Energy and water balance

The energy and water balance part, which is the most comprehensive, considers three pools of water, which are soil water, intercepted water residing on the vegetation, and snow. Precipitation can fall as rain directly on the ground, or hit the vegetation, filling up the interception reservoir up to 0.1 mm times leaf area index (LAI, see subsection on phenology) after which throughfall occurs, or it falls as snow and contributes to the snow reservoir. (There is no intercepted snow, so that snow is always assumed to lie beneath the vegetation.) Rainfall on the ground, throughfall and snow melt all fill up the ground water pool up to a value of if this value is exceeded, runoff occurs. This is essentially a bucket scheme, in which the total soil water content can vary between a minimum at the wilting point and a maximum where runoff and vertical drainage start to occur. The modelled soil water content, is equal to the actual soil water content minus the amount stored in the soil at wilting point; this quantity is sometimes called "plant-available" soil moisture content. To ensure conistency with the

ECHAM-4 climate model, the maximum plant-available soil moisture content, wmax, is taken everywhere as 65% of the saturated soil water content, a value derived from the same external input data as for the ECHAM-4 model (see below). The water balance is updated at a daily time step.

Apart from runoff, water leaves the surface as evapotranspiration, where it becomes part of the energy balance. Potential soil evaporation and snow evaporation are assumed equal to the equilibrium rate [Jarvis and McNaughton, 1986], until either the snow pool is depleted, or soil moisture becomes limiting. The limitation of soil evaporation is assumed to depend on the time since the last rainfall event, following Ritchie [1972]. Evaporation of intercepted water and transpiration, the latter by far the largest flux for most cases, are both computed with the Penman-Monteith formula [Monteith, 1965]. Energy balance and radiation are all computed hourly.

The Penman-Monteith formula requires the specification of the combined conductance of all stomata, or leaf pores, of the canopy, denoted G. This value is set to infinity for evaporation of intercepted water, and to Gc= Gct)l (1 + be Ae) for transpiration, where Gc (l is stomatal conductance at a standard leaf-internal concentration, assumed when water is not limiting, Ae the vapour pressure deficit above the canopy, and be a factor controlling stomatal closure in response to soil water. Gc „ is computed from the diffusion equation across the stomata from the photosynthesis rate without waterlimitation, Acj (see next subsection):

where Ca is the atmospheric C02 concentration, is assumed 65% of Ca for plants with C3, and 37% for plants with C4 photosynthesis, TK is the air temperature in Kelvin, p air pressure, and R the universal gas constant. be is set to a value that limits the transpiration rate at 13:00 h (assumed to represent the situation of highest atmospheric demand) to a root supply rate ofS = cww I wmax, with cw set to 1 mm/hour [Federer, 1982].

The other conductance term needed for the calculation of transpiration and evaporation of intercepted water is the aerodynamic conductance between the canopy and the free air. Its value varies between approximately 0.200 m/s for forests and 0.025 m/s for grasslands, and is computed from Brutsaert [1982]:

with k = 0.41 (von Karman constant), wind speed u, reference height z, roughness length zo, and zero plane displacement d. For vegetated surfaces, the following parameters have been fitted to data [Kelliher et al., 1993], assuming a globally fixed wind speed of u = 3 m/s: z(> = 0.1 hc, z = hc + 2 m, and taking vegetation dependent values for the canopy height,

Its values, chosen to deliver good agreement with typical values for forests and grasslands, are 30 cm for short grass, 1 m for shrubs, 2 m for long grass, 15 m for temperate, boreal and tropical deciduous trees, and 30 m for tropical evergreen trees; they are determined according to the vegetation map used as input (see below). Previous sensitivity tests with the BETHY scheme have shown that both carbon uptake and transpiration are rather insensitive against a variation of u between half and double the globally fixed value taken here.

The radiative balance considers incoming solar radiation at the surface, derived from incoming surface PAR (photosynthetically active radiation) with a conversion factor depending on cloudiness and solar angle [Pinker and Laszlo, 1992], outgoing longwave radiation based on a surface emissi-vity of 0.97, and sky radiation depending on air temperature, air vapour pressure, and cloudiness [Brutsaert, 1982, p. 137]. Daily averages of incoming PAR are interpolated linearly between monthly values from input data, which are then used to compute hourly PAR and broadband solar radiation, following a method by Weiss and Norman [1985]. Cloudiness is kept constant over a day and is inferred from the ratio of incoming to potential PAR. Net radiation is computed separately for vegetation and bare soil to computed separate evaporation rates from vegetation and soils. The broadband albedo of vegetation albedo is set to 0.15 globally, while soil albedo is prescribed from input data, taking 0.15, 0.20 and 0.35 for dry dark, medium and bright soils, respectively, and 0.07, 0.10 and 0.18 for the same soils when they are wet [Wilson and Henderson-Sellers, 1985]. A soil heat flux is also included, assumed to be a fixed fraction (0.036) of the overall net radiation [Verma et al., 1986].

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