HESSHydrology and Earth System SciencesHESSHydrol. Earth Syst. Sci.1607-7938Copernicus GmbHGöttingen, Germany10.5194/hess-19-1055-2015Mapping irrigation potential from renewable groundwater in Africa –
a quantitative hydrological approachAltchenkoY.y.altchenko@cgiar.orgVillholthK. G.International Water Management Institute, Pretoria, South AfricaMinistère de l'Agriculture, de l'Agroalimentaire et de la Forêt, Paris, FranceLaboratoire METIS, UMR 7619 UPMC/CNRS, Université Pierre et Marie Curie, Paris, FranceY. Altchenko (y.altchenko@cgiar.org)26February20151921055106715April201410June20143October201429October2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.hydrol-earth-syst-sci.net/19/1055/2015/hess-19-1055-2015.htmlThe full text article is available as a PDF file from https://www.hydrol-earth-syst-sci.net/19/1055/2015/hess-19-1055-2015.pdf
Groundwater provides an important buffer to climate variability in Africa.
Yet, groundwater irrigation contributes only a relatively small share of
cultivated land, approximately 1 % (about 2 × 106 hectares)
as compared to 14 % in Asia. While groundwater is over-exploited for
irrigation in many parts in Asia, previous assessments indicate an
underutilized potential in parts of Africa. As opposed to previous
country-based estimates, this paper derives a continent-wide, distributed
(0.5∘ spatial resolution) map of groundwater irrigation potential,
indicated in terms of fractions of cropland potentially irrigable with
renewable groundwater. The method builds on an annual groundwater balance
approach using 41 years of hydrological data, allocating only that fraction
of groundwater recharge that is in excess after satisfying other present
human needs and environmental requirements, while disregarding
socio-economic and physical constraints in access to the resource. Due to
high uncertainty of groundwater environmental needs, three scenarios,
leaving 30, 50 and 70 % of recharge for the environment, were implemented.
Current dominating crops and cropping rotations and associated irrigation
requirements in a zonal approach were applied in order to convert recharge
excess to potential irrigated cropland. Results show an inhomogeneously
distributed groundwater irrigation potential across the continent, even
within individual countries, mainly reflecting recharge patterns and
presence or absence of cultivated cropland. Results further show that
average annual renewable groundwater availability for irrigation ranges from 692 to
1644 km3 depending on scenario. The total area of cropland irrigable
with renewable groundwater ranges from 44.6 to 105.3 × 106 ha,
corresponding to 20.5 to 48.6 % of the cropland over the continent. In
particular, significant potential exists in the semi-arid Sahel and eastern
African regions which could support poverty alleviation if developed
sustainably and equitably. The map is a first assessment that needs to be complimented
with assessment of other factors, e.g. hydrogeological conditions,
groundwater accessibility, soils, and socio-economic factors as well as more
local assessments.
Introduction
Irrigation expansion is seen as a significant leverage to food security,
livelihoods, rural development, and agricultural and broader economic
development in Africa, especially in sub-Saharan Africa (SSA). National and
regional (CAADP, 2009; NEPAD, 2003) policies and plans stress irrigation
development, and more broadly sustainable land and water management, as a
key component to poverty alleviation and gains in food productivity.
FAO (2005) assessed the potential for irrigation development
Definition of irrigation potential in FAO (2005): area of land (ha) which is
potentially irrigable. Country/regional studies assess this value according
to different methods – for example some consider only land resources suitable
for irrigation, others consider land resources plus water availability, and
others include in their assessment economic aspects (such as distance and/or
difference in elevation between the suitable land and the available water)
or environmental aspects, and so forth.
in Africa to be 42.5 × 106 ha,
corresponding to 20.1 % of the cultivated area or 5.7 % of the
cultivable land. While still playing a secondary and minor role in national
and regional plans, groundwater is increasingly included as a viable and
suitable supplementary or sole source to develop for irrigation along with
traditional surface water resources (MoAC, 2004; MoFA and GIDA
In
the Ghana National Irrigation Policy, groundwater irrigation falls under the
category “informal irrigation”.
, 2011; MoFED, 2010; MoIWD, 2005; MoWEA,
2013). This is explained by evidence that farmers progressively embrace
groundwater irrigation (GWI) spontaneously and with own investments where
conditions permit (Villholth, 2013) and the notion that the groundwater
resources in Africa generally are plentiful as well as underutilized
(MacDonald et al., 2012).
Groundwater irrigation presently covers around 2 × 106 ha in
Africa, equivalent to 1 % of the cultivated land
Cultivated land
and cropland are here used interchangeably, to mean the combined arable land
area and the area under permanent crops
(http://www.fao.org/nr/water/aquastat/data/glossary/search.html?lang=en).
(Siebert et al., 2010). In Asia, similar figures amount to 38 × 106 ha
or 14 % of cultivated land (Siebert et al., 2010). Hence, it
is fair to assume that there is appreciable scope for further developing GWI
in the continent. Barriers to an expansion of groundwater-based irrigation
in Africa, and in particular SSA, include lack of knowledge of the resource
and best options for sustainable development. So, while present levels of
development are comparatively low and most development occurs in the
informal sector (Villholth, 2013), progress towards greater and long-term
benefits need to be informed by estimations of upper limits for sustainable
development and most appropriate geographic areas for development. The need
for qualified estimates of groundwater irrigation potential (GWIP) is
recognized at the national (MoFA and GIDA, 2011; Awulachew et al., 2010) as
well as regional scale (MacDonald et al., 2012). Qualitative, relative
groundwater potential was mapped for Ethiopia by MacDonald et al. (2001),
however, with no specific focus on the potential for irrigation. You et al. (2010)
estimated the potential contribution from small-scale irrigation
(including ponds, small reservoirs, rainwater harvesting, and groundwater) in
Africa to be 0.3 to 16 × 106 ha based on a continental
distributed mainly economic multi-criteria analysis at a 5 min. resolution.
