Crop Transpiration Rate Research Articles

A Review of Plastic Film Mulching on Water, Heat, Nitrogen Balance, and Crop Growth in Farmland in China

Plastic film mulching has been widely used to improve crop yield and water use efficiency, although the effects of plastic film mulching on water, heat, nitrogen dynamics, and crop growth are rarely presented comprehensively. This study investigated a large number of studies in film mulching fields from the past 10 years (mostly from 2019 to 2023) and summarized the impact of plastic film mulching, progress in modeling with film mulching, and future research directions. The effects of plastic film mulching were intricate and were influenced by film mulching methods, irrigation systems, crop types, crop growth stages, etc. Overall, plastic film mulching showed a positive effect on improving soil water, temperature, and nitrogen status, enhancing crop transpiration and photosynthetic rates, and promoting crop growth and yield, although film mulching may have negative effects, such as increasing rainfall interception, blocking water entering the soil, and reducing net radiation income. The crop yield and water use efficiency could increase by 39.9–84.7% and 45.3–106.4% under various film mulching methods. Coupled models of soil water and heat transport and crop growth under plastic film mulching conditions have been established by considering the effects of plastic film mulching on the upper boundary conditions of soil water and heat, energy budget and distribution processes, and the exchange of latent and sensible heat between soil and atmosphere. The models have good applicability in film mulched farmland of maize, rice, and potato for different regions of China. Further development is needed for soil water, heat, nitrogen migration, and crop growth models under different plastic film mulching methods, and the acquisition of soil and crop indicators under plastic film mulching conditions based on big data support. The study will provide reference for the subsequent development and innovation of plastic film mulching technology.

Evaluation of Crop Water Status and Vegetation Dynamics For Alternate Partial Root-Zone Drip Irrigation of Alfalfa: Observation With an UAV Thermal Infrared Imagery

Characterization of the spatiotemporal patterns of crop water status and vegetation dynamics are a prerequisite to overcome water scarcity and obtain precise agricultural water management. Based on high-resolution aerial thermal imagery, we estimated crop water stress index (CWSI), transpiration rate (T), and crop growth status. The research was conducted with conventional subsurface drip irrigation (abbreviated as R) and alternate partial root-zone drip irrigation (abbreviated as P) under four different irrigation quotas: 0 mm, 10 mm, 20 mm, and 30 mm, which are denoted as CK, R10 (P10), R20 (P20), and R30 (P30), respectively. Results indicate that the CWSI is a suitable tool to define alfalfa water status under different irrigation regimes. The CWSI values reflect CK > R10 > R20 > P10 > P20 > R30 > P30, with values of 0.57, 0.41, 0.26, 0.24, 0.18, 0.17, and 0.13, respectively. The T values show that CK < R10 < P10 < R20 < P20 < R30 < P30, with values of 0.46 mm/h, 0.61 mm/h, 0.70 mm/h, 0.71 mm/h, 0.76 mm/h, 0.77 mm/h, and 0.78 mm/h, respectively. In addition, under the same irrigation quotas, the CWSI presented as P10, P20, and P30 was lower than R10, R20, and R30, respectively, while the T was the opposite. Taking the improved transpiration rate as the amount of water saving, it was demonstrated that the alternate partial root-zone drip irrigation was a water-saving method, and each increase of 10 mm in alternating irrigation quotas could save 67.2, 18.4, and 4.6% of water, respectively. This evaluation contributes to a better understanding of the spatiotemporal variations of water and growth status and provides references and theories for the development of modern precise agriculture.

Exploring the Use of Vegetation Indices for Validating Crop Transpiration Fluxes Computed with the MOHID-Land Model. Application to Vineyard

The success of an irrigation decision support system (DSS) much depends on the reliability of the information provided to farmers. Remote sensing data can expectably help validate that information at the field scale. In this study, the MOHID-Land model, the core engine of the IrrigaSys DSS, was used to simulate the soil water balance in an irrigated vineyard located in southern Portugal during three growing seasons. Modeled actual basal crop coefficients and transpiration rates were then compared with the corresponding estimates derived from the normalized difference vegetation index (NDVI) computed from Sentinel-2 imagery. On one hand, the hydrological model was able to successfully estimate the soil water balance during the monitored seasons, exposing the need for improved irrigation schedules to minimize percolation losses. On the other hand, remote sensing products found correspondence with model outputs despite the conceptual differences between both approaches. With the necessary precautions, those products can be used to complement the information provided to farmers for irrigation of vine crop, further contributing to the regular validation of model estimates in the absence of field datasets.

