Estimating the groundwater storage for future irrigation schemes

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 2008). 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. 2014). 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 socio-economic 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 2003). There has been supply water for about 200 million hectares of irrigational activities in SSA over the decades (Giordano 2006). 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. 2006). Siebert et al. (2010), 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 2014). The low temperatures are experienced from December to late February, during which the North-East Trade winds (harmattan) greatly influence several activities in the district. Generally, high temperatures as well as the low humidity brought about by the dry harmattan winds favor high rates of evaporation and transpiration, leading to water deficiencies in the ground. Undoubtedly, these high temperatures force certain plants to give up water through evapotranspiration. The major controllers of groundwater occurrence may be geology, hydrogeology, climate change, and lack of systematic data on groundwater. The issues of climate change and variability supersede all other controllers of groundwater production. Vaccaro (1992), Bouraoui et al. (1999), Scibek & Allen (2006), Zhou et al. (2010), in separate studies identified that the impact of climatic variability could be the fluctuation in water levels; however, irrigation is another way of replenishing and restoring the groundwater abstracted.

The occurrence of groundwater depends on the existing geology. It occurs within fractures and spaces (pores) where these fractures have high interconnectivity in the rock. Sandstone and limestone formations (Paleozoic sedimentary) are recommended for groundwater development because of their potential gradient having an average borehole success rate of about 77% in the world. However, the Upper and lower Birimain (Wa Granite) has an average borehole success rate of 80% with a yield of ≥0.55 L/s. In the context of classification, the SSA falls under the Precambrian hydrogeological province. This kind of formation determines the nature and occurrence of groundwater. The study area is characterized by the Precambrian basement and the sedimentary fractured aquifers, which are both locally high in terms of groundwater production. For sustainable and successful extraction of groundwater for irrigation it is imperative to understand the hydrogeological and geological condition of the study area. The desired drilling method is determined by the geology and hydrogeology of the study area; for instance, mud drilling would be suitable for coastal aquifers compared to air drilling, which will be the option for crystalline formations. The dominant geologic units in the study area comprise shales, fresh granite, and meta-rocks mainly of a crystalline form. Air drilling would be absolute per the underlying geologic units in the area. This kind of formation is recognized for its high yielding of groundwater in fractured conditions (Abdul-ganiyu et al. 2020; Prosper et al. 2020).

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 2020). 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.

Figure 1 depicts the boundaries of the study area, which roughly comprises the upper-half of the country and is located between latitudes 7°58′N and 11°11′N and longitudes 2°57′W and 0°34′E. It consists of the three Northern regions of Ghana, namely the Upper East Region, Upper 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 km²). 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 2000). 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.

The geological units present in the study area are the basement crystalline rocks and the Paleozoic consolidated sedimentary formations. The overlying regolith is defined by a weathered layer of varying lithology found meters away from the unsaturated zone (Martin 2006). The regolith thickness within the study area ranges between 2.7 m and 44 m but can be up to 125 m in certain areas around the North West (Apambire et al. 1997), factors that influence the thickness of the regolith include structural characteristics, nature of the lithology, climate, vegetation, and erosion. The basement crystalline geology is sub-grouped into the Birimian, Tarkwan, Dahomeyan, Granite, Buem, and Togo formations. However, this geology of Precambrian age comprises quartzite, schist, granite-gneiss-greenstone rocks, anorogenic intrusion, and deformed metamorphosed rocks. The Voltaian formation also refers to the Paleozoic consolidated sedimentary, which consists mainly of sandstone, shale, mudstone, arkose, pebble beds, and limestone. The Voltaian formation is sub-grouped into the upper, middle, and lower Voltaian (Obuobie & Barry, 2010). The upper Voltaian is noted for its massive and thin-bedded quartzite sandstone, which is intercalated with shale and mudstone. The middle Voltaian (Obusum and oti beds) has a the dominance of sandstone, shale, mudstone, siltstone, and arkose. The lower Voltaian comprises massive quartzite sandstone and grit (Obuobie and Barry, 2010). The regolith is less thick compared to the Precambrian basement formation; the thickness varies from 4 m to 30 m.

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 2000). In the Mesozoic and Cenozoic (minor geologic formations), three aquifers can be found (Obuobie and Barry, 2010). 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 limestone formation aquifer, which has a depth between 130 m to 300 m. Groundwater under this formation is artesian (freshwater) with a yield varying from 5.5 m³/h to 58 m³/h (Dapaah Siakwan & Gyau-Boakye 2000). The geological cross-section of an unconfined aquifer is evident from the Atankwidi catchment of the Volta Basin in Ghana; this is indicative that groundwater in this formation is high (Martin 2006). Boreholes within the Northern Sector have depths of around 80 m and are believed to be sourcing from the productive zone (Martin 2006). 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 2006). 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.

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, 1994). 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.

MN = distance between potential electrodes (m), AN and AM = distance between current electrodes (m), = potential difference measured (volts) and I = applied current strength.

