A Mariotte-based verification system for heat-based sap flow sensors

Determination of the accuracy of commonly used techniques for measuring sap flux density in trees often presents a challenge. We therefore designed and built a verification system for heat-based sap flow sensors typically used at stem level. In the laboratory, a Mariotte's bottle device was used to maintain a constant flow rate of water through freshly cut stem segments of American beech (Fagus grandifolia Ehrh.). This verification system was used to determine the accuracy of three heat-based sap flux density techniques: heat pulse velocity, thermal dissipation and heat field deformation. All three techniques substantially underestimated sap flux density when compared against gravimetric measurements. On average the actual sap flux density was underestimated by 35% using heat pulse velocity, 46% using heat field deformation and 60% using thermal dissipation. These differences were consistent across sap flux densities ranging from 5 to 80 cm(3) cm(-2) h(-1). Field measurements supported the relative sensor performance observed in the laboratory. Applying a sensor-specific correction factor based on the laboratory test to the field data produced similar estimates of sap flux density from all three techniques. We concluded that a species-specific calibration is therefore necessary when using any of these techniques to insure that accurate estimates of sap flux density are obtained, at least until a physical basis for error correction can be proposed

Parameter subset selection for the dynamic calibration of activated sludge models (ASMs): experience versus systems analysis

In this work we address the issue of parameter subset selection within the scope of activated sludge model calibration. To this end, we evaluate two approaches: (i) systems analysis and (ii) experience-based approach. The evaluation has been carried out using a dynamic model (ASM2d) calibrated to describe nitrogen and phosphorus removal in the Haaren WWTP (The Netherlands). The parameter significance ranking shows that the temperature correction coefficients are among the most influential parameters on the model output. This outcome confronts the previous identifiability studies and the experience based approaches which excluded them from their analysis. Systems analysis reveals that parameter significance ranking and size of the identifiable parameter subset depend on the information content of data available for calibration. However, it suffers from heavy computational demand. In contrast, although the experience-based approach is computationally affordable, it is unable to take into account the information content issue and therefore can be either too optimistic (giving poorly identifiable sets) or pessimistic (small size of sets while much more can be estimated from the data). An appropriate combinations of both approaches is proposed which offers a realistic (doable) and sound approach for parameter subset selection in activated sludge modelling

Radial sap flux profiles and beyond: an easy software analysis tool

Sap flow measurements are nowadays used worldwide to validate forest-ecosystem estimates of water use and storage. Tall trees with large radial variability in sap flux across their sapwood still present a challenge in scaling single point sap flux measurements to whole-tree sap flow. Assessment of the radial sap flux profile is, hence, necessary to make a precise scaling possible. A sensor useful for assessing the radial sap flux profile in trees is the heat field deformation (HFD) sensor. This sensor records changes in the heat field around a linear heater at different radial positions in the xylem and links the heat field deformation to sap flow. The technical construction of the HFD-sensor, however, requires the configuration of multiple channels on the data logger (usually 8 to 12 channels per sensor or 48 to 72 channels per one “cross section” of the tree trunk when using 6 sensors in one cross section). Logging each channel with a small scan rate (e.g. 5 min) quickly results in a massive amount of data. The task of analysing these large data sets can become difficult and cumbersome with spreadsheet programs and the large file sizes often prevent efficient data interpretation. The objective of this paper was therefore to develop a software tool for easy analysis of HFD-data sets, including very large ones. Its primary use is to visualise in a fast way the measured radial sap flux profile in both 2D and 3D. The computed results can be saved in a compact form, drastically reducing the resulting file size in comparison to the output generated using classical spreadsheet programs. But the features of the software tool go beyond the analysis of radial sap flux profiles. Due to easy determination, K-values inherent to the sap flux density calculations can now be used to derive stem specific properties (e.g. relative water content) or raw signals can be used to quantify the sap flow index

A step towards new irrigation scheduling strategies using plant-based measurements and mathematical modelling

Because of the increasing worldwide shortage of freshwater and costs of irrigation, a new plant-based irrigation scheduling method is proposed. In this method, two real-time plant-based measurements (sap flow and stem diameter variations) are used in combination with a mathematical water flow and storage model in order to predict the stem water potential. The amount of required irrigation water is derived from a time integration of the sap flow profile, while the timing of the irrigation is controlled based on a reference value for the predicted stem water potential. This reference value is derived from the relationship between midday values of maximum photosynthesis rates and stem water potential. Since modelling is an important part of the proposed methodology, a thorough mathematical analysis (identifiability analysis) of the model was performed. This analysis showed that an initial (offline) model calibration was needed based on measurements of sap flow, stem diameter variation and stem water potential. Regarding irrigation scheduling, however, only sap flow and stem diameter variation measurements are needed for online simulation and daily model calibration. Model calibration is performed using a moving window of 4 days of past data of stem diameter variations. The research tool STACI (Software Tool for Automatic Control of Irrigation) was used to optimally combine the continuous measurements, the mathematical modelling and the real-time irrigation scheduling. The new methodology was successfully tested in a pilot-scale setup with young potted apple trees (Malus domestica Borkh) and its performance was critically evaluated

Model-assisted evaluation of crop load effects on stem diameter variations and fruit growth in peach

Key message: The paper identifies and quantifies how crop load influences plant physiological variables that determine stem diameter variations to better understand the effect of crop load on drought stress indicators. Stem diameter (D (stem)) variations have extensively been applied in optimisation strategies for plant-based irrigation scheduling in fruit trees. Two D (stem) derived water status indicators, maximum daily shrinkage (MDS) and daily growth rate (DGR), are however influenced by other factors such as crop load, making it difficult to unambiguously use these indicators in practical irrigation applications. Furthermore, crop load influences the growth of individual fruits, because of competition for assimilates. This paper aims to explain the effect of crop load on DGR, MDS and individual fruit growth in peach using a water and carbon transport model that includes simulation of stem diameter variations. This modelling approach enabled to relate differences in crop load to differences in xylem and phloem water potential components. As such, crop load effects on DGR were attributed to effects on the stem phloem turgor pressure. The effect of crop load on MDS could be explained by the plant water status, the phloem carbon concentration and the elasticity of the tissue. The influence on fruit growth could predominantly be explained by the effect on the early fruit growth stages