Pavelic et al. (2012, 2013) afforded a relatively simple water balance
approach to provide country or catchment scale estimates, respectively, of
gross GWIP in terms of irrigable cropland, taking into consideration the
crop irrigation water needs and disregarding existing irrigation
development. Water available for irrigation was constrained by renewable
groundwater resources, priority demands from domestic, livestock, industrial
uses as well as environmental requirements. They determined the GWIP of 13
semi-arid countries in SSA to be in the range of 13.5 ± 6.0 × 106 ha,
or between 0.1–3.9 × 106 ha per country. While
the previous estimations of GWIP in Africa were continental (You et al.,
2010), national (Pavelic et al., 2013), or sub-national (Pavelic et al.,
2012) in scope, the present paper builds on the latter approach providing a
fully distributed and consistent assessment of the gross GWIP for the entire
continent at a grid scale of 0.5∘. The concept of the approach is
to map crop area that can be irrigated with locally renewable groundwater
resources at a continental and distributed scale. By doing so, regional
differences across the continent become conspicuous and variability within
the countries also becomes apparent. The extent and distribution of GWIP is
subsequently compared with the existing GWI extent and distribution across
Africa to determine net GWIP, i.e. areas and regions with high and low
residual GWIP. Finally, the limitations and uncertainties related to the
methodology are assessed and discussed.
Methodology
Following the approach of Pavelic et al. (2013), the methodology assumes
groundwater as the sole source of irrigation water and hence gives an
estimate of the area that could potentially be irrigated by groundwater
disregarding any existing irrigation, whether from groundwater or surface
water. Importantly, the method considers sustainable GWI from a
resource perspective, i.e. the use of only renewable groundwater for human
needs (including irrigation) while partially satisfying environmental
requirements from this renewable resource. As a consequence, non-renewable
(fossil) groundwater is not considered available, preventing long-term
aquifer depletion.
The water balance assessment is based on a GIS analysis and mapping with a
final resolution of 0.5∘ assuming each cell (about 50 km × 50 km)
to be homogeneous and independent of other cells, i.e. no lateral
groundwater or irrigation water flows occur between cells. For each cell,
the GWIP [L2] is calculated as the potential cropland area that the
available groundwater resource can irrigate (Supplement):
GWIP=GWAvailableIrrig.WaterDemandmax,
where groundwater availability [L3 T-1] is calculated as any
excess of groundwater recharge, considering other groundwater demands from
humans (domestic uses, livestock, industry) and the environment:
GWAvailable=GWRecharge-HumanGWDemand-Environ.GWReq.
The gross irrigation water demand [L T-1], which represents the
groundwater abstraction needed to satisfy the deficit rainfall and the
irrigation losses, is determined by
Irrig.WaterDemand=∑i=1n∑j=1mCropWaterDemand-GreenWaterj×%ofAreaiIrrig.Efficiency.
The crop water demand [L T-1], which represent the monthly amount of water
needed by the crop to grow optimally during the months of its growing period,
independently of the water source and considering water as the only limiting
factor for optimal growth (FAO, 1986), is determined by:
Crop Water Demandj=Kc×E0,maxj.
The equation
parameters are given as follows:
E0,max [L] is the maximum reference evapotranspiration for each calendar
month.
Kc [–] is the crop coefficient.
Green Water [L T-1] is the water available for plants
naturally and indirectly
from the rainfall through soil moisture.
% of Area [–] is the areal fraction of a specific crop
relative to the total crop area within a grid cell.
n [–] is the number of crops grown within the grid cell.
m [–] is the number of months of the year (12).
Irrig. Efficiency [–] is the irrigation efficiency coefficient.
It is used to express the fraction of groundwater abstracted that is not
lost along the water transport from the abstraction point to the crop (FAO,
1989). The extracted groundwater quantity does not reach fully the crops
because of transport losses or losses in the field.
GW Recharge [L3 T-1] is the net groundwater recharge.
It corresponds to the total quantity of water from rainfall which reaches
the aquifer as diffuse recharge. Return flows from surface water irrigation
and other forms of artificial recharge as well as focused or induced
recharge from water surface bodies are disregarded.
Human GW Demand [L3 T-1] is the groundwater use for
anthropogenic activities, such as domestic and industrial water supply and
livestock watering. Domestic and industrial water requirement are assumed to
come partly from groundwater while livestock watering is assumed to be fully
supplied by groundwater (see also Sect. 3.3).
Environ. GW Req. [L3 T-1] is the quantity of water
coming from groundwater, which is directly linked to the environment for
maintaining ecosystems. This includes river baseflow and groundwater influx
to wetlands.
The proposed approach, taking annual water balances, yields an estimate of
GWIP with respect to historic hydrology when considering the assessment over a
number of years with varying rainfall and recharge over the continent. This
is described in more detail in the next section.
Data sources and preparationHydrological data
Data on recharge (GW Recharge, Eq. 2) and green water (Green Water, Eq. 3)
derive from model outputs from the PCR-GLOBWB global hydrological model (Van
Beek et al., 2011; Wada et al., 2011). Data for Africa from a global
simulation with 0.5∘ spatial resolution for a recent 41-year
period (January 1960 to December 2000) have been used (including Madagascar,
but excluding the smaller islands of Comoros, Mauritius, Seychelles and
Cape Verde). The model calculates for daily time steps the water storage in
two vertically stacked soil layers and an underlying groundwater layer, as
well as the water exchange between the layers and between the top layer and
the atmosphere (rainfall, evaporation and snow melt). The model also
calculates canopy interception and snow storage. During the simulation
period, land cover changes are not taken into consideration. For the green
water availability, the sum of the simulated actual transpiration of the two
soil layers under non-irrigation conditions (i.e. natural vegetation and
rainfed crops) was used (Van Beek et al., 2011). This conservative approach,
effectively reducing precipitation for surface runoff, percolation, soil
evaporation and interception, gives a measure of easily available soil
moisture for the plants, and ensures that the availability of water for the
crops is not overestimated. This approach is in agreement with the green
water definition by Savenije (2004) and the productive green water
definition by Falkenmark and Rockström (2006), who define transpiration
as the productive component of the green water, which is involved in biomass
production in terrestrial ecosystems as opposed to the unproductive part
attributable to soil evaporation (Supplement).
