The Water Crisis in Yemen

Most of Yemen is arid or hyper-arid (Chapter 1). Current water availability per head of the population is about 100 m³ annually, one-tenth of the regional average, a hundredth of the worldwide average.¹ Availability is projected to continue to decrease sharply with the rapid population increase (Table 4.1). As a rule of thumb, 100 m³ per person annually is required for human needs, and 1,000 m³ for food self-sufficiency. Overall, therefore, Yemen is classed as a very water-scarce country.

Yemen’s water resources comprise seasonal rivers,³ springs and groundwater. There are no significant permanent rivers but there is considerable seasonal ‘spate’ flow in wadis.⁴ Data are poor but adequate to build a general picture. The ‘internal renewable resource’ – i.e. the amount of precipitation that is available for use from rainfall running off into watercourses or percolating into groundwater – is estimated at about 2.1 BCM annually (see Table 4.2).

Overall use is estimated at about 2.8 billion m³ (BCM), with the difference between resources and uses being made up by drawdown of groundwater reserves. This ‘overdraft’ is unevenly distributed across basins, and the total actual drawdown of reserves is estimated at about 1.1 BCM (Table 4.2). The risks and costs entailed by this overdraft form a central theme of this book.⁵

In the 1990s, the then President of Yemen, Ali Abdullah Saleh, was taxed with Yemen’s ‘water crisis’. He responded: ‘Yemen is noted for its comparatively high rainfall. Why is such a small proportion of the rain that falls on Yemen available for use?’ This is a good question – 37 BCM of rain falls on Yemen annually, but only 2 BCM is available for use. Why?

Rain falling on Yemen will either evaporate directly from the surface, it will penetrate the soil horizon, or it will run off to join the drainage system. The balance between these outcomes is crucial in answering the President’s question. In fact, the main response to the President’s challenge is Yemen’s climate. As described in Chapter 1, Yemen has a largely arid climate, which means that the country’s low humidity and high temperatures cause a very large proportion of the rain simply to evaporate as soon as it falls.

Most of the rain falling in Yemen thus returns to the atmosphere within a few days directly through evaporation. Another share will penetrate the soil and be stored temporarily as soil moisture. The infiltration capacity of the soil horizons is a crucial factor in determining what proportion of rainwater will enter the soil profile. In Yemen, in general, the soils are not very permeable, so infiltration is limited.

Much of the water that does enter the soil profile will rise to the surface again by capillary action and evaporate. In higher rainfall areas where there is permanent vegetation or in agricultural areas, some of the rainwater in the soil profile is picked up by the roots of plants and is returned to the atmosphere through evapotranspiration action. Smaller portions may percolate deeper into the ground and join groundwater flows. This will occur where the geological structures underlying the soils are relatively porous.

Rain that does not directly evaporate or percolate into the soil will run off to the nearest branch of the drainage network. Rain falling on a dry permeable soil will initially produce little or no run-off. Rain falling on a saturated or impermeable surface such as rock will produce more run-off. In Yemen, run-off is increased by the violent rainfall events, the sparse vegetation and the limited permeability of soils in most catchment areas.

Typically less than one-tenth of rain that falls on Yemen is captured in streams and rivers

Despite the intense rainfall events and the limited permeability, typically, less than one-tenth of Yemen’s rainfall will be converted to stream flow.⁸ For larger catchments (more than a few thousand km²) run-off coefficients are generally less than 10 per cent, and in the largest catchments the mean is only 5 per cent (see Tables 4.3 and 4.4). Overall, rainfall effectively captured into streams as run-off is only a twentieth of the total. In Wadi Juaymah, out of 35 mm of rain, only 1 mm passes as run-off.

The terraces long employed in Yemen’s agriculture stem the run-off, and the water penetrates their ploughed level surfaces. In some regions, run-off areas on the slopes of hills are specially prepared to reduce infiltration and to channel the run-off to run-on fields. Run-off/run-on farming systems can be very efficient at capturing rainfall for beneficial use. One study of the Amran valley found that 25–50 per cent of the rain falling on the micro-catchment was effectively diverted onto the run-on area and was available for beneficial use. In addition, traditional water harvesting systems capture water in local structures – ponds, tanks, small reservoirs – for human and animal use.⁹

Rainfed agriculture increases local beneficial use but reduces the water available to run-off and groundwater recharge

The beneficial use of rainwater in the soil profile is also increased by direct use in rainfed agriculture in the large plains that characterize the coastal regions and the inter-montane basins of the highlands. Here evapotranspiration from the growing plants reduces both run-off and percolation to groundwater.

