Combining sap flow measurements and modelling to assess water needs in an oasis farmland shelterbelt of Populus simonii Carr in Northwest China

Farmland shelterbelts provide an ecological protection screen for an oasis but exhibit high mortality in the face of water shortage. It is necessary to understand farmland shelterbelt tree transpiration under different levels of water stress and stand ages for proper management. Sap flux measurement techniques and models are among the most useful method to detect water stress and to evaluate plant water consumption. The usefulness of both methods decreases, however, when applied to species, such as Populus simonii Carr, that have an outstanding tolerance to drought and a remarkable capacity to take up water from drying soils. Our hypothesis is that analysis using simultaneous measurements of sap flow and models in the same trees is useful for assessing the irrigation needs in farmland shelterbelts. To test our hypothesis, we analysed the relationships between canopy transpiration, canopy conductance, relative extractable water and atmospheric factors in a farmland shelterbelt and evaluated the effectiveness of the model. Measurements were made during one growing season. The time courses of sap flow measured and modelled on days of contrasting weather and soil water conditions were analysed to evaluate the usefulness of the method to assess the crop water needs. We calculated the daily tree water consumption from sap flow measurements and the parameterized modified Jarvis-Stewart model, and we evaluated the model’s usefulness to assess the final water needs under water stress and stand ages for farmland shelterbelt irrigation. The transpiration decreased as the soil drought increased, and it increased as the atmospheric drought increased. The time course of the water needs showed that the occurrence of water stress in the farmland shelterbelt trees had a large impact on their water consumption, which increased as the water stress decreased, following the equation y = 1/[1 + e−60.67 × (REWx−0.402)]. The simultaneous use of modelling and tree structural data increased the reliability of assessing water needs from youth to maturity. A similar analysis with the water consumption values, from which stand age values were derived, showed that water needs increased with the tree age following the equation y = 847 − 844/[1 + (x/87.9)1.9]. We conclude that compared to the use of sap flow records alone, the simultaneous use of sap flow records and model values provides more detailed information to assess water needs in a farmland shelterbelt, which has an important significance for farmland shelterbelt protection.