Interception

Abstract: . This paper outlines a new strategy to derive evaporation from satellite observations. The approach uses a variety of satellite-sensor products to estimate daily evaporation at a global scale and 0.25 degree spatial resolution. Central to this methodology is the use of the Priestley and Taylor (PT) evaporation model. The minimalistic PT equation combines a small number of inputs, the majority of which can be detected from space. This reduces the number of variables that need to be modelled. Key distinguishing features of the approach are the use of microwave-derived soil moisture, land surface temperature and vegetation density, as well as the detailed estimation of rainfall interception loss. The modelled evaporation is validated against one year of eddy covariance measurements from 43 stations. The estimated annual totals correlate well with the stations' annual cumulative evaporation (R=0.80, N=43) and present a low average bias (−5%). The validation of the daily time series at each individual station shows good model performance in all vegetation types and climate conditions with an average correlation coefficient of R =0.83, still lower than the R =0.90 found in the validation of the monthly time series. The first global map of annual evaporation developed through this methodology is also presented.

Abstract: The description of the evaporation of rainfall intercepted by forests in terms of a regression of evaporation loss on incident rainfall is discussed and some of the assumptions implicit in that method are re-examined. The two major factors which control the evaporation of intercepted rainfall are identified. These are: (i) the amount of time that the canopy spends saturated during rainfall and the evaporation rate applicable under these conditions; and (ii) the canopy saturation capacity and the number of times this store is emptied, by drying out after the cessation of rainfall. A model is then constructed which is conceptually similar to the Rutter model, but which replaces that model's numerical approach with an analysis by storm events. The evaporation from a saturated canopy during rainfall is estimated from the Penman-Monteith equation; the evaporation after rain has ceased, the effect of small storms insufficient to saturate the canopy, wetting-up the canopy and evaporation from the trunks are added as separate terms. The model has been tested against data from Thetford Forest in East Anglia, with satisfactory agreement between observation and estimation. It is suggested that the model may be capable of making useful estimates of the evaporation of intercepted rainfall, solely from rainfall measurements.

Abstract: Canopy structure and light interception were measured in an 18-m tall, closed canopy deciduous forest of sugar maple (Acer saccharum) in southwestern Wisconsin, USA, and related to leaf structural characteristics, N content, and leaf photosynthetic capacity. Light attenuation in the forest occurred primarily in the upper and middle portions of the canopy. Forest stand leaf area index (LAI) and its distribution with respect to canopy height were estimated from canopy transmittance values independently verified with a combined leaf litterfall and point-intersect method. Leaf mass, N and A max per unit area (LMA, N/area and A max/area, respectively) all decreased continuously by over two-fold from the upper to lower canopy, and these traits were strongly correlated with cumulative leaf area above the leaf position in the canopy. In contrast, neither N concentration nor A max per unit mass varied significantly in relation to the vertical canopy gradient. Since leaf N concentration showed no consistent pattern with respect to canopy position, the observed vertical pattern in N/area is a direct consequence of vertical variation of LMA. N/area and LMA were strongly correlated with A max/area among different canopy positions (r2=0.81 and r2=0.66, respectively), indicating that vertical variation in area-based photosynthetic capacity can also be attributed to variation in LMA. A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns. Empirical relationships between leaf and canopy-level characteristics may help resolve problems associated with scaling gas exchange measurements made at the leaf level to the individual tree crown and forest canopy-level.