Estimating the groundwater storage for future irrigation schemes

Presented in this paper is a feasibility study of groundwater for agricultural use (irrigation) in Northern Ghana. The study was conducted using geo-electrical data, boreholes drill logs, results of water quality, and results of the pumping test. The geo-electrical results were to unearth the lithology of the study area; it was found to be underlain with varying geology of both Precambrian and Paleozoic sedimentary formation. These formations consist of phyllite, schist, granite, meta-sediments, and meta-volcanics making up the Precambrian and sandstone, shale, siltstone, mudstones, granitiods also, of the Paleozoic sedimentary. Areas of low resistivity were targeted for drilling per the geophysical results of the profile, values between 24 and 100 ohm.m were zones of probable groundwater occurrence in the study. The groundwater storage capacity and the extractable storage capacity were estimated to be approximately 710,000 km and 290,000 km. The pumping test results acquired from 81 boreholes from the study were used to analyze the sustainability. However, groundwater depth was studied using the Static water level (SWL), areas of SWL around 22 m and 25 m are likely to have a shallow depth whereas areas of 17 m would have deeper groundwater depth.


GRAPHICAL ABSTRACT INTRODUCTION
In recent times, the search for water to address the needs of agriculture across Sub-Saharan Africa and many parts of the world have been retarded directly and indirectly by climate change. It has been established that over 150 million of the rural population in Africa rely on groundwater services for domestic and agricultural use (Adelana & MacDonald ). It necessary to value the quality of groundwater in this study as irrigation is the subject. An appropriate evaluation of the water quality before its use in irrigation will help to arrest any harmful effect on plant productivity and the hydraulic properties of irrigation soils. The suitability of water for irrigation is determined in several ways including the degree of acidity or alkalinity (pH), electrical conductivity (EC), residual sodium carbonate (RSC), sodium adsorption ratio (SAR), exchangeable sodium ratio (ESR), as well as the effects of specific ions (Sadat-Noori et al. ). The study area is known to solely rely on groundwater as a source of drinking water and also for agriculture during the dry season when there are no rains. The importance of agriculture in the socioeconomic conditions of these communities in the study area cannot be overemphasized. To safeguard these interests, there is an urgent need to carry out a comprehensive assessment of the groundwater resources for sustainable development and management for irrigation. This is because the sustainable development and management of a resource begin with an assessment of its quantity and quality. The adoption of groundwater for irrigation tends to elevate agricultural productivity to a greater height (Moench ).
There has been supply water for about 200 million hectares of irrigational activities in SSA over the decades (Giordano ). The use of groundwater has been robust, supporting most of the SSA countries despite the 1,000 mm/y annual precipitation recorded (Foster et al. ). Siebert et al. (), discovered Thailand and Myanmar to be the largest cultivators using groundwater-irrigation.
The area receives an annual rainfall averaging 1,000 mm, which is considered to be enough for a single farming season.
Temperatures are generally high, averaging 34 C. The maximum temperature could rise as high as 42 C and the minimum as low as 16 C (West Mamprusi District Assembly  The essence of this research is to study and understand the groundwater in the study area using hydro-geophysical techniques. This gives insight into the groundwater and hydrogeological behavior of the area. Furthermore, the quantity of groundwater stored and the extractable quantity was estimated to conclude on the feasibility of groundwater irrigation in the study area. The unanswered question is, can groundwater be resilient and also a better alternative to supplement irrigation? Similar studies have been done around the globe using different approaches to achieve a single goal, which is investigations into whether groundwater can supplement existing water resources for irrigation and also finding suitable alternatives in the absence of existing water resources. Findings from these previous works show that groundwater can indeed supplement the existing irrigational water with proper management practices. China, Thailand, Algeria, Libya, Europe, and some parts of the world have the largest irrigation systems and have put measures in place to minimize the complete mining of these resources (Abdul-ganiyu & Prosper ). Regionally, the groundwater storage capacity has been studied in some parts of Ghana, findings from these papers confirm that the presence of these high yielding aquifers is capable of boosting agriculture in the dry season.
The study area West Region, Northern Region, and the recent two regions created (Savannah and North-Eastern Region), which respectively account for 9, 19, and 72% of the total study area (108,671 km 2 ). The study area is bordered by Côte d'Ivoire to the West, Burkina Faso to the North, Togo to the East, and the Volta and Brong-Ahafo Region to the South. The study area is predominately flat and gently undulating with slopes of about 5% to 1% in many areas (Quansah ). It is characterized by three main ecological zones, which are: Guinea Savannah, Transition zone and the Sudan Savannah, defined in the context of climate and the natural vegetation influenced by the regolith and rainfall in the study area. Rainfall often decreases with increasing latitude and its distribution throughout the year is uneven; it is known for a single rainfall pattern, which is from late April to October. The annual precipitation within the study area is 900 to 1,200 mm. Temperatures within these regions are extensively high, ranging from 27 C to 29 C, and annual extremes of temperature between 17 C to 40 C.