Proportion of cropland per cell (0.5 x 0.5 degree) in
2000 (Ramankutty et al., 2008).
Crop and irrigation data
The necessary crop data to calculate irrigation water demand (Irrig. Water
Demand, Eq. 3) relate to the crop distribution across the continent, the crop
calendar over the year, encompassing one or a maximum of two crops per year
for any area, and the annually accumulated monthly crop water demand for
each crop in each cell. For the crop distribution, data for the 2000 crop
distribution have been used (Monfreda et al., 2008; Ramaunkutty et al., 2008).
Figure 1 shows the cropland (217 × 106 ha) distribution in
Africa. This includes the cultivated (i.e. harvested) cropland and
non-cultivated cropland in 2000.
Six major irrigated crop groups, accounting for an average of 84 % of the
total harvested cropland in 2000 (165.7 × 106 ha) over the
continent, were considered (Table 1).
These include: cereals, oil crops, roots, pulses, vegetables and sugar crops
(sugarcane mostly in Africa). The proportion of the land area occupied by
the different crop groups is shown in Fig. 2. It is assumed that the
cropping pattern is not influenced by introduction of groundwater. While it
is known that smallholder GWI may preferentially be applied to higher value
crops (like vegetables) in SSA (Villholth, 2013) and that the dominant crops
in irrigated and rainfed agriculture differ from region to region in Africa
(Portmann et al., 2010), no data on the larger scale and distributed impact
of crop pattern change as a result of GWI exist.
In certain areas, the aggregated crop group areas accounted for more than
84 % of the harvested cropland. This is because double cropping occurs.
Hence, in order to ensure that double cropping does not entail exaggerated
cropland areas, the crop group areas were downscaled by cell-by-cell
factors, making the aggregated crop group area for those cells equal to
84 % of the harvested cropland.
Areal proportion of crop groups cultivated in Africa for
the year 2000, adapted from Monfreda et al. (2008).
Type of cropArea (106 ha)Proportion ( %)Cereals79.447.92Oils19.611.83Roots17.810.74Pulses16.39.84Vegetables4.42.66Sugar crops1.40.84Fruit8.45.07Forage3.72.23Fiber4.22.53Tree nuts1.30.78Other crops9.25.56Total165.7100 %
Proportion of crop group area per cell (0.5∘× 0.5∘)
cultivated in 2000 of the six largest crop
groups (adapted from Ramankutty et al., 2008).
For the crop calendar, Africa can be divided into 23 irrigation cropping
pattern zones, within which crop calendar, irrigation method and cropping
intensity can be assumed to be homogeneous within the cropland (FAO, 1997)
(Fig. 3). This subdivision is applied in this study.
Delineation of the 23 irrigation cropping pattern zones in Africa (based on
FAO, 1997; http://www.fao.org/geonetwork/srv/en/main.home, last access: 1 April 2014).
The crop calendar data have been extracted from the FAO crop
calendar
http://www.fao.org/agriculture/seed/cropcalendar/welcome.do (last access: 31
March 2014).
and other sources (FAO, 1992, 1986) and compiled into a
calendar per crop group done for each irrigation cropping pattern zone. The
calendar indicates the specific crops present in the group for each
irrigation cropping pattern zone (Supplement). Up to two
specific crops from the same crop group can be cultivated per year on the
same cropland and allows year-round cropping and an annual cropping
rotation.
The monthly crop water demand for each crop group is determined by Eq. (4),
using the maximum monthly reference evapotranspiration for each calendar
month over the period 1960–2000 and crop coefficient (Kc) as determined in
the Supplement. Growth periods and corresponding Kc values for the various
crops are extracted from the literature (FAO, 1992, 1986) and are assumed to
be constant over the 41-year period while the reference evapotranspiration
data are extracted from inputs to the PCR-GLOBWB global hydrological model.
Since the crop calendar includes entries with more than one specific crop for
a crop group (e.g. millet/wheat for cereals) and they have similar, but not
equal monthly water demands (Supplement), a conservative approach
is applied, whereby the larger figure for the crops has been applied, unless
the difference between them is equal to or more than 0.05 and 0.1, in
which case the larger coefficient is reduced by 0.01 or 0.02,
respectively. The reason for applying the conservative
approach is to ensure that the GWIP is not overestimated.
Irrigation efficiency dependent on irrigation cropping
pattern zone (FAO, 1997).