As terrace agriculture and upstream diversion have become more efficient, lower base flows and less frequent spate events have been recorded in some wadis (Wadi Surdud, for example). This does not mean that more water is being lost to the hydrological system (as more may be percolating into groundwater), nor does it mean that water is being wasted (as upstream uses may be efficient compared to downstream uses).¹⁰

Overall, about 2 BCM of water are captured in the surface water system – about 5 per cent of total rainfall

A best estimate is that the total run-off – the effective rainfall going to stream beds and theoretically available for spate diversion and groundwater recharge – totals 2 BCM, i.e. just over 5 per cent of rainfall. The largest share of run-off – more than one-third of the total – discharges into the Red Sea wadis. The Gulf of Aden and Arabian Sea wadis each channel just over one-quarter of the total run-off (Table 4.3).¹¹

In time of rain, run-off moves rapidly to the nearest branch of the drainage system and rushes downstream into ever larger wadi beds. In the wadis, run-off forms quickflow: the seasonal flow of rainwater in the drainage system. The aggregate volume of run-off is determined by the size of the catchment area, the volume of rainfall, the permeability of the soils, and prevalence of vegetation and temperature. Around the mean of 5 per cent, run-off coefficients range from 1 per cent in Wadi Rasyan (around and below Ta’iz) to 11 per cent in Wadi Rima (Table 4.4).

In the wadis, much of the water infiltrates the groundwater system. Due to this infiltration, and in some cases because of diversion of quickflow for farming (spate irrigation, see below), water does not increase in volume steadily as the wadi flows downstream. The flow may be totally interrupted at some point and then pick up again several times as the wadi is recharged by side wadis, and then again lose water to percolation or diversion. Little flow reaches the sea.

Typically, wadi beds are dry for much of the year. However, some water that infiltrates groundwater reappears above ground to contribute to surface flows in stream beds as baseflow (Figure 4.1). Baseflow is the year-round component of stream flow fed by discharge from groundwater springs. Most wadis have baseflow, and baseflow is an important component of total flows in the Red Sea basin. Here infiltration to groundwater is higher in the mountain catchment zone, and evaporation losses are lower than in other surface water basins. All major wadis draining to the west coast have permanent baseflows in the foothills zone that may make up about 40 per cent of total flow. In some cases, this baseflow is fed by recent rains, in others the outflow from groundwater into the wadis is part of a system of underground storage. Water may be flowing from reserves laid down from rainfall of the distant past.¹³

Figure 4.1 Wadis that carry huge spate flows for a few days a year may also have a much smaller permanent baseflow. Photograph courtesy of Dorte Verner.

The Red Sea basin: this is Yemen’s most important basin, contributing 36 per cent of total run-off (Figure 4.2). A number of large wadis drain the steep western escarpment and lose most of their water in the permeable sediments of the coastal Tehama. All wadis have catchment areas larger than 1,000 km² – Wadi Mawr is the largest (approximately 8,000 km²). Rainfall in the highlands in spring and summer generates significant run-off in the upper and middle catchment. Flood peaks are high as rainfall in the catchment area is high and slopes are steep. In the coastal plain, some of the flow is diverted for spate irrigation and lost to evapotranspiration, some percolates below the soil profile into the aquifer and recharges groundwater, and some reaches the sea, generally by groundwater outflow.

Gulf of Aden basin: contributes 27 per cent of total run-off. The main wadis, all of which have catchments of more than 1,000 km², drain south from the Southern Highlands. The large spate flows are extensively diverted in the broad coastal plain, but more flow reaches the sea than in the Red Sea systems. In recent years, spate flows in the plain have dwindled as diversion upstream has increased.

