Performance evaluation of reservoir-based irrigation schemes in the Upper East region of Ghana

Abstract

The design of relevant adaptation strategies for water users in irrigation schemes in drylands of Sub-Saharan Africa requires up-to-date information about the current performance of these schemes in view of rapid changes in climate and land use, population growth, and competing water demands. The entire system of two (a small- and a medium-scale) irrigation schemes shared by multiple users in the Upper East region of Ghana were examined, including the water reservoir, water conveyance and distribution network, cropping fields, and management entity. First, multi-level performance indicators with relevance to water delivery, water utilization, and agricultural production were adapted and applied based on measurements of meteorological, soil and groundwater parameters, irrigation water inputs, crop management and yields for two rainy and dry seasons during 2014–2016 in prevalent cropping systems. For field-level evaluation, the FAO AquaCrop model was applied to develop an improved year-round irrigation schedule for dry-season cultivation of tomato and to assess a possibility for supplemental irrigation of maize in the rainy season under “wet” and “dry” climate scenarios. Finally, a scenario-based analysis of irrigation performance was conducted at scheme scale for the period of 2015–2030 using the Water Evaluation and Planning System (WEAP), a decision support modeling tool. These modeling scenarios considered the observed rainfall variability, introduction of supplemental irrigation in the rainy season, irrigable area expansion, and system efficiency improvement.<br /> Technical factors, such as underutilized reservoir storage capacity and deteriorated conditions of water delivery infrastructure strongly undermined the irrigation system performance. In particular, the medium-scale irrigation scheme utilized less than 40% of total storage, whereas the small-scale scheme utilized about 70% of the storage. The examination of field-level water management practices suggests that an application efficiency of 58–68% is achievable in both schemes by improving the irrigation scheduling of the major crops. Overall system efficiency can be increased from 50% to 68% by reducing water conveyance network losses and by eliminating over-irrigation of fields. The AquaCrop simulations show that improved irrigation schedule for dry-season tomato cultivation would result in a water saving of 130–1,325 mm compared to traditional irrigation practices, accompanied by approximately 4–14% increase in tomato yield. Supplemental irrigation of maize would require 107–126 mm of water in periods of low rainfall and frequent dry spells, and 88–105 mm in periods of high rainfall and rare dry spells. Therefore, year-round irrigation may be feasible, using water saved in dry-season tomato cultivation for supplemental irrigation of maize in the rainy season. However, as predicted by the WEAP analysis, supplemental irrigation in the small-scale scheme could be possible only if the rise in water demand is counterbalanced by about 10% increase in the system efficiency and by setting limits on the cultivation of the water-intensive tomato crop in the dry season. The unavailability of long-term historical data at present prevents the calibration and validation of the WEAP model in Ghana but the conducted scenario analysis sets the framework for further evaluation of the potential water scarcity adaptation options. Overall, the integrated, whole-system approach is essential in the assessment of suitable options for improving reservoir operations and adapting to water scarcity in Sub-Saharan Africa.