The answer to the classical question - how much water to apply? - requires information on ET. In some extent, the answer to - when to irrigate? - depends on that information. However, the currently available ET models are not refined enough to dependably predict ET of tall, uneven crops with incomplete canopies, such as woody perennials and further work on ET is needed. Yet, some methods of ET measurement currently used in annual crops are difficult to apply to woody crops because of the size of the roots (deep, sparse) or the shoots (tall and/or rough canopies). As a consequence, direct ET measurements required to build and test ET models have been scarce and some aspects related with ET modeling on these stands are not well developed. Some of the particularities of most woody crop stands compared to annual crops, related with the limitations described, are the following:
a) higher surface roughness, heterogeneity and/or important anisotropy of the canopy means efficient transport of water vapor from the surface, ability to act as a trap for sensible heat (advection) and reduced vertical gradients for heat, mass and momentum;
b) as a consequence of (a), stomatal control of transpiration plays an important role, as discussed below;
c) higher water interception in the canopy which, also due to (a), can result in evaporation losses much higher than maximum transpiration losses, because the evaporation of intercepted water is independent of stomatal control;
d) higher radiation interception, particularly in dominant or isolated trees;
e) higher biomass and canopy volume which implies that, at least in hourly estimates, the heat storage in vegetation cannot be neglected;
f) different zones for energy and momentum exchanges and, in some cases, different climates at the level of the crowns and near the soil, with the need to consider multilayer models, if the canopy is dense;
g) dispersed and deep root systems, especially in Mediterranean climate, resulting in difficult access to soil near roots and difficult characterization of root zone extent and soil properties.
Those differences have implications in general on water and energy balance (1), and particularly on the choice of methods to measure ET (2) and on ET modelling (3).
The starting point for the discussion of the implications of woody crops particularities on water and energy balance is described in (a) above. Turbulent diffusivity for heat, mass or momentum (Kh, Kv and Km) in tall or rough stands is, on average, at least one order of magnitude higher than that observed in low crops (Oke, 1990; Monteith and Unsworth, 1990). In the case of woody stands, the high diffusivity requires a separate analysis for wet and dry canopies. If wet, evaporation is mainly limited by the available energy, while, for a low crop, it could be limited by the low diffusivity of water vapor to the atmosphere. In some cases, ET is higher than net radiation during limited periods, as high and rough stands act as a sink for energy from advection. If these canopies are dry, transpiration is controlled by stomata; the stomatal conductance becomes the limiting resistance in dry woody stands. Conversely, in low crops, the low diffusivity of water vapor from the leaves implies that ET is, on average, less dependent upon stomatal behaviour than in the tall and rough woody crops.
Jarvis (1985) and Jarvis and McNaughton (1986) expressed these differences using a decoupling coefficient, Q (0<Q<1), defined by:
where the bulk stomatal resistance of the canopy (rc), can be estimated by (Jones 1992) rc = rs/LAI (LAI = leaf area index, rs = average leaf stomatal resistance), ra is the aerodynamic resistance of the canopy, j is the psychrometric "constant" and A is the slope of the saturation vapor pressure curve. The interpretation of these resistances and the problems of scaling-up, from the leaf to the canopy scale, were discussed by Stewart and Thom (1973), Lhomme (1991) and Baldocchi et al. (1991), among many others. According to McNaughton and Jarvis (1983), the value of the Q coefficient ranges from 0.1 to 0.2 for forests (vegetation coupled to the prevailing weather) to 0.8 to 0.9 for low crops (decoupled from the prevailing weather), and it decreases with increasing rc (water shortage) as illustrated in Figure 2, where aerodynamic resistances for different crop types were selected according to McNaughton and Jarvis (1983).
Stomata adjust their resistance in such a way that the transpiration rate becomes more stable than atmospheric variations. This can be observed both in time and space. For instance, in a large irrigated field subjected to advection, stomatal resistance decreases with the distance from the leading edge in response to decreasing gradient in vapor pressure deficit (VPD), while transpiration keeps a more constant value along the transept (Davenport and Hudson, 1967; Itier et al., 1994; Brunet et al., 1994).
Thus, the annual ET from a high and rough stand can be higher than ET from a well irrigated grass (Moore et al., 1976; McNaughton and Black, 1973; Stewart, 1977, followed by many others) or much lower (Berbigier et al., 1996), depending on the precipitation frequency and environmental conditions. In low crops, the variability is restricted because the limiting factor for ET is not as dependent upon the occurrence of precipitation or advection as it is in high crops. One of the advantages of the choice of a low crop as a reference for ET practical applications - the well-known reference evapotranspiration (ETo) - derives from the relative independence of its ET from the stomatal behavior. Yet, as McNaughton and Jarvis (1991) suggested, increasing scale leads to an increase in the number of negative feedbacks that contribute to reduce the sensitivity of T to changes in stomatal closure. The differences in limiting factors for different stands can be seen as artificial because they are a consequence of the spatial scale at which experimental evidence is obtained (Avissar, 1993).
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