ESTIMATION OF POTENTIAL EVAPOTRANSPIRATION WITH SHALLOW LYSIMETERS INA FOREST TREE NURSERY

Daily evapotranspiration values (ET(L)) were obtained during three con secutive summers from water balance measurements on eight shallow drai nable lysimeters in similarly planted nursery beds of either white spr uce [Picea glauca (Moench) Voss] or black spruce [Picea mariana (Mill) B.S.P.]. Values of ET(L) and of potential evapotranspiration (PET), c alculated with the Penman equation, were nearly equal for most of the sampled days during the first two years. During the last year of study , the ratio ET(L)/PET was reduced to 0.8 due to a lack of water input (precipitation and irrigation) to the lysimeters. Water balance measur ements from the surrounding seedling beds showed that values of actual evapotranspiration (AET) for the first two years were 0.61 times thos e of either PET or ET(L). Rates of evapotranspiration in the lysimeter s remained high because of the high water contents in the shallow lysi meters induced by the presence of a zero tension plane at the base of the lysimeters, above the drainage holes. Results indicate that well-w atered shallow lysimeters can be used to estimate values of PET and be used directly for the purpose of driving water balance models for irr igation scheduling.