NCAP Yemen: Results from Sadah Basin

Submitted by Ben Smith | published 4th Jan 2012 | last updated 13th Jan 2020
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Sadah Basin

An intermountain basin, the Sadah Basin is situated in Yemen’s western highlands about 250km north of Sana’a as shown in Figure 3.1 below. The basin is characterized by sparse rainfall and high evapotranspiration rates. The plain is surrounded by mountains reaching 2,750m above sea level (ASL). Elevations in the plain itself range from 1,840 to 2,050m ASL, with gentle gradients towards the southeast where water collects in the Marwan Wadi, which flows northerly into the Najran Wadi. There are no permanent or seasonal streams and only after heavy rains will surface water runoff discharge into the Marwan Wadi. As such, there are no major flood risk areas.

Current Vulnerability

Understanding the current vulnerability of water resources in the Sadah Basin area relied on input from stakeholder consultations and existing studies. Stakeholders consulted included farmers, NGO representatives, and officials in key government offices. The information collected helped to identify causes of water use inefficiency, how people were responding to growing scarcity, and potential obstacles to adopting alternative water management practices. The primary finding of these studies was that the high current vulnerability of Sadah Basin water resources to climate variability and change, according to stakeholder testimony, is largely due to the current rapid depletion of groundwater, low efficiency of water use, low coverage of water and sanitation services.

While there were no specific studies on the vulnerability of water resources to climate change in the Sadah Basin, the YOMINCO/TNO (1983), the DHV (1992) and Techniplan/NWRA (2002) studies addressed the issue indirectly. The YOMINCO/TNO study mainly investigated water resources through geophysical surveys, a well inventory and aquifer tests. The DHV study updated the well inventory and investigated other issues related to water management, such as the socioeconomic context. The Techniplan/NWRA study dealt with remote sensing aspects of the Sadah Basin and illustrated the dominant aquifers and aquicludes in the basin and outlined the most significant flow directions.

Further groundwater depletion poses large economic risks to farmers. Farmers are concerned that not only are existing wells’ yields continuously deteriorating, but water quality is also declining. Researchers found high rates of salinity and pollution from improper waste management were affecting the quality of well water.

Simultaneously with the above trends, groundwater recharge has declined. In 1983, net aquifer recharge was estimated at 10 million m³ by the YOMINCO/TNO study (1983). Between 1983 and 2001, total groundwater extraction grew from 45 to 90 million m³/year while net aquifer recharge dropped to just 7 million m³/year, or about a 30% decline from previous levels. By the end of 1998, the active zone of the aquifer had been exhausted.

In principle, Yemen’s national water management plan is 'basin co-management' whereby stakeholders and state institutions are considered partners in managing water resources at the catchments level. However, in practice the current Sadah Basin Committee is widely acknowledged to be ineffective. Attempts to establish more community-based water management organizations have been consistently constrained by limitations in the National Water Resource Authority’s resources and on-site presence. Regarding current coping strategies to deal with water scarcity, some water harvesting does occur for irrigation purposes. Compared to groundwater pumping, however, water harvesting is still a minor tool used by farmers. Moreover, there are no wastewater treatment plants, therefore reuse of wastewater (i.e. greywater) is not an option. In any event, overall stakeholder sentiment is critical of the notion of wastewater reuse as residents remain confused and skeptical of the potential benefits from a plant. This negative perception is related to problems experienced in other cities, e.g. the Sana’a wastewater treatment plant which most associate with pollution and odors.

Future Vulnerability

There are seven major hydrological sub-basins that have been identified using diagrams obtained from provincial reports. These sub-basins include Nushur, Razamat, A’kwan, Dammaj, A’lat and A’yn, Sabar and As Sa’id, and al Haniyah and A’wayrah.

Modeling of water supply accounted for groundwater and surface water resources. Groundwater for the entire basin was assumed to be 3,728 million m³ of stored water based on available statistics for the basin. Four existing surface storage structures (i.e. dams to capture flash flood waters) having a combined annual capacity of 1.7 million m³ were also modeled (i.e. dams on the Dammaj, the Sabar, the Nushur and the Marwan wadis).

The reference scenario repeated historical climate data for the Sadah Basin through to 2026. Two climate change scenarios simulated the magnitudes of changes in precipitation and temperature; the OSU Core and the UKH1 dry scenarios . The UKH1 serves as a worst case scenario in which annual precipitation decreases by up to 32% relative to the 1960 to 1990 baseline and that temperature would increase by up to about 2.2°C relative to the historical baseline by 2050. These assumptions were based on previous climate modeling work undertaken during the earlier NCCSAP program.

Without the identification and implementation of suitable adaptation measures, and assuming patterns of growth described above, the key finding of the Scenario analysis is that the aquifers underlying the Sadah Basin will be completely depleted by around 2024, essentially regardless of climate change. The predicted timing of aquifer depletion is roughly consistent with earlier studies by Al-Sakkaf (1996) which showed full depletion by 2032 and Tecniplan (2006) which showed full depletion by 2030. It is against this alarming conclusion that stakeholders and the research team consulted about potential strategies to mitigate or avoid this calamitous outcome.

Adaptation Strategies

In consultation with stakeholders in the region, three Adaptation Scenarios were developed to explore ways to reduce future vulnerability, as described below:

  • Improved irrigation efficiency: This adaptation scenario considered the introduction and aggressive penetration of drip irrigation techniques that dramatically increase irrigation efficiency.
  • Increased storage capacity: In this adaptation strategy scenario, five new dams are added to the wadis starting in 2010, each with a 0.5 million m³ storage capacity. These dams would be coupled with rain harvesting technologies to enhance groundwater recharge and increase rainfall runoff collection during dry months.
  • Greywater use: This scenario focuses on the construction and operation of a wastewater treatment plant with a capacity of 50,000 m³/day for Sadah city. The effluent from this plant is available for use as agricultural irrigation in the Alat and A’yn sub-basin as well as for reuse in the city beginning in 2008.

A key finding of the Adaptation Scenario analysis is that, taken in isolation, none of the options is able to delay the depletion of aquifers underlying the Sadah Basin by more than a few years. The drip irrigation scenario avoids total groundwater depletion by only an additional two years relative to reference conditions; building new dams delays depletion by only one year; and the greywater use option has a negligible impact (due to the limited volume of water produced and stakeholders benefited). The failure of these options to forestall the ruin of the region’s productive domestic and agricultural sectors merely underlines the urgency to consider multiple concurrent and/or more intensive options.

The adaptation strategies above fed into an MCA process in which the results were discussed with stakeholders in an effort to prioritize adaptation options. Of the initial three strategies, only the first two were prioritized as the use of treated wastewater was viewed as too limited in its ability to benefit a large section of the population of the Sadah basin. Given the meager impact of each option applied individually, stakeholders advised the implementation of all three options concurrently. The additive effect of the adaptation strategies includes the combined effect of grey water, drip irrigation, and new dams. It is important to note that even this aggressive adaptation scenario is not able to stabilize annual groundwater demand relative to annual groundwater supply, and primarily serves to delay the inevitable groundwater depletion by only a few years.

These findings illustrate that no matter the degree to which the capacities of regional water sector agencies are enhanced, or the rigor by which water demand mandates are monitored and enforced, it is too late for the region to achieve a sustainable water use trajectory through incremental improvements. That this situation exists regardless of climate change highlights the nature of the sustainable development crisis at hand in the region.

Other more intensive, more politically unpalatable strategies are likely to need exploring, and soon. Discussions of the Sadah Basin Committee and beyond would be more effective if all parties involved made a concerted effort to work together and avoid conflict, rather than acting individually.


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