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 microlysimeters were similar to those measured in the surronding soil. Estimated E, when the soil surface was wet, varied from 80% of reference ET (ET(o)) at 0.8 leaf area index (LAI) to 15% when LAI increased to four. A single relationship between E/ET(o) 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(o), in cotton was 1.1 for a LAI of three. If the surface soil was wetted by rain or irrigation , the calculated T/ET(o) 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.