Development of a transpiration model for precise tomato (Solanum lycopersicum L.) irrigation control under various environmental conditions in greenhouse

Transpiration can directly reflect the response of the crop growth and development, therefore irrigation design based on a transpiration model is an important factor towards establishing an efficient irrigation strategy. Thus, the purpose of this experiment is to develop and verify a tomato transpiration model by correcting the relationship between the transpiration rate and environmental factors by measuring the actual transpiration rate. The actual crop transpiration rate, which is measured using a load cell, and the weight changes calculated at 10-min intervals, are applied to the development of the transpiration model. The experimental results show that the transpiration rate has no linear relationship with the radiation amount (Rad) or vapor pressure deficit (VPD). The relationship between Rad and VPD with transpiration rate was fitted by the exponential rise to maximum, and gaussian peak curve, respectively. This allowed a transpiration model to be developed by compensating the Rad and VPD based on the existing Penman–Monteith (P-M) equation. The developed transpiration model showed higher regression constant values than the existing one. The developed transpiration model from the experiment can be utilized for precise irrigation control.

Higher forage yields under temperate drought explained by lower transpiration rates under increasing evaporative demand

In temperate regions, perennial forage-based cropping systems are expected to face an increasing frequency of summer droughts over the next decades prompting the need for more resilient cultivars. However, most efforts mainly focus on Mediterranean-type environments where the plant survival is often engaged. Under temperate environments, vapor pressure deficit (VPD) is a key component of drought, because its variation alters the crop transpiration rate (TR) and therefore its ability to fix carbon even in well-watered conditions. Despite this knowledge, there is no available data about the diversity of whole-plant TR responses to VPD and soil moisture among key forage crops such as alfalfa, red clover, cock’s foot and perennial ryegrass. Further, field-based evidence is lacking regarding the links between TR responses to VPD and yield under drought. Here, we combined experimental approaches characterizing gas exchange responses to VPD and soil moisture at scales that ranged from the growth chamber to the field, where yields were characterized both quantitatively and qualitatively over the course of 2 years on 8 genotypes from the 4 above species. A significant variability in TR responses to increasing VPD and soil water deficit was found among locally-adapted cultivars. More importantly, TR responses to VPD – but not to decreasing soil moisture – were found to be consistently correlated to relative yield performances under drought, in a way indicating that conservative water use under high evaporative demand promoted higher yield outputs. In contrast, yields under drought were unrelated to canopy temperature and leaf gas exchange measured in the field. Further, no link was found between TR responses to VPD and qualitative yield traits such as digestibility indicating that the hypothesized water saving strategy does not improve yield at the expense of forage quality. This study opens the way for future forage breeding and management strategies taking advantage of the diversity of TR responses to drought to implement climate-change resilient forage-based systems.

Assessing crop coefficients for Zea mays in the semi-arid Hailiutu River catchment, northwest China

To improve irrigation water-use efficiency, plant transpiration and soil evaporation in a maize (Zea mays L.) field in the Bulang sub-catchment of the Hailiutu River catchment in Northwest China were determined using in situ measurements. Crop transpiration (Tp) rates from Jul 15 to Oct 1, 2011 were measured with sap flow sensors, and soil evaporation (Ep) rates were measured with micro-lysimeters under an absence of water deficit. The two rates together gave the total evapotranspiration (ETc) of the maize field. Cumulative Tp and Ep were 245 and 85mm, accounting for 74 and 26% of total ETc (330mm), respectively. To calculate the total ETc rate of the maize field for the entire growing season, the Penman–Monteith equation combined with a single crop coefficient method (FAO-56) was used. The estimated crop coefficient (Kc) was calibrated using actual sap flow and soil evaporation data to provide accurate estimates of actual evapotranspiration. The total crop ETc of the maize field for the 2011 and 2012 growing seasons was 583 and 500mm, respectively, with a mean daily value of ∼4mmd−1. Groundwater contributed 33% of the maize ETc in 2011 (average groundwater table of 1.12m with full irrigation) and 27% in 2012 (average groundwater table of 0.89m with full irrigation). These results will improve precise planning and efficient management of irrigation for maize in this region.

