Seasonal variation in radiation and energy balances of permanent pastures at different altitudes

The radiation budget and energy balance of permanent managed grassland was analyzed at different altitudes (548m, 915m, and 1367m above sea lever) in Switzerland to relate temporal changes in energy fluxes to changes in canopy structure and soil moisture, and to altitude. A continuously operating Bowen-ratio measuring system was used during the growing seasons of 1993 and 1994, and evapotranspiration was calculated from energy balance and Bowen ratio measurements. Field management included fertilization and regular harvesting. Soil water content (theta(v)) was obtained either from direct measurements using time domain reflectometry (TDR) or from estimates based on soil matrix potentials and desorption curves. Leaf area index not including the stubble layer (L-m) was measured weekly with an optical canopy analyzer. Maximum L-m of between 5 and 7 and maximum standing biomass were similar at all sites, despite differences in the botanical composition. The reflectivity for short-wave radiation (albedo, A) ranged from 0.14 after a cut to 0.24. Independent of altitude, maximum A occurred with L-m of 3-4. Maximum evaporation rates were 0.74mm h(-1) and 5mm day(-1), and evaporation was strongly correlated with available energy (net radiation minus soil heat flux). With the warm and dry Fohn wind frequently occurring at the most elevated site, high nocturnal evaporation was observed. Variations in the partitioning of available energy were associated with differences in surface resistance (r(s)). Strong short-term increases in r(s) occurred in combination with low theta(v) and low L-m. Median daytime r(s) ranged from 24s m(-1) under favorable soil moisture conditions to 46s m(-1) during dry periods. Altitude had no effect on mean radiation and energy budgets, whereas aboveground dry matter yield and water use efficiency (WUE), calculated as the ratio between yield and total evapotranspiration, declined with altitude. This decline was accompanied by an increase in total root mass. In conclusion, with L-m > 2 surface energy fluxes of productive pastures at different altitudes are independent of canopy structure, and short-term variations in r(s) are related to changes in theta(v) especially in the presence of a small canopy. Aboveground yield and WUE declines with increasing altitude because of changes in dry matter allocation. (C) 1997 Elsevier Science B.V.