Partitioning evapotranspiration in sparsely vegetated rangeland using a portable chamber

A portable chamber was used to separate evapotranspiration (ET) from a sparse, mixed-species shrub canopy in southeastern Arizona, United States, into vegetation and soil components. Chamber measurements were made of ET from the five dominant species, and from bare soil, on 3 days during the monsoon season when the soil surface was dry. The chamber measurements were assembled into landscape ET using a simple geometric model of the vegetated land surface. Chamber estimates of landscape ET were well correlated with, but about 26% greater than, simultaneous eddy-correlation measurements. Excessive air speed inside the chamber appears to be the primary cause of the overestimate. Overall, transpiration accounted for 84% of landscape ET, and bare soil evaporation for 16%. Desert zinnia, a small (similar to 0.1 m high) but abundant species, was the greatest water user, both per unit area of shrub and of landscape. Partitioning of ET into components varied as a function of air temperature and shallow soil moisture. Transpiration from shorter species was more highly correlated with air temperature whereas transpiration from taller species was more highly correlated with shallow soil moisture. Application of these results to a full drying cycle between rainfalls at a similar site suggests that during the monsoon, ET at such sites may be about equally partitioned between transpiration and bare soil evaporation.