Modeling Study Of Evapotranspiration And Its Components In A Subtropic Planted Coniferous Forest

Evapotranspiration (ET) in the terrestrial ecosystem actually includes canopy transpiration (Ec), soil evaporation (Es), and interception evaporation (Eint). Partitioning of ET is an important basis in order to accurately assess biomass production and estimate water use efficiency in terrestrial ecosystem. Actually, Ec is the desired component within the water cycling, which being used to enhance plant productivity. Up to now, it is still difficult to accurately partition ET through observation methods in the long term. Instead, partioning ET with models are effective in a long period. However, most of them roughly partition ET into Ec and Es, ignoring Eint. This partition way could lead to deviation in  and Es simulation. In this study, a modified Shuttleworth-Wallace model was used to partition ET into Ec, Es and Eint in forest ecosystem. Monte Carlo, an approach of random parameterization, was performed to optimize the key parameters in the functions of estimating soil surface resistance and canopy stomatal resistance in the model. Based on the modified model, we simulated the evapotranspiration and its components in the planted coniferous forest ecosystem at the Qianyanzhou site combining the eddy covariance measurement data (including sensible/latent heat flux and CO2 flux) with routine meteorological data (including air temperature/humidity, CO2 concerntration, wind speed, net radiation, soil temperature/water content/heat flux and precipitation) in 2011.The results showed that the simulation amount of ET agreed well with the measurement data at 30 minutes temporal scale not matter on sunny days or rainy days. At the same temporal scale, the index of agreement, mean error, and root mean square error between simulation value and measurement data in whole year were 0.73,0.21 mmol m-2 s-1, and 1.55 mmol m-2 s-1 respectively. The proportion of ET in annual accumulative precipitation was more than 80%, which meant that the amount of ET contributing to the water outputs is the most in this ecosystem. The proportion of canopy transpiration in ET was about 85%, which suggested that water use efficiency in the forest ecosystem might be quite high in 2011. The seasonal variation of the transpiration was obvious. The environmental factors, vapor pressure deficit and air temperature, and leaf area index controlled transpiration. Transpiration correlated positively with the three factors. Soil evaporation was 5% of the ET and changed little with season. Interception evaporation accounted for 10% of the ET and showed seasonal dynamic. This term correlated positively with precipitation and negatively with rainfall storm events. This result indicated that the vegetation canopy couldn’t effectively intercept the heavy rainfall. The modified Shuttleworth-Wallace model is a powerful tool for studying the hydrological processes in terrestrial ecosystem with the advantages of a few parameters, high temporal resolution, and a good performance in simulating the characteristics of evapotranspiration and its components.It’s recommended to get the dynamic measurement of air CO2 concentration and leaf area index when selecting the inputs of the modified SW model. If eddy covirance technique is used to measure the fluxes above the canopy, it will be OK of replacing the mean wind speed inside the canopy with the measured wind speed at the referrence height. Also it is just fine to input the air temperature instead of the canopy skin temperature, if the latter is hard to access. Since the model is highly sensitive to the extinction coefficient, great attention should be paid when the model is applied to study a wide region.