Estimating maize ground cover using spectral data from Aqua-MODIS in Córdoba, Argentina

Ground cover by foliage is a biophysical property of vegetation linked both to the interception of photosynthetically active radiation and to the crop transpiration rate. The spectral information provided by the Moderate Resolution Imaging Spectroradiometer on board the Aqua (Aqua-MODIS) satellite, which has a spatial resolution of 250 m, is an observation and monitoring resource that may be appropriate for estimating the ground cover of maize when plots exceed 40 ha. In this research, 10 maize plots were monitored in the central region of the province of Córdoba, Argentina, during the 2005–2006 growing season, obtaining photographic records of ground cover and soil moisture data. The normalized difference vegetation index (NDVI) of the Aqua-MODIS images showed a significant linear relationship with maize ground cover which, when the complete cycle is taken into account, is sufficient to explain 87% of the variability of ground cover, with an RMSE of 9%, a level of accuracy that increases when the crop is in the vegetative stage and the moisture conditions of the soil are less limiting. Other vegetation indices and linear mixed models were assessed. In addition to using data from the red and near-infrared channels, they incorporate information about soil conditions, but they showed no predictive advantages compared to the NDVI, resulting in simple models that explained between 77% and 87% of the variability of ground cover, with RMSE values of between 9% and 14%.

Predicting the transpiration of lucerne

Crop transpiration rate is often predicted as the minimum of the supply of water from the roots and the demand for water from the shoots. A common method for predicting transpiration demand is the transpiration efficiency approach, which uses daily dry matter growth, constant transpiration efficiency coefficient (ω) and the vapour pressure gradient from the leaf to the atmosphere. The former is usually represented by daytime averaged atmospheric vapour pressure deficit (VPD′z), and assumes that the difference between air temperature and leaf temperature is small. In this paper we show that this assumption is likely to be incorrect in cool, moist environments where unstressed leaves may become substantially warmer than the surrounding air. We used VPD′z to calculate an apparent ω for field grown lucerne and found it increased from 1.5 to 3.5Pa across a mean daily temperature range of 7–18°C. We present an alternative method for calculating transpiration demand which we call the canopy conductance approach, which uses the product of VPD′z, intercepted radiation (Ri) and a canopy conductance coefficient (ΘT). Experimental data showed transpiration demand of lucerne was linearly related to Ri and VPD′z and, in contrast to the variability of the derived value of ω, a constant and stable ΘT of 0.45mmMJ−1kPa−1 was derived. Predictions of transpiration were compared with transpiration calculated from soil water balance measurements for dryland and irrigated lucerne crops grown over 3 years at Lincoln University, Canterbury, New Zealand. The canopy conductance approach accurately (±0.05mmd−1, RMSD=26% of observation mean) predicted transpiration from an independent data set, confirming its simplicity and efficacy. Calculations of transpiration using the transpiration efficiency approach with a constant ω of 5.0Pa underestimated transpiration by 1.4mm d−1 (RMSD=126%). However, using ω and changing it in proportion to radiation use efficiency (which changed in response to temperature) substantially improved the accuracy of the growth approach (±0.10mmd−1, RMSD=30%). The methods presented can be adopted to improve the generic applicability of transpiration predictions in crop simulation models.

