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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