Geological and hydrogeological description
The geological units present in the study area are the base- The Voltaian and Precambrian formations, which dominate the study area, both have low porosity, rendering the rocks impermeable, but groundwater occurrence in this study area is attributed to secondary porosity. However, groundwater potential in the Voltaian formation is of low gradient compared to the Precambrian formation (Dapaah Siakwan & Gyau-Boakye ). In the Mesozoic and Cenozoic (minor geologic formations), three aquifers can be found (Obuobie and Barry, ). First is the unconfined aquifer, with a depth ranging from 2 m to 6 m, which occurs in recent sand very close to the coast. The second is the leaky aquifer, made up of continental deposits of clay and gravel with aquifer depth from 8 m to 120 m. The third type is the Ghana; this is indicative that groundwater in this formation is high (Martin ). Boreholes within the Northern Sector have depths of around 80 m and are believed to be sourcing from the productive zone (Martin ). The productive zone is referred to as the areas below the regolith and above the fractured bedrock. The fractured zone is mainly sub-vertical and generally develops at a depth greater than 20 m below the ground surface (HAPS ). The fractured zone varies with thickness, the factors that account for these are the degree of weathering, structural history, lithology, and depth. Hydrogeologically, the Voltaian formation is the least understood formation because of its complexity.

Groundwater irrigation in Ghana
In the Upper Regions, dug-out and hand-dug well were means of extracting groundwater from alluvial channels along streams for vegetable production during the dry season.
Water was lifted from this well manually to irrigate 0.07 to 0.3 hectares of vegetable farms (Kortatsi, ). Research has shown that about 100 to 200 hectares were irrigated using groundwater in the Atankwidi catchment in the Upper East Region. Elsewhere in the study area, leafy vegetables are cultivated in urban and peri-urban areas of Tamale using shallow groundwater resources; these farmers irrigate land areas of about 1.2 hectares. Most of the crops are cultivated on small plots and made readily available after harvest on markets, these crops include okro, pepper, cabbage, onions, shallots, and carrots. Less than 5% of the groundwater usage is attributed to irrigation and livestock watering.

Geophysical technique
Groundwater is considered to be a hidden natural resource, taking into consideration its location and extraction process. However, each configuration has a maximum depth of study. The geophysical goal was to study the lithology and delineate zones of high productivity aquifers. Apparently, the Schlumberger configuration has a deeper depth of study and also can be used to study lateral variation between profiles concerning resistivity. This configuration was chosen in this research to accomplish the geophysical goal (groundwater and lithological survey). The average profile length was about 1.5 km with a current electrode spacing of 400 m. Communities around the Northern Region had a stretch of profile length of about 1.2 km as a result of complex and varying geology. The resistivity can be calculated by first estimating the geometric factor (K) using Equation (1b). The geometric factor is a constant that varies from one setting to another and also depends on the configuration used for the survey. The geometric factor (K) is then substituted into Equation (1a) to calculate the resistivity at each station interval.

Pumping test
The process by which a borehole is pumped at a controlled rate and the drawdown is measured against time in one or more surrounding wells.

Water quality analysis
All major ions (Na þ , K þ , Ca 2þ , Mg 2þ , Fe 2þ ), as well as minor elements, such as NO 3 À , and F À , were analyzed using a Dionex DX-120 ion chromatograph at the water quality laboratory of the Water Research Institute, Tamale. The bicarbonate ion concentration in the water was determined by titration. Following the international standards, results with an ionic balance of more than 5% were rejected. The TDS was estimated by summing up all the major cations and anions in the sample using Microsoft Excel software.
The sodium adsorption ratios (SAR) indicate the effect of relative cation concentration on sodium accumulation in the ground, this is calculated from the relationship in Equation (2): where Na þ , Ca 2þ and Mg 2þ are in Mg/L.