Reliability of current Spanish irrigation designs in a changed climate: a case study

SUMMARYA very serious effort to modernize irrigation systems is being made in Spain, to reduce water and energy losses in an environmentally sustainable way. This is expensive and it is important that the new irrigation systems work properly over a long period. The systems have been designed taking into account historical evapotranspiration (ET) averages during the months of maximum demand, as well as the crop-specific ET values (Kc coefficients) of typical crops. However, the increase in ET rates due to global warming could mean that the capacity of these new and expensive irrigation systems to fulfil the crop water requirements may be exceeded in the near future. However, the expected increase in CO2 concentration could diminish crop transpiration rates for similar water demands from the atmosphere, thereby reducing irrigation requirements. A methodology was developed in order to estimate crop water requirements under climate change conditions. The reliability of a new irrigation system designed in Valladolid, Northern Spain was tested. The regionalized climate change scenarios for Valladolid, provided by the National Institute of Meteorology, were used for the periods 2011–40, 2041–70 and 2071–2100 and the A2 and B2 emission scenarios were considered using the ECHAM and coupled general circulation model (CGCM) global circulation models. A historical series of daily meteorological data for Valladolid was used to generate statistical ET distributions through the LARS-WG generator. Simulations considered each of the above periods, global circulation models (GCM) and emission scenarios. Furthermore, the Kc of the typical irrigated crops of the zone (maize, potato and sugar beet) were reduced for each period, GCM and emission scenario according to the relationships between CO2 concentrations and transpiration obtained by Kruijt et al. (2008). The results indicated that, on average, historical ET rates provide a sufficiently robust indicator to enable estimations of the crop ET in the future, particularly considering the CO2 effect in reducing crop transpiration. However, ET variability is significantly increased after 2040, especially for the A2 emission scenario. The results show that ET variability rather than global increase is the most serious risk that current irrigation systems must face in the near future in Northern Spain, as consequence of climate change. Such variability should be included in irrigation designs.

Measurement and Simulation of the Ventilation Rates in a Naturally Ventilated Azrom-Type Greenhouse in Zimbabwe

A simple greenhouse ventilation model, based on the stack and wind effects (the main driving forces for natural ventilation) was adapted, calibrated, and validated using measured air renewal rates in a three-span naturally ventilated Azrom-type greenhouse in Zimbabwe. Crop transpiration rates were monitored using stem heat balance sap flow gauges installed on the main stems of rose plants to continuously monitor whole-plant transpiration (WPT). This allowed continuous and automatic determination of full scale air renewal and leakage rates using the water vapor balance method. The model was fitted to experimental data of ventilation rates, and discharge and wind effect coefficients were determined. The results show a good fit between measured and predicted values (R2 = 0.80 and 0.82 for winter and summer, respectively), although there is a general over-estimation of the greenhouse air renewal rates, particularly during the night. The model, nevertheless, adequately describes the natural ventilation process in the greenhouse all year round. The model can be used as a design tool to evaluate and optimize the effects of different ventilation configurations and strategies on greenhouse air renewal rates, and as a component in a greenhouse climate model in order to further evaluate the effects of ventilation strategies on the inside greenhouse and crop microclimate, and thus lead to better greenhouse climate control.

Response of an eggplant crop grown under Mediterranean summer conditions to greenhouse fog cooling

The effects of greenhouse cooling using a high-pressure fog system on greenhouse microclimate and on eggplant ( Solanum melongena) crop response were studied at the coastal area of western Greece. Measurements were carried out in two distinct greenhouse compartments involving: (1) no air humidity control and (2) a fog system operating in order to obtain a greenhouse air relative humidity of 80%. Fog cooling reduced mean fruit temperature by about 3 °C and maintained greenhouse air temperature below 32 °C, while maximum temperature without cooling reached 40 °C. Furthermore, fogging reduced air vapor pressure deficit by about 55% and increased crop stomatal conductance by about 73%. These changes, in combination with alterations in crop aerodynamic conductance, resulted in around 31% decrease of crop transpiration rate. Fog system cooling efficiency was relatively low (46%) resulting in relatively high water consumption for fog cooling, reaching an equivalent to about 60% of crop water uptake. Nevertheless, since fog cooling decreased crop water needs, total greenhouse water consumption with fog cooling was only 19% higher than with no air humidity control. These results indicate the need to increase the cooling efficiency of fog systems in order to reduce greenhouse water consumption. Furthermore, the fog system enhanced mean fruit weight and marketable fruits, but appreciably reduced total fruit number per plant. Finally, fog cooling did not affect fruit quality characteristics such as fruit resistance to penetration, skin colour, fruit titratable acidity and total soluble solids.

