two relatively disparate economic systems. However, severe shocks — including the return of approximately 850,000 Yemenis in 1990 from the Persian Gulf states, a subsequent major reduction of aid flows, and internal political disputes culminating in the 1994 civil war — hampered economic growth. Industries Agriculture and fishing Agriculture is the mainstay of Yemen's economy, generating more than 20 percent of gross domestic product (GDP) since 1990 (20.4 percent in 2005 according to the Central Bank of Yemen). Agriculture employs more than half (54.2 percent in 2003) of the working Yemeni population. However, a U.S. government estimate suggests that the sector accounted for only 13.5 percent of GDP in 2005. Numerous environmental problems hamper growth in this sector — soil erosion, sand dune encroachment, and deforestation — but the greatest problem by far is the scarcity of water. As a result of low levels of rainfall, agriculture in Yemen relies heavily on the extraction of groundwater, a resource that is being depleted. Yemen's water tables are falling by approximately two meters per year; it is estimated that Sanaa's groundwater supplies could be exhausted by 2008. The use of irrigation has made fruit and vegetables Yemen's primary cash crops. With the rise in the output of irrigated crops, the production of traditional rain-fed crops such as cereals has declined. According to the Central Bank of Yemen, the production of khat, a mildly narcotic and heavily cultivated plant that produces natural stimulants when its leaves are chewed, rose 6.7 percent in 2005 and accounted for 5.8 percent of GDP; the consumption of khat is widespread in Yemen. According to the World Bank and other economists, cultivation of this plant plays a dominant role in Yemen's agricultural economy, constituting 10 percent of GDP and employing an estimated 150,000 persons while consuming an estimated 30 percent of irrigation water and displacing land areas that could otherwise be used for exportable coffee, fruits, and vegetables. Although Yemen's extensive territorial waters and marine resources have the potential to produce 840,000 tons of fish each year, its fishing industry is relatively underdeveloped and consists largely of individual fishermen in small boats. In recent years, the government has lifted restrictions on fish exports, and production has reached one-quarter of capacity, yielding revenues valued at US$260 million in 2005. Fish and fish products constitute only 1.7 percent of Yemen's GDP but are the second largest export. In December 2005,