baselines for constructing localized, climate-adjusted FAO-56 K-cb curves for growth cycles under the seasonal climatic variations encountered in the semiarid, southwestern U.S. Daily ETc was measured for alfalfa in three weighing lysimeters subjected to varying irrigation regimes for eight cutting cycles in 1985 at a field site in Phoenix, Arizona. Daily K, data were determined from the measured ETc and calculations of the FAO-56 grass-reference evapotranspiration (ETo) using meteorological data obtained at the site. Daily K-c values were partitioned into the dual crop coefficients (K-cb and K-e) using back-calculations of the FAO-56 dual procedures. Examples are given to illustrate how the magnitudes Of K-cb, K-e and Kc varied with respect to crop development and wet and dry surface soil conditions. The effects of water stress on ETc were considered in the back-calculations using the FAO-56 water stress reduction coefficient (K-s). Linear-style FAO-56 K-cb curves were then constructed separately for each lysimeter for each of the eight cutting cycles based on the back-calculated K-cb data. Daily ETc based on the constructed curves and FAO-56 calculation procedures were highly correlated to the daily measured lysimeter ETc (r(2) = 0.98), and the mean daily difference between calculated and measured ETc (0.03 mm) was not significant (p > 0.7). Comparison of measured ETc for water-stressed and well-watered alfalfa indicated that calculated K-s described ETc reductions due, to soil water stress adequately when atmospheric evaporative demand was considered in the K-cb. Baseline K-cb values for constructing local climate-adjusted FAO-56 alfalfa K 0.30, 1.22, and 1.05 for the initial, mid-season, and end of late season, respectively.
- 出版日期2002-12
- 单位西北农林科技大学