MANAGEMENT OF GREENHOUSE CROP TRANSPIRATION: THE WAY FORWARD

The importance of management of greenhouse crop transpiration increases, being part of both the water and energy balance. A simulation model for crop transpiration can serve as a soft-sensor in an early warning system for the grower, and is an essential component of an energy model for a greenhouse with a crop. Published data on model validation of crop transpiration under commercial settings are scarce. In an effort to develop a model-based soft-sensor for crop transpiration, continuous and instantaneous rates of crop transpiration were obtained over a large part of 2006 from a tomato grower using a weighing gutter. The wide variation in environmental conditions caused similarly wide variation in crop transpiration rates, both among and within days. This enabled broad model validation. Validation gave over-estimation of crop transpiration, but parameters that relate the stomatal conductance to environmental conditions were successfully calibrated on the basis of total daily transpiration. Although seasonal calibration may in certain cases be sufficiently accurate to enable robust simulation of daily course of crop transpiration, a more robust approach is to calibrate for shorter time periods than an entire season. Robustness is a prerequisite for on-line management of the water and energy balances. Further data analysis can reveal structural patterns in the relations between model parameters and simulated transpiration. Built on this, on-line sensor information on transpiration can be used to continuously optimize the transpiration model, and increase its usefulness in information and early-warning systems.

STEERING OF FOGGING: CONTROL OF HUMIDITY, TEMPERATURE OR TRANSPIRATION?

Fogging systems are increasingly used to cool greenhouses and prevent water stress. More recently, fogging systems are applied also in relatively low radiation environments (such as The Netherlands), for a better control of product quality than whitewashing and to reduce need for natural ventilation ? thus allowing for higher CO2 concentrations to be maintained in the greenhouse. Most commonly the steering of such systems is done by setting an upper limit to the deficit of specific humidity that, whenever exceeded, triggers the fogging system. In both cases, however, one may wonder whether static and pre-fixed set points are the most effective choice. In the experiment presented in this paper, fogging and venting were controlled with the purpose of steering crop transpiration. The desired transpiration rate was the input of an algorithm that calculated on-line the required humidity and air temperature set points in view of the current weather factors. The set points were then the input of a standard P-controller that calculated vent opening and time of operation of the fogging system. In this paper, the resulting climate and actuator control operations are discussed and compared with a similar greenhouse controlled in a traditional fashion. The study concluded that a desired crop transpiration rate (an all-round indicator of crop well-being) could be used to select dynamic set points for the climate control in a greenhouse equipped with a fogging system.

Greenhouse Microclimate and Soilless Pepper Crop Production and Quality as Affected by a Fog Evaporative Cooling System

The influence of greenhouse humidity control on greenhouse microclimate, crop transpiration rate, and yield and fruit quality of a soilless grown pepper crop was studied in a greenhouse located at the coastal area of western Greece. Measurements were carried out during summer and autumn in two distinct greenhouse compartments involving: (1) no air humidity control and (2) a fog system operating when the relative humidity of the greenhouse air was lower than 80%. Under fog conditions, the greenhouse air and the crop leaf temperature were about 3°C lower than those measured under no fog conditions. The fog system allowed maintenance of the greenhouse air temperature under 30°C, while a maximum value of about 35°C was reached under no fog conditions. In addition, under fog conditions, the air vapor pressure deficit was lower than 2 kPa, even during the warmest part of the day, while under no fog conditions it reached values near 4 kPa. However, the transpiration rate was not affected to such an extent by fog as the air vapor pressure deficit, and under fog conditions it decreased by about 26%. This is attributed to the values reached by the bulk stomatal conductance, which were about 1.5 times higher under fog than under no fog conditions. The crop leaf area index values observed after the middle of the experimental period and later, under fog conditions, were higher than that observed under no fog conditions. Finally, the fog system enhanced the mean fruit weight and the percentage of marketable fruits but reduced appreciably the total number of fruits per plant. The free (titratable) acidity and the total soluble solids in the pepper fruit sap were slightly reduced by the fog cooling system, while the fruit size was increased. The use of a fog cooling system proved to be beneficial for summer crops of pepper grown under Mediterranean climatic conditions due to a favorable impact of the reduced vapor pressure deficit on both the mean fruit weight and the quality in terms of fruits graded Class I. Nevertheless, the high air humidity imposed by a cooling fog system may reduce the number of fruits per plant, thereby offsetting the benefits from the increased mean fruit weight in terms of total yield.

