Fig. 1.

Effect of vegetation on the soil water balance. (A) Dependence of evapotranspiration on (shallow) soil moisture. Bare-soil (i.e., V = 0) evaporation linearly increases from zero at the hygroscopic point, s_h, to potential evaporation, PE, at field capacity, s_fc (24), and remains constant for s ≥ s_fc. In completely vegetated plots (i.e., V = V_max), unstressed evapotranspiration occurs for s greater than a critical value s^*. For s < s^*, evapotranspiration linearly decreases, due to stomata regulation (19, 20, 25). E = 0 when s is less than the permanent wilting point, s_w. Soil parameters are for sandy soils (s_h ≅ s_w = 0.13; s^* = 0.33; s_fc = 0.42). PE is 4 mm/d, and the maximum rate of evapotranspiration from the top 10 cm of soil (Z = 10 cm) is ε·PE with ε = 0.3. The use of different values of ε does not alter the general results of this study. (B) Average seasonal soil moisture calculated with Eq. 1 as a function of R for bare (〈s〉_v=0) and completely vegetated (〈s〉_v=1) soils. The other parameters (n = 0.38; K_s = 2,000 cm·d^-1; β = 12.5, α = 10 mm·d^-1; ε = 0.3, growing season length of 180 d) are typical of sandy soils and arid climates (e.g., ref. 20).