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Universitat Leipzig (2015)

Performance of Large-Scale Gezira Irrigation Scheme and its Implications for Downstream River Nile Flow

Al Zayed, Islam

Titre : Performance of Large-Scale Gezira Irrigation Scheme and its Implications for Downstream River Nile Flow

Auteur : Al Zayed, Islam

Université de soutenance : Universitat Leipzig

Grade : Doctor rerum naturalium (Dr. rer. nat.) 2015

Résumé partiel
Policy makers adopt irrigated agriculture for food security, since irrigation doubles crop production. Therefore, the development of large irrigation systems has a long history in many places worldwide. Although large-scale irrigation schemes play an important role in improving food security, many schemes, especially in Africa, do not yield the expected outcomes. This is related to poor water management, which is generally due to a lack of effective evaluation and monitoring. The objective of this study, therefore, is to propose a new methodology to assess, evaluate and monitor large-scale irrigation systems. Information on irrigation indicators is needed to enable the evaluation of irrigation performance. The evaluation is the first and the most significant step in providing information about how it is performing. After reviewing extensive literature, a list of indicators related to the performance of irrigation, rainwater supply and productivity is suggested. The irrigation efficiency indicators Relative Irrigation Supply (RIS) and Relative Water Supply (RWS) are selected. Potential rainwater supply to crops can be tested based on the Moisture Availability Index (MAI) and the Ratio of Moisture Availability (RMA). Water productivity can be assessed by Crop Yield (Y) and Water Use Efficiency (WUE). However, the central problem facing large-scale irrigation schemes is always the lack of data, which calls for the development of a new method of data acquisition that allows evaluation and monitoring. Remote Sensing (RS) technology makes it possible to retrieve data across large areas. Two different approaches via RS, the Normalized Difference Vegetation Index (NDVI) and Actual Evapotranspiration (ETa), can be utilized for monitoring. The well-known Vegetation Condition Index (VCI), derived from the NDVI, is modified (MVCI) to allow a qualitative spatio-temporal assessment of irrigation efficiency. MVCI takes into account crop response to water availability, while ETa indicates whether water is used as intended. Furthermore, the assessment of the possible hydrological impact of the irrigation system should be considered in the evaluation and monitoring process. The Sudanese Gezira Scheme of 8,000 square kilometers in the Nile Basin, where performance evaluation and monitoring are absent or poorly conducted, is no exception. This research takes the large-scale irrigation of the Gezira Scheme as a case study, as it is the largest scheme, not only in the Nile Basin but also in the world, under single management. The first long-term historical evaluation of the scheme is conducted for the period 1961–2012 rather than only on a short-time scale as is the common practice. An increase in RIS and RWS values from 1.40 and 1.70 to 2.23 and 2.60, respectively, since the 1993/94 season shows decreasing irrigation efficiency. MAI and RMA for summer crops indicate a promising rainfall contribution to irrigation in July and August. The Gezira Scheme achieves low yield and WUE in comparison to many irrigation schemes of the globe. Low productivity is mainly due to poor distribution and irrigation mismanagement. This is indicated by the 15-year MVCI spatio-temporal analysis, which shows that the northern part of the scheme experiences characteristic drought during the summer crop season. Although MVCI can be considered a monitoring tool, the index does not deduct the soil water content, and water could be wasted and available in other ways (e.g. water depressions). Spatio-temporal information for ETa is required to better quantify water depletion and establish links between land use and water allocation. However, several RS models have been developed for estimating ETa. Thus, improving the understanding of performance of such models in arid climates, as well as large-scale irrigation schemes, is taken into account in this study. Four different models based on the energy balance method, the Surface Energy Balance Algorithm for Land (SEBAL), Mapping EvapoTranspiration at High Resolution with Internalized Calibration (METRIC™), Simplified Surface Energy Balance (SSEB) and MOD16 ET are applied in order to determine the optimal approach for obtaining ETa. Outputs from these models are compared to actual water balance (WB) estimates during the 2004/05 season at field scale. Several statistical measures are evaluated, and a score is given for each model in order to select the best-performing model. Based on ranking criteria, SSEB gives the best performance and is seen as a suitable operational ETa model for the scheme. SSEB subsequently is applied for summer and winter crop seasons for the period 2000–2014

Mots clés  : Bewässerung Leistung ; Bewässerung Wasserwirtschaft ; Fernerkundung ; Tatsächliche Evapotranspiration (ETA) ; SEBAL ; METRIC ; SSEB ; NDVI ; VCI ; MVCI ; Wassernutzung ; Überwachung ; Auswirkungen auf die flussabwärts. — Irrigation performance ; Irrigation water management ; Remote sensing ; Actual evapotranspiration (ETa) ; SEBAL ; METRIC ; SSEB ; NDVI ; VCI ; MVCI ; Water harvesting ; Monitoring ; Implications on downstream.

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Page publiée le 22 décembre 2015, mise à jour le 2 décembre 2018