WATER LOSSES IN THE PATAGONIAN STEPPE - A MODELING APPROACH

In this paper we sought to answer questions related to the long-term soil water dynamics of the Patagonian steppe: What are the magnitude and seasonal dynamics of transpiration, evaporation, and deep percolation? How do these fluxes respond to fluctuations in annual precipitation? What is the pattern of soil water availability? We developed a soil water model for the steppe with a daily time step. The model gives weekly cumulative values of transpiration, evaporation, and drainage, maximum and minimum water content for the different soil layers in each week, and the weekly frequency of days with soil water potential higher than -1 MPa for each layer. The model was tested against three sets of experimental data. Simulated data of total water losses were significantly correlated with observed data, and the slope did not differ significantly from 1 nor the y-intercept from 0. On a long term basis, evaporation accounted for 56% of total water loss, transpiration 34%, and deep percolation the remaining 10%. Transpiration and evaporation had asynchronic dynamics. Evaporation was high during the coldest and wettest months of the year (mainly winter months). Transpiration, on the contrary, reached maximum values when energy and water availability were simultaneously high in late spring-early summer. Drainage took place during the coldest months, when most of precipitation occurred, and the soil remained near field capacity. Both evaporation and transpiration had a positive response to an increase in precipitation. However, the proportion of total water loss following these pathways decreased with increasing precipitation. Drainage had a positive exponential relationship with winter precipitation. Probabilities of soil water potential higher than -1 MPa in the upper soil layer were very low during most of the warm season (P < 0.15). At the beginning of the growing season the wettest layer was located at an intermediate depth (10-20 and 20-40 cm), and moved downward so at the end, only deep roots had high soil water availability.