MEASUREMENT AND ESTIMATION OF EVAPORATION FROM SOIL UNDER SPARSE BARLEY CROPS IN NORTHERN SYRIA

Increased yields of rain-fed barley grown in northern Syria have been produced by the application of phosphate. It has been suggested that enhanced early growth of the fertilised plants restricts evaporative loss from the soil surface, improving water-use efficiency by decreasing the ratio of evaporation from the soil to transpiration (E/T). The objective of this study was to evaluate the use of micro-lysimeters for direct measurements of E from the soil beneath barley canopies. The lysimeters were made from sections of 10-cm diameter plastic pipe, 15 cm in length. Daily E from fallow and cropped (leaf area index (LAI) ≤ 2) soil surfaces was measured for 100 days to crop maturity. Evapotranspiration (ET) was determined by water balance from neutron probe readings, allowing the performance of micro-lysimeters in the bare soil treatment to be checked. Over rain-free periods, the micro-lysimeter and water balance measurements agreed to within ± 0.25 mm day-1. Over periods when rain fell, E measured by micro-lysimeter was 0.1-1.1 mm day-1 less than E from water balance. A simple method was devised to estimate E in all treatments on wet days; these estimates were used to replace micro-lysimeter measurements on days when rainfall exceeded 1 mm. Fertiliser application had little impact on total ET, but total E was reduced by 10%; E accounted for 67 and 77% of total ET in the fertilised and unfertilised treatments, respectively. A large fraction of total E occurred on days when rain fell, emphasising the need to achieve accuracy over rainy periods before micro-lysimetry can be used successfully to monitor E through the duration of a rain-fed crop. The dataset of daily E was used to examine two previously published methods for estimating E from crop, soil and meteorological data. The performance of both methods is discussed. © 1990.