Arabian Sea basin: contributes 28 per cent of total run-off. In ancient times, this large complex basin supported agriculture for the old city states. Its topography would allow water to flow from the eastern slopes of the highlands, down through the Ramlat al-Sabatayn to Wadi Hadramawt, and out towards the sea via Wadi Masila. However, as rainfall rates are low and the soils allow for rapid infiltration, the ‘basin’ is more a series of discontinuous segments. Run-off volumes can be large and cause flooding.

Rub’ al-Khali basin: the northern and north-eastern slopes of the highlands drain into the sands of the Empty Quarter, where run-off and wadi flows infiltrate into groundwater.

The Mountain basins: scattered through the highlands, a series of plains surrounded by mountains constitutes self-contained basins, with little or no surface water draining outside the basin.¹⁵ The recharge of groundwater in these small basins is limited, but they are centres of high population concentration and hence of heavy water use.¹⁶

The concentration of population in the relatively water-scarce highland basins – especially Sana’a and Ta’iz – is leading to extreme water constraints in towns and to aggravated competition for water between town and country. Coastal areas are better supplied through run-off from the highlands, and the eastern provinces are sitting on vast but very deep fossil aquifers that have been scarcely touched up to now.

Figure 4.2 Main surface water systems in Yemen. ‘The Water Resources of Yemen: A Summary and Digest of Available Information’, Report WRAY-35, Sana’a, Republic of Yemen, March 1995.

Only for the shared fossil aquifers is there is a possible issue of transboundary riparian rights

The Umm al-Ruduma and Dammar aquifers underlie Yemen’s north-east border with Saudi Arabia and Oman, with a small natural outflow from Yemen to its neighbours estimated at about 50 MCM annually. Yemen is mercifully free from the transboundary water rights issues that dog other nations in the region.¹⁷

As in all arid zones, rivers in Yemen tend to be ephemeral. Typically, the wadi beds are dry for most of the year, and floods come and go quickly. Some wadis may remain dry for several years and then a huge flash flood arrives. Flood peaks are often quick and torrential, because rainfall events are violent, slopes are steep, and infiltration in the catchment area is reduced by sparse vegetation and limited soil permeability.¹⁸

The nature of a wadi spate can be illustrated by the rainfall event on 17 June 1984 in Wadi Surdud, measured at Faj Al Hussein just at the bottom of the escarpment before the wadi enters the Tehama coastal plain. The flow increased in just a quarter of an hour from 1.1 cusecs to 87 cusecs.¹⁹ The flow then receded to less than 2 cusecs within four hours.²⁰

The time taken for the flood water to peak is short, usually less than 30 minutes. This produces the characteristically dangerous rush of spate water. As the wadis are wide, travellers are often caught in the flow, unable to reach the farther side as the waters rapidly rise. Box 4.1 tells of my own experience of this. Peak flows are huge. The highest recorded peak flows were Wadi Bana, which reached 3,810 cusecs in 1982 – 3,810 tons of water passing by every second – and Wadi Sarr, which reached 2,160 cusecs in 1989. Average flows are more modest, reflecting the sudden peaks associated with the flows. In the 1974 event described above, the peak flow in Wadi Surdud was 87 cusecs, but the average flow for the day was only 3.3 cusecs.²¹

Box 4.1: Near-death by spate in the Tehama.

Travelling in the Tehama in 1999, I spotted a cloud way off above the highlands, 50 miles distant. By the time we arrived at the next wadi crossing, there was a trickle of water in the main bed, which swelled in five minutes to ankle depth across the 100 m span of the wadi.

Prudently we stopped, but a couple of trucks, pressed for time, lumbered on. When they reached the middle of the wadi, the water had risen above the level of their chassis, and both trucks stalled in the wadi bed. The waters continued to rise, nudging the trucks downstream towards the sea, a mile distant. Soon the fast-rising torrent pushed the larger truck up against the smaller. Both drivers clambered onto the roof of their truck, and then as the smaller truck wobbled in the force of the flow, its driver leaped onto the cab of the larger truck. The two drivers clung together as the smaller truck slipped away and was borne out towards the sea.