Evaporative cooling of a ventilated greenhouse rose crop

Greenhouse crops may suffer from excessive heat load in warm weather. A procedure is developed to evaluate latent heat cooling by means of crop transpiration and free water evaporation from a wet pad and fan system. The procedure uses concurrent external climatic factors as input data. It treats construction characteristics (dimensions and radiometric properties of the roof cover), plant foliage (leaf area, stomatal conductance) and ventilation rate as parameters to calculate heat transport coefficients. Measurements in a greenhouse rose crop ( Rosa indica L cv ‘Mercedes Long’) show that the numerical solution of the energy balance equation predicts accurately crop transpiration, foliage temperature, air temperature and humidity inside the greenhouse. With ventilation rates of 30 volume changes per hour and external air humidity below 50%, transpiration of a plant well supplied with water, cools the foliage and the air in the greenhouse below external temperature even when solar radiation is at its maximum value. Cooling obtained with an evaporative wet pad at the air inlet lowers vapour pressure deficit in the greenhouse and decreases transpiration rate. Still, total latent heat dissipation added to pad evaporation and crop transpiration is higher than that obtained by crop transpiration without the wet pad. The combined solution of the energy balance of the air passing through the evaporative pad and of the crop predicts accurately transpiration of the rose crop and internal temperature and humidity. The evaporative pad cools the air considerably; but the lowering of transpiring leaf temperature is only minor. Evaporation from the pad decreases when external humidity increases. Crop transpiration rate when the wet pad operates is nearly independent of external humidity and ventilation rate.

The effects of different irrigation regimes on cucumber ( Cucumbis sativus L.) yield and yield characteristics under open field conditions

A study was conducted to determine the effects of different drip irrigation regimes on yield and yield components of cucumber ( Cucumbis sativus L.) and to determine a threshold value for crop water stress index (CWSI) based on irrigation programming. Four different irrigation treatments as 50 (T-50), 75 (T-75), 100 (T-100) and 125% (T-125) of irrigation water applied/cumulative pan evaporation (IW/CPE) ratio with 3-day-period were studied. Seasonal crop evapotranspiration (ET c) values were 633, 740, 815 and 903 mm in the 1st year and were 679, 777, 875 and 990 mm in the 2nd year for T-50, T-75, T-100 and T-125, respectively. Seasonal irrigation water amounts were 542, 677, 813 and 949 mm in 2002 and 576, 725, 875 and 1025 mm in 2003, respectively. Maximum marketable fruit yield was from T-100 treatment with 76.65 t ha −1 in 2002 and 68.13 t ha −1 in 2003. Fruit yield was reduced significantly, as irrigation rate was decreased. The water use efficiency (WUE) ranged from 7.37 to 9.40 kg m −3 and 6.32 to 7.79 kg m −3 in 2002 and 2003, respectively, while irrigation water use efficiencies (IWUE) were between 7.02 and 9.93 kg m −3 in 2002 and between 6.11 and 8.82 kg m −3 in 2003. When the irrigation rate was decreased, crop transpiration rate decreased as well resulting in increased crop canopy temperatures and CWSI values and resulted in reduced yield. The results indicated that a seasonal mean CWSI value of 0.20 would result in decreased yield. Therefore, a CWSI = 0.20 could be taken as a threshold value to start irrigation for cucumber grown in open field under semi-arid conditions. Results of this study demonstrate that 1.00 IW/CPE water applications by a drip system in a 3-day irrigation frequency would be optimal for growth in semiarid regions.

