Evapotranspiration partitioning in a semiarid shrubland and its relation to spring precipitation

ET partitioning is crucial to examine how water and carbon cycles are coupled and to understand the impact of climate change and human activities on ecosystems and water resources. In this study, an optimality-based ecohydrological model is validated and applied for ET partitioning in a Chihuahuan Desert shrubland site in south-eastern Arizona, USA. The ratio of transpiration to evapotranspiration is 49% for the whole period. Evaporation and plant transpiration mainly occur in growing season following the precipitation events. Evaporation responds immediately to rainfall events, while transpiration shows a lagged response of several days to those events. T/ET ratio dynamic in growing season demonstrates different patterns. Some years show low T/ET ratio at the beginning of the growing season. The peak of the T/ET ratio lags behind the rain events. Other years demonstrate stable and relatively high T/ET ratio T/ET ratio is higher than 60% during monsoon when vegetation is active. We find out that spring precipitation especially the size of the precipitation have a significant influence on shrub growth and the T/ET ratio in growing season. These years have dry spring with extremely low spring precipitation, shrubs remain inactive and there is no evident CO₂ uptake during the spring. Under such circumstance, when summer rainfall event happens, shrub has not grown yet, so the most rains are consumed by soil evaporation. In contrast, these years with high T/ET during the growing season all have high amount of spring precipitation. As a consequence, shrubs have developed a certain number of roots and leaves in spring, shrubs recover quickly after the first rain event during the growing season.

Keywords: evapotranspiration partitioning; evaoration; transpiration; spring precipitation; semiarid shrubland; Chihuahuan Desert.