## 1 Introduction
[29] found that daily evapotranspiration was mainly dominated by soil moisture in the hot-dry season and was controlled by environmental factors in the mid-to-late rainy season and cool-dry season at a tropical rain forest site in Xishuangbanna, southwestern China. The main environmental factors affecting the sap flow of _Salix psammophila_ [PERSON]. [PERSON] et [PERSON]. [PERSON] in semi-arid regions are Rn, relative humidity, and wind speed [30]. However, there were no significant correlations between T and the soil water content in _Populus euphration_ Oliv. and _Tamarix chinensis_ Lour., 1790 forests in arid regions [31]. As a result, the impact of each environmental factor on sap flow varies with climatic region, species, and the age of trees [19]. T is closely related to environmental factors. Atmospheric factors and the soil water content are the two main components that affect transpiration [32]. Among atmospheric factors, Rn and VPD have direct effects on tree water use [23; 33; 34]. Insemi-arid areas, an in-depth understanding of the relationship between forests and water is not yet achieved [35]. This requires determining the forest water consumption capacity and understanding the regulatory mechanisms by which trees respond to environmental stress. Various methods of obtaining forest T are available, and field observations and modeling are the two most commonly used methods. However, complete and accurate field data are often limited. Simple empirical models have been widely used for T estimation in different forest types because of their simple structures and easily acquired key parameters, for example, [PERSON] et al. [36] used a modified Jarvis-Stewart modeling approach that was previously used [37] to calculate canopy conductance to calculate T for the native tree species _Eucalyptus cerebra_ [PERSON] and _Callitris glaucophylla_ [PERSON]. [PERSON]. in Australia. Three environmental variables, namely, solar radiation, VPD, and the soil moisture content, plus LAI, were used to calculate T, using measured rates of tree water use to parameterize the model. Functional forms for the model were derived with the use of a weighted non-linear least squares fitting procedure. The model is able to give T estimates comparable to sap flow measurements. [PERSON] et al. [38] established an improved Jarvis-Stewart model to quantify the independent contribution of RSWC (relative soil water content) in different soil layers to T. The impacts of soil moisture that are coupled in the Jarvis-Stewart model can genuinely reflect the environmental influence and can be used to quantify the contributions of soil moisture to T. [PERSON] et al. [39] investigated transpiration and its main driving factors in a hybrid poplar plantation with the clone _Populus maximowiczii_ A. [PERSON] at a site in the hilly loess region of Saxony (Germany). They discovered the main controlling factors for T included the evaporative demand, water availability, and soil temperature. The information was implemented into a simple empirical model for the prediction of transpiration. Thus, the development of a simple empirical T model, which can reflect the response mechanisms of T to dominant factors, is expected to accurately quantify the impacts of major factors. White birch (_Betula platyphyllla_ [PERSON]) is one of the typical tree species in the studied semi-arid region of Northeast China and plays an important role in soil and water conservation. White birch was chosen because it is one of the typical forest species in the region and is extremely susceptible to drought. This study focused on the environmental controls on transpiration. Our goals are (1) to understand the tree transpiration variation characteristics and tree size-specific responses in dry years with varying intensities and (2) to determine the contribution of environmental factors (Rn, VPD, and REW) to T in dry years with varying intensities.