Irrigation scheduling of confectionery groundnut (Arachis hypogeaea L.) in Senegal using a simple water balance model

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Abstract

The sustainability of the groundnut production in North of Senegal hydro-agricultural systems depends on the development of irrigation methods which guarantee yield, quality and meet best water management practices. To achieve this goal, a water balance model taking into account plant development and soil water status was developed. The proposed model expresses evapotranspiration as a function of the observed leaf area index (LAI) and simulated soil water status. Compared to classical tools, mostly based on crop coefficients, the conceptual advantage of the developed model is to take into account not only the impact of water availability on leaf development of the crop but also their incidence on transpiration. Irrigation scheduling consists of determining net irrigation levels by adjusting the fraction of transpirable soil water (FTSW). The model was first calibrated and validated using experiments carried out in 1994, 1996 and 2000, on two groundnut varieties, Fleur 11 and GH 119-20, at the experimental station of Bambey in Senegal, using sprinkler irrigation methods. The model was then applied in 2000 during the dry season to the Senegal river valley under drip irrigation conditions so as to schedule irrigation according to diverse situations of water resource availability. Evidence was made of the model capacity to devise a sustainable irrigation method which guarantees yield, quality and appropriate water management practices.

Introduction

The recent development of the Senegal river basin (Gay and Dancette, 1995) presents a new opportunity for groundnut production under irrigated conditions. The increase in irrigated areas should be accompanied by the search for export markets such confectionery groundnut production. However, the increase in the used amount of irrigation water could lead to a reduction in available water resources, which should be paid for and shared by more users. Agricultural research is therefore faced with the question of developing appropriate sustainable production techniques.

The effect of soil water availability on a number of plant development mechanisms has been considerably reported. For instance on groundnut or on pea, water stress reduces leaf area through leaf number and leaf size (Ong et al., 1985, Lecoeur and Guilioni, 1998, Lecoeur et al., 1995). In bean, sorghum and cotton, water deficit has also been manifested by an earlier leaf senescence (Karamanos, 1978, Rosenthal et al., 1987). Moreover, in a review on the effect of limiting available soil water to many cultivated species, Sadras and Milroy (1996) reported that leaf expansion showed higher sensitivity to soil water limitation compared to transpiration. Yet, when transpiration is affected by soil water status through the stomata closure (Jarvis and McNaughton, 1986, Comstock, 2002), the combination of the long-term effects on leaf area and of the short-term effects on stomatal conductance reduces the gaseous exchange and leads to a reduction of the canopy water consumption (Jones, 1992). Based on these results, groundnut yield may be regulated by the amount of water available to the plant during its development (Hammer et al., 1995). Concerning the quality management of the crop, Ramamoorthy and Basu (1996) reported that occurrence of water stress during flowering affects the number of mature pods per plant and the seed size. Nevertheless, reducing irrigation at the end of the peak-flowering stage generally results in yield (Nageswara Rao et al., 1985) and pod quality increases, in particular concerning their commercial grade (Nautiyal et al., 1991). These results show that the effect of water deficits depends on its intensity and time of occurrences. This points out the need to master irrigation scheduling.

Most available water balance models are based on the use of characteristics for each phase of plant cycle (Doorenbos and Pruit, 1977). In fact, water regime and farmer practices (plant density) have an important impact on vegetative plant development which renders ineffective the use of invariable crop coefficients for each developmental stage. Hammer et al. (1995) proposed an interesting simple model which included estimate of phenology, leaf expansion, biomass accumulation and soil water balance. However, most of the proposed parameters have been estimated for a temperate climate. Moreover, daily transpiration estimations are based on the calculation of the daily biomass increment involving the soil water module which could not be used independently.

The aim of this study is to develop and evaluate a model to simulate the fraction of transpirable soil water (FTSW) as proposed by Sinclair and Ludlow (1986) for two groundnut varieties cultivated in Senegal. The developed water balance model uses a set of simple equations which allows daily estimation of available soil water based on daily meteorological data, a succinct description of the soil characteristics as well as an estimation of leaf and root development. Based on the hypothesis that soil water status is the only factor that significantly influences the transpiration of the crops studied, this model provides a tool that could be used to schedule irrigation. From daily climatic data, hydrodynamic characteristics of the soil and LAI, the model permits to calculate the amount of water to be applied in order to reach a targeted FTSW. The results using the model to test targeted soil water status paths on confectionery groundnut production and water availability, under drip irrigation conditions are included in this paper.

Section snippets

Model development

The adopted original model is based on a pea model (Lecoeur and Sinclair, 1996), in which it is possible to simulate the fraction of transpirable soil water, which accounts for the amount of soil water available for the plant in the root zone area (George et al., 2000). The model considers the soil as a reservoir with two compartments, whose relative sizes vary in time with root growth (Fig. 1). The first compartment, explored by the roots, contains the soil water reserve. This compartment is

LAI growth and senescence curves assessment

Results obtained in 1994 and 1996 showed that, whatever the water treatment applied or the variety, observed phase 1 and phase 2 LAI data could satisfactorily be adjusted by a three parameters sigmoid curve and a straight line, respectively (treatment nos. 1–12 from Table 1). For Fleur 11 and GH 119-20 phase 1 (all treatments together), R2 values were 0.823 and 0.861, corresponding to RMSD of 0.81 and 0.60, respectively. For phase 2, R2 values were 0.712 and 0.631, with RMSD of 1.11 and 0.64

Conclusion

The developed model has an advantage in simulating soil and crop water balance based on data related to climate, soil, and onsite specific development status of the groundnut crop. This approach permitted to take into account all the environmental effects on the changes in plant leaf area. Consequently we limited as much as possible the model development to physic processes by integrating the plant architecture plasticity, one of the major adaptive responses of plants to their environment

Acknowledgements

The authors wish to thank the technicians at CERAAS for their assistance in collecting field data. They also wish to thank Dr. Harold Roy-Macauley and Dr. Serge Braconnier for suggestions to improve the paper and Yaye Couna Sylla for informatic assistance. They are also grateful to the “Comité National Interprofessionnel de l’Arachide” (CNIA) of Senegal and the European Union (EU) for their financial support.

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