Winter wheat (Triticum aestivum L.) is one of most important procrops in the North China Plain. However, soil water deficit (SWD) often occurs due to lack of precipitation in its growing season. In this study, we introduce two semiempirical approaches, a recharge mode and the crop coefficient (Kc)-reference evapotranspiration (ET0) approach, to estimate wheat actual evapotranspiration (ETa) under no SWD and slight and severe SWD conditions. The recharge model allocated ET0 to reference evaporation and reference transpiration as a function of leaf area index. In the model, ETa is limited by soil water content, and crop water extraction for ETa is distributed through the soil profile as exponential functions of soil and root depth. The Kc-ET0 approach regardedETa under theSWDcondition as a logarithexmic function of soil water availability. Under no SWD condition, the recharge model simulated 10-d ETa with a root mean square error (RMSE) of 5.58 mm and a bias of 0.95 mm compared with measurements from a large-scale weighing lysimeter. The two approaches both estimated seasonal evapotranspiration (ET) well compared with the adjusted ET (from the soil water balance and the recharge model-simulated deep drainage). The recharge model, which simulated the seasonal ET with the RMSE of 27.8 mm and the bias of -8.0 mm, was better than the Kc-ET0 approach (RMSE =31.7 mm and bias= -33.1 mm). The seasonal pattern of soil water stress coefficient (Ks) showed that there were faster water losses at grain-filling stage than at other stages.

Mass and energy fluxes between the atmosphere and vegetation are driven by meteorological variables, and controlled by plant water status, which may change more markedly diurnally than soil water. We tested the hypothesis that integration of dynamic changes in leaf water potential may improve the simulation of CO2 and water fluxes over a wheat canopy. Simulation of leaf water potential was integrated into a comprehensive model (the ChinaAgrosys) of heat, water and CO2 fluxes and crop growth. Photosynthesis from individual leaves was integrated to the canopy by taking into consideration the attenuation of radiation when penetrating the canopy. Transpiration was calculated with the Shuttleworth-Wallace model in which canopy resistance was taken as a link between energy balance and physiological regulation.A revised version of the Ball-Woodrow-Berry stomatal model was applied to produce a new canopy resistance model, which was validated against measured CO2 and water vapour fluxes over winter wheat fields in Yucheng (36◦57'N, 116◦36'E, 28m above sea level) in the North China Plain during 1997, 2001 and 2004. Leaf water potential played an important role in causing stomatal conductance to fall at midday, which caused diurnal changes in photosynthesis and transpiration. Changes in soil water potential were less important. Inclusion of the dynamics of leaf water potential can improve the precision of the simulation of CO2 and water vapour fluxes, especially in the afternoon under water stress conditions.

This paper presents a new analysis of inequality of eddy diffusivities for sensible heat (KH) and water vapor (KW). It is shown that the same set of equations, established on the principle of dual-source diffusion, can be applied to both local and regional advection. Various advection scenarios are discussed using a formula that relates KH/KW to the Bowen ratio of the advective source and the observed gradient Bowen ratio (βg) near the ground surface. A similar analysis can also be performed for eddy diffusivities for trace gases. The ratio KH/KW, observed at a well-watered wheat field in the North China Plains, was mostly greater than unity when βg was negative and smaller than unity when βg was positive. The pattern was consistent with the theoretical analysis of the ratio under the influence of regional advection. Some degree of local advection was also suggested by the data. Despite inequality of the eddy diffusivities, there was little systematic bias between the evapotranspiration rates measured with a Bowen ratio/energy balance and an eddy covariance system.

A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pesudo first-order pocess, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance(gs) model that combines net photosynthetic rate (PN), transpiration rate (E), and gs and also the leaf energy balance, The model was run to simulate the diurnal courses of PN, E, gs, photochemical efficiency, i.e, ratio of intercellular CO2 concentration and CO2 concentration over leaf surface (C/Cs), and leaf temperature (T1) under different irradiances, air temperature, and humidity separately with fixed time courese of others. When midday depression occurred under high temperature, gs decreased the most and E the least. The duration of midday depression of gs wasthe longest and that in E the shortest. E incerased with increasing vapour pressure deficit (VPD) initially. but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in gs. When air temperature exceeded a certain value, an increase in solar irradiance raised T1 and gs showed reasonable decreases under conditions causing photoinhibition compared with non-photonihibition condition under high irradiance. The T1 under photoinhibition was higher than thae under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in Ci/Cs at midday implies that stomatal closure is a factor causing midday depression of photosynthesis