Irrigation cropping pattern zoneNumberZone nameIrrigation efficiency (%)1Mediterranean coastal zone602Sahara oases703Semi-arid to arid savanna West-East Africa504Semi-arid/arid savanna East Africa505Niger/Senegal rivers456Gulf of Guinea507Southern Sudan508Madagascar tropical lowland509Madagascar highland5010Egyptian Nile and Delta8011Ethiopian highlands5012Sudanese Nile area8013Shebelli–Juba river area in Somalia5014Rwanda – Burundi – Southern Uganda highland5015Southern Kenya – Northern Tanzania5016Malawi – Mozambique – Southern Tanzania4517West and Central African humid areas4518Central African humid areas below equator4519Rivers effluents on Angola/Namibia/Botswana border5020South Africa – Namibia – Botswana desert & steppe6521Zimbabwe highland6022South Africa – Lesotho – Swaziland6023Awash river area50
The irrigation efficiency (Irrig. Efficiency, Eq. 3) takes into
consideration the water lost during the irrigation path from the water
abstraction point to the water reaching the plants. Water losses occur
mainly during water transport (i.e. pipe leakage or evaporation/leakage in
open canals) and in the field (i.e. water running off the surface or
percolating past the root zone). Each irrigation cropping pattern zone has
an irrigation efficiency coefficient based on figures found in the
literature, type of crops irrigated and intensification level of the
irrigation techniques (FAO, 1997) (Table 2). The coefficient is mainly based
on surface water irrigation and it is here assumed applicable to GWI. This
assumption implies a conservative estimate of GWIP as open canal water
transport from rivers or lakes is typically found to be less efficient than
groundwater, which is abstracted more locally and in a distributed fashion
(Foster and Perry, 2010).
Other groundwater uses
Irrigation is only one of the groundwater uses and it is necessary to take
into account the other anthropogenic and environmental groundwater uses.
They are divided into four categories: domestic, industrial, and livestock
demands as well as environmental requirements. Irrigation from groundwater
is possible only after the groundwater demands of these uses have been
satisfied.
Other groundwater uses (adapted from Pavelic et al., 2013).
Portion assumedDaily waterto come fromUses/unit need (L)groundwater (%)DomesticInhabitant5075IndustrialInhabitant2575LivestockBig ruminant40100Small ruminant20100Pig30100Poultry0.2100
Groundwater demand of anthropogenic activities is calculated for each cell
using the density map of population and livestock from 2000 (FAO, 2007a, b)
and data in Table 3. Domestic, industrial and livestock water demand is
assumed constant over the period 1960–2000.
The environmental groundwater requirement remains highly uncertain. To
account for this, three scenarios have been applied: with environmental
groundwater requirements representing 70 % (Scenario 1), 50 % (Scenario 2),
and 30 % (Scenario 3) of the recharge, respectively over the continent
(Pavelic et al., 2013).
Calculation of groundwater irrigation potential
The GWIP (Eq. 1) is calculated using the maximum annual estimate of irrigation
water demand (Irrig. Water Demand) over the 1960–2000 period for each cell.
Hence, a conservative estimate of the irrigation potential is obtained. However,
rather than equally using the maximum values of groundwater availability (GW
Available), a constant averaged annual value of this parameter was used. This
in essence corresponds to smoothing out the variability in groundwater
availability (and recharge) and accounting for the buffering effect of the
resource. Hence, in low groundwater availability years, regular water
availability is assumed. If the average GW Available is negative in a cell
(due to persistent low recharge years or high human and environmental
demand), the availability is set to zero for that cell.
For the Irrig. Water Demand (Eq. 3), annual values were processed from
aggregated monthly data of crop water demand for the individual crop
groups within each cell, after reducing by available green water for each particular month. The
share of each crop group within the crop group area is accounted for
(% of Area, Eq. 3). Since for each crop group, up to two
specific crops can be grown in rotation on the same area but never
concurrently (Supplement), the number of crops (n, Eq. 3) in this
case refers to the number of crop groups, rather than specific crops.
Similarly, the Crop Water Demand refers to the sum of the crop water demand
of the actually grown crops in the crop group.
Estimated average net irrigation water demand
(1960–2000) for the cropland in Fig. 1: (a) expressed in 106 m3 year-1
cell-1 (0.5∘× 0.5∘)
and (b) in mm year-1.
Results
The net irrigation water demand (Irrig. Water Demand × Irrig. Efficiency)
is shown in Fig. 4. It is seen (Fig. 4a) that the
irrigation demand reflects primarily the density of cropland (Fig. 1) and
the aridity of the regions (Fig. 4b). It also reflects the green water
availability, which is higher in the equatorial regions, except in East
Africa (Supplement).
Average groundwater availability for irrigation
(1960–2000), expressed in 106 m3 year-1 cell-1
(0.5∘× 0.5∘), for various levels of
environmental groundwater requirements as a fraction of recharge:
(a) Scenario 1: 70 %, (b) Scenario 2: 50 %, (c) Scenario 3: 30 %.
The groundwater available for irrigation is the surplus recharge after
satisfying human and environmental groundwater needs (Eq. 1). This varies
according to the three scenarios (Fig. 5). The total renewable groundwater
availability for irrigation across the continent ranges from 692 (Scenario
1) to 1644 km3 year-1 (Scenario 3). Not surprisingly, the
availability is greater along an equatorial band across the continent where
rainfall and recharge are highest. It is also seen that large parts of
northern and southern Africa are devoid of excess recharge to enable
irrigation from renewable groundwater resources.
(I) Total area irrigable with groundwater inside a cell
(0.5∘× 0.5∘) in 103 ha and (II)
proportion of cropland irrigable with groundwater, for various levels of
environmental groundwater requirements as a fraction of recharge:
(a) Scenario 1: 70 %, (b) Scenario 2: 50 %,
(c) Scenario 3: 30 %.
