Combining stable isotopes, Eddy Covariance system and meteorological measurements for partitioning evapotranspiration, of winter wheat, into soil evaporation and plant transpiration in a semi-arid region

Abstract

A good knowledge of the partitioning of evapotranspiration (ET) is particularly important for improving crop water productivity, especially in semi-arid regions where irrigated agriculture uses up to 85% of total available water. In this paper, stable isotopic method and micrometeorological data were combined to assess the ET partition into plant transpiration and soil evaporation of an irrigated winter wheat field lying in the center of Morocco characterized by a semi-arid climate. Atmospheric, vegetation and soil samples were collected during two consecutive growing seasons (2011–2012 and 2012–2013). The sampling campaign was carried out after and before two irrigation events and lasted for five days.Additionally, an Eddy covariance system and a standard meteorological station were installed. Isotope turbulent mixing relationships, isotopic values of transpired water from plants (T) and that of evaporating water vapor from soil surface (E) were used to estimate fractions of transpiration and evaporation contributing to the total ET. The partition of ET was only performed by using δ2H. In fact, results obtained during 2011–2012 period, showed that this method worked well for the deuterium in comparison to oxygen-18 (δ18O). After irrigation, the ratios T/ET were about 0.73, 0.59 and 0.74 respectively in Days Of Year (DOY) 53, 54 and 55, averaging 0.69. Prior to irrigation, when the soil was dry, the T/ET ratio was 0.83 and 0.77, respectively in two days (DOY 101 and 102), averaging 0.80. Afterward, these fractions have been used in conjunction with Eddy Covariance measurements for converting the obtained percentages to mmd−1 values which are required by the farmers and the water managers. The obtained averages of evaporation and transpiration were respectively: 1.76 and 3.85mmd−1 for the 2011–2012 and 1.34 and5.45mmd−1for the 2012–2013. Our approach is useful for partitioning ET in an irrigated winter wheat under semi-arid climate at an ecosystem scale on short time steps.