Appendix B - Calculation of total wilting levels

The area- weighted mosaic tile soil moisture for the WISM initial state over Eurasia is shown in Figure B.1. Full wilting levels for each tile were calculated using the equation for the evapotranspiration from a vegetation canopy given the leaf water potential between leaf and combined surface and root zones. We convert the leaf water potential to a soil moisture value using the parameterized leaf water potential which results in complete wilting. The leaf water potential for which ET ceases is specified for each vegetation type. We can use the relation between root zone moisture potential and root zone soil moisture to determine equivalent soil moistures required in the surface and root zones to result in total wilting (after Koster and Suarez, 1995b):

(B.1)

where yi is the root zone moisture potential, ys is the moisture potential of the soil at saturation, b is a soil parameter based on the distribution of pore sizes in the soil associated with each vegetation type, Wi is the soil moisture in the root zone, and Wi-sat is the soil moisture at saturation in the root zone. All values on the right-hand side of equation B.1 are parameterized in the Mosaic model. This equation is substituted into the leaf water potential equation:

(B.2)

where yl is the leaf water potential of the canopy, yr is the root zone moisture potential, Z is the canopy height, rplant and rsoil are the average resistance to moisture transport provided by plant and soil/root system, respectively, rw is the density of water and g is the gravitational acceleration. Because we are interested in the critical leaf water potential y2 and soil moisture for total wilting, the ET term drops out, and we are left with:

(B.3)

where all terms but Wi are parameters given by the LSM. We solve for Wi and obtain:

(B.4)

If we substitute critical leaf water potential parameter y2 for yl, we obtain the critical soil moisture at and below which ET from the canopy leaves shuts down completely. The resulting values are then applied as the initial soil

moisture condition over the Eurasian continent as defined in section 6.1.

Figure B.1: The Eurasian total wilting levels used to initialize the wilted initial soil moisture condition ensemble members. Units are fraction of volumetric saturation. The calculation method is discussed in the text. The bar below the figure indicates fractional values.