The crowds on the bank were exhorting a local man who had a Caterpillar front-loader with a shovel to go in. He was terrified but eventually he agreed. The big machine edged up to the remaining truck, which was shifting horribly. The two drivers scrambled into the shovel at the very moment when the spate torrent dislodged the big truck and swept it out to sea. All three men got back safely to our side. They were shaking. We bought them a hot dinner.

Spate flows rarely last more than two days, and an entire season’s water in a wadi may be delivered in that short period. This has implications for the size and cost of the works designed to divert spate flows for agriculture – a weir may have to be built that can accommodate a huge daily flow, but the works operate for just a few days each year.

Baseflow is crystal clear in all cases. The level of dissolved solids tends to increase as the flow slackens. Measurements on Wadi Surdud showed 500–600 micromho/cm during baseflow, dropping to around 300 during floods. In Wadi Bana, baseflow showed 1,500–1,800 micromho/cm, dropping to 400–500 during spate flows.²²

Springs occur in many parts of Yemen, particularly in the mountain and plateaux areas. In the Ahjar area, for example, in the upper reaches of Wadi Surdud, hundreds of small springs run (or used to run) where the sandstone outcrops above less porous formations. Flows are usually small – a few litres per second. Many of these springs have now dried up.

Groundwater is water that has penetrated beneath the surface into the pores and fissures of geological strata. Water typically percolates down through a zone where the pores and fissures also contain air (the unsaturated zone) until it reaches a zone where all pores and fissures are entirely filled with water (the saturated zone or aquifer). Groundwater flows underground, largely by gravity i.e., straight down or downhill along an impermeable layer. Its extraordinary prevalence and value are described in Box 4.2.²³

Most aquifers are sedimentary, i.e. the rocks are formed from erosion and deposition of other rocks. Sedimentary rocks tend to have porous characteristics that allow water to seep in and flow through. Aquifers are usually extensive, both laterally and in depth. By contrast, igneous and metamorphic rocks, which are formed under conditions of high temperature and pressure, generally have few interconnected pore spaces, and hence have only low water-bearing capacities.

Fractured rocks of all kinds can contain substantial amounts of groundwater that flows into the fissures. However, this water is only available locally, whereas water can flow down and along through sedimentary rocks and unconsolidated deposits of sand and shale, providing extensive groundwater resources with good yields. Below about 10 km depth, all rocks are impermeable. Box 4.3 describes the three main types of aquifer: unconfined, perched and artesian.

Box 4.2: Groundwater: the predominant freshwater resource of the world.

Excluding ice sheets and glaciers, groundwater accounts for 94 per cent of all fresh water in the world. Half of this is more than 800 m below ground. In total, if this water were above ground, it would cover the entire land surface of the earth to a depth of 60 m.

Groundwater is a vast regulator of the world’s hydrological balance: the average ‘residence time’ of water in aquifers is 300 years. Many aquifers contain water that is thousands of years old. Some groundwater in England and in Libya has been dated to 20,000–30,000 years ago. There is groundwater 1.4 million years old in Central Australia.

Groundwater is a high-quality source of clean water. It provides more than 90 per cent of drinking water in Germany, Austria and Demark.

Box 4.3: The three aquifer types.²⁴

Unconfined aquifer, which is usually close to the surface. There is no impermeable rock above the aquifer, whose upper boundary is the water table. The water table tends to follow the contours of the overlying ground surface. These aquifers are easily recharged by vertical percolation through permeable strata. The water level may vary a lot according to how much is recharged by water flowing down through the soil. These aquifers may have a high concentration of dissolved solids, like salts or nitrogen, picked up as the water passes through the soil.

Perched aquifer, which also may have a permeable upper bound but which sits over a local impermeable bed.

Confined or artesian aquifer, which has only a limited inflow area, and impermeable or low permeability rock both below and above it. Water typically enters from the side – for example, rain falls on the mountains and is channelled down into the aquifer – but an overlying impermeable layer prevents upward movement and creates confined conditions. A hydraulic pressure can be created such that the piezometric surface (height to which water will rise if free to do so) is above ground level, which will force water up any well tapping into the aquifer (artesian pressure).

