# Grapevine Phenology in Four Portuguese Wine Regions: Modeling and Predictions
## 1 Introduction  
Phenology can be understood as the study of the phenological events, or the stages of plant development that occur during their active lifecycle, in response to climatic conditions [1]. This isparticularly important in agriculture, as crops are subject to continuously evolving environmental conditions. For a specific site and year, the farmer may vary strategies, such as irrigation, to adapt a particular crop to the current environmental conditions [2], aiming to maintain or even improve the annual productivity and income. The development of grapevines is mainly influenced by climatic elements [3], such as temperature, solar radiation, and precipitation, which also influence the production and the quality of grape berries [4; 5]. In effect, temperature is the main forcing element on the phenology of grapevines [6; 7]. The projected increases in temperature under future likely climate change scenarios may lead to the advancement of 6 to 25 days for different grapevine varieties in Europe [8]. In addition, accelerated growth and a shortening of the phenological phases are expected [9], which may also affect the composition and the quality of grape berries and of the wine produced in a given region. Grapevine varieties have a vast genetic variability within the species _Vitis vinifera_ L., which is manifested in their different phenological timings. Although the varieties are usually well adapted to the specific climatic conditions of the sites where they traditionally grow [10; 11], they do differ in the thermal accumulation required for a given phenological stage [12; 13]. As an illustration, in a study carried out by [PERSON] et al. [14], the thermal requirement of ten varieties were compared, being within a certain range of base temperatures suitable for budburst (0.4 \({}^{\circ}\)C to 4.6 \({}^{\circ}\)C). Still in another study for five varieties, different responses to growth and development were found after budburst within a temperature range between 15 \({}^{\circ}\)C and 35 \({}^{\circ}\)C [15]. Temperature-driven phenological models can be used for forecasting and thus to better understand the temporal evolution of grapevines [16]. The models that integrate a dormancy phase typically include the sum of daily temperatures above a certain level at which the grapevine breaks dormancy. These models differ in the articulation between the thermal effects of the dormancy phase and the grapevine vegetative phase, being designated as alternating [17; 18], sequential [19], parallel [20; 21], unified [22], deepening rest [23], and four phases [21; 24]. The thermal models of the vegetative cycle are based on the occurrence of a given phenological stage through a sum of temperatures, starting on a pre-defined date and needing reach a given threshold for each variety. For the phenological models on the basis of growing degree-days (GDD) or heat units, the temperatures are accumulated from a certain value called the base temperature (lower thermal limit). Although this value is conventionally considered of 10 \({}^{\circ}\)C for grapevine [25], [PERSON] et al. [26] recommended 0 \({}^{\circ}\)C for the calculation of heat units. Furthermore, some authors suggest that the thermal accumulation should start on the first day of the year [27; 28]. Despite being a very simple model, the degree-day model also has important constraints [29]. The two-parameter sigmoid model (SM henceforth) proposed by [PERSON] [21] overcomes the limitation of imposing an artificial threshold for heat accumulation, as it allows a gradual change in the weights given to different temperatures within a transition range. The [PERSON] and [PERSON] [30] model also considers the potential negative effects of high temperatures on the development of a given crop. Nevertheless, an accurate estimation of these model parameters requires a sufficiently large sample of data on the upper side of the curve, which is not always feasible in temperate climates.