I would like to find a good function to fit kinetic data of leaf senescence (few and meaningful parameters). So far I have chosen a Gompertz function to explain green area index over thermal time. Do you use the function properties (Gompertz or any else), or do you estimate senescence onset as the time for an empiric percentage of senescence threshold?
Dear François,
The following paper might be helpful.
A simple algorithm for yield estimates: Evaluation for semi-arid irrigated winter wheat monitored with green leaf area index Benoıˆt Duchemin a, *, Philippe Maisongrande b , Gilles Boulet a , Iskander Benhadj a
http://www.cesbio.ups-tlse.fr/data_all/SAFY/008_Duchemin_EMS.pdf
Received 27 July 2006; received in revised form 11 June 2007; accepted 3 October 2007
Available online 3 December 2007
Abstract
In this study we investigated the perspective offered by coupling a simple vegetation growth model and ground-based remotely-sensed data for the monitoring of wheat production. A simple model was developed to simulate the time courses of green leaf area index (GLAI), dry aboveground phytomass (DAM) and grain yield (GY). A comprehensive sensitivity analysis has allowed addressing the problem of model calibration, distinguishing three categories of parameters: (1) those, well known, derived from the present or previous wheat experiments; (2) those, phenological, which have been identified for the wheat variety under study; (3) those, related to farmer practices, which has been adjusted field by field. The approach was tested against field data collected on irrigated winter wheat in the semi-arid Marrakech plain. This data set includes estimates of GLAI with additional DAM and GY measurements. The model provides excellent simulations of both GLAI and DAM time courses. GY space variations are correctly predicted, but with a general underestimation on the validation fields. Despite this limitation, the
approach offers the advantage of being quite simple, without requiring any data on agricultural practices (sowing, irrigation and fertilisation). This makes it very attractive for operational application at a regional scale. This perspective is discussed in the conclusion.
Keywords: Crop model; Sensitivity; Irrigated wheat; Production; Yield; Leaf area index
Rafik
A couple of refs applied a modified Gompertz to fit decay as in http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7348.2000.tb00011.x/abstract or http://jxb.oxfordjournals.org/content/early/2015/08/04/jxb.erv378.full, and a rather simple excel tool has been introduced for basic growth functions with essential refs inhttp://aob.oxfordjournals.org/content/90/4/485.full
We have used photothermal time in preference to just thermal time. The PhD thesis of Rizaldi Boer at Sydney University deals with this aspect. The thesis was entitled 'Climatic Constraints on anthesis of wheat in a major wheat growing area of Australia'.
In eastern Australia, the rate of senescence is fairly linear even with thermal time; it deviates if very hot conditions prevail say 35+ C.
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