Lynn J. Brun

Transpiration and evapotranspiration from maize as related to leaf area index

Abstract Evapotranspiration and transpiration from maize ( Zea mays L.) were measured with precision weighing lysimeters in 1983 and 1984. Climatic parameters were measured to calculate potential evapotranspiration, while leaf area index was measured twice weekly. Crop coefficients were determined from daily values of evapotranspiration (or transpiration)/potential evapotranspiration. Non-linear models were tested and showed that 72–86% of the variation in crop coefficient for evapotranspiration could be described knowing leaf area index. Model testing with transpiration showed that 90–95% of the variation in crop coefficient could be described knowing leaf area index.

Evaporation and energy balance for bare and stubble covered soil

Daily solar, reflected, and net radiation, air and dew-point temperature, wind speed, and soil heat flux were compared with daily evaporation for a bare and stubble covered soil surface for 45 days during spring 1984. Surface temperatures and radiation fluxes were monitored every 15 min from sunrise to sunset on five of these days. Daily net radiation was always greater for the bare soil surface, averaging 11 443 kJ m−2 compared to 10 483 kJ m−2 for the stubble covered soil. Temperature of the bare soil surface was always greater than the stubble covered surface except for overcast conditions with wet surfaces when they were about equal. Maximum differences were 14.5°C. Evaporation was always greater from the bare surface until it was dry (<8% water). Evaporation was then greater from the stubble covered surface because of its greater available water. There was no correlation between daily evaporation from either surface and the net or solar radiation. Daily evaporation was correlated only with the number of days after precipitation and with wind speed. Reduced evaporation from the stubble covered soil was related to reduced wind speed and lower surface temperatures in the stubble.

Evaluation of energy balance and water use by spring wheat during a normal and a dry season

The energy balance for spring wheat (Triticum aestivum L.) was related to growing season water availability. In 1981, with favorable soil moisture, evapotranspiration (ET) was about 92% of net radiation (Rn). Under dry conditions in 1982, ET was about 60% of Rn and sensible heat (A) was 30% of Rn. Midday canopy temperatures tended to be warmer than air temperature in 1982 and cooler than air temperature in 1981. Evapotranspiration was highly correlated to Rn in 1981 (R2 = 0.80 and 0.81). Net radiation was also highly correlated to solar radiation (R2 = 0.99 and 0.98) although Rn was a smaller fraction of solar radiation under the drier conditions of 1982. Net radiation alone was a good predictor of ET under the favorable soil water conditions of 1981 in this subhumid environment.