170,023 research outputs found
Uncertainty in the determination of soil hydraulic parameters and its influence on the performance of two hydrological models of different complexity
No full textData of soil hydraulic properties forms often a limiting factor in unsaturated zone modelling, especially at the larger scales. Investigations for the hydraulic characterization of soils are time-consuming and costly, and the accuracy of the results obtained by the different methodologies is still debated. However, we may wonder how the uncertainty in soil hydraulic parameters relates to the uncertainty of the selected modelling approach. We performed an intensive monitoring study during the cropping season of a 10 ha maize field in Northern Italy. The data were used to: i) compare different methods for determining soil hydraulic parameters and ii) evaluate the effect of the uncertainty in these parameters on different variables (i.e. evapotranspiration, average water content in the root zone, flux at the bottom boundary of the root zone) simulated by two hydrological models of different complexity: SWAP, a widely used model of soil moisture dynamics in unsaturated soils based on Richards equation, and ALHyMUS, a conceptual model of the same dynamics based on a reservoir cascade scheme. We employed five direct and indirect methods to determine soil hydraulic parameters for each horizon of the experimental profile. Two methods were based on a parameter optimization of: a) laboratory measured retention and hydraulic conductivity data and b) field measured retention and hydraulic conductivity data. The remaining three methods were based on the application of widely used Pedo-Transfer Functions: c) Rawls and Brakensiek, d) HYPRES, and e) ROSETTA. Simulations were performed using meteorological, irrigation and crop data measured at the experimental site during the period June – October 2006. Results showed a wide range of soil hydraulic parameter values generated with the different methods, especially for the saturated hydraulic conductivity <i>K</i><sub>sat</sub> and the shape parameter &alpha; of the van Genuchten curve. This is reflected in a variability of the modeling results which is, as expected, different for each model and each variable analysed. The variability of the simulated water content in the root zone and of the bottom flux for different soil hydraulic parameter sets is found to be often larger than the difference between modeling results of the two models using the same soil hydraulic parameter set. Also we found that a good agreement in simulated soil moisture patterns may occur even if evapotranspiration and percolation fluxes are significantly different. Therefore multiple output variables should be considered to test the performances of methods and models
Entanglement of two blocks of spins in the critical Ising model
No full textWe compute the entropy of entanglement of two blocks of L spins at a distance d in the ground state of an Ising chain in an external transverse magnetic field. We numerically study the von Neumann entropy for different values of the transverse field. At the critical point we obtain analytical results for blocks of size L =1 and 2. In the general case, the critical entropy is shown to be additive when d\to\infty. Finally, based on simple arguments, we derive an expression for the entropy at the critical point as a function of both L and d. This formula is in excellent agreement with numerical results
Utilizzo di un sistema di simulazione idrologico per l’analisi di scenari di cambio di uso del suolo e riallocazione della risorsa idrica
No full textNella memoria vengono presentati i risultati dell’applicazione di un sistema di simulazione idrologico integrato acque superficiali – acque sotterranee a scenari di cambio di uso del suolo e di limitazione alla disponibilità della risorsa idrica utilizzabile a fini irrigui per il territorio afferente al Comprensorio di Bonifica Muzza Bassa Lodigiana (porzione centro-meridionale della pianura lombarda), onde valutarne gli effetti sull’assetto complessivo delle risorse idriche, nonché sulla distribuzione del fabbisogno e del consumo irriguo da parte delle colture. Il sistema di simulazione utilizzato per lo studio è basato sull’accoppiamento di due modelli distribuiti: ALHyMUS, che opera il bilancio idrologico per il sistema suolo-pianta-atmosfera, e MODFLOW, per la riproduzione delle dinamiche del sistema acquifero e dell’interazione tra acque sotterranee e reticolo idrico superficiale. I due modelli sono accoppiati tramite un’interfaccia sviluppata ad hoc e integrati in un GIS. Gli scenari sono stati creati in accordo con: (1) i possibili effetti di Agenda2000 sulle colture della pianura lombarda, (2) la variabilità delle disponibilità idriche derivabili per uso irriguo, come conseguenza di fattori naturali (variabilità del regime idrologico) e/o artificiali (revisioni delle concessioni di derivazione). I risultati sono stati confrontati con quelli ottenuti per uno scenario di riferimento, costruito mantenendo uso del suolo, pratiche colturali e disponibilità idrica a fini irrigui attuali. I risultati mostrano chiaramente che la disponibilità irrigua attuale è sufficiente per soddisfare il fabbisogno delle colture, mentre modifiche diffuse degli attuali ordinamenti colturali possono determinare variazioni significative sia del fabbisogno irriguo comprensoriale che della ricarica al sistema di acquiferi. La riduzione della disponibilità alla derivazione porta ad un incremento dell’aliquota effettivamente utilizzata delle colture; tuttavia, per riduzioni significative, il fabbisogno irriguo delle colture non riesce ad essere completamente soddisfatto, soprattutto nei mesi di luglio e agosto, critici per il mais, coltivazione molto diffusa nell’area
Simulation of maize irrigation requirements at the regional scale : comparison between results obtained with measured and FAO-56 crop coefficient
No full textThe FAO-56 “single crop coefficient” or “double crop coefficient” approaches are the most recommended and widely adopted procedures for the estimation of crop irrigation requirements. In these methods crop evapotranspiration in well-watered conditions is calculated by multiplying the grass reference evapotranspiration ET0 determined by the Penman-Monteith FAO-56 equation and a crop coefficient Kc depending on the crop type and its growing stage. In particular, the “double crop coefficient” allows the separation of soil evaporation and crop transpiration, splitting Kc in two different terms: a basal crop coefficient Kcb and a soil evaporation coefficient Ke.