The storage capacity of an aquifer is determined by the porosity of its host rock. Porosity ranges from above 50 per cent for loose surface soils down to 1 per cent for certain crystalline rocks (see Table 4.5). Some rocks have high specific retention, i.e. the rock has poor transmissivity or hydraulic conductivity, and the water does not flow very fast within the aquifer.

Hydraulic conductivity – the speed at which water moves through the strata – can vary from less than 1 mm a day up to 5.5 km a day (as in fissured chalk aquifers in England). Water can in practice travel huge distances in an aquifer. The Nubian Sandstone aquifer, for example, is charged by rainfall in Northern Chad and flows over 1,000 km to discharge into the Qattara and Siwa Depressions in Egypt. The 11 major aquifers identified in the Arabian Peninsula transport water charged in the mountains in the west across the peninsula to discharge into the Persian Gulf. Much of the water in these aquifers is fossil, laid down up to 35,000 years ago and travelling at a glacial pace.

As groundwater cannot be seen, and it is often deep below ground, gathering information is difficult and data give a vague picture. Knowledge of geology and hydrology is then used to guess the rest.²⁶

In Yemen, there are two main types of aquifer: solid rock aquifers and alluvial aquifers. The principal aquifers are found in the sandstones of the c Wajid and Tawilah groups (Table 1.1). The limestones and volcanics of the Amran Group and the Umm al-Ruduma formation are less permeable, but are very widespread and extremely thick, and hence contain large quantities of groundwater.

Solid rock aquifers of sandstone and limestone tend to be deep and extensive. These aquifers may contain ‘fossil water’, or may be recharged partially each year. As in all arid areas, recharge to a deeper aquifer may result from groundwater inflow from higher level groundwater basins within a complex system. Four major complexes contain most of Yemen’s groundwater reserves (Table 4.6).²⁷

• The Tehama Quaternary Aquifer, between the mountains and the Red Sea, recharged by mountain rivers. Here the quaternary deposits of the Tehama plain contain Yemen’s most productive aquifer system, extending 400 km north to south, and up to 60 km from coast to foothills. The thickness is greater than 50 m. An estimated 250 BCM of fresh groundwater is stored here, but recharge is very slow – perhaps 0.2 per cent a year – and the aquifers are sensitive to saline intrusion. Depth to groundwater is usually 10–50 m. The flow is from east to west, and the aquifers either discharge underground to the sea or into sebkhas (coastal salt flats).

• The Southern Coastal Aquifer, in the Gulf of Aden. The deltas of the southern coast (Tuban, Abyan, Ahwar) and the Maifa’ah Plain have thicknesses of quaternary deposits that are easily accessible not far below ground (50–100 m). Reserves are estimated at 70 BCM, with recharge of about 0.5 per cent each year. Again there is the risk of saline intrusion.

• Under the Ramlat as Sabatayn lies the Mukalla sandstone formation and, further east, the vast but less productive Umm al Rudhuma formation of calcareous rocks that extend over large distances in Saudi Arabia and Oman. The Mukalla sandstone aquifer averages 300–400 m thick, at depths between a few metres in parts of Ramlat as Sabatayn and 300 m towards the east, where it lies beneath the Jol. This is the largest groundwater system in Yemen, storing huge quantities of water. Transmissivity is high.

• Aquifers in the sediments of the basins of the highland plains are small but have high transmissivities and favourable recharge conditions, and are easily exploited. Here the small reserves (50 BCM) are being overdrawn, and groundwater levels are rapidly declining.²⁹

Alluvial aquifers – the commonest and smallest of Yemen’s aquifers – are formed from unconsolidated deposits of sand and gravel. The water they contain is locally very important. These aquifers, which occur throughout Yemen, contain the percolation from spate flows and spring inflows. Typically, these aquifers are unconfined and close to the surface and rarely more than a few dozen metres thick. It is cheap and easy to extract the water. The water table fluctuates rapidly, depending on seasonal inflows and discharges.

