Water‐limited ecosystems occupy nearly 30% of the Earth, but arguably, the controls on their ecosystem processes remain largely uncertain. We analyzed six site years of eddy covariance measurements of evapotranspiration (ET) from 2008 to 2010 at two water‐limited shrublands: one dominated by winter precipitation (WP site) and another dominated by summer precipitation (SP site), but with similar solar radiation patterns in the Northern Hemisphere. We determined how physical forcing factors (i.e., net radiation (Rn), soil water content (SWC), air temperature (Ta), and vapor pressure deficit (VPD)) influence annual and seasonal variability of ET. Mean annual ET at SP site was 455 ± 91 mm yr−1, was mainly influenced by SWC during the dry season, by Rn during the wet season, and was highly sensitive to changes in annual precipitation (P). Mean annual ET at WP site was 363 ± 52 mm yr−1, had less interannual variability, but multiple variables (i.e., SWC, Ta, VPD, and Rn) were needed to explain ET among years and seasons. Wavelet coherence analysis showed that ET at SP site has a consistent temporal coherency with Ta and P, but this was not the case for ET at WP site. Our results support the paradigm that SWC is the main control of ET in water‐limited ecosystems when radiation and temperature are not the limiting factors. In contrast, when P and SWC are decoupled from available energy (i.e., radiation and temperature), then ET is controlled by an interaction of multiple variables. Our results bring attention to the need for better understanding how climate and soil dynamics influence ET across these globally distributed ecosystems.
[1] The study of air-sea CO 2 fluxes (FCO 2 ) in the coastal region is needed to better understand the processes that influence the direction and magnitude of FCO 2 and to constrain the global carbon budget. We implemented a 1 year (January through December 2009) paired study to measure FCO 2 in the intertidal zone (the coastline to 1.6 km offshore) and the near-shore ($3 km offshore) off the north-western coast of Baja California (Mexico); a region influenced by year-round upwelling. FCO 2 was determined in the intertidal zone via eddy covariance; while in the near-shore using mooring buoy sensors then calculated with the bulk method. The near-shore region was a weak annual net source of CO 2 to the atmosphere (0.043 mol CO 2 m À2 y À1 ); where 91% of the outgassed FCO 2 was contributed during the upwelling season. Sea surface temperature (SST) and DpCO 2 (from upwelling) showed the strongest relationship with FCO 2 in the near-shore, suggesting the importance of meso-scale processes (upwelling). FCO 2 in the intertidal zone were up to four orders of magnitude higher than FCO 2 in the near-shore. Wind speed showed the strongest relationship with FCO 2 in the intertidal zone, suggesting the relevance of micro-scale processes. Results show that there are substantial spatial and temporal differences in FCO 2 between the near-shore and intertidal zone; likely a result of heterogeneity. We suggest that detailed spatial and temporal measurements are needed across the coastal oceans and continental margins to better understand the mechanisms which control FCO 2 , as well as reduce uncertainties and constrain regional and global ocean carbon balances.
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