Converting the groundwater availability into GWIP in terms of irrigable
area, a similar pattern is found (Fig. 6). The white areas in central Africa
with zero potential correspond to areas with no cropland, essentially areas
covered by permanent forest. Appreciable hydrological potential exists for
GWI across much of Africa, except for the most arid regions and in the most
southern part where demand from other sectors compete with GWI (data not
shown). Hence, most regions in the Sahel and the eastern tract of the
continent, from Ethiopia down to Zimbabwe, may provide significant
unexploited opportunities for groundwater development for agriculture, with
up to all cropland, and sometimes more, being irrigable from renewable
groundwater. This benefit accrues from mostly supplementary GWI in the wet
season as well as mostly full GWI in the dry season. The maps also indicate
that relatively large disparities in GWIP exist within individual countries,
e.g. Ethiopia, Mozambique, Angola and Tanzania. Potential hotspot areas should
be further explored in terms of other factors governing the potential for
GWI development. Aggregating the GWIP across the continent, values range
from 44.6 × 106 ha to 105.3 × 106 ha for the three
scenarios, corresponding to 20.5 to 48.5 % of the cropland.
Gross groundwater irrigation potential and cultivated area
per country in Africa.
Area of cropland irrigable with Siebert etFAO –groundwatera (103 ha)al. (2010)AQUASTATbScenario (PercentageArea equippedof groundwaterfor irrigationfor environmentalirrigated withCultivatedrequirements)groundwater (103 ha)land (103 ha)1 (30 %)2 (50 %)3 (70 %)Algeria1409449362.18465Angola70325016300116.05190Benin5183682182.23150Botswana6646270.7287Burkina Faso2681881083.06070Burundi214149840.01450Cameroon7019500529901.07750Central African Republic6961496929780.01880Chad5664012376.04932Côte d'Ivoire2920207812360.07400Democratic Republic of Congo23 06016 45098400.07810Djibouti5321.02Egypt221331.93612Equatorial Guinea6344532710.0180Eritrea107416.2692Ethiopia4336306417932.616 488Gabon5884420225200.0495Gambia2417100.0445Ghana1426101059412.07400Guinea2751196211720.53700Guinea-Bissau176125754.9550Kenya5123551991.06130Lesotho211580.1285Liberia223815979560.0710Libya261810464.02055Madagascar6753481428750.04110Malawi6404542680.03885Mali7875593311.07011Mauritania5237224.8411Morocco1459749677.29403Mozambique217115469210.65950Namibia9870411.6809Niger191261.416 000Nigeria62874446260666.841 700Republic of Congo7420529531700.0600Rwanda148102560.11432Senegal38227116010.23415Sierra Leone155111076620.21897Somalia51352010.01129South Africa27018195127.312 413South Sudan3042216412860.22760cSudan42929916969.013 893cSwaziland211581.0190Tanzania30072135126317.516 650Togo3002131260.12850Tunisia26179257.05249Uganda5713992280.19150Western Sahara0000.04cZambia3952281816846.73836Zimbabwe37025914820.04100
a Errors up to 35 % for small countries (due to the
cell size, the projection used in GIS and the shape of the countries, i.e.
Gambia); bhttp://www.fao.org/nr/water/aquastat/countries_regions/index.stm;
c estimated.
The GWIP for the 13 countries estimated by Pavelic et al. (2013)
(13.5 × 106 ha) is here calculated to 17.1 × 106 ha,
showing good correspondence between the methods, though the present method
does indicate the distributed extent of GWIP across the countries and for
the whole continent. In Table 4, the GWIP for the individual countries in
Africa are given. The results show that the GWI area in Africa can safely be
expanded by a factor of 20 or more, based on the conservative renewability
and environmental requirements of the resource and the present human
demands, possibly with wide livelihood benefits for smallholder farmers in
many Sahel and semi-arid regions of eastern Africa. Comparing the GWIP with
the overall irrigation potential of 42.5 × 106 ha estimated by
FAO (2005), it is clear that groundwater can play a significant role in food
production and food security in large parts of Africa. While in such
comparison, figures for irrigation potential may not be simply additive due
to overlap of the resources and lack of cropland or other constraints, it is
clear that opportunities exist in the concurrent development of both sources
and some benefits are achievable in planning schemes that are conjunctive
(Evans et al., 2012).
Some blue areas with very high potential relative to the cropland area (Fig. 6i),
as seen in arid parts of South Africa, Mali and Sudan can be explained
by very small cropland areas relative to the cell size. Hence, accumulated
recharge over the cell, albeit low in nominal terms, may be sufficient to
irrigate these areas.
(a) Area irrigated with groundwater in 2005 expressed in
ha. per cell adapted from Siebert et al. (2010) and (b) groundwater
irrigation potential for Scenario 2 (the environmental groundwater
requirements represent 50 % of the recharge) for the year 2000 expressed
as the percentage of the area irrigated with groundwater in 2005.
In order to further analyse the GWIP, and explore the untapped part of the
potential, the results are compared with existing data on the present
development of GWI across Africa (Fig. 7). The map in Fig. 7a presents the
best available continent-wide data for areas equipped for GWI (Siebert et
al., 2010), while Fig. 7b shows the relative GWIP (in terms of area) in
Scenario 2 (the environmental groundwater requirements represent 50 % of
recharge), expressed as the percentage of the data from Siebert. While this
approach only captures and compares areas having non-negative values for
present GWI development, it gives a clear indication of the contrast across
the continent with respect to the areas with and without further GWIP (the
non-red areas versus the red areas). In northern and southern Africa
the untapped development potential is very limited or patchy, while in
western Africa and the eastern belt, still appreciable GWI development
potential exists. These results also indicate that presently GWI is mostly
developed in regions with limited potential, and significantly in areas
where groundwater is non-renewable (like in northern Africa) or where limited
uncommitted renewable groundwater resources exist. In fact, the method also
gave indications of where groundwater is already over-allocated, based only
on the human needs (let alone irrigation and the environment) relative to
the recharge. This is generally not the case, but occurrences appear in arid
high-density livestock or populated parts of northeastern South Africa and
southeastern North Sudan (data not shown). An apparent artefact is
discernible in the horn of Africa. Here, appreciable GWI exists (Fig. 7a),
while Fig. 1 shows no cropland. The explanation could be that areas in this
region are mostly irrigated pasture land, or pasture land converted into
irrigated cropland after the 2000 map of cropland (Fig. 1) was produced.
