Evaporation from natural land surfaces often exhibits a strong variation during the course of a day, mostly in response to the daily variation of radiative energy input at the surface. This makes it difficult to derive the total daily evaporation, when only one or a few instantaneous estimates of evaporation are available. It is often possible to resolve this difficulty by assuming self‐preservation in the diurnal evolution of the surface energy budget. Thus if the relative partition of total incoming energy flux among the different components remains the same, the ratio of latent heat flux (LE) and any other flux component can be taken as constant through the day. This concept of constant flux ratios is tested by means of data obtained during the First ISLSCP Field Experiment (FIFE); the instantaneous evaporation values were calculated by means of the atmospheric boundary layer (ABL) bulk similarity approach with radiosonde profiles and radiative surface temperatures. Good results were obtained for evaporative flux ratios with available energy flux (LE/(Rn ‐ G)), with net radiation (LE/Rn), and with incoming shortwave radiation (LE/S ↓).

This paper presents results of 1 year (from March 25, 2003 to March 24, 2004 of continuous measurements of net ecosystem CO 2 exchange (NEE) above a steppe in Mongolia using the eddy covariance technique. The steppe, typical of central Mongolia, is dominated by C 3 plants adapted to the continental climate. The following two questions are addressed: (1) how do NEE and its components: gross ecosystem production (GEP) and total ecosystem respiration (R eco ) vary seasonally? (2) how do NEE, GEP, and R eco respond to biotic and abiotic factors? The hourly minimal NEE and the hourly maximal R eco were À3.6 and 1.2 lmol m À2 s À1 , respectively (negative values denoting net carbon uptake by the canopy from the atmosphere). Peak daily sums of NEE, GEP, and R eco were À2.3, 3.5, and 1.5 g C m À2 day À1 , respectively. The annual sums of GEP, R eco , and NEE were 179, 138, and À41 g C m À2 , respectively. The carbon removal by sheep was estimated to range between 10 and 82 g C m À2 yr À1 using four different approaches. Including these estimates in the overall carbon budget yielded net ecosystem productivity of À23 to 1 20 g C m À2 yr À1 . Thus, within the remaining experimental uncertainty the carbon budget at this steppe site can be considered to be balanced. For the growing period (from April 23 to October 21, 2003), 26% and 53% of the variation in daily NEE and GEP, respectively, could be explained by the changes in leaf area index. Seasonality of GEP, R eco , and NEE was closely associated with precipitation, especially in the peak growing season when GEP and R eco were largest. Water stress was observed in late July to early August, which switched the steppe from a carbon sink to a carbon source. For the entire growing period, the light response curves of daytime NEE showed a rather low apparent quantum yield (a 5À0.0047 lmol CO 2 lmol À1 photons of photosynthetically active radiation). However, the a values varied with air temperature (T a ), vapor pressure deficit, and soil water content.

No abstract

The data set obtained during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) allowed an assessment of several expressions originally proposed to estimate daily downward longwave radiation Rid under clear sky. These expressions with the original values of the constants were found to yield instantaneous Rld values with a standard error of estimate of the order of 10-20 W m -2, but with some bias, when applied to the instantaneous values of the input data set obtained under clear sky. Calibration of the constants for the FIFE area and for the instantaneous R lc• estimation under clear sky improved the results with a standard error smaller than 10 W m -2 and practically without bias. Inclusion of the cloudiness and cloud type information into the formulation allowed the expansion of its usage for both clear and cloudy sky conditions; the resulting standard error of the Rlc• estimate was found to be around 15-17 W m -2 if only the cloudiness data were utilized, and around 12 W m -2 if both cloudiness and cloud types were considered. These results were some 5 and 10 W m -2, respectively, better than the error in the estimation of R la without cloudiness information. Payne, R. E., Albedo of the sea surface, J. Atmos. Sci., 29,959-970, 1972. Satterlund, D. R., An improved equation for estimating long-wave radiation from the atmosphere, Water Resour. Sellers, The FIFE information system, IEEE Trans. Geosci. Remote Sens., 28, 703-710, 1990. Sugita, M., and W. Brutsaert, Wind velocity measurements in the neutral boundary layer above hilly prairie, J. Geophys. Res., 95, 7617-7624, 1990. Tuller, S. E., The relationship between diffuse, total and extraterrestrial solar radiation, Sol. Energy, 18,259-263, 1976.

This sex-linked difference in anatomical variations may be correlated to the well-known sex-linked difference in aneurysm distribution.