Primarily, cuttings from each drilling process were sampled after every 5 m of penetration through the formation. The process aids in studying and correlating the lithological units to results from the geophysical survey. The lithological sampling helps in determining the nature of the aquifer and possible related characteristics of the aquifer, which can only be revealed after the pumping test. The machinery and techniques involved in borehole drilling vary depending on the intended use, geological condition of the study area, and the manufacturer. The Indian Drill (Ashok) and German Rigs are purposely used for air drilling, which is the preferred drilling method in Northern Ghana. The main differences between these rigs are the length of drill rods and the nature of the compressors. Data was collected from mounting to dismounting of the drill rig. The data collected during the drilling phase assisted in computing some parameters for defining the groundwater in the area. However, the Schoeller (1967) equation was used in estimating the groundwater storage of the basin.

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. The importance of carrying out a pumping test cannot be overlooked in a borehole development stage. In the pumping test, the common techniques are the Constant Rate Test (CRT) and the Step Drawdown Test (SDT). The distinction between the CRT and SDT is that SDT involves a sequence of constant-rate steps at the control well to estimate borehole performance such as borehole loss and efficiency (Abdul-ganiyu et al. 2020). The CRT was the pumping method used here; the groundwater in the aquifer was pumped at a controlled rate constantly, to show the storage capacity of the basin can supplement the irrigation water in the area. The aim of conducting a pumping test is to determine transmissivity, hydraulic conductivity, and storativity which helps predict the aquifer characteristics. The pumping test was done with the help of a 1.5 horsepower (Hp) Superdub submersible pump, a diesel generator, 80 m HD pipes, a water level indicator, and some pumping test kits. Prominent data generated from the pumping test include discharge rate, drawdown, and static water level. The data gathered was plotted on an Excel pumping test software package; the constant discharge was plotted with the recovery test.

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 the sandstones, mudstones, shale, siltstones, and highly metamorphosed materials (Martin 2006). Areas of resistivity between 47 ohm.m and 200 ohm.m are mainly clay dominant zones (Karaga, Nanumaba North, Kpandai). A community within Salaga (Kafaba) showed different resistivity values uniquely from the other communities (ranges from 67 to 210 ohm.m), which accounted for the mottled nature (completely weathered) during the drilling. In regolith geology, continuous weathering of felsic mineralized rocks turns to clay, thus the high presence of clay in the Northern Region suggests dominant felsic mineral rock and also laterites as the surface materials thus occur as a result of frequent transportation and sorting. Kpandi, Nanumba North, Tatale, Karaga and Yendi showed promising results of the available groundwater resources even before data analysis (Prosper et al. 2020). Information from the drilling results would be the only proof to validate the presence of groundwater in the Northern Region.

Table 2 and Figure 2 shows values of VES in the Upper Region, which falls under the Pre-cambrian formation popularly known as the Birimain, this geological formation is made up of meta-volcanics rocks, meta-sedimentary formation, metamorphic rocks, and basaltic flows. The Wa municipality generally has thick granitoids as base-rock of mostly mica-homblende-boitite mineralization, these kind of rocks under high pressure undergo shearing which results in fractures and joints within it. These fractures serve as pathways and high reserves for underground resources, the VES values for Wa municipality and central depict meta-sedimentary rocks, these can be evident from the drill log of boreholes around kpaguri, Dondoli, limanyiri and Bamahu. The drill logs show a formation of phyllitic sediments which accounts for the VES values of 38 ohm.m – 540 ohm.m and 34 ohm.m – 250 ohm.m. Tegbera, Wadhi, and Loho (Jirapa road) show defined geology of the Birimain, these areas are predominantly meta-volcanics and metarmphosed granitoids. The VES values ranges from 63 to 190 ohm.m, depicting crystalline formation with isolated areas of high clay deposit. The Birimain formation has a borehole success rate of 96% with an average depth of 65 m (Okrah et al. 2012). The average profiling length for the Upper East area was 200 m, the Zebilla catchment falls under the Birimain formation which is known to have high groundwater potential. The VES values for these areas were uniform, but there was an anomaly in one area; which is the Tempane JHS where the resistivity values range from 120 ohm.m to 710 ohm.m. The geology between the Upper East and Upper West is not widely different this accounts for the uniqueness in geology and the high groundwater potential within these Regions.

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 2013). Several attempts have been made to estimate the quantity of groundwater available in storage around the world for different activities. Some of these studies have been conducted around China and some parts of Europe to determine the potential of groundwater irrigation (MacDonald & Davies 2000). However, investigation has been done into quantifying the storage of groundwater in some parts of Ghana and the study area. Research has proven that groundwater is one of the abundant water sources in this part of the world which has the tenacity of meeting agriculture, especially in irrigation. The quantity of groundwater within the area was computed as well as the extractable storage capacity; the essence of these computations was to quantify the amount of groundwater in storage as well the amount that can be extracted economically and legally for irrigation (Prosper et al. 2020). Also, this research assessed high performing boreholes to minimize the gap that may result in the computation to reflect the true potential of groundwater in the study area.

