Bulk Surface Resistance and Its Effect on Evapotranspiration Rates in Irrigated Wheat
- A. Luchiari and
- S. J. Riha
Use of the combination model to predict evapotranspiration (ET) from a crop has been limited by uncertainty in the value and behavior of the bulk sulface resistance (rb). The purpose of this study was to characterize rb for irrigated wheat (Triticum aestivum L.) and evaluate its use for predictive purposes. Hourly Bowen ratio measurements were made over a field of irrigated ‘Anahuac’ wheat during the entire growing season of 1983. Data from 4 d were used to calculate rb using both the energy balance and residual methods. Crop surface temperatures predicted from measurement of wind speed and air temperature at one height above the canopy were within 0.4 °C of infrared measured canopy temperatures. Both the energy balance and residual methods gave similar values for rb. From 0830 h to 1430 h the day before irrigation, values for rb ranged from 50 to 100 s m−1 whereas after irrigation, rb for the same period ranged from 15 to 50 s m−1. In both cases, rb rapidly increased after 1430 h to values above 200 s m−1 by 1630 h. At the same time, latent heat loss in proportion to net radiation increased and canopy temperature decreased more rapidly than air temperature, suggesting that the increase in rb at this time is not due to a lag in the supply of water to the canopy. The combination model was used to predict ET on both an hourly and daily basis for several periods during another crop growing season. Mean predicted values were within 6% of Bowen ratio measurements and were highly correlated with hourly and daily measured rates of ET, suggesting that the combination model can be used for predictive purposes when rb is well characterized.
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