Spatiotemporal analysis of lake chlorophyll-a with combined in situ and satellite data.

We estimated chlorophyll-a (Chl-a) concentration using various combinations of routine sampling, automatic station measurements, and MERIS satellite images. Our study site was the northern part of the large, shallow, mesotrophic Lake Pyhäjärvi located in southwestern Finland. Various combinations of measurements were interpolated spatiotemporally using a data fusion system (DFS) based on an ensemble Kalman filter and smoother algorithms. The estimated concentrations together with corresponding 68% confidence intervals are presented as time series at routine sampling and automated stations, as maps and as mean values over the EU Water Framework Directive monitoring period, to evaluate the efficiency of various monitoring methods. The mean Chl-a calculated with DFS in June-September was 6.5-7.5 µg/l, depending on the observations used as input. At the routine monitoring station where grab samples were used, the average uncertainty (standard deviation, SD) decreased from 2.7 to 1.6 µg/l when EO data were also included in the estimation. At the automatic station, located 0.9 km from the routine monitoring site, the SD was 0.7 µg/l. The SD of spatial mean concentration decreased from 6.7 to 2.9 µg/l when satellite observations were included in June-September, in addition to in situ monitoring data. This demonstrates the high value of the information derived from satellite observations. The conclusion is that the confidence of Chl-a monitoring could be increased by deploying spatially extensive measurements in the form of satellite imaging or transects conducted with flow-through sensors installed on a boat and spatiotemporal interpolation of the multisource data.

Field measurements of ultrasonic acoustic emissions and stem diameter variations. New insight into the relationship between xylem tensions and embolism.

We examined interrelated xylem water tensions and embolism dynamics under field conditions by simultaneously monitoring ultra-acoustic emissions and changes in stem xylem diameter. Variation in stem xylem diameter was measured with linear displacement transducers to estimate variation in sap tension. Measured ultrasonic acoustic emissions coincided well with changes in xylem diameter, indicating that individual peaks in embolism occurred simultaneously with peaks in water tension. The good time resolution between measurements makes this method especially suitable for observing embolism dynamics on a short timescale. Longer lasting measurements can also be made to monitor inter-daily patterns in water tension and embolism because the techniques are non-destructive. Ultra-acoustic emissions occurred mainly during periods of decreasing stem xylem diameter, i.e., increasing water tension, when the water tension was high enough. Embolism also occurred during periods of increasing xylem diameter, i.e., decreasing water tension, but the number of embolizing conduits under these conditions was small.

Thin-film thermoelectric devices with high room-temperature figures of merit.

Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

Tree stem diameter variations and transpiration in Scots pine: an analysis using a dynamic sap flow model.

A dynamic model for simulating water flow in a Scots pine (Pinus sylvestris L.) tree was developed. The model is based on the cohesion theory and the assumption that fluctuating water tension driven by transpiration, together with the elasticity of wood tissue, causes variations in the diameter of a tree stem and branches. The change in xylem diameter can be linked to water tension in accordance with Hookeâ s law. The model was tested against field measurements of the diurnal xylem diameter change at different heights in a 37-year-old Scots pine at Hyytiälä, southern Finland (61 degrees 51' N, 24 degrees 17' E, 181 m a.s.l.). Shoot transpiration and soil water potential were input data for the model. The biomechanical and hydraulic properties of wood and fine root hydraulic conductance were estimated from simulated and measured stem diameter changes during the course of 1 day. The estimated parameters attained values similar to literature values. The ratios of estimated parameters to literature values ranged from 0.5 to 0.9. The model predictions (stem diameters at several heights) were in close agreement with the measurements for a period of 6 days. The time lag between changes in transpiration rate and in sap flow rate at the base of the tree was about half an hour. The analysis showed that 40% of the resistance between the soil and the top of the tree was located in the rhizosphere. Modeling the water tension gradient and consequent woody diameter changes offer a convenient means of studying the link between wood hydraulic conductivity and control of transpiration.

A photographic system for measuring shoot elongation in tree seedlings and relation to solar radiation and temperature.

An automatic system has been developed to measure the variations in the rate of shoot elongation of tree seedlings. A camera equipped with 50 mm macro-objective lens, with automatic flash and winder is attached to a motor-operated rotatable stand. The camera is connected to an intervalometer enabling exposures of various seedlings placed around the stand to be taken in sequence at a given time interval. The time interval between the exposures of each seedling is adjustable between 5 min and one week. A paper scale with 0.25 mm rulings is placed behind each shoot. The negatives are projected onto a screen with a X 10 enlargement and the shoot lengths measured. The results show that the resolution of the device is +/- 0.05 mm. The system was tested under field conditions in June 1979 at the Forestry Field Station of the University of Helsinki with containerized Scots pine (Pinus sylvestris L.) seedlings. The daily course of the shoot elongation rate closely followed the variations in ambient temperature. The system can be applied to the measurement of other plant species.