Abstract:More than 40 years of revegetation using mainly Caragana korshinskii at Shapotou Desert Experimental Research Station in China has established a dwarf-shrub and herbaceous cover on the stabilized sand dunes. The evapotranspiration (ET) of the dwarf-shrub C. korshinskii community and the evaporation (E) of the bare soil were measured by the auto-weighing lysimeter method during the growing seasons from 1990 to 1995. The average ET rate was 0Ð83 mm d 1 , which varied from 0Ð71 to 1Ð06 mm d 1 during the 6-year period. Monthly ET ranged from 0Ð14 to 2Ð35 mm d 1 , with a monthly precipitation (P) range of 0 to 3Ð12 mm d 1 . Monthly ET/P ratios varied from 0Ð94 to 1Ð28 and averaged 97Ð4% overall. Most of the precipitation in this area usually falls between June and September. The lowest precipitation was recorded in 1991, with most of it falling during April and May. This temporal anomaly in the precipitation resulted in the highest ET/P ratio (by 0Ð33) and soil moisture depletion (S D 31Ð5 mm) for the 6-year period. ET from the revegetated sand dunes around Shapotou under natural precipitation may account for 12Ð2% of the potential evaporation. Cumulative P and cumulative ET and E were highly correlated, with a well-defined linear relationship of ET D 0Ð80P C 10Ð0 and E D 0Ð55P C 5Ð89, with regression R 2 ranging from 0Ð953 to 0Ð985. In 1991, however, the regression equations for both ET and E were considerably different than the other years, with a steeper slope and a negative intercept resulting from the anomaly in the precipitation pattern.

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Abstract:The arctic environment, and in particular the Mackenzie Basin, displays a very dynamic interrelationship between the atmosphere and the surface for the different ecosystems represented. The Canadian Twin Otter research aircraft flew a total of 24 grid and long regional transects, over tundra, forest and delta ecosystems, during the period of snow melt (late May-early June) and early summer (early July) as part of the 1999 Mackenzie Area GEWEX (Global Energy and Water Cycle Experiment) Study (MAGS) field campaign. Observations over tundra showed a sharp rise in the sensible heat flux at the onset of melt, reaching a maximum at the end of the melting period similar to those observed in early summer. The latent heat flux showed a more gradual rise from snowmelt to early summer with a Bowen ratio (sensible heat/latent heat) of two during melt. The forested system demonstrated a similar gradual rise in the latent heat flux, whereas the sensible heat flux was already high with Bowen ratios reaching three at the start of the observation period in late May. The gradual rise in latent heat flux can be tied to gradual thawing of the root zone and the onset of photosynthesis activity. The relatively low solar elevation angle and earlier start of snow melt along the regional transect may account for the much larger sensible heat flux. An analysis of the turbulent coherent structures indicates that the spatial density of structures for both latent heat and sensible heat increases strongly with season, from snow melt into the early summer conditions. This has implications for sampling criteria and optimum flux averaging period.There are distinct differences in energy partitioning between the various arctic ecosystems. At the beginning of the observation period, almost all the net radiation over the delta and tundra regions is utilized in non-turbulent form, whereas the forested areas use less than 50%. Model simulations of the ground heat flux showed observed diurnal imbalances and suggest that the magnitudes depend on the position of the permafrost table and may partially account for the large non-turbulent energy. Preliminary results from the Canadian MC2 model using the MAGS aircraft data for diagnosis and validation have identified sensitive model components that may merit further investigation. The findings from this study will help to fill gaps in our knowledge about surface-atmosphere interactions in arctic environments, particularly during snow melt, and broadens our contemporary view of evapotranspiration dynamics of wet surfaces.