Many authors in the last fifteen years showed that the FAO Kc and Kcb tabulated coefficients, even if adjusted using the specific procedure based on local meteorological, irrigation and crop data suggested by FAO-56, tend to underestimate the observed crop coefficients in arid and semi-arid environments, while an overestimation often occurs for humid and semi-humid regions. In the literature differences up to ±40% especially during the middle growth cycle are reported, mainly due to the complexity of the crop coefficient which actually integrates several physical and biological factors.
The purpose of our research was to measure the Kc pattern for maize grown in the Lombardy Region (Northern Italy) and to evaluate the difference in crop irrigation requirements at a regional scale considering the measured Kc instead of the FAO tabulated values using a spatially distributed hydrological model.
Kc was calculated for two experimental maize fields for years 2006, 2010 and 2011 as the ratio between actual crop evapotranspiration (ET) in well watered conditions and ET0. ET was measured using eddy-covariance technique while ET0 was determined from agro-meteorological data registered by the two standard meteo stations closest to the experimental areas. The second step of the research was achieved by using the distributed model IDRAGRA, which allows the computation of crop irrigation requirements on the basis of the “double crop coefficient” FAO-56 approach. This model has been adopted in various projects carried out in collaboration with the Lombardia Regional Authority. In the simulations, the spatial variability of soil types and the spatial and temporal variability of meteorological inputs was taken into account.
Observed Kc and Kcb patterns showed that the mid-season stage tabulated values overestimate the observed values by around 18%; if adjustments with local data are considered for FAO crop coefficients, the average overestimation reduces to 14%. Results of the spatially distributed model application illustrated the effect of this overestimation on the crop irrigation requirements over the regional territory. Considerations on its repercussion in term of water resources planning were finally made
Performance of evapotranspiration models for a maize agro-ecosystem : from bare soil to maximum coverage in irrigated and rainfed conditions
No full textTo assure an efficient management and planning of irrigation water resources, an accurate computation of actual evapotranspiration (ET) from cropped surfaces is needed. ET models can be classified in two categories: “direct” methods, based on the original Penman-Monteith (P-M) equation, in which the canopy resistance rc is modelled, and “indirect” methods, based on the calculation of ET for a well-watered reference grass (ET0) with constant rc multiplied by a crop coefficient that represents the relative rate of ET from a specific crop and condition to that of the reference.
This last procedure, standardized by FAO-56 bulletin, is the most widely adopted for the estimation of ET. However, in literature there are evidences that direct methods (P-M models with rc modelled) are still the most performing. In fact, for indirect methods, errors introduced by the calculation of ET0 considering a constant rc for reference crop and by the estimation of the crop coefficient, which actually integrates several physical and biological factors, can be relevant.
This study evaluates the performance of different models for the estimation of ET for a maize agro-ecosystem in the Padana Plain (Northern Italy). The following models have been considered: 1) the “one-step” P-M model using a constant daily canopy resistance following the classical Monteith approach; 2) the “one-step” P-M model using a variable canopy resistance based on the approach of Katerji-Perrier, in which rc is calculated as a function of climate variables, aerodynamic resistance, vegetation type and its water status; 3) the “two-step” Shuttleworth model as updated by Shuttleworth and Gurney (1990), which combines one-dimensional models of crop transpiration and of soil evaporation, where canopy and soil surface resistances regulate the heat and mass transfer at the plant and soil surfaces, and aerodynamic resistances regulate those between these surfaces and the atmospheric boundary layer; 5) the indirect “single crop coefficient” method proposed by FAO-56; 6) the indirect “double crop coefficient” method proposed by FAO-56, which allows the separation of soil evaporation and crop transpiration.
Latent heat fluxes measured in 2006, 2010 and 2011 in an experimental maize field by eddy-covariance are used to evaluate the models accuracy. Crop, soil and meteo data monitored contextually are used for different models implementation. Data from the closest standard agro-meteorological station are adopted in the ET0 calculation for indirect methods.
Results of this work confirm what reported by other authors in the literature, demonstrating that the calculation of crop evapotranspiration by direct method is more accurate than the use of indirect methods for both irrigated (2006, 2010) and rainfed (2011) conditions
Evaluation of four models to estimate evapotranspiration under well-watered conditions for a maize agro-ecosystem in northern Italy
No full textTo ensure efficient management and planning of irrigation water resources, an accurate computation of actual evapotranspiration (ET) from cropped surfaces is needed. ET for well irrigated crops may be evaluated using models adopting a Penman-Monteith type formula (direct) or models based on a “crop coefficient approach” (indirect). This study evaluates the performance of different models for the estimation of ET for a maize agro-ecosystem grown in the Padana Plain (Northern Italy): 1) the original “onestep” PM model; 2) the “two-step” Shuttleworth model combining one-dimensional models of crop transpiration and soil evaporation; 3-4) the indirect “single crop coefficient” and “double crop coefficient” methods proposed by FAO-56, the latter allowing the separation of soil evaporation and crop transpiration. Latent heat fluxes measured in 2006 by eddy-covariance are used to evaluate the models accuracy. Results show that: i) the calculation of ET by direct methods is more accurate, confirming what found by many authors in the literature; ii) the PM “one-step” model underestimates ET during the initial and development growth stages, while it shows a good performance for LAI>3; iii) the SW “two-step” model successfully simulates ET; iv) the “single” and “double crop coefficient” FAO-56 models with generalized crop coefficients overestimate ET for the entire agriculture season; v) further research is needed to make the direct models suitable for operational use
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