Geo-electrical (VES) results
The results collected from the geophysical profiling aided in studying the geology of the study area (geo-electricity) and also giving insight into the petrography of the rocks. In irrigation, the quantity and quality of water define the life span of the scheme and hence investigating the geology, which controls the water source is an important step of sustaining an irrigational scheme. Moreover, the existing geology gives an overview of how rich the regolith (soil) will be and determines suitable crops to be cultivated if detailed soil research is carried out. Table 1 and Figure 2 show the range of resistivity values from the VES results within the districts of the research area describing the geology of some parts in the Northern Region. It falls within the Voltaian basin, which consists of

Groundwater storage capacity
The accessibility of water, especially for drinking and agriculture, has been of great concern to underprivileged communities around the world. The demand for water around African countries is not just a major challenge for the beneficiaries but also on the duty bearers and researchers to investigate the potential of groundwater resources for consumption. This study brings to light the groundwater available in the study area and its feasibleness in supporting current irrigation water. However, groundwater is 15% of African renewable water resources, accounting for the total water in the aquifer (ACPC ). Several attempts have been made to estimate the quantity of groundwater available in storage around the world for different activities.   But S y ¼ V wd =V t , Where V wd is the volume of water drained and V T is the total rock or material volume (Bear, 1979).
The capacity of groundwater was computed based on the data generated from the boreholes using Equation (3a) and (3b), taking specific yield and effective porosity to be 2 and 5% respectively (Asomaning ) for an area of 108,671 km 2 . After computation of the total groundwater storage capacity and extractable storage was approximately 710, 000 Km 3 and 290, 000 Km 3 respectively.

Pumping test results
The discharge rate is referred to as the discharge the shows the quantity of groundwater that has been pumped out over a specific period of time (dt). Is computed by dividing the time taken for pumping by drawdown (da) as it is in Equation (4a), the drawdown in context is the level of water drop in a borehole during pumping at successive time intervals.
Tables 4 and 5 shows information that was gathered during the pumping test (CRT); specifically, Table 3 depicts ranges of borehole depth and static water level (SWL) while Table 5  which accounts for the bore depth (Table 4), while the Northern Region has high SWL, accounting for the borehole depth (Table 4) Water quality standards Table 6 shows the results of water samples confirming accep-    their importance to irrigation for the agricultural sector in northern Ghana. The largest irrigation dam in Ghana is the Tono irrigation scheme, which is found within the study area, also some prominent irrigation systems in the area include; the Golinga scheme, Libiga, Botanga irrigation scheme, Pwalugu irrigation scheme, and some small irrigation schemes scattered around the study area. The limiting factor on the performance of these dams and water resources in the study area is the siltation of reservoirs and high temporal variability. The siltation mostly occurs in the rainy season, making the dams less beneficial for irrigation, whereas high temporal situations tend to dry up dams during the dry season (harmattan) leading to a shortage of water for irrigation. It is in this regard that the study of groundwater resource and its tendency for supplementing the existing irrigation water becomes a target for agricultural productivity not only in Ghana but the whole of Africa and the world at large.
The field water balance gives more insight into why irrigation and groundwater may be practicable. This is an equation used to establish change in water storage in the soil. The value for the change in storage can either be a positive or negative value, suggesting where it should be added or removed from the soil. At saturated state, the water balance equation is given as: where Ig ¼ gross irrigation requirement, Pe ¼ effective rainfall, L in ¼ lateral Inflow, Ca ¼ capillary rise, Etc ¼ evapotranspiration, Ge ¼ deep percolation,  Critically, it can directly be seen that making P inflows, the subject will estimate a quantity of water that is been sent into the ground, as shown in Equation (5c) and (5d). This water that is been sent into the ground is a source of replenishment to the existing water-table and also maintains the hydrological cycle. The components for the inflows include effective rainfall and gross irrigation requirement, which is excess water in the soil that is been leached into the aquifer.
In summary, the water balance suggests that groundwater that is been used for irrigation goes a long way to replenish the aquifer and hence not to waste.

CONCLUSION
The data generated was from a hydrogeological perspective, analyzing the potential of groundwater within the study and its feasibility to add to the current agricultural work to boost

DATA AVAILABILITY STATEMENT
Data cannot be made publicly available; readers should contact the corresponding author for details.