Direct recharge is very low in Yemen, as rainfall infiltrating the soil horizon rapidly evaporates (see section 4.2). Aquifer recharge is thus almost entirely from the beds of streams and wadis. In agricultural areas, particularly the highland plateaus, there is nowadays also recharge from infiltration losses from groundwater use in irrigation. Diversion of wadis for surface irrigation and damming of wadis also encourages infiltration, although there may be a net overall loss of infiltration as a part of the water that could have percolated from the wadi bed is instead lost to evaporation.³⁰

A generation ago, groundwater abstraction from wells was a minor water source. Nowadays it is far and away the dominant source of water for use – and a key element of Yemen’s economy (Figure 4.3). The reasons for the popularity of the resource are clear. It is a year-round source, providing water security for both farming and human uses. It is a reserve that smoothes out the effect of drought, because of the existence of stocks held over from previous rainy seasons. It is an ‘individual’ resource which can be developed and used by an individual without the bothersome constraints of cooperation. And under Yemeni conditions, groundwater has been cheap, with no resource charge and with some handsome subsidies. The attendant risks and stresses, and the difficulty of reducing current levels of abstraction, form a large part of the water challenge facing Yemen today (see Figure 4.3, Chapters 6 and 9 passim).

Figure 4.3 With the advent of the tubewell in the 1970s, groundwater abstraction increased very rapidly (Wadi Hadramawt). Photograph courtesy of Matthias Grueninger.

Although reserves may look ample, the economically exploitable share is only a fraction

It may seem from Table 4.6 above that there is very ample groundwater storage and that current rates of depletion can be sustained for many years. However, this is not the case in most locations, for several reasons.

First, depletion is concentrated in densely populated areas where reserves are limited (Figure 4.4). Already groundwater levels have declined in major agricultural areas in the vicinity of cities, in some cases at alarming rates. In the highlands, in particular, annual drops of 2–6 m are commonly observed, and for many locations physical exhaustion of the aquifers is already in view. Abstraction costs rise sharply with pumping depth. These physical events are having deep socioeconomic repercussions ranging from pauperization to conflict.

Figure 4.4 Changing groundwater levels in the Sana’a basin. ‘The Water Resources of Yemen: A Summary and Digest of Available Information’, Report WRAY-35, Sana’a, Republic of Yemen, March 1995.

Second, rapid depletion of groundwater storage has quality implications. In the coastal aquifers, for example, saline water may upcone under the wells or sea water may intrude. In highland confined aquifers, salination occurs as the resource dwindles.

Third, falling aquifer levels affect the hydraulic balance. The increased groundwater abstractions in Yemen are altering the regimes and conditions of the main groundwater systems profoundly. Associated springs are drying up. Over-extraction of groundwater may result in permanent compaction, especially with aquifers with a high clay content. This may result in some surface subsidence.³¹

Finally, most groundwater reserves are far from population centres or are very deep, and so uneconomic to exploit. This is particularly true of the vast Mukalla Complex, which underlies desert areas. As a result, economically usable groundwater reserves have recently been estimated at no more than 35 BCM.³²

Groundwater quality in Yemen is usually good, of low mineralization. However, as groundwater flows down, mineralization increases, and there is some salination near the coast. Nowadays, groundwater is affected by human influence. Aquifers are vulnerable to the vertical percolation of pollutants from the land surface. Reflows of groundwater used in irrigation are affecting the quality of shallow groundwater with salts and nitrates. In urban areas, groundwater pollution from effluent is occurring. In the Sana’a area, nitrates from the sewage plant, from cesspits and from agriculture are polluting groundwater.³³

Floods are hard to predict in dry lands because of the sharp, intense rainfall events and the rapid run-off. The shape of the catchment is important. A branched set of channels (‘dendritic pattern’) tends to bring high flood peaks, whereas a single channel with short side feeders (‘attenuated pattern’) should increase infiltration. The attenuated pattern is predominant in Yemen. However, infiltration is constrained by the impermeability of many soils. Certainly, floods have been a distinctive force throughout Yemeni history, and their frequency and intensity may be increasing. In October 2008, for example, Wadi Hadramawt was affected by severe flooding that caused many fatalities, destroyed 450,000 palm trees, damaged farmland, killed livestock, destroyed houses, and contaminated water wells. Wetter conditions or more intense rainfall events due to climate change are likely to increase the frequency of flooding.