Comparison of estimations of groundwater recharge for
selected African countries.
ahttp://www.fao.org/nr/water/aquastat/main/index.stm
(last accessed: 2 April 2014).
b Data as provided in Margat and Gun (2013).
c Data calculated from the PCR-GLOBWB model (Van Beek et al.,
2011).
DiscussionUncertainty and variability of recharge and environmental
requirements
In assessing the confidence of the methodology presented, the uncertainty
and temporal variability of recharge as well as the uncertainty of the
environmental requirements need to be taken into consideration. Table 5
summarizes estimations of groundwater recharge for a number of African
countries from different sources. It shows that the annual recharge
estimation from the hydrological model PCR-GLOBWB (this paper) is quite
similar to the one estimated from the WaterGAP Global Hydrology Model (WGHM)
(Döll and Fiedler, 2008) while there is more discrepancy with the FAO
data set. Since the GWIP is strongly dependent on the recharge, this
uncertainty will be reflected in the GWIP.
Average annual recharge (mm year-1) and (b)
its coefficient of variation (%), both over the period 1960–2000 (data
from Van Beek et al., 2011).
Aggregated groundwater available (km3 year-1)
for the three environmental scenarios.
∗ Min. and Max. refers to minimum and maximum annual values over the 41
years.
The maps in Fig. 8 present the average annual recharge (Fig. 8a) and the
coefficient of variation of the recharge (Fig. 8b) of the 41-year simulation
period. The coefficient of variation shows clearly that the areas where the
recharge is smaller (say less than 50 mm per year) also have the highest
variability over the years. In these areas, recharge can vary from zero to
double of the average recharge (dark red colour). The results indicate that
where groundwater recharge is sufficient to support GWI in these areas, it
is likely to be a very strategic resource in buffering seasonal and
inter-annual climate variability. Secondly, the actual buffering capacity of
groundwater, which is governed by the longer-term storage capacity of the
aquifers, more so than the recharge, becomes equally important in these
areas and needs to be addressed in further and more detailed assessments. In
the present approach, buffering of the groundwater is only considered by
using the long-term average GW Available in Eq. (), as explained in the
Sect. 4. Similarly, the buffering capacity of groundwater in a spatial sense
was applied in assuming that all recharge in a cell can be captured anywhere
in that cell.
The uncertainty associated with the environmental requirements relates to
the lack of knowledge of the location and functioning of ecosystems
dependent on groundwater throughout Africa and their groundwater
requirements in quantitative terms. Such ecosystems and their requirements
may depend on the hydrogeological setup of an area, the scale of the
aquifers, and the climate (Tomlinson, 2011). However, in the absence of better
understanding and tested approaches, the three scenarios approach was used
(Pavelic et al., 2013). When comparing the uncertainty related to the
scenarios in terms of the GW Available (Table 6) (about 480 km3
year-1, as calculated from the difference between the averages of Scenario
2 and 1, and Scenario 2 and 3, respectively), and the uncertainty related to
the recharge (estimated from the range between the average and min. and
average and max. annual GW Available for Scenario 2, which is 417 and 496 km3)
it is apparent that the uncertainty on groundwater availability
related to the environmental requirements is of the same order of magnitude
as the effect of the temporal variability of recharge.
Limitations of approach
The water balance approach considers locally renewable groundwater
availability as the major controlling parameter for GWIP and assumes
non-limiting conditions in terms of other fundamental physical properties,
e.g. soil and water quality, terrain slope, and groundwater accessibility
(as determined by e.g. depth of the usable aquifer, storage available for
recharge, and well yields) for the implementation of GWI. Considering an
average landholding size of 1 ha with a single well, or alternatively 1 well
per hectare for landholdings larger than 1 ha, over the continent and the gross
irrigation water demand per year varying between 235 and 3000 mm per year,
with the cropping pattern applied in the present study, an average cropping
season of 240 days of daily irrigation for 8 h, this translates into a
required well yield varying from 0.3 to 4.3 L s-1. Comparing this with
continental-wide maps of well yields (MacDonald et al., 2012), it is evident
that in certain geological formations, like the basement rock aquifers, that
occupy 34 % of the continent (Adelana and MacDonald, 2008), the yield of
the geological substrata may in places be limiting for larger scale or very
intensive GWI development.
The GWIP was conceived strictly in terms of the quantitative availability of
renewable groundwater. Possible constraints related to hydrogeology as well
as water quality and socioeconomic conditions, such as infrastructure
(roads, markets, energy/electricity) or intuitional/farmer capacities may
further reduce this potential or hamper its realization as will be further
analysed in a companion paper. Rapid assessment of borehole yields indicated
a possible limitation due to the hydrogeological transmitting properties in
certain regions.
Furthermore, climate trends and progressive water demands from growing human
and livestock populations have not been considered. For these reasons, it is
suggested to apply the most conservative estimates (i.e. Scenario 1) for a
robust estimate of hydrological GWIP. Likewise, historic and potential
future changes in cropping patterns and irrigation efficiencies have not
been considered though they could significantly enhance the groundwater
availability, through increasing the green water availability (by shifting
the unproductive part to productive), and hence the potential for
irrigation. In essence, the method is a snapshot continental distributed
view of present or most recent GWIP, based on averaged hydrological
conditions and best available most recent coherent data sets. However, the
influence of cropping choice was clearly demonstrated in Pavelic et al. (2013).
They showed that going from a 1000 mm year-1 irrigation demand
to a 100 mm year-1 crop, everything else being equal, entailed an order
of magnitude higher GWIP.
