CONTRAINTES HYDRIQUES POUR LA CULTURE DE PRINTEMPS

Study region and general methodology

The mountainous area of Vietnam is a typical case where the government encourages the development of spring crops. Rice, aerobic rice (i.e. able to withstand periods when the rice field is not flooded, (Bouman et al., 2005), soybean and maize are the main options promoted by agricultural extension services who provide, additionally to Month of the year technical training, seeds and fertilizers at attractive prices. Yet so far, in several municipalities of this area, double cropping became the most common practice among farmers. However, it failed to develop in many other locations, climatic constraints and the lack of adapted cultivars being possible causes. The summer season begins at the end of June with direct sowing or establishment of nurseries, transplanting taking place around 10- 15 July at the latest. Between the spring and summer crops, farmers need around 10 to 15 days for harvesting, threshing and transport, and for soil preparation. So, in order to introduce a spring crop without hampering the summer rice, cultivars and sowing dates have to allow harvesting the spring crop by July, 5 at the latest.
Irrigated rice fields in the northern highlands of Vietnam are found not only along valley floors, with elevation ranging from 15m to 500m (e.g. in Pho Yen, Thai Nguyen and Muong Thanh, Dien Bien Phu, respectively), but also on terraces carved along slopes, at elevation over 1400 m (e.g., Sa Pa, Lao Cai). Apart from the effects of elevation and slope, the climate of the region is known to vary greatly across space. Latitude and the position relatively to the main axis of the relief are key factors of this variation. The cultivars currently available in spring season in Vietnam for the previously mentioned crops (rice, aerobic rice, soybean, and maize) are insensitive to photoperiod, so that their cycle duration is mainly determined by temperature, the time interval between sowing and maturity increasing when temperature decreases (Rezaul, 1997). As a consequence, at a given location and for a given cultivar insensitive to photoperiod, the likeliness of completing the spring cycle before the 5th of July is expected to increase with the earliness of the sowing date.
The methodology adopted to meet the objectives of this study consists in two main stages. The first stage was devoted to calibrating the TRYSim model which simulates phenological development and crop growth for rice (irrigated and in ‘aerobic’ condition), maize and soybean without growth limitations from water, nutrients, pests or diseases. The purpose was therefore to estimate potential yield as defined by Lobell et al. (2009) for irrigated systems. TRYSim was built following the guidelines proposed by Sinclair and Seligman (1996) for ad hoc modelling. The model was specifically constructed for the study, using as much as possible simple relationships and model components that have proven to be robust after numerous tests in various environments including environments and crops similar to those of this study. In the second stage, the model was used for virtual experiments in which cropping systems were simulated using long term series of temperature and radiation data for a set of locations of contrasted climates, under scenarios combining varied elevations and sowing dates.

Model description

TRYSim was written in VBasic under Microsoft Access, so as to facilitate virtual experiments based on principles of interfacing between models and databases described by Affholder et al (2012). It uses a daily time step and simulates the duration of a crop cycle depending on thermal time, leaf area index (LAI) dynamics during the cycle, total aboveground biomass production resulting from the interception of incident solar radiation by the LAI, its conversion into biomass depending on the temperature, and yield through the allocation of a share of that biomass to the grain during a grain filling development phase, also determined by thermal time. The model also simulates the mortality of crops exposed to excessively cold temperatures. It accounts for transplanting and its impact on crop development and growth. The crop is assumed to be sown at a “standard” stand density corresponding to the recommendation made by the farmer support service in the region (see section 2.3).

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Table des matières

TABLE DES ILLUSTRATIONS
1. Liste de figures
2. Liste de tableaux
ABRÉVIATIONS ET SIGLES
LISTE DES PARAMÈTRES DU MODÈLE (TRYSIM ET WATRYSIM)
INTRODUCTION GÉNÉRALE
CHAPITRE 1. CONTEXTE, PROBLÉMATIQUE, MÉTHODOLOGIE
1. Contexte agricole
1.1. La zone montagneuse du Nord du Vietnam : un fort besoin de développement dans
un environnement naturel très contraint
1.2. Face au manque de surface agricole, le développement agricole passe par
l’intensification de l’agriculture
1.3. Implanter une culture de printemps dans les vallées impose de lever de lourdes
contraintes climatiques
2. Problématique
3. Démarche générale
4. Méthodologie : modèle agro-climatique et démarche expérimentale
4.1. Choix des localités et définitions des variables du modèle agro-climatique
4.2. Matériels et méthodes des dispositifs expérimentaux de terrain
4.3. Etapes de construction et d’utilisation de notre modèle agro-climatique
CHAPITRE 2. CONTRAINTES DE TEMPÉRATURE ET DE RAYONNEMENT POUR LA CULTURE DE PRINTEMPS
ABSTRACT
1. INTRODUCTION
2. MATERIALS AND METHODS
2.1 Study region and general methodology
2.2. Model description
2.3. Experiments for model calibration
2.4. Model calibration
2.5. Virtual experiment
3. RESULTS
3.1. Growth conditions established in the experiment
3.2. Model calibration
3.3. Virtual experiment
4. DISCUSSION
5. CONCLUSION
CHAPITRE 3. CONTRAINTES HYDRIQUES POUR LA CULTURE DE PRINTEMPS DANS
LES SYSTÈMES DE CULTURE DES TERRES IRRIGABLES DES MONTAGNES DU NORD DU VIETNAM
1. Introduction
2. Matériels et méthodes
2.1. Le modèle utilisé
2.2. Le dispositif expérimental
2.3. Calage du modèle
2.4. Simulation des scénarios
3. Résultats
3.1. Caractéristiques des sols expérimentaux
3.2. Conditions hydriques des vallées de montagne du Nord du Vietnam
3.3. Calage du modèle
3.4. Scénarios de cultures de printemps sous contrainte hydrique
4. Discussion
4.1. Modélisation du bilan hydrique
4.2. Modélisation de la germination et de la levée
4.3. Modélisation du LAI, de la biomasse et du rendement
4.4. Poids relatif de la contrainte hydrique et des autres contraintes climatiques pour la faisabilité d’une culture pluviale au printemps
5. Conclusion
CHAPITRE 4 : MARGES DE MANOEUVRE TECHNIQUE POUR RÉDUIRE LES CONTRAINTES CLIMATIQUES
1. Analyse de sensibilité du modèle
2. Evaluation des principales techniques envisageables
2.1. Sélection variétale
2.2. Irrigation
2.3. Serres tunnels pour les pépinières de riz irrigué conduit en repiquage
2.4. Réduction de l’évaporation par des mulchs en culture pluviale
3. Conclusion
CHAPITRE 5 : DISCUSSION GÉNÉRALE ET CONCLUSION
1. Rappel des partis pris méthodologiques et des principaux résultats
2. Méthode employée
3. Validité des résultats et leurs conséquences pour la problématique agricole
4. Perspectives pour la recherche agricole
RÉFÉRENCES
ANNEXES
Annexe 1. Plan du dispositif
Annexe 2. Méthodes de mesure et d’échantillonnage sur le terrain
Annexe 3. Structure de la base de données
Annexe 4. Données observées sur le dispositif
Annexe 5. Résultats d’expérimentation virtuelle