Precipitation event distribution in Central Argentina : spatial and temporal patterns

The annual amount of precipitation inputs received by a site during a full year is considered a dominant spatial and temporalcontrol of primary productivity and other related process in arid to subhumid ecosystems. However, to be effectively used byplants, these inputs have to escape runoff, favoured by large and less frequent precipitation events, and evaporation losses,favoured by small and more frequent events. Thus, available water for plant transpiration is not only influenced by the annualsum of precipitation events but also by their frequency-size distribution. In this paper, we characterize this distribution and itsassociation to total annual precipitation inputs through space (five sites along a tenfold precipitation gradient across 1000 km) andtime (1961–2010) in the plains of central Argentina. We decomposed total precipitation into two structural components, whichare the frequency and mean size of events, showing that they have similar contributions (log–log slopes≈0·5) explainingprecipitation shifts in space. Over time, however, we found a preponderance of mean event size explaining precipitationfluctuations, particularly towards wetter sites (log–log slopes increasing from 0·61 to 0·88). The relative variability of event sizes,independent of their mean size (i.e. inequality), was numerically characterized with Gini coefficients derived from Lorenz curves,which showed highly constant values in space and time. Assuming fixed event-size thresholds for evaporation and runoff, andignoring other controls beyond precipitation structure, the proportion of water potentially available for plant transpiration grewwith total precipitation, raising from 0·45 to 0·71 from the driest to the wettest sites, but displaying stronger responses to totalprecipitation in time, particularly in drier sites. No long-term trends in any of the precipitation structure variables were detected.Response functions of frequency and mean size of events to annual precipitation together with Lorenz curves appeared to berobust descriptors of precipitation regimes that, not requiring any a priori assumptions, are useful to assess how spatial andtemporal shifts in total precipitation may concurrently affect its relative availability for plant transpiration.