Conclusions
The present study has estimated the extent and distribution of groundwater
irrigation potential (GWIP) across the African continent (0.5∘
resolution), based on the hydrologically available and renewable groundwater
over a 41-year recent historic period and using crop and cropland data from
the beginning of the century. The GWIP is assessed to be between
44.6 × 106 ha and 105.3 × 106 ha, depending on the
proportion of recharge assumed allocated preferentially to the environment
(30–70 %), while assuming constant human needs for groundwater. This is
a gross estimate, disregarding existing groundwater irrigation (GWI).
However, with the present GWI area amounting to approximately 2 × 106 ha,
the difference between net and gross potential is small.
However, comparing GWIP to existing maps of GWI, it is clear that present
GWI has been primarily developed in northern and southern Africa where the
development potential is relatively limited, and where it is governed by
abstraction from non-renewable or already stressed resources, from recharge
from larger rivers like the Nile, or return flows from surface water
schemes, while the rest of the continent (except for the Sahara region)
still has appreciable potential, especially and most relevantly for
smallholder and less intensive GWI in the semi-arid Sahel and east Africa
regions. This could significantly increase the food production and
productivity in the region from a reliable and renewable resource.
The Supplement related to this article is available online at doi:10.5194/hess-19-1055-2015-supplement.
Acknowledgements
Rens Van Beek, Geosciences – Utrecht University, kindly provided data from
the PCR-GLOBWB model. Agnès Ducharne, Laboratoire METIS – Université Pierre et Marie Curie – Paris,
provided critical view, specifically on Crop Water Demand. The work has been partially funded by the French
Ministry of Agriculture and by USAID (Enhanced Regional Food Security
through Increased Agricultural Productivity to Sustainably Reduce Hunger
project of the USAID Southern Africa Feed The Future (FTF) programme
(2012–2013)). It was also supported by CGIAR Strategic Research Program on
Water, Land and Ecosystems (WLE).
Edited by: M. Vanclooster
References
Adelana, S. M. A. and MacDonald, A. M.: Groundwater research issues in
Africa, in: Applied Groundwater Studies in Africa, edited by: Adelana, S. M.
A. and MacDonald, A. M., IAH Selected Papers on Hydrogeology, no. 13, Ch. 1,
CRC Press, Taylor & Francis Group, London, UK, 507 pp., 2008.Awulachew, S. B., Erkossa, T., and Namara, R. E.: Irrigation Potential in
Ethiopia – Constraints and Opportunities for Enhancing the System,
International Water Management Institute, available at:
http://www.ata.gov.et/wp-content/uploads/Ethiopia-Irrigation-Diagnostic-July-2010.pdf (last access: 20 May 2014),
59 pp., 2010.CAADP (Comprehensive African Agriculture Development Programme):
Sustainable Land and Water Management, The CAADP Pillar I Framework,
available at:
http://www.caadp.net/pdf/CAADP Pillar 1 Framework.pdf (last
access: 21 May 2014), 76 pp., 2009.Döll, P. and Fiedler, K.: Global-scale modeling of groundwater recharge,
Hydrol. Earth Syst. Sci., 12, 863–885, 10.5194/hess-12-863-2008, 2008.
Evans, W. R., Evans, R. S., and Holland, G. F.: Conjunctive use and
management of groundwater and surface water within existing irrigation
commands: the need for a new focus on an old paradigm. Thematic Paper 2.
Groundwater Governance: A Global Framework for Country Action, GEF ID
3726, 48 pp, 2012.
Falkenmark, M. and Rockström, J.: The new blue and green water paradigm:
Breaking new ground for water resources planning and management, J. Water
Resour. Plann. Manage., 132, 129–132, 2006.
FAO: Irrigation Water Management: Irrigation Water Need, Training Manual no.
3, edited by: Brouwer, C. and Heibloem, M., Food and Agriculture
Organization of United Nations, Rome, Italy, 102 pp., 1986.
FAO: Irrigation Water Management: Irrigation Scheduling, Training Manual no.
4, edited by: Brouwer, C., Prins, K., and Heibloem, M., Food and Agriculture
Organization of United Nations, Rome, Italy, Land and Water Development
Division, 66 pp., 1989.FAO: Crop Water Requirements, FAO Irrigation and Drainage Paper no. 24,
edited by: Doorenbos, J., Pruitt, 5 W. O., Aboukhaled, A., Damagnez, J.,
Dastane, N. G., Van den Berg, C., Rijtema, P. E., Ashford, O. M., and Frere,
M., Food and Agriculture Organization of United Nations, Land and Water
Development Division, Rome, Italy, 144 pp., 1992.
FAO: Irrigation Potential in Africa: a Basin Approach, FAO Land and Water
Bulletin 4, Food and Agriculture Organization of United Nations, Land and
Water Development Division, Rome, Italy, 177 pp., 1997.FAO: Irrigation in Africa in Figures, AQUASTAT Survey – 2005, FAO Water
Reports no. 29, Food and Agriculture Organization of the United Nations,
Rome, available at: ftp://ftp.fao.org/agl/aglw/docs/wr29_eng.pdf (last access: 20 May 2014), 74 pp., 2005.FAO: Population density (persons/km2), available at:
http://www.fao.org/geonetwork/srv/en/resources.get?id=30586&fname=poprecl_ASCII.zip&access=private (last access: 31 March 2014), 2007a.FAO: Gridded livestock of the world, available at:
http://www.fao.org/ag/againfo/resources/en/glw/GLW_dens.html
(last access: 31 March 2014), 2007b.
Foster, S. and Perry, C.: Improving groundwater resource accounting in
irrigated areas: a prerequisite for promoting sustainable use, Hydrogeol.
