On the separate effects of soil and land cover on mediterranean ecohydrology: two contrasting case studies in Sardinia, Italy

Natural mixed ecosystems (grass and woody vegetation) and managed grasslands are the dominant contrasting ecosystems of semiarid regions. These two types of land covers are known to differ in their responses to water stress, as the trees demonstrate both greater stress resistance and greater ability to tap into deeper water sources. In this study, the contrasting influences of vegetation differences (grassland versus mixed ecosystems) and soil differences (deeper alluvial valley soils versus shallow upland soils) on evapotranspiration (ET) and CO2 exchange dynamics have been examined. Data from two representative case study sites within the Flumendosa river basin on Sardinia were obtained. At both sites, land-surface and CO2 fluxes were estimated by eddy covariance instruments on micrometeorological towers. Fluxes at the two ecosystems were compared, and the effect of the vegetation cover was examined with the help of an ecohydrologic model to control for the different soil influences. The results show that the water and carbon fluxes in these ecosystems are more controlled by soil differences during the late spring, when the deeper soil depth leads to a doubling of the available moisture and an increase of 48% in the mixed natural vegetation transpiration. The system then switches to vegetation control in the summer as the presence or absence of drought-tolerant trees is the dominant imprint on continued transpiration and photosynthesis. In fact, total grassland ET in the summer is only 20% as large as the mixed vegetation ET in the summer

Parameter estimation of coupled water and energy balance models based on stationary constraints of surface states

[1] We use a conditional averaging approach to estimate the parameters of a land surface water and energy balance model and then use the estimated parameters to partition net radiation into latent, sensible, and ground heat fluxes and precipitation into evapotranspiration and drainage plus runoff. Through conditional averaging of the modeled fluxes with respect to soil moisture and temperature, we write an objective function that approximates the temperature- and moisture-dependent errors of the modeled fluxes in terms of atmospheric forcing (e.g., precipitation and radiation), surface states (moisture (S) and temperature (Ts)), and model parameters. The novelty of the approach is that the error term is estimated without comparison to measured fluxes. Instead, it is inferred from the deviation of the conditionally averaged tendency terms (expectation equation image and expectation equation image) from zero since each of these terms equals zero in stationary systems but diverges from zero in the presence of misspecified parameters. Minimization of the approximated error yields parameters for model applications. This strategy was previously studied for simple water balance models using soil moisture conditional averaging. Here we extend the idea to include energy balance fluxes and surface temperature conditioning. The method is tested at two AmeriFlux sites, Vaira Ranch (California) and Kendall Grassland (Arizona). The estimated fluxes (using only observed forcing and state variables) are in reasonable agreement with field measurements. Because this method is based on conditional averages, it can be applied to situations with subsampled or missing data; that is, continuous integration in time is not required.United States. National Aeronautics and Space Administration (NASA Terrestrial Hydrology Program, grant NNG06GE48G)United States. National Aeronautics and Space Administration (NASA Terrestrial Hydrology Program, grant NAG5-11695

Reflex circuits and their modulation in motor control: a historical perspective and current view

Sensorimotor reflexes have long been, and continue to be, an area of tremendous research in movement neuroscience. Here I aim to provide an account of some studies that have been crucial in advancing our understanding of the organization of reflex circuits, their function and their modulation during motor control. I review research ranging from early experiments in reduced animal preparations that investigated the basic building blocks of reflex circuits to more recent studies in humans that demonstrate remarkable tunability of reflexes in response to variety of contingencies related to the task, the body and the environment. By providing such an integrated account of the historical aspects and current view on reflex function, I attempt to bring out the stunning complexity of reflex machinery, as well as the incredible adaptability of this machinery despite its underlying complexity.by Pratik K. Muth