Sustainability of irrigated agriculture under salinity pressure – A study in semiarid Tunisia

In semiarid and arid Tunisia, water quality and agricultural practices are the major contributing factors to the degradation of soil resources threatening the sustainability of irrigation systems and agricultural productivity. Nowadays, about 50% of the total irrigated areas in Tunisia are considered at high risk for salinization. The aim of this thesis was to study soil management and salinity relationships in order to assure sustainable irrigated agriculture in areas under salinity pressure. To prevent further soil degradation, farmers and rural development officers need guidance and better tools for the measurement, prediction, and monitoring of soil salinity at different observation scales, and associated agronomical strategy. Field experiments were performed in semi-arid Nabeul (sandy soil), semi-arid Kalâat Landalous (clay soil), and the desertic Fatnassa oasis (gypsiferous soil). The longest observation period represented 17 years. Besides field studies, laboratory experiments were used to develop accurate soil salinity measurements and prediction techniques. In saline gypsiferous soil, the WET sensor can give similar accuracy of soil salinity as the TDR if calibrated values of the soil parameters are used instead of standard values. At the Fatnassa oasis scale, the predicted values of ECe and depth of shallow groundwater Dgw using electromagnetic induction EM-38 were found to be in agreement with observed values with acceptable accuracy. At Kalâat Landalous (1400 ha), the applicability of artificial neural network (ANN) models for predicting the spatial soil salinity (ECe) was found to be better than multivariate linear regression (MLR) models. In semi-arid and desertic Tunisia, irrigation and drainage reduce soil salinity and dilute the shallow groundwater. However, the ECgw has a larger impact than soil salinity variation on salt balance. Based on the findings related to variation in the spatial and temporal soil and groundwater properties, soil salinization factors were identified and the level of soil “salinization risk unit” (SRU) was developed. The groundwater properties, especially the Dgw, could be considered as the main cause of soil salinization risk in arid Tunisia. However, under an efficient drainage network and water management, the soil salinization could be considered as a reversible process. The SRU mapping can be used by both land planners and farmers to make appropriate decisions related to crop production and soil and water management

Fonctionnement du drainage au sein d'une oasis modernisée du sud tunisien. Cas de l'oasis de Fatnassa Nord à Kébili, Tunisie

Un dispositif pérenne de suivi de l'irrigation et du drainage a été mis en place par l'INRGREF et le Cemagref dans l'oasis de Fatnassa Nord. Ce dispositif expérimental permet un suivi temporel des volumes et des concentrations en sels des eaux d'irrigation et de drainage au niveau parcellaire et de l'ensemble du périmètre (l'oasis) et ce en vue de décrire et d'évaluer l'efficience de ces systèmes « récents » dans ce contexte oasien. Ceci nous a amené à caractériser et à quantifier les bilans hydrique et salin au niveau de l'oasis (114 ha) et d'une parcelle (0,8 ha), et de faire le lien avec les modalités de gestion de l'irrigation au sein des exploitations. Dans ce contexte, quelques résultats obtenus entre avril 2003 et septembre 2005 sont présentés dans cette communication. A l'échelle de la parcelle, l'analyse piézométrique met en évidence une très forte dynamique de la nappe superficielle suite aux irrigations, qui témoignent d'une remontée systématique de cette nappe à chaque irrigation et d'une vitesse de tarissement assez liée à la demande climatique. En termes de sels apportés, ces cumuls correspondent respectivement à 48,4 ; 30,4 et 23,3 t/ha de sels. Cela montre l'importance cruciale du drainage et de l'apport d'une fraction de lessivage pour mieux contrôler cette salinité. Les mesures au niveau de la sortie ont montré une variation saisonnière de la fraction drainée et en fonction du volume apporté à chaque tour d'eau. Par ailleurs, on a estimé l'exportation de sels due au pompage au niveau de cette parcelle vers les extensions et qui représente un ordre de grandeur quadruple de celui du drainage, soit de 60 tonnes /ha /an. Cette pratique traduit une gestion hydrosaline complexe entre l'oasis et les extensions et il est nécessaire de mieux estimer le poids de cette gestion dans l'établissement du bilan notamment à l'échelle du périmètre. A l'échelle de l'oasis, les différents termes du bilan hydrosalin (entrée - sortie) révèlent un bilan déficitaire sur toute la période de mesure. Les quantités d'eau et de sels sont estimées en entrée et en sortie du système sur deux années de mesure. Les entrées d'eau et de sels sont mesurées à l'échelle de la parcelle et extrapolées à l'échelle de l'oasis, et sont confirmées par les informations recueillies au niveau des forages. Les sorties par drainage souterrain sont mesurées indépendamment à deux exutoires. En revanche, les sorties de sel par l'écoulement naturel de la nappe vers le Chott n'ont pas été estimées. Au niveau des deux exutoires, 15 t/ha de sels ont été évacués par drainage sur 48 t/ha apportés pour l'année 1 et 5 t/ha ont été évacués sur 30 t/ha apportés pour l'année 2 avec une bonne répétabilité entre les deux exutoires. Le rapport correspondant entre volume de drainage et d'irrigation est de 11 % et 6 % respectivement aux deux années de mesure avec un coefficient de concentration quasi-constant de l'ordre de 2,8. Ces résultats traduisent un déficit respectif de 69 % et 84 % en termes de quantités de sels évacués par drainage. Ceci nous amène à nous interroger sur les performances techniques et économiques d'un tel système de drainage souterrain et sur sa pérennité dans ce milieu vu sa sensibilité au colmatage, constatée sur le terrain, par ensablement et pénétration des racines

