Root water uptake under non-uniform transient salinity and water stress

The study described in this thesis focuses on the quantitative understanding of water uptake by roots under separate and combined salinity and water stresses. The major difficulty in solving Richards' equation stems from the lack of a sink term function that adequately describes root water uptake. From the existing microscopic and macroscopic sink term functions, the empirical macroscopic approach was chosen because it requires the least number of parameters whose values can readily be determined. All existing reduction functions as well as those newly developed in this study are used in the macroscopic model and tested against experimental data. The experimentally obtained data are used to derive the parameter values needed for the simulation model HYSWASOR. The experiments cover root water uptake by alfalfa under salinity stress, water stress, and combined salinity and water stress. This order is followed with the analysis of the data and the simulation.Under salinity stress , both experimental and simulated results indicate that the well-known linear crop response function can be used as a reduction function. The parameter values available in the literature for different reduction functions cannot provide acceptable agreement with the experimental data. When experimentally derived parameters are used in the simulation model, the agreement becomes much closer, but calibration is still needed. The parameter values obtained by calibration differ slightly from the experiments, because the experimentally derived parameter values are based upon mean soil solution salinity. Both experimental and simulation results indicate that different salinity reduction functions can provide almost the same results if the parameter values are well specified. For practical use the linear reduction function with the least number of parameters appears to be adequate.Under water stress , all existing reduction functions as well as the one developed in this study are tested on the experimental data. Since the trend of the experimental relative transpiration versus mean soil water pressure head is nonlinear, the linear reduction function cannot fit the data. The existing nonlinear reduction functions can fit only half of the data range satisfactorily. The best agreement is obtained with the newly developednonlinear two-threshold reduction function .The parameter values obtained by calibration differ only slightly from those of the experiments. Soil water pressure head heterogeneity over the root zone does not play an important role in water uptake. The roots appear to take up water from the relatively wetter parts of the root zone to compensate for the water deficit in the drier parts. On the first day after irrigation both relative transpiration and relative leaf water head are almost the same for the stressed and non-stressed plants. While the simulated transpiration agrees closely with the experimental data, the main reason for the discrepancy between the simulated and actual water contents appears to be water uptake during the night.Under combined water and salinity stress , the additive and multiplicative reduction functions are first tested against the experimental data and then inserted in the simulation model. A new combination reduction function is introduced that differs conceptually from the additive and multiplicative functions. Both the experimental and simulated results show that the newly proposed model fits the data best, while the worst results are obtained with the simple additive model.</p

Microgrid working conditions identification based on cluster analysis. A case study from lambda microgrid

This article presents the application of cluster analysis (CA) to data proceeding from a testbed microgrid located at Sapienza University of Rome. The microgrid consists of photovoltaic (PV), battery storage system (BESS), emergency generator set, and different types of load with a real-time energy management system based on supervisory control and data acquisition. The investigation is based on the area-related approach - the CA algorithm considers the input database consisting of data from all measurement points simultaneously. Under the investigation, different distance measures (Euclidean, Chebyshev, or Manhattan), as well as an approach to the optimal number of cluster selections. Based on the investigation, the four different clusters that represent working conditions were obtained using methods to define an optimal number of clusters. Cluster 1 represented time with high PV production; cluster 2 represented time with relatively low PV production and when BESS was charged; cluster 3 represents time with relatively high PV production and when BESS was charged; cluster 4 represents time without PV production. Additionally, after the clustering process, a deep analysis was performed in relation to the working condition of the microgrid

Simulation of root water uptake. I. Non-uniform transient salinity using different macroscopic reduction functions

A macroscopic root extraction model was used with four different reduction functions for salinity stress in the numerical simulation model HYSWASOR. Most of the parameter values originally proposed for these functions did not provide good agreement with the experimental data. Therefore, the parameter values were derived from extensive measurements of one of five salinity treatments of alfalfa experiments in the greenhouse and then validated with the four remaining treatments. The simulation results indicated that a well-known crop yield response function can be used as a water uptake term, using the same crop-specific slope and a modified salinity threshold value. The most sensitive part of this reduction function appeared to be the threshold value; while for the non-linear reduction function, without a threshold, the major sensitivity lies in its shape parameter. The simulated actual cumulative transpirations are rather close to the experimental values, while the simulated soil water contents and soil solution osmotic heads indicate some discrepancies with the actual data, but the mean values of these variables are very close to the measured data. While the non-linear two-threshold reduction function provides better agreement with the experimental data for most treatments, all other functions provided close results. This observation suggests the use of the simple linear reduction function in simulation model

Simulation of root water uptake. III Non-uniform transient combined salinity and water stress

Six different reduction functions for combined water and salinity stress are used in the macroscopic root water extraction term. The reduction functions are classified as linear additive, non-linear multiplicative, and that which is neither additive nor multiplicative. All these reduction functions are incorporated in the numerical simulation model HYSWASOR. The relation between the experimental relative transpiration and the joint soil water osmotic and pressure heads appears to be linear (with an exception for the salinity near the threshold value). As the mean soil solution salinity increases, the trend becomes more linear. The simulations indicated that for most treatments the newly proposed reduction term provides the closest agreement with the experimental transpiration. Soil water content, and particularly soil solution salinity simulated with this equation agree reasonably with the experimental data: in spite of the observed differences, the trend of the simulated data is good. A reason for the disagreement between the simulated and experimental water contents can be attributed to the influence of roots and the soil solution concentration on the soil hydraulic conductivity. The input soil hydraulic parameters were obtained from soil samples without roots and salinity and assumed constant during the simulations

Simulation of root water uptake. II. Non-uniform transient water stress using different reduction functions

The macroscopic root water uptake approach was used in the numerical simulation model HYSWASOR to test four different pressure head-dependent reduction functions. The input parameter values were obtained from the literature and derived from extensive measurements under controlled conditions in the greenhouse. The simulation results indicated that the linear reduction function cannot fit the data satisfactorily. Most of the existing non-linear reduction functions can fit only half of the data range, while the best agreement is obtained with the non-linear two-threshold reduction function. The parameter values obtained by calibration differ only slightly from those of the experiments. Soil water pressure head heterogeneity over the root zone does not play an important role in water uptake. The roots appear to take up water from the relatively wetter parts of the root zone to compensate for the water deficit in the drier parts. While the simulated transpiration agrees closely with the experimental data, the main reason for the discrepancy between the simulated and actual water contents appears to be water uptake during the night