J., 18, 291–294, 2010.MacDonald, A. M., Calow, R. C., Nicol, A. L., Hope, B., and Robins, N. S.:
Ethiopia: water security and drought, Technical Report WC/01/02, British
Geological Survey, Nottingham, available at:
http://nora.nerc.ac.uk/501045/1/Ethiopia_map.pdf (last
access: 20 May 2014), 2001.MacDonald, A. M., Bonsor, H. C., Dochartaigh, B. É. Ó., and Taylor,
R. G.: Quantitative maps of groundwater resources in Africa, Environ. Res.
Lett., 7, 024009, 10.1088/1748-9326/7/2/024009, 2012.
Margat, J. and Van der Gun, J.: Groundwater Around the World: a Geographic
Synopsis, CRC Press, Taylor & Francis Group, London, UK, 376 pp., 2013.MoAC (Ministry of Agriculture and Co-operatives): The Republic of Zambia:
National Agriculture Policy 2004/2015, 54 pp., available at:
http://www.gafspfund.org/sites/gafspfund.org/files/Documents/5. Zambia_strategy.pdf (last access: 4 April 2014), 2004.
MoFED (Ministry of Finance and Economic Development): The Federal
Democratic Republic of Ethiopia: Growth and Transformation Plan (GTP)
2010/11–2014/15 Draft, September 2010, Addis Ababa, 85 pp., 2010.MoFA and GIDA (Ministry of Food and Agriculture and Ghana Irrigation Development
Authority): National Irrigation Policy, Strategies and Regulatory
Measures, GIDA, Accra, available at:
http://mofa.gov.gh/site/wp-content/uploads/2011/07/GHANA-IRRIGATION-DEVELOPMENT-POLICY1.pdf (last access: 20 May 2014), 37 pp., 2011.MoIWD (Ministry of Irrigation and Water Development): The Republic of
Malawi: National Water Policy, Lilongwe, available at:
http://www.moafsmw.org/Key Documents/National Water Policy FINAL.pdf
(last access: 4 April 2014), 2005.MoWEA (Ministry of Water and Environmental Affairs): The Republic of South
Africa: National Water Resource Strategy, Department of Water Affairs,
available at:
http://www.dwaf.gov.za/nwrs/LinkClick.aspx?fileticket=u_qFQycClbI%3d&tabid=91&mid=496
(last access: 4 April 2014), 2013.
Monfreda, C., Ramankutty, N, and Foley, J. A.: Farming the planet: 2.
Geographic distribution of crop areas, yields, physiological types, and net
primary production in the year 2000, Global Biogeochem. Cy., 22, GB1022,
10.1029/2007GB002947, 2008.NEPAD (New Partnership for Africa's Development): Comprehensive Africa
Agriculture Development Programme, available at:
http://www.nepad.org/system/files/caadp.pdf (last access: 20 May 2014), 102
pp., 2003.Pavelic, P., Smakhtin, V., Favreau, G., and Villholth, K. G.: Water-balance
approach for assessing potential for smallholder groundwater irrigation in
Sub-Saharan Africa, Water SA, 38, 399–406, 10.4314/wsa.v38i3.5, 2012.Pavelic, P., Villholth, K. G., Smakhtin, V., Shu, Y., and Rebelo, L. M.:
Smallholder groundwater irrigation in Sub-Saharan Africa: country-level
estimates of development potential, Water Int., 38, 392–407,
10.1080/02508060.2013.819601, 2013.Portmann, F. T., Siebert, S. and Döll1, P.: MIRCA2000 – Global monthly
irrigated and rainfed crop areas around the year 2000: A new high-resolution
data set for agricultural and hydrological modelling, Global Biochem.
Cy., 24, GB1011, 10.1029/2008GB003435, 2010.Ramankutty, N., Evan, A., Monfreda, C., and Foley, J.: Farming the planet:
1. Geographic distribution of global agricultural lands in the year 2000,
Global Biogeochem. Cy., 22, GB1003, 10.1029/2007GB002952, 2008.
Savenije, H. H. G.: The importance of interception and why we should delete
the term evapotranspiration from our vocabulary, Hydrol. Process., 18,
1507–1511, 2004.Siebert, S., Burke, J., Faures, J. M., Frenken, K., Hoogeveen, J., Döll,
P., and Portmann, F. T.: Groundwater use for irrigation – a global
inventory, Hydrol. Earth Syst. Sci., 14, 1863–1880,
10.5194/hess-14-1863-2010, 2010.
Tomlinson, M.: Ecological Water Requirements of Groundwater Systems: a
Knowledge and Policy Review, Waterlines Report Series No. 68, National Water
Commission, Canberra, Australia, December 2011, 134 pp., 2011.Van Beek, L. P. H., Wada, Y., and Bierkens, M. F. P.: Global monthly water
stress: I. Water balance and water availability, Water Resour. Res., 47,
W07517, 10.1029/2010WR009791, 2011.Villholth, K. G.: Groundwater irrigation for smallholders in Sub-Saharan
Africa – a synthesis of current knowledge to guide sustainable outcomes,
Water Int., 38, 369–391, 10.1080/02508060.2013.821644, 2013.Wada, Y., Van Beek, L, Viviroli, D., Dürr, H., Weingartner, R., and
Bierkens, M.: Global monthly water stress: 2. Water demand and severity of water
stress, Water Resour. Res., 47, W07517, 10.1029/2010WR009792, 2011.You, L., Ringler, C., Nelson, G., Wood-Sichra, U., Robertson, R., Wood, S.,
Guo, Z., Zhu, T., and Sun, Y.: What Is the Irrigation Potential for Africa?
A Combined Biophysical and Socioeconomic Approach, IFPRI Discussion Paper
00993, June 2010, IFPRI, available at:
http://www.ifpri.org/sites/default/files/publications/ifpridp00993.pdf (last
access: 20 May 2014), 2010.