NCAP Yemen: Results from Aden City

Submitted by Ben Smith | published 4th Jan 2012 | last updated 13th Jan 2020
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Aden City

Aden City is the chief port of Yemen and is one of the largest natural harbors in the world with an area of about 70km² of sheltered water surrounded by Jebel Shamsan, Khoremakser, and the shore extending to the hills of Little Aden. With a population of almost 600,000 in 2004, it then represented about 3% of total population of Yemen.

Current Vulnerability

Aden City is currently highly vulnerable to water scarcity issues. As a growing urban center and a regional hub of economic activity, it is plagued by water consumption far in excess of available freshwater supplies. The most vulnerable communities are middle-income and lower-income households, particularly those in coastal areas dependent on agriculture.

Indeed, Aden City is in the midst of a water crisis. As no perennial rivers flow through Yemen, the city relies predominantly on diminishing groundwater supplies to meet its domestic, agricultural, and industrial sector demands. The only surface water available is from wadis near the Gulf of Aden that originate from rainfall in the southern escarpment of the country. The city depends on four well fields to extract groundwater, and in peri-urban communities groundwater is provided by an extensive system of wells. Groundwater sources are also located in other governorates, but accessing those supplies is a potential source of acute political tension.

Despite these constraints, existing economic development plans call for agricultural sector expansion as well as the attraction of foreign investment for industrial development. Without adequate planning, such plans are likely to be highly maladaptive due to the fact that groundwater resources, already heavily exploited, are constrained by weak natural recharge rates that depend on wadi bed inflow. While previous studies have identified sustainable extraction rates that would prevent future deterioration of water quantity and quality, by and large, these measures have not been implemented.

The water crisis that Aden City faces results from a number of interrelated causes. Overexploitation of groundwater results in an average drop of 1m in the water table per year. The domestic sector consumes 60% of available water – while commercial (12.6%), industrial (2.7%), and institutional (24.1%) uses account for the remaining 40%. There is also widespread structural collapses of old water wells, active and unplanned construction of irrigation wells, and serious water pollution associated with the increased use of fertilizers and pesticides.

Future Vulnerability

The WEAP model for the Aden area divides the system into two major sub-basins representing the Abyan and Tuban deltaic systems upstream from the city of Aden. These sub-basins comprise the ephemeral Tuban and Bana (Abyan) wadis which convey flash floods during the rainy season from uplands located outside of the model boundaries. A groundwater aquifer for each of these two sub-basins was included in the model.

Water demand was simulated for three urban centers: Aden, Lahej, and Abyan, which were given annual growth rates of 3.8%, 2.6%, and 2.4%, respectively, through the period of analysis to 2026. Additional urban infrastructure incorporated in the model included a wastewater treatment plant that serves Aden and discharges effluent to the sea, and a desalination plant with a capacity of 8.4 million m³/day, but that due to cost, does not currently supply any water to Aden. Irrigation demand for agriculture was also simulated in each of the two sub-basins and for the peri-urban area around Aden.

Results for two climate projections based on the OSU Core and UKH1 Dry climate models were incorporated in the model. The OSU Core projection predicts a 10% increase in precipitation for the area by 2050, while the UKH1 Dry data projects a 16% decrease. The OSU Core climate simulation was assumed to be the 'expected' level of climate change while the UKH1 Dry simulation captures a possible 'worst case' drier climate trajectory. These projections were superimposed on a reference climate sequence that was based on historical precipitation values over the period 1952 to 1980. A simplified hydrological simulator was used for the Aden model due to lack of data availability for constructing models similar to those used for Sadah and Sana’a. As such, temperature was not included as a variable in this model.

The changes in precipitation represented by the OSU Core and UKH1 Dry climate projections had relatively small impact on groundwater resources in the two sub-basins surrounding Aden. Under reference climate conditions, both aquifers are nearly fully depleted by 2025 (Figures 3.11a and 3.11b), although depletion of the Lahej aquifer is more rapid (2015 compared to 2019 for Abyan) because of a greater reliance on groundwater compared to wadi flow in Lahej. The UKH1 climate projection, for example, shifts the point of depletion only several months earlier in Abyan and no substantial shift is observed for Lahej.

Adaptation Strategies

Several adaptation strategies were developed and applied in the Basin EAP model for comparison study as briefly outlined below.

  • Desalinization: This strategy involves supplying Aden city with desalinated water from the Alhiswa hydropower plant at a capacity of approximately 22,000 m³/day.
  • Improved irrigation efficiency: Enhancing irrigation efficiency can be accomplished by several means: implementing drip irrigation technology; conveying irrigation water through plastic piping, and rehabilitation of traditional earth and sand irrigation channels used with spate irrigation methods to transfer surface and groundwater to fields.
  • Use of greywater for recharge: With this strategy, treated wastewater from the Aden wastewater treatment plant would be injected into the Lahej aquifer. This strategy would provide an effective method to store treated water before use, minimizing evaporative losses of the treated wastewater. It would also involve injecting treated wastewater from the Abyan and Lahej wastewater treatment plants (planned to go into operation in 2009) into the Abyan and Tuban aquifers.
  • Use of greywater for irrigation: This involves the direct use of treated wastewater from the Aden wastewater treatment plant for irrigation in the Lahej catchment area. This water would be stored in a surface container while awaiting use. It would involve the direct use of treated wastewater from the planned Abyan and Lahej treatment plants for irrigation in the Lahej and Abyan catchments.
  • Improved water distribution systems: Distribution system losses for the urban centers of Aden, Abyan, and Lahej would be reduced in this scenario. Initial loss rates of 34.2%, 33.1%, and 33.8% for Aden, Abyan, and Lahej, respectively (Komex, 2002) are decreased by 2.5% each year in each of the city starting in 2007.

The adaptation strategies simulated aid in mitigating depletion of the two aquifers on which the Aden, Lahej, and Abyan urban area and surrounding agricultural areas are dependent. The individual impacts to Abyan and Lahej groundwater storage from each of the adaptation strategies analyzed are shown in Figures 3.12 and 3.13, respectively. Note that for the Abyan aquifer (Figure 3.12a), use of improved irrigation technologies (here, a 25% saving from drip irrigation is represented) mitigates groundwater depletion much more so than all the other strategies, which shift the time of full depletion by only several months.

In contrast, three strategies provide similar levels of improvement to groundwater storage in the Lahej aquifer (Figure 3.13a) improved irrigation technology (again, the 25% savings from drip irrigation is represented) and either re-using or recharging Aden wastewater. The improvement due to re-use or recharge of Aden wastewater is substantial compared to the Reference condition where wastewater (WW) is discharged directly to the ocean. Therefore, in the 'WW Re-use (Aden WWTP)' and 'WW Recharge (Aden WWTP)' scenarios, the treated Aden wastewater becomes an additional source of water for meeting demands – a source that is not available in the reference scenario.

Implementing all strategies in parallel benefits the Abyan aquifer more than such a plan would benefit the Lahej aquifer. For the Abyan aquifer, the cumulative impact of all strategies results in a sustainable trajectory (Figure 3.12b), while for Lahej aquifer, storage continues to decline, although much less dramatically compared to the reference condition (Figure 3.13b). Note that for this comparison, only one permutation of wastewater use (recharge to the aquifer) was included because the scenarios were developed with the intent of simulating either re-use or recharge, but not both occurring simultaneously. Including both permutations would erroneously double the water savings from use of wastewater.

 

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