Study Of Parameterization Scheme On Regional Evapotranspiration Flux Over Heterogeneous Land Surface

Land surface hydrologic process is one of the crucial processes which controls the land surface process and the land-atmosphere interaction. However, to hydrological cycle which contains the processes of preciptation, run-off, evaporation and storage of soil water, the heterogeneity of physical and ecological characteristics of soil surface and their description method and the hydrologic flux to land surface is still a challengeable problem in modeling of land surface. In this paper, starting with the physical mechanics of land surface hydrologic process and the introduction of the theory in probability statistic distribution, an analytical expression of heterogeneous evapotranspiration efficiency is given which takes heterogeneous soil water content's subgrid scale spatial variability into account, through which the parameterization scheme of subgrid scale soil surface's evapotranspiration efficiency(which is generally called mosaic method) is modified to a statistic-dynamic scheme in which a whole heterogeneity of the grid region is considered. The scheme is on the basis of the heterogeneity of soil water content's horizontal spatial distribution in the region and the evaporative power of relatively saturated region. If the probability density function(PDF) of soil water content's horizontal spatial distribution and the evaporative power of relatively saturated region are attainable, the mean evapotranspiration flux of the region can be calculated.To demonstrate the heterogeneity of hydrological flux and the significant influence on regional evaporation, firstly, it is proved theoretically that because of soil water content's horizontal spatial variability(heterogeneous distribution) an additional heterogeneous disturbance which is equivalent to mesoscale flux is induced. Numerical experiment shows that this heterogeneous disturbance arising from soil water content's spatial variability cannot be neglected when regional mean evaporation fluxis estimated. Secondly, based on NOAA_AVHRR satellite remote sensing data, by means of thermal-inertia method and semi-experiential semi-theoretical soil water content model, the soil water content's spatial distribution of Changjiang Delta is retrieved and the corresponding PDF is fitted. The results show that at least in summer halfyear(from April to September), the soil water content's spatial distribution obeys B -distribution or Weibull-distribution, and the soil water content's PDF is suitable for moist climate region.So as to estimate evaporation power of heterogeneous soil surface more efficiently, the K-B model is improved. The experiment shows that the improved model is applicable to the calculation of evaporation in moist climate region. Compared with hydrologic balance method, the improved model is more credible in calculating evaporation of Changjiang Delta.According to the regional evapotranspiration analytical expression of heterogeneous soil surface applicable to moist climate region which is put forward in this paper, on one hand, soil water absorption is calculated under the consideration of the hydrological condition and hydraulic characteristic of soil surface; on the other hand, using improved K-B model which is based on Penman formula, the potential evaporation power is calculated. Thereby the complicated evaporation process of heterogeneous soil surface is simplified into a flux integral expression which contains two-pail contributions from soil water relatively saturated region and unsaturated region, and it is proved that the calculation of mean evapotranspiration efficiency in grid region can be simplified into weighted average of heterogeneous regions. It is demonstrated by experiment that this statistic-dynamic parameterization scheme has more superiorities.