Numerical Estimation of Evapotranspiration Rate in a Greenhouse

A simulation model constructed in this study consisted of three sub models, i.e. the radiation environment model, the crop transpiration model, and the soil water and heat transfer model. The radiation model was used for simulating the distribution of short- and long-wave radiation in a greenhouse. Amounts of short-wave radiation in several processes such as transmission on a plastic film, interception by crop plant, arrival on soil surface were estimated. The distribution of short-wave flux on soil surface was obtained by reproducing the process that short-wave radiation passes through foliages. Crop transpiration rate was calculated via a heat balance on surfaces of a single big-leaf. The processes of water and heat transfer in the soil were calculated for the two-dimensional plane. The accuracy of the sub models was confirmed by comparing estimates with measurements. The short-wave radiations arriving on a canopy and soil surface were estimated well. Then, the reproduction of a short-wave radiation environment around a canopy could be successful. The estimates of temperature of leaf and soil surfaces also agreed with measurements. This simulation model would be useful to estimate evapotranspiration rate by distinguishing into transpiration and soil evaporation.

Simple Indirect Estimation of Ventilation and Crop Transpiration Rates in a Greenhouse

A simple linear model has been developed, based on the greenhouse crop heat and mass balances allowing for the calculation of inside air temperature and humidity together with crop temperature. This model is valid for a mature non-stressed crop with low-temperature difference between inside and outside. It can numerically be inverted in order to identify the greenhouse ventilation function together with the soil heat storage parameter, by fitting measured and calculated data of crop temperature and inside air temperature and humidity, and by deducing crop transpiration rate. This model was tested in summer in a ‘classically ventilated tunnel’ with a small opening surface (6%), and in a ‘largely opened tunnel’ with a large opening surface (18%). Measurements were carried out when the tomato crop was mature. On the basis of the experimental measurements, the coefficients of efficiency for ventilation were determined and used to validate the model with respect to inside air measurements. The identified values for the ventilation coefficients are in agreement with the values reported in the literature. Likewise, calculated inside air speed deduced from ventilation in both tunnels was also in good agreement with the measured values. It is shown that this approach allows for a precise estimation of ventilation and transpiration rates using only simple measurement devices such as temperature and air humidity sensors.

Natural Ventilation of a Greenhouse with Continuous Roof Vents: Measurements and Data Analysis

Although natural ventilation plays an important role in affecting greenhouse climate, as defined by temperature, humidity and CO2 concentration, particularly in Mediterranean countries, little information and data are presently available on full-scale greenhouse ventilation mechanisms. In this paper, we present the main results of a 4-yr study (1988-1992) involving both experiments and modelling of natural ventilation.Various experimental techniques were used to make full-scale measurements of the air exchange rate in a 400 m2 plastic-greenhouse, equipped with roof ventilators. Theses included two tracer gas techniques, (i) the decay rate method using N2O as tracer gas in the presence of a crop and (ii) the constant enrichment technique using CO2 but without a crop and (iii) the greenhouse water vapour balance based on measurements of inside and outside air humidity and tomato crop transpiration rates. Good agreement was found between these different experimental approaches.Two models based on a non-dimensional analysis of the flux of ventilation were derived and compared with the measurements and with other results already published in the literature. It was found that the wind effect is the main driving force of ventilation. The air exchange rate is linearly dependent on the wind speed and on the effective opening of the vents. No effect of the wind direction on ventilation performance was found.

Experimental Investigation and Modelling of Heat and Mass Transfer between a Tomato Crop and the Greenhouse Environment

Experimental investigation and modelling of heat and mass transfer between a tomato crop and the greenhouse environment is elaborated. The transfer of sensible and latent heat between the canopy and the ambient air is assumed to take place via an exchange area equal to the total leaf area across two resistances, the internal and the external, which are properly defined. The external resistance is determined as a function of the Nu number. A method is proposed to parameterize the internal resistance as a function of the canopy temperature, the canopy full spectrum net radiation and the crop-air vapour pressure deficit. A model is proposed for the calculation of the crop temperature and crop transpiration rate as a function of time and the environmental variables. The calculated canopy temperature compared well with the measured one, which was found to be lower than that of the greenhouse air. Calculated canopy transpiration rates are presented as a function of time and the environmental variables. The canopy transpiration flux was found to be higher than that of the full spectrum crop net radiation on a 24 h basis. The comparison of calculated crop transpiration on a daytime basis with those obtained by two other models was satisfactory only at moderate solar radiation intensities. During days with high radiation intensities the present model compared well with one model only.