AN IMPROVED TENSION-PLATE SYSTEM FOR MEASURING FIRST-STAGE EVAPORATION UNDER STRAW MULCH

Accurate measurements of the evaporation rate are crucial if turbulent transfer within mulches and effects of mulching on soil moisture and thermal regimes are to be understood. Arkin et al. (1974) [Soil Sci, S ec. Am. Proc., 38: 951-954] described a tension-plate system that meas ures first-stage evaporation under mulches. A detailed mass-flow analy sis of their system shows that it does not operate properly if the eva poration rate is low and if the water column air temperature and atmos pheric pressure vary. To solve this problem, we added a second flow li ne in parallel with that to the tension plate, in which the flow rate can be accurately monitored using a simple drop-counting technique. Th e additional line increases the flow from the water column so that the system functions properly, independent of variations in the water col umn air temperature and atmospheric pressure. Measurement errors were within 5 W m(-2). Field tests using micro-lysimeter and Bowen ratio/en ergy balance techniques confirmed the ability of the improved system t o measure accurately the evaporation rates from bare soils and under m ulches, including the negative evaporation rate (condensation) which o ften occurred in our studies. (C) 1997 Published by Elsevier Science B .V.