رسائل دكتوراة
English
2022
Field experiment and numerical modelling of water flow, solute fate under micro irrigation system in oasis arid climate south part of Tunisia
Fethi Bouksila, Nessrine Zemni, Rachida Bouhlila, Slama Fairouz
المدرسة الوطنية للمهندسين - تونس-جامعة تونس المنار · تونس
In southern Tunisia, underground water is the only resource available to cover irrigation and different domestic needs. Until the beginning of the last century, aquifers exploitation was carried from artesian springs or foggara hollowed on the mountainside. The ancient oases owed their existence to the water of several artesian springs from fossil aquifers common to Tunisia, Algeria and Libya. In the Nefzaoua area, the CT aquifer is the main source for irrigation and is therefore showing signs of over exploitation, particularly in terms of salinity levels reaching 10g/L. Indeed, over time, the excessive use of these resources had led to a serious impact on groundwater. The extension of oases, the creation of boreholes and private agricultural activity as well as illicit wells have led to an overexploitation of the fossil water resources, resulting in a deterioration of the water quality. On the other hand, in this part of southern Tunisia, most of the phreatic aquifers are supplied by drainage water and irrigation water return flow, with a high salinity that varies between 3 and 10 g/l. In the Nefzaoua area, the Chott Jerid, which has always constituted the natural discharge of the SASS aquifers, now represents a risk of salinisation of these reservoirs by inversion of the hydraulic gradients due to overexploitation. Furthermore,in southernTunisia, irrigated farmland is dominated by the flood irrigation methods, which cause significant losses of percolation due to drought, irrigation intervals and poor irrigation management. Indeed, several scientific research undertaken in these regions showed an alarming situation and degradation of oasis resources (e.g. Salts accumulation in the root zone, saline water irrigation, aquifers overexploitation, etc.) Indeed, socio-economic changes and development policy options have largely make irrigation methods the focus of water and land resource saving. Authorities’ strategies should be focused on farmers’ practices. Oasis development can only be consolidated with a view to ensure its sustainability only with optimizing irrigation management and taking into account the needs of plants, water quality and soil salinityas well as the adequate irrigation technique. Inarid environment, a water saving technique, such “micro-irrigation system” with its different ways (drip irrigation, bubbler irrigation or buried diffusers) seems to be an appropriate solution in conserving water by reducing evaporation and deep percolation. The objective of this study is to quantify, through in situ monitoring and various models, the efficiency of micro-irrigation systems in a context of aridity and poor quality of available water. At present, Jemna site belonged to AVFA Kebeli (Extension Agency and the Agricultural Training), which is located at Nefzaoua region, Kbeli gouvernorat, South part of Tunisia, is equipped with two different system of micro-irrigation: the bubbler and sub-surface (Chahbani Tunisian system).In the study area, the water table is 30 m deep. The irrigation water comes from the CT with a relatively high salinity of about 4.5 dSm-1. The phenomenon of solute return flow from irrigation does not seem to be a short-term problem in our case study, but overexploitation of the CT, water lossthrough poor irrigation management and land salinisation is the focus of our research. Indeed, climatic variables, irrigation, and soil monitoring on Jemna oasis was done from April 2018. Real time monitoring of soil properties (salinity, water content and temperature) was done using frequency domain reflectometry (FDR-5TE sensor) from April 2018 at different soil depth under two micro-irrigation system.The approach is to calibrate water transfer and solute fate models in porous media under two micro-irrigation system using HYDRUS software. Therefore, the present work deals with the performance of two micro-irrigation system to preserve oasis from soil salinity degradation and to test their use for date palm root water uptake under farmer practices and different irrigation strategies. To achieve these tasks, in the firstpart of the study (chapter 2 and 3), we tested the performance of two FDR sensors the 5TE and WET. The choice of soil moisture sensor for real time monitoring was based on calibrationand validation results. The5TE smart irrigation sensor was selected because it gave better accuracy of measurement. The second part of this work (chapter 4) deals with modelling of root water uptake, salinity distribution and yield production under subsurface irrigation system using HYDRUS-2D model. The focus of this partis testing the innovative Tunisian irrigation system “CHAHBANI system” under oases condition and discussing the different buried diffuser position regarding soil salinity distribution, root water uptake and yield production of date palm.To full these objectives, Hydrus 2D model was calibrated and validated using data of two-crop growth season (2018 and 2019). Subsequently, management scenarios corresponding to the position of the Chahbani buried diffuser (distance from the palm trunk and soil depth) were studied using 100% ETc. For the second irrigation system, the “bubbler system” a Hydrus1d model was used after calibration and validation for datepalm root uptake and soil salinity distribution. Moreover, a long-term oases sustainability under Tunisian future climate change was investigated. The results showed that the large amounts of water applied during flood irrigation were successful in soil leaching, but did not increase yield. For both observed cropping seasons, the amount of root water uptake was less than 100% of the crop water requirement. This result could be explained by the poor performance of the irrigation schedule (dose and frequency) practiced by the technical staff of the AVFA centre. The impact of irrigation management is significant on yield and consideration of the salinity problem is necessary to obtain better results. The scenarios varying the positions of the buried diffuser showed that its installation at the level of the date palm canopy (X=1.5 m), resulted in the lowest water uptake by the roots and the lowest calculated relative yield (402 mm and 34%, respectively). The T4opt scenario (X=0.5 m, Z=0.6 m), whose position corresponds to the maximum root density of the date palm, showed an increase inwater uptake efficiency by 37%. Furthermore, an increase in salinity at the soil surface layer (0-0.40 m) was noted along the entire lateral length, except in the wetted area near the emitter. This result is related to the high evaporation combined to thelack of spreading surface irrigation water. In addition, the fraction required for leaching did not reduce salinity for the first 0.40 m deep. In view of these results, for an arid environment and brackish water, we recommend using the Chahbani irrigationsystem considering occasional or alternate surface irrigation. The objective is to avoid the accumulation of salts on the soil surface and at the edge of the water bulb, which can have a negative impact on the sustainability of the oases. Long-term studies on subsurface irrigation are therefore strongly needed. The challenge is to consider not only yield production and water saving, but also environmental aspects such as soil and groundwater degradation. It is also important to assess the impacts of climate change related to increasing temperature and decreasing precipitation on the oasis system when using subsurface systems.