NCAP Yemen: Results from Sana'a Basin

Submitted by Ben Smith 4th January 2012 12:07

Sana’a Basin

The Sana'a Basin is located in the Central Highlands of Yemen and includes the capital city of Sana'a. At an elevation ranging from 2,000 to 2,200m ASL, the basin is characterized by localized rainfalls with intense precipitation events of relatively short duration. Rainfall is the source of runoff in the wadis and of ground water recharge.

Current Vulnerability

Hydrologically, the Sana’a Basin can be divided into an upper (northern) unit referred to as the Wadi Al Kharid Hydrological Unit and a lower (southern) unit referred to as the Musyareka Hydrological Unit. On the basis of surface water drainage systems and topography, a total of 22 distinct sub-basins have been identified for planning purposes, as shown in Figure 3.5a. There are six major aquifers in the Sana’a Basin (Figure 3.5b): Northwestern, Northeastern, Central Plains, Eastern, Southwestern, and Southern. The urban area surrounding Sana’a draws water from the Central Plains aquifer.

Water consumption in urban areas has increased from 57 million m³ to 70 million m³ between 1995 and 2000, which implies an average annual growth rate of 4.2%. In rural areas, beneficiaries of water supply projects have grown from 6.8 million to 7.7 million over the same period, implying an average annual growth rate of 2.5%. It is important to note that current water supply sustains probably no more than half the amount demanded, as there is substantial unmet demand in the region.

Trends in water consumption have put enormous strain on the basin’s limited groundwater resources. Indeed, the Sana’a basin experienced dramatic declines in its groundwater table in recent years. The decline in groundwater is partially due to technological advancement. Well have been dug deeper and deeper over the years, with most being more than 400m deep today.

Future Vulnerability

To assess future vulnerability of groundwater resources using a WEAP process, groundwater in the basin was divided into five major aquifer systems according to a characterization conducted by WEC (2001). These comprised the Central Plain, Southwestern, Southeastern, Eastern, and Western aquifers. Modeling the aquifer systems in this way made it possible to represent differences in groundwater storage and availability across the basin.

A number of assumptions were integrated into the reference scenario. Population growth in Sana’a city was assumed to continue at high rates, 5% per year through to the end of the planning period in 2026. Agriculture in sub-basins 9 and 16 (surrounding Sana’a city) was assumed to continue recent patterns of a decline of about 3% per year, and a 1% per year decline in sub-basins south of Sana’a. On the other hand, agriculture in the sub-basins north of Sana’a was assumed to increase by about 3% annually, consistent with recent patterns and government plans. In addition, the treatment capacity of the Sana’a waste water treatment plant (WWTP) was assumed to increase to 0.1 million m3/day in 2012, consistent with expansion plans.

To gauge the impact of climate change on water resources in the Sana’a Basin, two climate change scenarios were developed to represent possible changes in precipitation and temperature; the OSU Core and the UKH1 Dry scenarios . Compared to the reference scenario, basin-wide annual precipitation under the OSU Core scenario increases by approximately 17 million m³, or 2%, by 2025.In contrast, annual precipitation decreases by as much as 73 million m³ (approximately 10% of the reference scenario value) during the same period for the UKH1 Dry scenario. Temperature increases, relative to the reference, in both scenarios by an average of between 1.0 and 1.3°C for the OSU Core and UKH1 Dry scenarios, respectively.

The effect of climate change on groundwater depletion in the Central Plain aquifer, for example, is also very small; groundwater is depleted by the year 2017 under all of the climate trajectories explored in this study. Full groundwater depletion also occurs in the Eastern and Northwestern aquifers while the remaining three aquifers in the Sana’a Basin (i.e. Northeastern, Southwestern, and Southern aquifers) are, in comparison, not particularly vulnerable to projected future water supply/demand conditions as they show relatively stable storage capacities over time.

Without the identification and implementation of suitable adaptation measures – and assuming patterns of growth described above – the key finding of the analysis is that supply/demand patterns are the primary drivers of vulnerability. Climate change exacerbates this vulnerability, rendering water scarcity issues more acute, particularly regarding the need for additional irrigation.

Adaptation Strategies

Several adaptation strategies were developed and explored in the WEAP model for the basin, as briefly outlined below. It is important to note that not all of these options were simulated uniformly across all the sub-basins or aquifers. Several strategies were more localized in nature, informed by both physical and strategic considerations (e.g. improving water distribution systems is relevant for the urban area of Sana’a).

  • Drip irrigation: Improve irrigation efficiency by introducing drip irrigation techniques.
  • Improved indigenous methods for wadi flow use/infiltration: Simulate the impact of increased small dam construction as a means to improve wadi flow infiltration and decrease losses to evaporation. A 20% increase in the infiltration rate of wadi flow is implemented in 2010 in each of the major wadis in the basin.
  • Alternative crop production: Shift from qat production to a crop less water intensive and one that can enhance local food security. A 3% per year decrease in the areas cultivated with qat by replacing it with wheat was implemented starting in 2008.
  • Improved water distribution systems: Implement measures to reduce the high losses in the municipal water distribution system and introduce water saving fixtures on the demand side in sub-basin 16. This strategy is modeled in the WEAP by reducing the annual water use rate in Sana’a by 1% per year starting in 2008 and reducing losses in the distribution system from an initial value of 30% to 10% over the planning period.
  • Promotion of lower population growth in Sana’a city: The population growth rate in this scenario is simulated as 2%, compared to 5% in the reference scenario.
  • Use of greywater: Use wastewater treatment plant (WWTP) effluent for agricultural irrigation requirements in sub-basin 9. In this scenario, treated wastewater is applied to irrigation areas rather than letting the water flow into the Al Khared wadi.

Under reference conditions, the groundwater situation appears sustainable, despite a drastic decline, when considering the basin aquifer storage as a whole, which levels off at approximately 65% of total groundwater storage at year 2020 and beyond (i.e. about 3.4 million m³ in 2020 and beyond compared to about 5.2 million m³ in 2007). However, when results for each of the six aquifers are represented individually, the disparities among trends in groundwater storage become apparent and pose major adaptation policy implications. Whereas groundwater storage in the Northeastern, Southeastern and Southwestern aquifers appears to be sustainable under the conditions simulated in the reference scenario, groundwater drops precipitously and becomes fully depleted in the Central Plains, Eastern and Northwestern aquifers over the time period studied.

For these three vulnerable aquifers, the adaptation strategies identified above were modeled individually to assess their potential role in stabilizing water demand. The adaptation options implemented independently fail to substantially mitigate the calamitous nature of future water/supply conditions in the Central Plain aquifer within the study time period. Only when the strategies are superimposed (i.e. simulated together) do the aggregate water savings enable groundwater decline to level off. Note though that with all six strategies in place, groundwater storage in the Central Plains aquifer still continues to decline with time, albeit far more gradually. This indicates that additional measures would need to be implemented in order to produce a flat, or sustainable, groundwater storage trajectory.

The effect of implementing the adaptation strategies on unmet demand shows clear differences regarding the effectiveness of individual options. Unmet demand across the entire basin diminishes from the implementation of each strategy. Those strategies that are implemented in all of the sub-basins (e.g. the introduction of irrigation efficiency and, promoting improved indigenous wadi flow infiltration) rather than targeted on specific sub-basins (e.g. improving urban efficiency in sub-basin 16 and using WWTP effluent for irrigation in sub-basin 9) provide greater savings of total groundwater reserves.

 

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