Soil water content and salinity determination using different dielectric methods in saline gypsiferous soil

Abstract in UndeterminedMeasurements of dielectric permittivity and electrical conductivity were taken in a saline gypsiferous soil collected from southern Tunisia. Both time domain reflectometry (TDR) and the new WET sensor based on frequency domain reflectometry (FDR) were used. Seven different moistening solutions were used with electrical conductivities of 0.0053-14 dS m(-1). Different models for describing the observed relationships between dielectric permittivity (K-a) and water content (theta), and bulk electrical conductivity (ECa) and pore water electrical conductivity (ECp) were tested and evaluated. The commonly used K-a-theta models by Topp et al. (1980) and Ledieu et al. (1986) cannot be recommended for the WET sensor. With these models, the RMSE and the mean relative error of the predicted theta were about 0.04 m(3) m(-3) and 19% for TDR and 0.08 m(3) m(-3) and 54% for WET sensor measurements, respectively. Using the Hilhorst (2000) model for ECp predictions, the RMSE was 1.16 dS m(-1) and 4.15 dS m(-1) using TDR and the WET sensor, respectively. The WET sensor could give similar accuracy to TDR if calibrated values of the soil parameter were used instead of standard values

Soil water and salinity distribution under different treatments of drip irrigation.

In this study, field experiments and numerical simulations for different drip irrigation treatments in a sandy loam soil were conducted to investigate soil water and salinity distribution as well as dye infiltration patterns. Three treatments, surface drip irrigation without and with plastic mulch (T1 and T2, respectively); and subsurface drip irrigation (T3), were used. In addition, daily and bi-weekly irrigation regimes were considered during performing each treatment. After ceasing the designed irrigation schedule of each treatment, horizontal soil sections were dug with 10 cm intervals. Then, dye patterns were captured using a digital camera and soil water and pore water electric conductivity were measured by a WET-sensor. Experiments results revealed that maximum dye infiltration depth and maximum dye coverage volume occurred during the bi-weekly irrigation regime and in T3. Daily irrigation regime kept the top soil layer moist with adequate amount of soil water as compared to bi-weekly regime. Moreover, T2 provided higher soil water content within the soil domain as compared to other treatments. The simulation results also demonstrated that model prediction for soil moisture distribution within the flow domain was excellent. Furthermore, T2 and daily irrigation showed lower salinity levels in the flow domain as compared to other irrigation treatments and regimes. In sum, mulching treatment with daily irrigation regime is recommended for arid areas over other drip irrigation treatments and regimes. In addition, HYDRUS-2D/3D can be used as a fast and cost effective assessment tool for water flow and salt movement for specific sites having similar soil conditions

Modelling the Impact on Root Water Uptake and Solute Return Flow of Different Drip Irrigation Regimes with Brackish Water

Water scarcity and quality degradation represent real threats to economic, social, and environmental development of arid and semi-arid regions. Drip irrigation associated to Deficit Irrigation (DI) has been investigated as a water saving technique. Yet its environmental impacts on soil and groundwater need to be gone into in depth especially when using brackish irrigation water. Soil water content and salinity were monitored in a fully drip irrigated potato plot with brackish water (4.45 dSm−1) in semi-arid Tunisia. The HYDRUS-1D model was used to investigate the effects of different irrigation regimes (deficit irrigation (T1R, 70% ETc), full irrigation (T2R, 100% ETc), and farmer’s schedule (T3R, 237% ETc) on root water uptake, root zone salinity, and solute return flows to groundwater. The simulated values of soil water content (θ) and electrical conductivity of soil solution (ECsw) were in good agreement with the observation values, as indicated by mean RMSE values (≤0.008 m3·m−3, and ≤0.28 dSm−1 for soil water content and ECsw respectively). The results of the different simulation treatments showed that relative yield accounted for 54%, 70%, and 85.5% of the potential maximal value when both water and solute stress were considered for deficit, full. and farmer’s irrigation, respectively. Root zone salinity was the lowest and root water uptake was the same with and without solute stress for the treatment corresponding to the farmer’s irrigation schedule (273% ETc). Solute return flows reaching the groundwater were the highest for T3R after two subsequent rainfall seasons. Beyond the water efficiency of DI with brackish water, long term studies need to focus on its impact on soil and groundwater salinization risks under changing climate conditions