The relationship was analysed between the vegetation cover factor expressed as a percentage and the area-averaged normalized difference vegetation index (NDVI). On selected days the NDVI was calculated from channel 1 and 2 reflectance data of the National Oceanic and Atmospheric Administration (NOAA-11) satellite's advanced very high-resolution radiometer (AVHRR) for five test areas under agricultural and forestry use. No ground-based reflectance measurements could be made for validation of these data. Therefore the land surface NDVI, which varied with time, and percentage vegetation cover of the test areas were deduced from time-independent but site-specific statistical land use data updated by temporal phenological observations, and from surfacespecific reflectance curves published in the literature. The result indicated that the area-averaged NDVI, as obtained from the NOAA-I 1 radiometer, was less than the value calculated from the land surface NDVI. After correction to reduce the offset of the data, the values would be a suitable indicator of the fraction of vegetation cover.

The cup plant (Silphium perfoliatum L.) is discussed as an alternative energy crop for biogas production in Germany due to its ecological benefits over continuously grown maize. Moreover, a certain drought tolerance is assumed because of its intensive root growth and the dew water collection by the leaf cups, formed by fused leaf pairs. Therefore, the aim of this study was to estimate evapotranspiration (ET), water-use efficiency (WUE) and the relevance of the leaf cups for the cup plant's water balance in a 2-year field experiment. Parallel investigations were conducted for the two reference crops maize (high WUE) and lucerne-grass (deep and intensive rooting) under rainfed and irrigated conditions. Root system performance was assessed by measuring water depletion at various soil depths. Transpiration-use efficiency (TUE) was estimated using a model approach. Averaged over the 2 years, drought-related above-ground dry matter reduction was higher for the cup plant (33 %) than for the maize (18 %) and lucerne-grass (14 %). The WUE of the cup plant (33 kg ha À1 mm À1 ) was significantly lower than for maize (50 kg ha À1 mm À1 ). The cup plant had a lower water uptake capacity than lucerne-grass. Cup plant dry matter yields as high as those of maize will only be attainable at sites that are well supplied with water, be it through a large soil water reserve, groundwater connection, high rainfall or supplemental irrigation.

Simple equations are given which describe the relationships between the land-surface emissivity, the normalized difference vegetation index and the fractional vegetation cover. The empirical equations are validated using data taken from the literature. Adequate agreement is found between the formulas used in this study and those proposed by other authors.& k w d : Key words Land-surface emissivity · Normalized difference vegetation index · Vegetation cover factor& b d y :Some simple relationships between land-surface emissivity, greenness and the plant cover fraction for use in satellite remote sensing & m i s c :

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An arable crop rotation (winter barley-sugar beet-winter wheat) was exposed to elevated atmospheric CO(2) concentrations ([CO(2) ]) using a FACE facility (Free-Air CO(2) Enrichment) during two rotation periods. The atmospheric [CO(2) ] of the treatment plots was elevated to 550 ppm during daylight hours (T>5°C). Canopy transpiration (E(C) ) and conductance (G(C) ) were measured at selected intervals (>10% of total growing season) using a dynamic CO(2) /H(2) O chamber measuring system. Plant available soil water content (gravimetry and TDR probes) and canopy microclimate conditions were recorded in parallel. Averaged across both growing seasons, elevated [CO(2) ] reduced E(C) by 9%, 18% and 12%, and G(C) by 9%, 17% and 12% in barley, sugar beet and wheat, respectively. Both global radiation (Rg) and vapour pressure deficit (VPD) were the main driving forces of E(C) , whereas G(C) was mostly related to Rg. The responses of E(C) and especially G(C) to [CO(2) ] enrichment were insensitive to weather conditions and leaf area index. However, differences in LAI between plots counteracted the [CO(2) ] impact on E(C) and thus, at least in part, explained the variability of seasonal [CO(2) ] responses between crops and years. As a consequence of lower transpirational canopy water loss, [CO(2) ] enrichment increased plant available soil water content in the course of the season by ca. 15 mm. This was true for all crops and years. Lower transpirational cooling due to a [CO(2) ]-induced reduction of E(C) increased canopy surface and air temperature by up to 2 °C and 0.5 °C, respectively. This is the first study to address effects of FACE on both water fluxes at canopy scale and water status of a European crop rotation.

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The rise of atmospheric CO 2 concentration ([CO 2 ]) affects stomatal conductance and thus transpiration and leaf temperature. We evaluated the effect of elevated [CO 2 ] levels under different water supply on daily sap flow and canopy microclimate (air temperature (Tc) and vapour pressure deficit (VPD)) of maize. The crop was cultivated in circular field plots under ambient (AMB, 378 lmol mol À1 ) and elevated [CO 2 ] (FACE, 550 lmol mol À1 ) using free-air CO 2 enrichment with sufficient water in 2007, while in 2008 a DRY semicircle received only half as much water as compared to the WET semicircle from mid of July. In 2007, sap flow was measured in WET simultaneously under AMB and FACE conditions and was significantly decreased by elevated [CO 2 ]. In 2008, sap flow was measured in all four treatments but not simultaneously. Therefore, data were correlated with potential evaporation and the slopes were used to determine treatment effects. Drought reduced whole-plant transpiration by 50 % and 37 % as compared to WET conditions under AMB and FACE, respectively. Moreover, CO 2 enrichment did not affect sap flow under drought but decreased it under WET by 20 % averaged over both years. The saving of water in the period before the drought treatment resulted in a displacement of dry soil conditions under FACE as compared to AMB. Under WET, CO 2 enrichment always increased Tc and VPD during the day. Under DRY, FACE plots were warmer and drier most of the time in August, but cooler and damper short after the start of drought in July and from the end of August onwards. Thus, the CO 2 effect on transpiration under drought was variable and detectable rather easy by measuring canopy microclimate.