Extractable Groundwater (⁠⁠) = percentage groundwater coverage (P) × Specific Yield (⁠⁠) × Saturation Depth (H) × Extent of Study Area (A).

But ⁠, Where is the volume of water drained and 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 1993) for an area of 108,671 km². After computation of the total groundwater storage capacity and extractable storage was approximately and respectively.

Therefore transmissivity (T) increases with an increase in hydraulic conductivity (K) of an aquifer. If an aquifer has high transmissivity then it may possess a characteristic of high yielding. In Table 5, it is shown that the Upper Regions have high transmissivity (0.81–1.7 m²/day and 0.72–1.5 m²/day) compared to the Northern Region (0.85–1.4 m²/day) which correlates to the yields of boreholes. These attributes of the aquifers in the study area confirm that the groundwater available is of appreciable characteristics to support the idea of groundwater irrigation in years to come.

Table 6 shows the results of water samples confirming acceptable standards of parameters as to whether it may be suitable for irrigation or not. The limits indicate that the aquifer is of maximum quality and has no adverse effects on plants. Results from pH, EC, TDS, and SAR are of recommendable quality for irrigational activities in the future. From Table 6, it can be seen that pH ranges from 5.6 to 9.8, indicating that a few of the samples might either be acidic or basic but not too excessive to render the water unsuitable, while SAR ranges from 0.47 to 2.3, which is wholesome for irrigation. The evaluation of groundwater quality for irrigation in this research is based on the following factors: salinity (total amount of dissolved salts in water), sodium hazard (the amount of sodium in the water compared to the summation of calcium and magnesium), and Magnesium hazard (MH). The most influential water quality guideline for crop productivity is the water salinity hazard as measured by electrical conductance (Yidana et al. 2012). The high concentration of salinity/EC in irrigation water affects crop yield through the inability of the plant to compete with ions in the soil solution for water, which results in an osmotic effect or physiological drought). The severity of an osmotic effect may vary with the plant growth stage and in some cases may go unnoticed because of a uniform yield decline over the plant (Abanyie et al. 2020). Sodium hazard is defined separately because of sodium's specific detrimental effects on soil physical properties. Excessive Na⁺ content in irrigation water renders it unsuitable for soils containing exchangeable Ca²⁺ and Mg²⁺ ions, as the soil takes up Na⁺ in exchange for Ca²⁺ and Mg²⁺ causing deflocculation (dispersion) and impairment of the tilth and permeability of soils. The sodium hazard is typically expressed as the sodium adsorption ratio (SAR). Sodium adsorption ratio (SAR) is an important parameter for determining the suitability of groundwater for irrigation because it is a measure of alkali/sodium hazard to crops.

Figure 3 denotes the surface hydrology (streams, tributaries of the white and black Volta) of the study area. The study area is noted for its high seasonal overflow variation in some rivers and this can be attributed to climate, vegetation, and topography. The main rivers in the study area are the black and white Volta which sources from lake Volta. The study area is noted to have several drainage systems comprising dams, canals, streams, and few springs. The number of drainage systems in the area is invalid if these systems are not of high functionality, it is appropriate to investigate the effectiveness of these systems to an extent as it may not be beneficial. The relationship between groundwater and surface hydrology to this research is of great importance (i.e. balancing irrigation water for high productivity). The study area is estimated to have over 570 dams with the ongoing 300 dams funded by the Ghana government under the ‘one village one dam’ project (Tamale Metropolitan Assembly 2016). Lake Volta has created several tributaries, which have extended towards the Northern sector. The notable tributaries within the area, including Nasia, Mole, Sissli, Mawli, Kulpawn, and many more. Previous study has outlined the functionality of these surface water resources and 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.

Critically, it can directly be seen that making ⁠, 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.

Figure 4(a)–4(c) are results from the pumping test (depth and yield) plotted on a graph to correlate the borehole depth and yield. Graphically, the plotted feature depicted a weak correlation between groundwater depth and yield in the study area. This is a result of the underlying parent rock. Also, this shows that the weathering process and groundwater behavior in the Precambrian and Paleozoic sedimentary formation are the same.

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 irrigation in the study area. Northern Ghana has varying geology comprising the Precambrian and the Paleozoic sedimentary formation; the Upper Regions and part of the Northern Region of the Precambrian formation are made up of phyllite, schist, granite, meta-sediments, and meta-volcanics, while 90% of the Northern Region is made up of sandstone, shale, mudstones, and granitiods. The total groundwater storage capacity and the extractable storage after computation were approximately and respectively. The pumping test results show that the quantity of groundwater pumped out attests to the storage capacity of the study area and hence the tendency of groundwater to be more reliable and also a backup to support irrigation in the dry season.

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