An evaluation of surface evaporation losses using undercanopy micro Bowen-Ratio measurements

The present micrometeorological study was designed to evaluate evaporation from soil surface (E) and transpiration (T) from plants separately during various growth stages of a tomato crop. In a separate study, losses by evaporation from wet, bare soil were determined by a large sensitive lysimeter to evaluate the reliability of micro Bowen Ratio systems in estimating E from a soil surface. Results of the latter study showed excellent agreement of estimated and measured E during eight days with moist soil-surface conditions, as well as one day of second stage drying conditions.; The primary approach in the tomato study was the use of three of the micro systems to determine temperature, humidity gradients and net radiation very close to the soil surface (1-6 cm). The analysis and reporting of results were directed at: (1) an analysis of under-canopy microclimatic conditions; and (2) the partitioning of evapotranspiration (ET) into its two components E and T. Under (1) large variations were noted, both spatially and temporally in the various microclimatic parameters air temperature, near-surface soil temperature, net radiation (Rn) and evaporation.; The results of the partitioning efforts for the earliest days of the study with only 34% plant cover, clearly indicate high evaporation losses at an exposed mid-row position with lower losses at two locations closer to the plant rows. As the plant cover increases, differences in E for the three sites became minor. The effect of irrigation timing and plant cover on partitioning of ET was considerable. With 34% crop cover an approximate even split of ET into E and T was indicated. With a ground cover of 43% at a later date, E was only 28.3 percent of ET but the reduction was likely due in part to drying of at least portion of the under-canopy-surfaces. However, total ET showed only a slight reduction with increased transpiration making up much of the difference.; As the growing season continued the percentage of evaporation decreased with transpiration representing a larger and larger percentage of ET. When the plant cover had reached its peak value (85%), T appears to represent some 95% plus of total ET. This was true even though soil surfaces under the plant canopy were moist, e.g. after irrigations.