Estimation of potential evapotranspiration with shallow lysimeters in a forest tree nursery

Abstract

Daily evapotranspiration values (ETL) were obtained during three consecutive summers from water balance measurements on eight shallow drainable lysimeters in similarly planted nursery beds of either white spruce [Picea glauca (Moench) Voss] or black spruce [Picea mariana (Mill) B.S.P.]. Values of ETL and of potential evapotranspiration (PET), calculated 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 ETL/PET was reduced to 0.8 due to a lack of water input (precipitation and irrigation) to the lysimeters. Water balance measurements from the surrounding seedling beds showed that values of actual evapotranspiration (AET) for the first two years were 0.61 times those of either PET or ETL. Rates of evapotranspiration in the lysimeters remained high because of the high water contents in the shallow lysimeters induced by the presence of a zero tension plane at the base of the lysimeters, above the drainage holes. Results indicate that well-watered shallow lysimeters can be used to estimate values of PET and be used directly for the purpose of driving water balance models for irrigation scheduling. Key words: water relations, irrigation scheduling, lysimeters, evapotranspiration, white spruce, black spruce