In this paper a numerical evaluation of the variability of soil moisture and surface runoff due to global warming is carried out. An analytical model of the soil moisture balance based on the authors' previous work is used to evaluate the probability distribution of the soil moisture concentration and resulting surface runoff. The input of hydroclimatic values is based on point precipitation models but modifications were carried out to account for the other variables following the approach suggested by C.W. Richardson. Preliminary results show that not only the mean of the distribution of both soil moisture and runoff charge, as expected, but the variability of the values around the means also does.