Study On The Mechanical Models For Simulating Transpiration Under A Wide Range Of Water Condition

Plant transpiration depends on environmental conditions?e.g.micrometeorological factors and root zone soil moisture?,and soil water deficit is considered the dominant environmental constraint for plant transpiration?and photosynthesis?under water-limited conditions.Buckley,Turnbull and Adams?2013?proposed a simplified process-based model,which is noted as BTA based on the first name of authors,and widely used to estimate plant transpiration and sap flow under stable soil water conditions with simple formulation and less variables?solar radiation?R_s?and vapor pressure deficit?VPD??.However,it fails to simulate transpiration when soil moisture varies widely,as water stress index does not included in the BTA model.In this study,to improve the performance of BTA under a wide range water condition,we modified the BTA model based on soil water potential???,soil water content???and crop water stress index?CWSI?,and name them as BTA-?,BTA-?and BTA-CWSI,respectively.All the proposed models were examined in simulating transpiration rates at 5 sites under subtropical humid climate zone and Mediterranean climate conditions at both daily and hourly resolutions.The results demonstrated that all proposed models improve the performance of BTA significantly and simulate transpiration effectively under different water conditions,providing useful tools for estimating plant water use at different temporal and spatial scales and under different soil moisture conditions.The detailed results are shown as follows:?1?The parameter E_max?maximum transpiration or sap flow under optimal environmental conditions?in the BTA model was replaced by an equation including soil water potential???and other two parameters to explicitly represent the dependence of plant transpiration on root-zone moisture conditions.The improved model was denoted as BTA-?.The performance of the BTA and BTA-?models were assessed at two sites in Changsha,South China?a subtropical monsoon climate?and one site in Adelaide,South Australia?a Mediterranean climate?with different levels of water stress.The BTA model performed reasonably in estimating daily and hourly transpiration under sufficient water conditions,but it failed during dry periods.Overall,the BTA-?model provided a significant improvement for estimating transpiration under a wide range of soil moisture conditions.Although both models could estimate transpiration?sap flow?at night,BTA-?was superior to BTA in this regard.Species differences in the calibrated parameters of both models were consistent with leaf-level photosynthetic measurements on each species,as expected given the physiological basis of these parameters.?2?Process-based models often suffer from a lack of data?such as soil water potential for the BTA-?model?;empirical models are difficult to transfer the optimal form of environmental stress functions across sites?e.g.the modified Jarvis-Stewart model,MJS?.A hybrid model is proposed to address these two limitations.The model is a combination of the BTA model and an empirical function of volumetric soil water content???,and referred to as BTA-?.Three different water stress functions were adopted for BTA-?and MJS to test the transferability of optimal response function across species and climatic zones.Overall,BTA-?estimated transpiration reliably with the Nash-Sutcliffe coefficient of efficiency being larger than 0.5 for both wet and dry periods and outperformed BTA significantly under various soil moisture conditions.BTA-?with different water stress functions performed comparably at two climatic zones.The various forms of water stress function in BTA-?had negligible effect on parameterization of the BTA equation.However,the MJS model constructed with three water stress functions performed variably and had a big influence on parameterization of other stress functions.These results suggest that the hybrid BTA-?model is superior to the process-based BTA model in simulating transpiration under different levels of soil water deficit and is more robust than the purely empirical MJS model in selecting appropriate stress functions.?3?Thermal indices based on canopy temperature?Tc?measurements have been widely used to monitor plant water stress.Such applications have mainly been on homogeneous agriculture crops.Few studies of continuous observations have been conducted on plants in natural ecosystems.Here we examine if the commonly used crop water stress index?CWSI?applies for vegetation in non-managed ecosystems.Two types of parameterization,determined empirically from vapour pressure deficit?CWSI?e?and theoretically from Penman-Monteith equation?CWSI?t?,are evaluated on the potential of indicating plant water stress based on Tc and concurrent air temperature?T_a?measurements on three sites under a Mediterranean climate.Results show that although requiring more weather data inputs,CWSI?t does not perform better than CWSI?e,which only requires Tc,Ta and vapor pressure deficit?VPD?.Midday is the optimal time for Tc measurements to indicate water stress for most of the trees.Weather conditions of wind speed less than 3.0 m s-1 and solar radiation larger than 600 W m^-22 are recommended for T_c for the index calculations.An exponential relationship is found between CWSI?e and soil water content???,and a linear relationship exists between CWSI?e and soil water potential???.Values of?and?inverted from CWSI?e agree well with the observations?R² is about 0.7?,indicating that with careful selection of Tc,CWSI?e provides a reliable and non-contact manner for assessing plant water stress in natural ecosystems.?4?The measurements of soil water potential???and soil water content???are destructive and labor-intensive and localized,limiting the wide application of BTA-?and BTA-?.To solve these limitations,we estimated??and??based on the relationship between CWSI?e and??and??,and applied BTA-?and BTA-?with simulated?and?to estimate transpiration rates.Also,we directly adopted CWSI as a water stress index to modify the BTA model and named it as BTA-CWSI.By comparing with observations,BTA-?and BTA-?with simulated?and?can simulate transpiration rates under a wide range of water conditions and outperformed BTA,though they performed slightly poor to those with observed?and?.BTA-CWSI is able to estimate transpiration rates under different soil moisture conditions,demonstrating the effectiveness of CWSI as a water stress index in transpiration modeling.As canopy temperature can be obtained by multiple methods at different temporal and spatial scales,BTA-?,BTA-?and BTA-CWSI are likely to be applied in simulating plant water use and partitioning transpiration and evaporation at large scale.We proposed three methods with multiple water stress indices to improve the simplified process-based BTA model when root zone soil moisture varies widely.All of the proposed models,BTA-?,BTA-?and BTA-CWSI,performed very well at different climate zones under different levels of water stress.In the application,BTA-?is suggested when soil water potential data are measured in terms of the good performance and physiological characteristics.BTA-?can be adopted if there are only soil water content data.Without soil water potential and soil water content,the BTA-CWSI model can be used to estimate transpiration rates via canopy temperature measurements.Continuous data of??or??can be estimated based on the relationship between??or??and CWSI,thus BTA-??or BTA-??can be used to simulate transpiration under different levels of water stress.Overall,the research findings are beneficial for estimating plant transpiration under different levels of water stress,forest and agriculture water management and understanding the processes of ecohydrology.