Small Reservoirs in Northern Ghana: Monitoring, Physical Processes, and Management

The importance of small reservoirs for the livelihoods of people in the Upper East Region of Ghana cannot be over-emphasized. They are used for many purposes which include fishing, livestock watering, construction, irrigation, recreation, drinking water, and other domestic uses. The reservoirs were built most often close to communities to support them with dry season water use since the region has a mono-modal rainfall pattern (April – October). The best time to realise the full extent or capacity of small reservoirs is therefore at the beginning of November.
This study was carried out in the Volta basin focusing on the Upper East Region as part of a larger Challenge Program for Water and Food and the EU H2020 TWIGA project. The shallowest (with a maximum depth less than 2m) reservoirs in the northern part of the Volta basin are often dry at the start of the Harmattan season (December - February) when they are most needed. The perception was that this was mainly a result of high rates of evaporation because of high temperatures (going up to 41oC) in that part of the basin in the dry season (Nov – April). Unfortunately, most of the reservoirs are ungauged making their management challenging. Remote Sensing methods have been used to monitor the reservoirs but mainly with regards to their distribution and capacities (surface areas).
In this research, we studied the filling and emptying of the reservoirs with a combination of remote sensing and in situ data, offering better insights into the components of the water balance and energy budget for small reservoirs and thereby the possibility to manage them better. Aside the usage of water in reservoirs, evaporation is considered to be the main component of the water balance of a reservoir. Accurate estimation of evaporation is required for irrigation management and water resources planning. Knowledge of hydrologic fluxes, including evaporation, is required for monitoring, and understanding hydrological and ecological processes. It is however expensive to directly measure evaporation energy fluxes in the field continuously for a long period of time using the Eddy Covariance method. Following this study, a cost effective and reliable way of measuring evaporation flux is proposed using a TAHMO-like meteorological station and the FAO-56 Penman-Monteith method in CropWat.
The main findings from the research are as follows:
• Water abstraction for irrigation, including through small reservoirs of up to 10m3/s from the Volta river, will have minimal impact on hydropower generation at Akosombo and Kpong. However, increasing irrigation and small reservoir abstraction (or storage) rates to about 38m3/s would mean that the water demand for hydropower for some years will not be fully met (about 0.1 percent shortage may be experienced). This means the one-village-one dam project might not create many problems for hydropower generation downstream if they are well-managed (gains not offset by high water losses).
• Evaporation from small reservoirs is not as high as expected. Average actual rate of evaporation is about 5mm/day instead of the reference evaporation of about 10mm/day estimated using meteorological variables from distant (> 3km) weather and climate stations.
• Even though evaporation in small reservoirs is low, the rate of evaporation is higher in shallow and smaller reservoirs. The management of the small reservoirs will therefore require better landuse planning and water allocation to make them fit for the purpose for which they have been constructed for use in the dry season.
• A combination of hydro-meteorological data from TAHMO-like stations and remote sensing offer a better way to monitor and manage the water use in small reservoirs.
• Small reservoirs are good for community water management and not as inefficient as often thought.