Evaporation Measurements beneath Corn, Cotton, and Sunflower Canopies

Experiments were conducted at Cordoba (southern Spain) between 1986 and 1988 to obtain independent estimates of evaporation from the soil ( E ) and crop transpiration ( T ) in irrigated corn ( Zea mays L.), cotton ( Gossypium hirsutum L.), and sunflower ( Helianthus annuus L.). Microlysimeters that allowed root exploration were used to estimate E by differential weighing. Cotton and sunflower evapotranspiration ( ET ) were measured with a weighing lysimeter (3 by 2 by 1.5 m), while corn ET was estimated using a water balance technique. Root density and soil water content inside the open microlysinteters were similar to those measured in the surrounding soil. Estimated E , when the soil surface was wet, varied from 80% of reference ET ( ET 0 ) at 0.8 leaf area index ( LAI ) to 15% when LAI increased to four. A single relationship between E/ET 0 and LAI fit the data for the three crops. The estimated E values for periods spanning 30 to 40 d, when LAI increased from 0.7 to 5, was 20 to 27% of measured ET , for an irrigation frequency of 7 to 8 d. The T values, calculated as the difference between ET and E , varied for a given LAI depending on the E level. When the soil surface was dry and E was low, the ratio, T/ET 0 , in cotton was 1.1 for a LAI of three. If the surface soil was wetted by rain or irrigation, the calculated T/ET 0 ratio dropped to 0.58 the 1st d after wetting. The results highlight the interdependence of the E and T processes in the consumptive use of water by row crops.