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[1] Soil moisture control on evapotranspiration is poorly understood in ecosystems experiencing seasonal greening. In this study, we utilize a set of multi-year observations at four eddy covariance sites along a latitudinal gradient in vegetation greening to infer the ET-q relation during the North American monsoon. Results reveal significant seasonal, interannual and ecosystem variations in the observed ET-q relation directly linked to vegetation greening. In particular, monsoon-dominated ecosystems adjust their ET-q relation, through changes in unstressed ET and plant stress threshold, to cope with differences in water availability. Comparisons of the observed relations to the North American Regional Reanalysis dataset reveal large biases that increase where vegetation greening is more significant. The analysis presented here can be used to guide improvements in land surface model parameterization in water-limited ecosystems. Citation: Vivoni, E. R., H. A.

Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO sink. However, such analyses are poorly constrained by measured CO exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water-limited Southwest region of North America with observed ranges in annual precipitation of 100-1000 mm, annual temperatures of 2-25°C, and records of 3-10 years (150 site-years in total). Annual fluxes were integrated using site-specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from -350 to +330 gCm across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest-dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (R ) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and R were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross-site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS-based models captured only 20-30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO exchange may be up to 3-5 times larger than current estimates.

[1] In the North American monsoon (NAM) region, in-phase seasonality in precipitation and radiation should lead to corresponding changes in the catchment hydrologic response and its spatiotemporal variability. Nevertheless, relatively little is known on the catchment response in the NAM region because of the paucity of observations. Numerical watershed models, tested against field and remote sensing data, can aid in identifying catchment hydrologic patterns and the controls exerted by climate, soil, vegetation, and terrain properties. In this study, we utilize a distributed hydrologic model to explore the soil moisture and evapotranspiration distributions in a semiarid mountain basin. Results indicate a reliable and consistent model performance at the point and catchment scales for a set of tested hydrologic states and fluxes. Distributed model simulations reveal that soil, vegetation, and terrain controls on catchment spatial patterns vary according to the wetness state in a manner similar to that found across a wider range of climate conditions. Spatiotemporal variations in soil moisture and evapotranspiration exhibit hysteresis as an emergent pattern induced by climate variability and the underlying hydrologic interactions in the catchment.Citation: Vivoni, E. R., J. C. Rodríguez, and C. J. Watts (2010), On the spatiotemporal variability of soil moisture and evapotranspiration in a mountainous basin within the North American monsoon region, Water Resour.

The large-aperture scintillometer (LAS) is by now a generally accepted device for routinely obtaining the area-averaged sensible heat flux, H, on a scale of up to 10 km. It is an optical instrument that consists of a transmitter and receiver. In practice, the LAS beam height often varies along the path due to a variety of reasons. This study will explain what effective height to use in such situations, when analyzing scintillometer data to derive H. Several aspects are covered: a slanted path over flat terrain, structured terrain, and varying path height due to the curvature of the earth's surface. To test the derived effective height formulation the authors present LAS data taken in September and October 1996 at a rangeland site in Sonora, Mexico. In experiment 1, the LAS was set up over a slant path, ranging roughly between 10 and 45 m above the surface over a 3200-m path. In experiment 2, a horizontal LAS path was used at approximately 30 m over a pathlength of 1100 m. The resulting sensible heat fluxes were compared with eddy-covariance data and show satisfactory results for both the full and one of the approximate formulations of the effective height.