We investigated how the distribution of precipitation over a growing season influences the coupling
of carbon and water cycle components in a semiarid floodplain woodland dominated by the deep-rooted velvet mesquite (Prosopis velutina). Gross ecosystem production (GEP) and ecosystem respiration (Reco) were frequently uncoupled because of their different sensitivities to growing season rainfall. Soon after the first monsoon rains, Reco was high and was not proportional to slight increases in GEP. During the wettest month of the growing season (July), the system experienced a net carbon
loss equivalent to 46% of the carbon accumulated over the 6-month study period (114 g C m)2;
May–October). It appears that a large CO2 efflux and a rapid water loss following precipitation early
in the growing season and a later CO2 gain is a defining characteristic of seasonally dry ecosystems.
The relative contribution of plant transpiration (T) to total evapotranspiration (ET) (T/ET) was
0.90 for the entire growing season, with T/ET reaching a value of 1 during dry conditions and
dropping to as low as 0.65 when the soil surface was wet. The evaporation fraction (E) was equivalent
to 31% of the precipitation received during the study period (253 mm) whereas trees and
understory vegetation transpired 38 and 31%, respectively, of this water source. The water-use
efficiency of the vegetation (GEP/T) was higher later in the growing season when the C4 grassy
understory was fully developed. The influence of rain on net ecosystem production (NEP) can be
interpreted as the proportion of precipitation that is transpired by the plant community; the water-use
efficiency of the vegetation and the precipitation fraction that is lost by evaporation.
